Ding, x et al 2015 higher rates of manure application lead to greater accumulation of both fungal an

Soil Biology & Biochemistry 84 (2015) 137e146

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Higher rates of manure application lead to greater accumulation of both fungal and bacterial residues in macroaggregates of a clay soil Xueli Ding a, b, Chao Liang c, d, **, Bin Zhang a, Yaru Yuan e, Xiaozeng Han b, * a

College of Land and Environment, Shenyang Agricultural University, Shenyang 110866, PR China Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Harbin 150081, PR China c State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, PR China d DOE Great Lakes Bioenergy Research Center, University of Wisconsin, Madison 53706, USA e College of Geographical Science, Harbin Normal University, Harbin 150025, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 October 2014 Received in revised form 9 February 2015 Accepted 10 February 2015 Available online 3 March 2015

Microbial residues represent a significant soil organic matter pool and participate in soil aggregation. The addition of organic manure is known to modify soil aggregation and strongly influence soil microbial residues. How manure application influences the spatial distribution of microbial residues in soil aggregates is largely unknown. This study attempts to determine the effect of manure application at various rates on the content and distribution of microbial residues among aggregates of different sizes. We used a long-term manure application experiment in a Mollisol in northeastern China, where manure has been applied since 2001 at rates of 0, 7.5, 15, and 22.5 Mg ha 1 yr 1 (dry weight). The abundance of microbial residues was indicated by amino sugar analysis. Glucosamine and muramic acid were used as biomarkers for fungal and bacterial residues, respectively. Amino sugars were examined within four aggregate fractions: large macroaggregate (>2000 mm), small macroaggregate (250e2000 mm), large microaggregate (53e250 mm) and small microaggregate (<53 mm). Application of manure at 15 and 22.5 Mg ha 1 yr 1 provided significantly higher proportions of macroaggregates and mean weight diameter (MWD) than non-manure treatment and manure applied at 7.5 Mg ha 1 yr 1. Manure application, especially at higher rates, significantly stimulated the accumulation of total amino sugars in both macroaggregates and large microaggregates and more amino sugars were found in >250 mm macroaggregates compared with microaggregates. However, effects of manure application rates on amino sugar accumulation in larger aggregates were limited when manure rate was increased from 15 to 22.5 Mg ha 1 yr 1. The response of fungal- and bacterial-derived amino sugars to manure application rates differed among aggregate fractions, i.e., glucosamine associated with macroaggregates increased more than that of microaggregates, whereas the enhancement of muramic acid was prominent in both macroaggregates and large microaggregates. The mass proportions of macroaggregates and MWD showed significant positive correlations with amino sugar contents, indicating that these microbial residues are involved in the formation and stabilization of aggregates. Manure applications greatly increased the contribution of microbial residues to soil organic C (SOC) in small macroaggregates and large microaggregates (P 0.05). We conclude that higher manure input may promote soil aggregation and higher SOC storage, which is closely related to a greater microbial residues-mediated improvement of soil aggregate stability. Our results also suggest that measurement of amino sugar content is a useful approach to assess fungal and bacterial contributions to soil aggregation. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Manure Aggregate Soil structure Amino sugar Mollisols

1. Introduction * Corresponding author. Tel.: þ86 451 86601048; fax: þ86 451 86603736. ** Corresponding author. State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, PR China.

E-mail addresses: cliang823@gmail.com (C. Liang), xzhan@neigaehrb.ac.cn (X. Han). http://dx.doi.org/10.1016/j.soilbio.2015.02.015 0038-0717/© 2015 Elsevier Ltd. All rights reserved.

Sequestration of carbon (C) in soil is critical for agriculture and the environment, in particularly sustainability of agroecosystem and food security (Lal, 2004). Soil C dynamics are closely related to catabolic and anabolic activities of microorganisms (Liang et al.,

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2011; Schimel and Schaeffer, 2012). The microbial contribution to soil C pool is directly related to microbial community dynamics and the balance between production and degradation of microbial products (Six et al., 2006). The great relevance of microbial residues (i.e., necromass) within soil organic matter (SOM) has been increasingly recognized within the last decade (Miltner et al., 2012).

Reports indicate that microbial residues represent a significant source of stable C pool and may play a greater role in long-term C sequestration in soils than traditionally believed (Simpson et al., 2007; Liang et al., 2011). Increasing the potential for agricultural soils to sequester C, therefore, requires a better understanding of contribution of microbial necromass to soil organic C (SOC).

Manure application effects on the amount and size of waterstable aggregates and organic matter associated with aggregates have been reported by Haynes and Naidu (1998) and other researchers (Aoyama et al., 1999; Mikha and Rice, 2004; Mellek et al., 2010). Soil aggregates governs the spatial heterogeneity of soil physical and chemical properties, and consequently, the heterogeneous distribution of microorganisms and their activity among aggregates of different sizes (Gupta and Germida, 1988; Schutter and Dick, 2002). Studies regarding the effects of organic matter input (manure or crop residues) on soil microbes mainly focus on changes in the biomass and composition of microbial communities (Kong et al., 2011; Le Guillou et al., 2012; Hurisso et al., 2013). The effects of manure application on microbial residues, particularly their distribution in aggregates, have received very limited attention (Simpson et al., 2004; Ding and Han, 2014). Nevertheless, it was suggested that soil microbial residues may help form or stabilize soil aggregates and their effects on soil aggregates may be more persistent than living biomass (Tisdall and Oades, 1982; Chantigny et al., 1997). In turn, the degree to which microbial residues accumulate in soil depends largely on the extent of physical protection of the soil aggregate structure (Six et al., 2006). Therefore, it is important to isolate the microbial residues from soil and determine their distribution within soil aggregate structure. This information will improve our mechanistic understanding of the impacts of agricultural management practices on SOC storage and turnover (Simpson et al., 2004; Six et al., 2006), and further predict the sustainability of particular crop management systems (Liang et al., 2013).

The dynamics of microbial residues and their contribution to SOM can be indicated by soil amino sugar analysis (Zhang et al., 1999; Amelung, 2001). Previous studies have shown that amino sugars are rather stable against fluctuations in living microbial biomass and primarily occur in dead microbial cells (Chantigny et al., 1997; Guggenberger et al., 1999a; Glaser et al., 2004).

Amino sugars also serve as a time-integrated biomarker to indicate microbial community structure (Glaser et al., 2004). Muramic acid is uniquely synthesized by bacteria, whereas glucosamine predominantly originates from fungal cell walls (Parsons, 1981; Amelung, 2001; Appuhn and Joergensen, 2006). Galactosamine constitutes a significant fraction of the total amino sugar pool (Glaser et al., 2004), but its origin in soil is still debated (Amelung, 2001; Engelking et al., 2007). Glucosamine and muramic acid has been used to differentiate between fungal and bacterial contributions to soil aggregation (Chantigny et al., 1997). Simpson et al.

(2004) observed a preferential accumulation of fungal-derived amino sugars in macroaggregates under no-tilled versus conventional-tilled soils. Manure application can enhance soil aggregation and provide a protective mechanism for organic matter even in annually tilled systems (Aoyama and Kumakura, 2001).

Although some knowledge exists regarding the effects of manure application on fungal and bacterial residues in bulk soils (Scheller and Joergensen, 2008; Joergensen et al., 2010; Ding et al., 2013), the information on the long-term manure effects on content and distribution of amino sugars in aggregates and association of these

microbial residues with aggregate stability is very scare. Recently, we found that long-term manure application (15 Mg ha 1 yr 1) combined with chemical fertilizer significantly stimulated the accumulation of total amino sugars in all aggregate-size fractions as compared to chemical fertilization in Mollisols (Ding and Han, 2014). The unknown questions so far are: 1) whether higher level of manure inputs could further lead to a corresponding increase in microbial residues within all soil aggregate size classes? 2) how fungal and bacterial residues along aggregates of different size respond to higher levels of manure input?

The objective of this study was to investigate the influence of 11 years of continuous manure addition under a broad range of input rates (from 0 to 22.5 Mg ha 1 yr 1) on distribution of microbial cell wall residues within the aggregate structure of a clay soil in China.

Our hypotheses were: 1) amino sugar concentrations in all aggregate-size fractions should be significantly higher in soils received higher manures versus those in non- or lower manure additions, with a greater magnitude of responses in macroaggregate-associated amino sugars, and 2) enhancement dynamics of fungal and bacterial residues in macro- and microaggregates with higher manure input would differ and the both ultimately approach to a saturation capacity.

2. Materials and methods 2.1. Study site The study site is located at the National Observation Station of Hailun Agro-ecology System (47 260 N, 126 380 E) in Heilongjiang province, China. The area experiences a typical temperate continental monsoon climate. The mean annual air temperature is 1.5 C. The mean annual precipitation is 550 mm, with approximately 65% occurring from June to August. The soils in this site are classified as Udolls according to the USDA Soil Taxonomy (Soil Survey Staff, 2010) and the respective WRB soil types are Phaeozems (World Reference Base for Soil Resources, 2006). The surface soils (0e20 cm) had an average texture of 258 g sand kg 1, 332 g silt kg 1, and 410 g clay kg 1.

2.2. Experimental setup The field experiment was initiated in the fall of 2001 using a randomized complete block design with three replicates and five treatments, which resulted in a total of 15 plots. Each plot was 12 m long and 5.6 m wide. The treatments selected for the present study were: (1) chemical fertilization with no manure application; (2) chemical fertilization with manure applied at 7.5 Mg ha 1 yr 1; (3) chemical fertilization with manure applied at 15 Mg ha 1 yr 1; and (4) chemical fertilization with manure applied at 22.5 Mg ha 1 yr 1.

Urea and ammonium hydrogen phosphate (30 kg N ha 1 and 36 kg P ha 1 for soybean, 150 kg N ha 1 and 33 kg P ha 1 for maize, and 75 kg N ha 1 and 33 kg P ha 1 for wheat) were applied as basal fertilizers. Potassium sulfate was used as K fertilizer (30 kg K ha 1 for all crops). Chemical fertilizers were all applied in one time as basal fertilizer. The manure was collected from an open, commercial pig feedlot and stored for 1 year before application. Although all the manure came from the same feedlot, there were some variations in manure characteristics among years. The moisture content, pH, organic C, and nutrient composition of the manure applied during the last 3 years were reported in Table S1. On average, the total C input in the form of manure was equivalent to 2467, 4935 and 7402 kg C ha 1 yr 1 at rates of 7.5, 15, and 22.5 Mg ha 1 yr 1, respectively, the amounts of added nitrogen (N) were 182.8, 365.5 and 548.3 kg N ha 1 yr 1, respectively and the amounts of added phosphorus (P) were 19.6, 39.2, 58.8 kg P ha 1 yr 1, respectively.

Each year manure was applied in one time by manually spreading

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Table 1 Selected soil characteristics under fertilization treatments with manure applied at different rates.

Treatmenta

Bulk density (g cm 3)

0 7.5 15 22.5

1.03 0.97 0.93 0.96

± ± ± ±

0.02 0.02 0.04 0.03

Soil organic C (g kg 1)

pH (H2O)

ab b b b

5.84 5.94 6.03 6.13

± ± ± ±

0.03 0.02 0.02 0.04

d c b a

26.4 28.8 29.4 30.2

± ± ± ±

0.37 0.22 0.51 0.43

c b ab a

Total N (g kg 1) 2.08 2.20 2.34 2.61

± ± ± ±

0.05 0.07 0.09 0.11

c b b a

Total P (g kg 1) 0.80 1.07 1.01 1.10

± ± ± ±

0.03 0.04 0.07 0.05

b a a a

Microbial biomass C (mg C kg 1 soil) 280 294 330 408

± ± ± ±

8.15 12.1 16.9 25.1

c c b a

Total amino sugars (mg kg 1) 1893 1908 2029 2058

± ± ± ±

35.8 27.1 5.83 45.0

b b a a

a 0, chemical fertilizer alone; 7.5, chemical fertilizer plus organic manure added at 7.5 Mg ha 1 yr 1; 15, chemical fertilizer plus organic manure added at 15 Mg ha 1 yr 1; 22.5, chemical fertilizer plus organic manure added at 22.5 Mg ha 1 yr 1. b Numbers within each column followed by different letters indicate significant differences among treatments at P 0.05 level.

on the soil surface followed by immediate incorporation into the soil by plowing before sowing. The cropping system was a 3-year maize (Zea mays L.)-soybean (Glycine max L.)-wheat (Triticum aestium L.) rotation. Maize and soybean were sowed in early May and harvested in early October and wheat was sowed in late April and harvest in late July. All above-ground crop residues were removed from the field plots after crop harvest. The plots were subjected to yearly conventional tillage, which involved autumn moldboard plowing to a depth of 20 cm, spring disking before planting, and harrowing during crop growing season.

2.3. Soil sampling Soil samples were collected from the plots with no manure added and manure applied at 7.5, 15, and 22.5 Mg ha 1 yr 1 on 16 October 2012 after soybean harvest. Eight randomized soil cores (3.4 cm in diameter) were taken from the 0e20 cm depth of each plot with a stainless steel soil probe. The soil cores from the same plot were placed in a clean plastic bucket and mixed thoroughly to form a composite sample. Composite samples were transferred immediately into polyethylene bags and kept in cold storage boxes for about 4 h until delivered to the laboratory. Once in the laboratory, all visible roots and plant fragments were removed manually from the soil samples. The field-moist soil samples were sieved to pass through an 8-mm sieve by gently breaking soil clods along natural breaking points. One part of the field-moist soil samples was passed through a 2-mm sieve and stored at 4 C for analysis of microbial biomass. The other part was air-dried and stored at room temperature for fractionation of soil aggregates. Air-dried subsamples were ground to pass through a 2-mm sieve for soil pH analysis and a 0.25-mm sieve for soil total C and nitrogen (N) determination.

2.4. Aggregate size fractionation Four classes of water-stable aggregates were isolated using a wet-sieving procedure: large macroaggregates (>2000 mm), small

macroaggregates (250e2000 mm), large microaggregates (53e250 mm), and small microaggregates (<53 mm). Briefly, 100 g of air-dried soils were spread on the top of three-tiered, nested sieves of progressively finer mesh (2000, 250, and 53 mm) and rewetted softly with a fine spray bottle. The rewetted samples were then submerged in deionized water for 5 min, after which different aggregate-size fractions were separated by gently moving the sieves up and down 50 times over a period of 2 min. The isolated aggregate fractions were backwashed into aluminum pans for drying. The soil particles and aggregates which passed through the 53-mm sieve were centrifuged (2576 g) to remove water. All aggregate fractions were oven-dried at 50 C and weighed. The recovery of soil mass after fractionation was 98% on average. The mean weight diameter (MWD) of water-stable aggregates was calculated according to Kemper and Rosenau (1986).

2.5. Soil analysis The total C and N contents in bulk soils and aggregate fractions were quantified by dry combustion with a VarioEL CHN elemental analyzer (Heraeus Elementar Vario EL, Hanau, Germany). Since the studied soils were free of carbonate, the total C content was equivalent to the soil organic C (SOC) content. Soil microbial biomass C in bulk soils was determined by the chloroform fumigation-extraction method (Vance et al., 1987). The organic C in the K2SO4 extracts was measured as CO2 by infrared absorption after combustion at 850 C with a TOC analyzer (Analytik Jena AG, Germany). Microbial biomass C was calculated as the difference in C concentrations between the fumigated and unfumigated samples with an efficiency constant of 0.45 (Wu et al., 1990). Soil pH was determined in a 1:2.5 soil/water suspension. Bulk density determined using a known volume steel cylinder (5 cm inner diameter and 20 cm height) driven into 0e20 cm soil layer. Soil cores were oven-dried at 105 C for 48 h. The bulk density was calculated by dividing the weight of the oven-dried soil by the volume of the soil.

Selected characteristics of bulk soils under different manure treatments were listed in Table 1.

Table 2 Mass proportion and organic C content of water-stable aggregate fractions as well as mean weight diameter (MWD) under fertilization treatments with manure applied at different rates.

Treatmenta

Mass proportion (%)

0 7.5 15 22.5

8.08 8.28 9.11 8.99

>2000 mm ± ± ± ±

0.50 0.19 0.27 0.12

250e2000 mm bb b a a

65.6 66.4 70.1 72.8

Organic C content (g kg 1 fraction)

MWD

± ± ± ±

1.93 1.80 2.10 3.04

c bc ab a

53e250 mm 14.5 14.1 11.9 11.0

± ± ± ±

1.27 1.35 1.15 1.06

<53 mm a a b b

5.12 5.18 4.83 4.95

± ± ± ±

0.91 0.63 0.69 1.20

>2000 mm a a a a

1.04 1.06 1.13 1.15

± ± ± ±

0.03 0.01 0.02 0.03

b b a a

33.6 37.2 42.4 45.7

± ± ± ±

0.53 1.57 0.83 1.64

250e2000 mm d c b a

29.8 30.9 32.8 33.4

± ± ± ±

0.12 0.80 1.07 1.44

c b ab a

53e250 mm 28.7 29.2 30.1 30.8

± ± ± ±

0.35 1.16 0.71 0.62

<53 mm b ab a a

19.0 19.7 19.4 19.7

± ± ± ±

0.48 0.65 0.13 0.37

a a a a

a 0, chemical fertilizer alone; 7.5, chemical fertilizer plus organic manure added at 7.5 Mg ha 1 yr 1; 15, chemical fertilizer plus organic manure added at 15 Mg ha 1 yr 1; 22.5, chemical fertilizer plus organic manure added at 22.5 Mg ha 1 yr 1. b Means ± standard errors. Numbers within each column followed by different letters indicate significant differences among treatments at P 0.05 level.

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Fig. 1. Concentrations of total amino sugars (A), glucosamine (B), muramic acid (C), and galactosamine (D) in different aggregate fractions under fertilization treatments with manure applied at different rates. Error bars indicate standard errors. Different lower-case letters indicate significant differences among fertilization treatments. Different uppercase letters indicate significant differences across aggregate fractions. 0, chemical fertilizer alone; 7.5, chemical fertilizer plus organic manure added at 7.5 Mg ha 1 yr 1; 15, chemical fertilizer plus organic manure added at 15 Mg ha 1 yr 1; 22.5, chemical fertilizer plus organic manure added at 22.5 Mg ha 1 yr 1.

2.6. Amino sugar analysis Amino sugar content in bulk soils and aggregate fractions was determined according to the method of Zhang and Amelung (1996).

Briefly, finely ground soil samples (containing approximately 0.3 mg N) were mixed thoroughly with 10 mL of 6 mol L 1 HCl. To avoid oxidation of amino sugars during hydrolysis, the mixture was bubbled with N2 gas and hydrolyzed at 105 C for 8 h. The hydrolysate was filtered with Whatman 2 Qualitative Circles (125 mm diameter), dried using a rotary evaporator, and redissolved in deionized water. The pH of samples was adjusted to 6.6e6.8 with 1 mol L 1 KOH and 0.01 mol L 1 HCl. Samples were then centrifuged (1006 g, 10 min) in 50-mL glass tubes. The supernatant was freeze-dried, after which amino sugars were washed out from the residues with methanol. The recovered amino sugars were transformed into aldononitrile derivatives, which were extracted with 1.5 mL dichloromethane from the aqueous solution. Excess anhydride was removed with 1 mol L 1 HCl and deionized water. The amino sugar derivatives were redissolved in 300 mL hexane and ethyl acetate solvent (v:v ¼ 1:1) for final analysis. The amino sugar derivatives were separated on an Agilent 6890A gas chromatography (GC, Agilent Tech. Co. Ltd., USA) equipped with an HP-5 fused silica column (25 m 0.32 mm 0.25 mm) and a flame ionization detector. The concentrations of individual amino sugars were quantified based on the internal standard myo-inositol which was added prior to hydrolyzation. The concentration of total amino

sugars was calculated as the sum of the glucosamine, galactosamine, and muramic acid. Data interpretation is based on the assumption that glucosamine represents fungal cell-wall residue and muramic acid is representative of bacterial cell-wall residue in soil (Chantigny et al., 1997; Zhang et al., 1999; Amelung et al., 2001).

2.7. Statistics To test the main effects of manure treatment and aggregate-size class as well as their interactions on measurements of aggregates, general linear model analysis of variance was conducted with manure treatment and aggregate-size class as fixed factors and replicate as random factor. Tukey's honestly significant difference test (Tukey's HSD) was performed when the effects and interactions of manure treatment and aggregate-size class were significant. One-way analysis of variance was used to compare measurements of bulk soils. Difference at P 0.05 level was considered to be statistically significant. Homogeneity of variance and normality assumption were tested using Levene's Test. Simple Pearson's correlation analysis was performed to determine relationships between concentrations of amino sugars and mass proportions of aggregate-size fractions, as well as relationships between amino sugar concentrations in bulk soils and MWD. Statistical analysis was performed with the software package SPSS 13.0 for Windows (SPSS Inc., Chicago, IL). Figures were generated by Sigmaplot 10.0 (Systat Software Inc., San Jose, CA).

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Fig. 2. Proportions of total amino sugars (A), glucosamine (B), muramic acid (C), and galactosamine in soil organic C in different aggregate fractions under fertilization treatments with manure applied at different rates. Error bars indicate standard errors. Different lower-case letters indicate significant differences among fertilization treatments. Different upper-case letters indicate significant differences across aggregate fractions. 0, chemical fertilizer alone; 7.5, chemical fertilizer plus organic manure added at 7.5 Mg ha 1 yr 1; 15, chemical fertilizer plus organic manure added at 15 Mg ha 1 yr 1; 22.5, chemical fertilizer plus organic manure added at 22.5 Mg ha 1 yr 1.

3. Results 3.1. Mass proportions and organic C contents in soil aggregate fractions The mass proportions were highest for the 250e2000 mm fraction, intermediate for the 53e250 and >2000 mm fractions, and lowest for the <53 mm fraction regardless of treatments (P 0.05, Table 2). The effect of manure application rate on mass proportions of aggregates depended on aggregate size (Table 2). Higher rates of manure input (15 and 22.5 Mg ha 1 yr 1) generally increased mass proportions of the >2000 and 250e2000 mm fractions, but decreased mass proportions of the 53e250 mm fraction compared with non-manure treatment and manure applied at 7.5 Mg ha 1 yr 1 (P 0.05). There were no differences in mass proportions of aggregates between the non-manure treatment and manure applied at 7.5 Mg ha 1 yr 1, or between the treatments with manure applied at 15 and 22.5 Mg ha 1 yr 1. No manure treatment effect was observed for mass proportions of the <53 mm fraction (P > 0.05). The MWD values were significantly higher in plots with manure applied at 15 and 22.5 Mg ha 1 yr 1 than those in plots with no manure addition and manure applied at 7.5 Mg ha 1 yr 1 (P 0.05).

Manure treatment had significant effects on organic C contents in soil aggregate fractions (P 0.05, Table 2). The organic C contents

in the >2000 mm fraction were significantly increased by 36.0, 26.2, and 10.7% with manure applied at 22.5, 15, and 7.5 Mg ha 1 yr 1, respectively, compared to the non-manure treatment (P 0.05).

The 250e2000 mm fraction also had significantly higher organic C contents under manure treatments compared to the non-manure treatment (P 0.05). For the 53e250 mm fraction, the organic C contents were only significantly increased when manure was added at 15 and 22.5 Mg ha 1 yr 1 compared to the non-manure treatment (P 0.055). No significant treatment effect was observed for organic C contents in the <53 fraction (P > 0.05).

3.2. Aggregate-associated total amino sugars and their proportions in SOC The concentrations of total amino sugars were highest in the >2000 mm fraction, intermediate in the 250e2000 and 53e250 mm fractions, and lowest in the <53 mm fraction irrespective of manure treatments (P 0.05, Fig. 1A). Manure treatments generally increased the concentrations of total amino sugars in three largest aggregate fractions, but had no effect on that in the <53 mm fraction compared to the non-manure treatment (Fig. 1A). For the >2000 mm fraction, significantly higher total amino sugars were observed under manure applied at 22.5 Mg ha 1 yr 1 than manure applied at 15 and 7.5 Mg ha 1 yr 1 (P 0.05, Fig. 1A). For the 250e2000 mm fraction, manure applied at both 15 and

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22.5 Mg ha 1 yr 1 resulted in significantly higher total amino sugars than manure applied at 7.5 Mg ha 1 yr 1 (P 0.05, Fig. 1A).

The proportions of total amino sugars in SOC were found to increase with decreased aggregate size in a range from 56 to 70 g kg 1 SOC (Fig. 2A). Manure application significantly increased the proportions of total amino sugars in SOC in the 250e2000 and 53e250 mm fractions with minor exception (P 0.05, Fig. 2A).

3.3. Individual amino sugars and their proportions in SOC for different aggregate fractions Manure treatment had significant influences on glucosamine concentrations in all aggregate fractions. The glucosamine concentrations in the >2000 mm fraction increased with increasing manure application rate (Fig. 1B). For the 250e2000 mm fraction, the glucosamine concentrations were significantly higher with manure applied at 22.5 and 15 Mg ha 1 yr 1 than with manure applied at 7.5 Mg ha 1 yr 1 and the non-manure treatment (P 0.05, Fig. 1B). Significant differences in glucosamine concentrations were also found between treatment with manure at 7.5 Mg ha 1 yr 1 and the non-manure treatment in this fraction (P 0.05, Fig. 1B). The manure treatments resulted in significantly higher glucosamine concentrations in the 53e250 mm fraction compared to the non-manure treatment (P 0.05). For the <53 mm fraction, the glucosamine concentrations were highest with manure applied at 7.5 and 15 Mg ha 1 yr 1 and lowest under the non-manure treatment (Fig. 1B). When expressed on an SOC basis, the glucosamine concentrations were significantly different among manure treatments only in the 250e2000 and 53e250 mm fractions (P 0.05, Fig. 2B).

The concentrations of muramic acid were highest in the >2000 mm fraction, intermediate in the 250e2000 and 53e250 mm fractions, and lowest in the <53 mm fraction (P 0.05, Fig. 1C).

Manure applied at 7.5 Mg ha 1 yr 1 resulted in significantly higher concentrations of muramic acid than the non-manure treatment in all aggregate fractions except the >2000 mm fraction (P 0.05, Fig. 1C). Manure applied at rates of 15 and 22.5 Mg ha 1 yr 1 led to significantly higher concentrations of muramic acid than the nonmanure treatment in all aggregate fractions except the <53 mm fraction (P 0.05, Fig. 1C). The proportions of muramic acid in SOC for the 250e2000 mm fraction were generally higher under manure treatments than the non-manure treatment (Fig. 2C). The proportions of muramic acid in SOC for the <53 mm fraction were significantly lower with manure applied at 15 and 22.5 Mg ha 1 yr 1 compared with the non-manure treatment and manure applied at 7.5 Mg ha 1 yr 1 (P 0.05, Fig. 2C).

Manure treatment had significant influences on the concentrations of galactosamine in all aggregates except the <53 mm fraction (Fig. 1D). For the >2000 mm fraction, manure application significantly increased the concentrations of galactosamine only when added at a rate of 22.5 Mg ha 1 yr 1 compared to the non-manure treatment (P 0.05). For the 250e2000 and 53e250 mm fractions, Table 3 Correlation coefficients (R) between amino sugar concentrations and mass proportions in different aggregate fractions, as well as between amino sugar concentrations in bulk soils and mean weight diameter (MWD).

Amino sugars

Aggregate fractions

MWD

>2000 mm 250e2000 mm 53e250 mm <53 mm Total amino sugar Glucosamine Muramic acid Galactosamine a

0.774 0.924a 0.940a 0.310

0.919a 0.942a 0.835 0.870

0.828 0.462 0.876 0.916a

0.359 0.047 0.899a 0.390

Indicate significant correlation between variables at P 0.05 level.

0.907a 0.956a 0.377 0.216

Table 4 Enrichment factors for total and individual amino sugars in different aggregate fractions.

Treatmenta

Aggregate fractions >2000 mm

250e2000 mm

53e250 mm

<53 mm

Total amino sugars 0 1.06b 7.5 1.08 15 1.12 22.5 1.17

0.91 1.03 1.06 1.08

0.91 1.01 0.98 0.96

0.69 0.72 0.64 0.66

Glucosamine 0 7.5 15 22.5

1.12 1.15 1.19 1.20

0.97 1.00 1.04 1.04

1.01 1.02 0.95 0.91

0.67 0.71 0.66 0.61

Muramic acid 0 7.5 15 22.5

1.14 1.11 1.27 1.28

1.02 1.01 1.16 1.11

1.03 1.00 1.07 1.09

0.84 0.80 0.75 0.74

Galactosamine 0 7.5 15 22.5

1.09 1.13 1.04 1.21

0.99 1.13 1.07 1.14

0.98 1.20 1.08 1.16

0.68 0.73 0.64 0.81

a 0, chemical fertilizer alone; 7.5, chemical fertilizer plus organic manure added at 7.5 Mg ha 1 yr 1; 15, chemical fertilizer plus organic manure added at 15 Mg ha 1 yr 1; 22.5, chemical fertilizer plus organic manure added at 22.5 Mg ha 1 yr 1. b Enrichment factors were calculated as the amino sugar concentration in an aggregate-size fraction divided by that in the corresponding bulk soil.

the concentrations of galactosamine were significantly higher with manure applied at both 15 and 22.5 Mg ha 1 yr 1 than manure applied at 7.5 Mg ha 1 yr 1 (P 0.05). The proportions of galactosamine in SOC were significantly increased under manure treatments than the non-manure treatment in both 250e2000 and 53e250 mm fractions (Fig. 2D).

There were no significant differences in the ratios of glucosamine to muramic acid among the four treatments in both >2000 and 250e2000 mm fractions (data not shown). The ratios of glucosamine to muramic acid were decreased from 11.5 under nonmanure treatment to 10.5 under manure applied at 22.5 Mg ha 1 yr 1 in the 53e250 mm fraction, while generally increased from 9.6 to 10.5 in the <53 mm fraction. On average, the ratios of glucosamine to muramic acid decreased from 11.6 in the >2000 mm fraction, to 11.3 in the 250e2000 mm fraction, to 11.1 in the 53e250 mm fraction, and finally to 10.2 in the <53 mm fraction.

4. Discussion 4.1. Manure effect on amino sugar accumulation within soil aggregates The amounts and distributions of microbial residues within different aggregate fractions were strongly influenced by manure application rates (Fig. 1). A greater increase in the total amino sugar contents was observed in the macroaggregates than that in the microaggregates, and most pronounced in soils that received 22.5 Mg manure ha 1 yr 1. This result corroborates our hypothesis and suggests that microorganisms in larger aggregate fractions were most affected by agricultural management practices. Moreover, there was a tendency for increase in the proportions of macroaggregates with increase in manure rates, concomitant to a tendency for decrease in the proportions of microaggregates, reflecting an increased consolidation of microaggregates into

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macroaggregates. Together, our results suggest that higher additions of manure (15 and 22.5 Mg ha 1 yr 1) significantly support macroaggregate formation and promote total amino sugar accumulation in these aggregates over an 11-year period (Table 2; Fig. 1A). This could be linked with increased microbial activity in the larger fractions following high organic matter input and consequently the high production of metabolites, including microbial cell wall residues. Our results indicate that the microbial residues are most likely produced wherever substrate source is located in soil matrix. According to Bronick and Lal (2005), heterogeneous distribution of organic C may lead to “hot-spots” of aggregation. We proposed that higher manure inputs created the microhabitats enriched in more available substrates, which may act as ‘hot-spots’ for rapid growth and metabolite production of microbes. As a result, the substantial amounts of microbial products and other organics are accumulated and serve to stabilize the macroaggregates, so increasing the MWD (Table 2). This is in accordance with the findings of several studies that addition of recently available organic materials led to higher microbial activity and more organic compounds serving as binding agents for macroaggregate formation (Le Guillou et al., 2012; Andruschkewitsch et al., 2014). It is worthy to note that large macroaggregate was the only aggregate fraction responsible for the apparent increase in total amino sugar concentrations with elevated manure applications, indicating a low efficiency of microbial C sequestration by high manuring. The present result indicates that amino sugar pools in the other fractions (<250 mm aggregates) may have reached saturation level at the highest manure application rate. Moreover, it appears that saturation of microbially sequestered C occurs in a hierarchical fashion, i.e., the amino sugar pools in smaller aggregate fractions saturate before the larger aggregates. This is supported by the observations that the amino sugar contents of the small microaggregates did not show significant differences between the three manure application rates, and large microaggregates only increased significantly at the lowest manure application rate (7.5 Mg ha 1 yr 1), and small macroaggregates at the second lowest manure application rate (i.e., 15 Mg ha 1 yr 1) (Fig. 1A).

The location of bacteria and fungi within the pore network is a key factor in controlling their survival and activity. The larger size of fungi may well grow in macroaggregates, while bacterial populations are consistently high in small pores (i.e., microaggregates) (Simpson et al., 2004; Mummey et al., 2006; Strickland and Rousk, 2010). This provides a theoretical basis for the changes in the distribution of fungal and bacterial residues among aggregate sizes.

Similarly, we found that fungal residues were primarily concentrated in macroaggregates (Table 4). Moreover, there exists a stronger effect of manure input on fungal residues in larger aggregates, as indicated by the consistently higher amounts of fungal residues in macroaggregate fractions under the higher rates of manure application. Not only were the concentrations of fungal residues higher but also those of MurA and GalN were significantly increased in larger aggregate fractions after long term greater manure application. Interestingly, we found that bacterial residues were significantly enriched not only in microaggregates but also in macroaggregates, as shown by enrichment factor (Table 4). With assuming that living bacterial biomass was relatively dominate in microaggregate compared with fungal biomass, our results showed that the allocation of microbial residues as indicated by amino sugar may differ from living microbial community within soil aggregate structures. This could be possibly due to long residence time of microbial residues than their living producers (Amelung, 2001). Moreover, it is worthy to mention that, the present result was inconsistent with our previous study in a field experiment of the same area, in which we found that MurA was only preferentially

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enriched in microaggregates after an 18-year manure application (Ding and Han, 2014). We attribute the divergences between two studies to an unbalanced state of amino sugar accumulation in the studied soils, i.e., the dynamics of amino sugar have not reached a state of equilibrium in soil matrix after 11 years' continuously manuring. As the amount of nutrients is a main factor determining microbial accumulation of amino sugars in soils (Liang et al., 2007), and higher levels of easily decomposable substrates favored the growth of bacteria (Guggenberger et al., 1999b). A more dynamic of muramic acid can be expected in soils annually receiving high amounts of manure and consequently a dynamic distribution patterns throughout the aggregate fractions. Galactosamine accounts for more than 30% of total amino sugar content and has a relatively high stability in soil (Glaser et al., 2004); the enrichment of galactosamine in larger aggregates implies an increased contribution of microbial-derived stable C to SOM accumulation due to long-term manure application. Joergensen et al. (2010) found a close relationship between galactosamine and bacterial C and suggested that galactosamine is mainly of bacterial origin. However, in this study, we could not link its dynamics with specific microbial residue responses to manure additions, since galactosamine accumulation pattern within aggregates was different from that of glucosamine or muramic acid (Fig. 1D). Further research is still required to clarify this phenomenon.

4.2. Microbial residue contribution to soil aggregation According to Tisdall and Oades (1982), living microbial cells can produce an external layer of carbohydrates able to bind soil particles into microaggregates or microaggregates into macroaggregates. Our study demonstrates that the relatively stable cell wall constituents are also involved in the formation and stabilization of aggregates, as shown by a significant positive correlation between total or individual amino sugars and mass proportions of aggregates as well as MWD (P 0.05, Table 3). This finding provides additional evidence for the proposal that after microbial cell dies most carbohydrates would be rapidly decomposed while cell wall fragments persist longer in the soil and develop new bonds with soil particles, which help preserve aggregate cohesion and stabilization (Chantigny et al., 1997). In our study, the close relationship between glucosamine and the mass proportion of macroaggregates as well as MWD is probably resulted from the association of macroaggregates with fungal hyphal cell wall residues. As to bacterial residues, muramic acid has been reported to have a close correlation with clay content (Liang et al., 2013), because it can be stabilized by physical association with clay particles (Zhang et al., 1999).

Bacteria are believed to be responsible for the aggregation of clay particles mainly by excreting polysaccharides (Tisdall and Oades, 1982). This study highlights that, in a clay soil (mollisols in our case), bacteria play an important role in soil aggregation as indicated by significant positive correlation of muramic acid with <53 mm and >2000 mm aggregates. We postulated that bacterial amino sugars are actively involved in soil aggregation serving as binding agent when sufficient organic matter is available in soils, i.e., they firstly bound together with soil primary particles to form microaggregates which then bound into macroaggregates. Further research should be conducted to elucidate these potential relationships. Our work also suggests that measurement of glucosamine and muramic acid is a potentially useful approach to investigate fungal and bacterial contributions to soil aggregation.

The soil-microbe system is self-organized, i.e., the aggregate structure determines the niche conditions for soil microorganisms; meanwhile, microbial activity also influences soil structure (Young and Crawford, 2004). Findings from this study indicate that soil aggregation is closely related to the buildup of microbial residue

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Fig. 3. A stylized illustration of the mechanical framework showing that microbial residue storage capacity and aggregation formation in soils can be enhanced with manure inputs.

It depicts a hierarchical relationship between microbial (fungal and bacterial) residues and different aggregate-size fractions. The solid arrows show the transfer of microbialderived organic matter into different aggregate-size fractions. The width of the solid arrows and the area of the circle shapes indicate the relative transfer rate and pool magnitude, which are not sized to accurate scale. F denotes fungal residues; B denotes bacterial residues; MWD denotes mean weight diameter of water-stable aggregates; LA denotes large macroaggregates; SA denotes small macroaggregates; LI denotes large microaggregates; SI denotes small microaggregates.

pool following long-term manure treatments. It seems that there is a positive feedback loop between the formation of soil macroaggregates and the accumulation of amino sugars in soils. Organic inputs stimulate the production and accumulation of microbial residues which have positive effect on soil aggregate formation; aggregates in turn physically protect microbial cell wall residues from degradation. These two processes are intrinsically interrelated with each other. The addition of higher quantities of manure accentuates the positive interaction to this feedback and shows the possibility to continuously sequestrate C and N in the form of microbial residues in Mollisols even under annual tillage. Nevertheless, with increasing manure input, the function of this positive feedback loop weakens and ultimately the system reaches to a saturation point, i.e., soil C carrying capacity with regarding to microbial residue pool, which provides a valuable ecological reference regarding the development of mitigation strategies and effective policies for soil C stock management. The soil C carrying capacity will differ as regarding that for various aggregate fractions.

The absence of signiďŹ cant changes in amino sugar contents within the microaggregate fractions <250 mm with elevating manure rate supports those insights.

4.3. Indicative signiďŹ cance of amino sugar proportion in SOC within aggregates The importance of microbial residues to the formation of stable SOM has been recognized increasingly (Liang and Balser, 2011). The proportion of amino sugars in SOC can be used to quantify the contribution of microbial residues to SOC (Amelung et al., 2001).

However, understanding the contribution of microbial residues to SOM formation and stabilization requires a closer look at its distribution in various aggregate fractions due to their different turnover rates. Amino sugar proportions in SOC showed an increasing trend with decreasing aggregate size irrespective of manure treatments (Fig. 2), indicating an increased microbial contribution to SOM. Manure application at higher rates greatly promoted microbial contribution to SOM in small macroaggregates (250e2000 mm) and large microaggregates (53e250 mm), in which

more than eighty percent of amino sugars were located. This indicates that amount of manure application strongly inďŹ&#x201A;uenced SOM quality with respect to microbial residues. It was suggested that macroaggregates are the most dynamic (Six et al., 2000) and they are oversaturated with SOC with only a smaller amount of the added organic matter being stabilized for longer periods within macroaggregates in the soil (Andruschkewitsch et al., 2014).

Therefore, greater proportions of amino sugars in SOC in macroaggregate fraction are important for the SOM pool's capacity to function as an active nutrient sink and source reservoir. In another aspect the increased contribution of amino sugar to SOC in microaggregates may present a more stable SOM pool as microaggregate stability is higher than macroaggregate stability (Six et al., 2000). Taken together, our results indicate that the Mollisols after an 11-year period with manure at higher rates held both an active pool of relatively labile SOM and also a stable pool of relatively stable SOM in terms of microbial-derived organic matter.

Accordingly, we propose to differentiate soil amino sugars into two pools: the Stable Pool (SP) and Active Pool (AP) according to their allocations in soil aggregates. The SP is mainly located in microaggregates, which size is mainly determined by intrinsic characters of a soil; and thus it does not respond sensitively to changing nutrient conditions. However, it may play an important role in soil aggregate formation through a shift from smaller size aggregate fractions to the largest aggregate fraction with increasing organics input. On the other hand, the AP, mainly located in macroaggregates, represents a newly-built pool and has a relatively greater potential to sequester C under increased organic matter input. Our data and interpretations suggest that changes in microbial residues are mainly occurred in the AP, which play an active role in SOM cycling and turnover to some extent.

5. Conclusions This study made progress towards understanding the relationship between microbial residue accumulation and soil aggregation under an annually-tilled cropping system with high organic input.

Our results suggest that manure application at higher rates (e.g.,

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22.5 Mg ha 1 yr 1) stimulates formation and stabilization of macroaggregates in a clay soil, and this process was positively correlated with increased contents of microbial residues. Significant correlations between aggregate parameters (such as mass proportions of macroaggregates and MWD) and total or individual amino sugar contents demonstrate that the stable microbial cell wall residues are highly involved in the formation and stabilization of soil aggregates over 11 years long term. Our work implies that the accumulation of microbial residues is an important process in the studied Mollisols receiving higher manure, which led to an improved aggregation and was largely related to an increased microbial-mediated improvement of soil aggregate stability.

We synthesize the present results into a framework (Fig. 3).

Fungal and bacterial residues conjunctly but distinctly exert soil aggregate formation as evidenced by an increased ratio of glucosamine to muramic acid with increasing aggregate sizes. The rate of increase in amino sugar concentrations decreased with increasing manure application, and maintained at a certain level when it reaches the saturation point. During this process, amino sugar accumulation followed a hierarchy fashion, i.e., the saturation of microbial residues in smaller sized aggregates occurs earlier than larger ones. Consequently, additional C will only accumulate in large aggregates that have a relatively faster turnover C pool. Our work provide evidence of the occurrence of soil C carrying capacity, which is ecologically important for the SOM stock when serving as a sink or source of C.

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (41371295), Science Foundation of the Chinese Academy of Sciences (KZZD-EW-TZ-16-02), and Excellent Young Talent of Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences (DLSYQ12002). Many thanks to the entire staff at the National Observation Station of Hailun Agroecology System for their invaluable help with field management and soil sampling. We also thank two anonymous reviewers for their insightful comments on previous versions of our manuscript.

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.soilbio.2015.02.015.

References Andruschkewitsch, R., Geisseler, D., Dultz, S., Joergensen, R.G., Ludwig, B., 2014. Rate of soil-aggregate formation under different organic matter amendmentsda short-term incubation experiment. Journal of Plant Nutrient and Soil Science 177, 297e306.

Amelung, W., 2001. Methods using amino sugars as markers for microbial residues in soil. In: Lal, R., Kimble, J.M., Follett, R.F., Stewart, B.A. (Eds.), Assessment Methods for Soil Carbon. Lewis, Boca Raton, FI, pp. 233e272.

Amelung, W., Miltner, A., Zhang, X., Zech, W., 2001. Fate of microbial residues during litter decomposition as affected by minerals. Soil Science 166, 598e606.

Aoyama, M., Angers, D.A., N'Dayegamiye, A., Bissonnette, N., 1999. Protected organic matter in water-stable aggregates as affected by mineral fertilizer and manure applications. Canadian Journal of Soil Science 79, 419e425.

Aoyama, M., Kumakura, N., 2001. Quantitative and qualitative changes of organic matter in an Ando soil induced by mineral fertilizer and cattle manure application for 20 years. Soil Science and Plant Nutrition 47, 241e252.

Appuhn, A., Joergensen, R.G., 2006. Microbial colonisation of roots as a function of plant species. Soil Biology and Biochemistry 38, 1040e1051.

Bronick, C.J., Lal, R., 2005. Soil structure and management: a review. Geoderma 124, 3e22. vost, D., Ve zina, L.P., Chalifour, F.P., 1997. Soil agChantigny, M.H., Angers, D.A., Pre gregation and fungal and bacterial biomass under annual and perennial cropping systems. Soil Science Society of America Journal 61, 262e267.

145

Ding, X., Han, X., Zhang, X., Qiao, Y., Liang, Y., 2013. Continuous manuring combined with chemical fertilizer affects soil microbial residues in a Mollisol. Biology and Fertility of Soils 49, 387e393.

Ding, X., Han, X., 2014. Effects of long-term fertilization on contents and distribution of microbial residues within aggregate structures of a clay soil. Biology and Fertility of Soils 50, 549e554.

Engelking, B., Flessa, H., Joergensen, R.G., 2007. Shifts in amino sugar and ergosterol contents after addition of sucrose and cellulose to soil. Soil Biology and Biochemistry 39, 2111e2118. n, M.B., Alef, K., 2004. Amino sugars and muramic acid biomarkers Glaser, B., Turrio for soil microbial community structure analysis. Soil Biology and Biochemistry 36, 399e407.

Guggenberger, G., Frey, S.D., Six, J., Paustian, K., Elliott, E.T., 1999a. Bacterial and fungal cell-wall residues in conventional and no-tillage agroecosystems. Soil Science Society of America Journal 63, 1188e1198.

Guggenberger, G., Elliott, E.T., Frey, S.D., Six, J., Paustian, K., 1999b. Microbial contributions to the aggregation of a cultivated grassland soil amended with starch.

Soil Biology and Biochemistry 31, 407e419.

Gupta, V.V.R.S., Germida, J.J., 1988. Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation. Soil Biology and Biochemistry 20, 777e786.

Haynes, R.J., Naidu, R., 1998. Influence of lime, fertilizer and manure applications on soil organic matter content and soil physical conditions: a review. Nutrient Cycling in Agroecosystems 51, 123e127.

Hurisso, T.T., Davis, J.G., Brummer, J.E., Stromberger, M.E., Mikha, M.M., Haddix, M.L., Booher, M.R., Paul, E.A., 2013. Rapid changes in microbial biomass and aggregate size distribution in response to changes in organic matter management in grass pasture. Geoderma 193e194, 68e75. €der, P., Fließbach, A., 2010. Long-term effects of organic farming Joergensen, R.G., Ma on fungal and bacterial residues in relation to microbial energy metabolism.

Biology and Fertility of Soils 46, 303e307.

Kemper, W.D., Rosenau, R.C., 1986. Aggregate stability and size distribution. In: Klute, A. (Ed.), Methods of Soil Analysis. ASA and SSSA, Madison, WI, pp. 425e442. rdova-Kreylos, A.L., Holmes, W.E., Six, J., 2011. Microbial Kong, A.Y.Y., Scow, K.M., Co community composition and carbon cycling within soil microenvironments of conventional, low-input, and organic cropping systems. Soil Biology and Biochemistry 43, 20e30.

Lal, R., 2004. Soil carbon sequestration impacts on global climate change and food security. Science 304, 1623e1627.

Le Guillou, C., Angers, D.A., Maron, P.A., Leterme, P., Menasseri-Aubry, S., 2012.

Linking microbial community to soil water-stable aggregation during crop residue decomposition. Soil Biology and Biochemistry 50, 126e133.

Liang, C., Cheng, G., Wixon, D., Balser, T., 2011. An absorbing Markov chain approach to understanding the microbial role in soil carbon stabilization. Biogeochemistry 106, 303e309.

Liang, C., Balser, T.C., 2011. Microbial production of recalcitrant organic matter in global soils: implications for productivity and climate policy. Nature Reviews Microbiology 9, 75.

Liang, C., Duncan, D.S., Balser, T.C., Tiedje, J.M., Jackson, R.D., 2013. Soil microbial residue storage linked to soil legacy under biofuel cropping systems in southern Wisconsin, USA. Soil Biology and Biochemistry 57, 939e942.

Liang, C., Zhang, X., Rubert, K.F., Balser, T.C., 2007. Effect of plant materials on microbial transformation of amino sugars in three soil microcosms. Biology and Fertility of Soils 43, 631e639.

Mellek, J.E., Dieckow, J., da Silva, V.L., Favaretto, N., Pauletti, V., Vezzani, F.M., de Souza, J.L.M., 2010. Dairy liquid manure and no-tillage: physical and hydraulic properties and carbon stocks in a Cambisol of Southern Brazil. Soil and Tillage Research 110, 69e76.

Mikha, M.M., Rice, C.W., 2004. Tillage and manure effects on soil and aggregateassociated carbon and nitrogen. Soil Science Society of America Journal 68, 809e816. €stner, M., 2012. SOM genesis: Miltner, A., Bombach, P., Schmidt-Brücken, B., Ka microbial biomass as a significant source. Biogeochemistry 111, 41e55.

Mummey, D.L., Holben, W.E., Six, J., Stahl, P.D., 2006. Spatial stratification of soil microbial populations in diverse soils: Rubrobacteria and Gemmatimonads are abundant in water-stable microaggregate interiors while Acidobacteria are primarily associated with macroaggregates. Microbial Ecology 51, 404e411.

Parsons, J.W., 1981. Chemistry and distribution of amino sugars in soils and soil organisms. In: Paul, E.A., Ladd, J.N. (Eds.), Soil Biochemistry, vol. 5. Marcel Dekker, New York, pp. 197e227.

Scheller, E., Joergensen, R.G., 2008. Decomposition of wheat straw differing in nitrogen content in soils under conventional and organic farming management.

Journal of Plant Nutrition and Soil Science 171, 886e892.

Schimel, J., Schaeffer, S.M., 2012. Microbial control over carbon cycling in soil.

Frontiers in Microbiology 3, 348.

Schutter, M.E., Dick, R.P., 2002. Microbial community profiles and activities among aggregates of winter fallow and cover-cropped soil. Soil Science Society of America Journal 66, 142e153.

Simpson, R.T., Frey, S.D., Six, J., Thiet, R.K., 2004. Preferential accumulation of microbial carbon in aggregate structures of no-tillage soils. Soil Science Society of America Journal 68, 1249e1255.

Simpson, A.J., Simpson, M.J., Smith, E., Kelleher, B.P., 2007. Microbially derived inputs to soil organic matter: are current estimates too low? Environmental Science and Technology 41, 8070e8076.

146

X. Ding et al. / Soil Biology & Biochemistry 84 (2015) 137e146

Six, J., Elliott, E.T., Paustian, K., 2000. Soil macroaggregate turnover and microaggregate formation: a mechanism for C sequestration under no-tillage agriculture. Soil Biology and Biochemistry 32, 2099e2103.

Six, J., Frey, S.D., Thiet, R.K., Batten, K.M., 2006. Bacterial and fungal contributions to carbon sequestration in agroecosystems. Soil Science Society of America Journal 70, 555e569.

Soil Survey Staff, 2010. Keys to Soil Taxonomy, 11th ed. USDA Natural Resources Conservation Service, Washington, DC, USA. Strickland, M.S., Rousk, J., 2010. Considering fungal:bacterial dominance in soils e Methods, controls, and ecosystem implications. Soil Biology and Biochemistry 42, 1385e1395.

Tisdall, J.M., Oades, J.M., 1982. Organic matter and water-stable aggregates in soils.

Journal of Soil Science 62, 141e163.

Vance, E.D., Brookes, P.C., Jenkinson, D.S., 1987. An extraction method for measuring soil microbial biomass C. Soil Biology and Biochemistry 19, 703e707.

World Reference Base for Soil Resources, 2006. A Framework for International ClassiďŹ cation, Correlation and Communication. World Soil Resources Reports, p. 103.

Wu, J., Joergensen, R.G., Pommerening, B., Chaussod, R., Brookes, P.C., 1990. Measurement of microbial biomass C by fumigation-extraction d an automated procedure. Soil Biology and Biochemistry 22, 1167e1169.

Young, I.M., Crawford, J.W., 2004. Interactions and self-organization in the soilmicrobe complex. Science 304, 1634e1637.

Zhang, X., Amelung, W., Yuan, Y., Zech, W., Samson-Liebig, S., Brown, L., Zech, W., 1999. Land-use effects on amino sugars in particle-size fractions of an Argiudoll.

Applied Soil Ecology 11, 271e275.

Zhang, X., Amelung, W., 1996. Gas chromatographic determination of muramic acid, glucosamine, mannosamine, and galactosamine in soils. Soil Biology and Biochemistry 28, 1201e1206.