Evaluation of biochar

Water Air Soil Pollut (2010) 213:47–55 DOI 10.1007/s11270-010-0366-4

Evaluation of Biochar Effects on Nitrogen Retention and Leaching in Multi-Layered Soil Columns Ying Ding & Yu-Xue Liu & Wei-Xiang Wu & De-Zhi Shi & Min Yang & Zhe-Ke Zhong

Received: 5 July 2009 / Accepted: 17 February 2010 / Published online: 10 March 2010 # Springer Science+Business Media B.V. 2010

Abstract Biochar can play a key role in nutrient cycling, potentially affecting nitrogen retention when applied to soils. In this project, laboratory experiments were conducted to investigate the adsorption properties of bamboo charcoal (BC) and the influence of BC on nitrogen retention at different soil depths using multilayered soil columns. Results showed that BC could adsorb ammonium ion predominantly by cation exchange. Ammonium nitrogen (NH4+-N) concentrations in the leachate of the soil columns showed significant differences at different depths after ammonium chloride application to the columns depending on whether BC had been added. Addition of 0.5% BC to the surface soil layer retarded the downward transport of NH4+-N in the 70-day experiment, as indicated by measurements made during the first 7 days at 10 cm, Ying Ding and Yu-Xue Liu contribute equally to this paper.

Y. Ding : W.-X. Wu Ministry of Agriculture Key Laboratory of Non-point Source Pollution Control, Hangzhou 310029, People’s Republic of China Y. Ding : Y.-X. Liu : W.-X. Wu (*) : D.-Z. Shi : M. Yang Institute of Environmental Science and Technology, College of Environment and Resource Science, Zhejiang University, Hangzhou 310029, People’s Republic of China e-mail: weixiang@zju.edu.cn Z.-K. Zhong China National Research Center of Bamboo, Hangzhou 310012, People’s Republic of China

and later, in the experimental period at 20 cm. In addition, application of BC reduced overall cumulative losses of NH4+-N via leaching at 20 cm by 15.2%.

Data appeared to suggest that BC could be used as a potential nutrient-retaining additive in order to increase the utilization efficiency of chemical fertilizers. Nonetheless, the effect of BC addition on controlling soil nitrogen losses through leaching needs to be further assessed before large-scale applications to agricultural fields are implemented.

Keywords Bamboo charcoal . Nitrogen leaching .

Nitrogen retention . Ammonium nitrogen . Adsorption

1 Introduction Excessive application of nitrogen fertilizers to agricultural land is regarded as a major contributor to various ecological problems, such as nitrogen leaching, which may pose a threat to the quality of surface and groundwater (Thomsen et al. 1993; Fraters et al.

1998; Alva et al. 2003; Camargo and Alonso 2006; Bakhsh et al. 2007; Yu et al. 2007). In addition, nitrogen leaching has become an important limitation to improving the utilization efficiency of nitrogen in agricultural production. In order to alleviate this problem, techniques must be developed to keep applied nitrogen in the topsoil and therefore increase its utilization efficiency. Either applying slow-release fertilizers (Gentile et al. 2009) or increasing adsorption

48

sites (Lehmann et al. 2003) can meet this demand to some extent. How effective slow-release fertilizers are in reducing leaching of applied nitrogen under high leaching conditions is not well known. An alternative option could be the application of biochar to soils (Lehmann et al. 2003).

The environmental impacts of biochar use in agriculture, including its effects on global climate change and the soil ecosystem, have resulted in a growing interest in the fields of atmospheric science, geology, and environmental science in recent years (Cheng et al. 2006; Forbes et al. 2006; Liang et al. 2006; Cheng et al. 2008; Mathews 2008). Biochar, namely biomass-derived charcoal, refers to the highly aromatic substance remaining after pyrolysis and carbonification of biomass under complete or partial exclusion of oxygen, which can be produced from a wide range of biomass sources including woody materials, agricultural residues such as corncobs and crop straw, livestock manures, and other waste products. When applied to soil, biochar has the characteristics of higher stability against decomposition and excellent ability to absorb ions as compared to other forms of soil organic matter, due to its greater surface area, negative surface charge, and charge density (Liang et al.

2006; Lehmann 2007). Recent research found that biochar was of great importance in increasing soil carbon storage, improving soil fertility, as well as maintaining the balance of soil ecosystems, and it could act as a kind of soil fertilizer or amendment to increase crop yield and plant growth by supplying and retaining nutrients (Glaser et al. 2000; Major et al. 2005; Steiner et al. 2007; McHenry 2009).

Conversion of bamboo into charcoal is of great significance when considering potential raw materials for biochar production. With a 10–30% annual increase in biomass versus 2–5% for trees, bamboo can yield 20 times more timber than trees on the same land area. Bamboo can be selectively harvested and regenerates without replanting. Therefore, bamboo can replace other wood products as a higher value crop for biochar production. Bamboo charcoal (BC) has a highly microporous structure, with an adsorption efficiency about ten times higher than that of traditional wood charcoal (Hua et al. 2009). Prior to this work, the ability of BC to adsorb heavy metal ions (such as Pb2+, Cu2+, Cr3+, etc.) has been studied (Wang et al. 2008). Some authors have also reported

Water Air Soil Pollut (2010) 213:47–55

the adsorption behavior of other kinds of biochar in soils, such as woody charcoal (Oya and Iu 2002; Iyobe et al. 2004) and the biochar derived from the residues of rice straw or wheat straw (Qiu et al. 2008).

Furthermore, biochar was shown to increase the cation exchange capacity of sandy soils in laboratory experiments (Tryon 1948). According to these reports, BC may be a potential amendment in soils for nutrient retention due to its adsorption properties and can theoretically retard nitrogen-leaching losses.

However, little information is available about the effect of BC on inorganic nitrogen adsorption and cation exchange capacity in agricultural soil under leaching conditions. Therefore, the influence of BC on nitrogen retention and leaching to groundwater during application of chemical fertilizers in soils needs to be further studied.

The objectives of this study are (1) to detect the effect of BC on ammonium ion adsorption, and (2) to investigate the potential capability of BC to retain ammonium nitrogen and decrease inorganic nitrogen loss under simulated leaching conditions during the application of ammonium fertilizers to the soil. We believe that the results of this study will provide practical information to guide the exploitation of a novel amendment to improve nitrogen utilization efficiency in agricultural applications.

2 Materials and Methods 2.1 Soil and Bamboo Charcoal The experimental soil for this study was sampled from two depths, 0–20 cm and 20–40 cm, of a profile at six different sites within the Haining Agricultural Development Zone in Jiaxing City (120.8° E, 30.8° N), Zhejiang Province. The soil is classified as a permeable type with a texture of sandy silt. This soil was relatively uniform and permeable along depth.

The soil was air-dried, passed through a 2-mm nylon sieve, and mixed to get a homogeneous sample of each original soil depth to be used in the column experiments. The bamboo charcoal particles (10–20 mesh) used in this study were purchased from Lin’an Yaoshi Charcoal Production Limited Company located in Hangzhou City. The characteristics of the experimental soil and the bamboo charcoal are shown in Table 1.

Water Air Soil Pollut (2010) 213:47–55 Table 1 Characteristics of the tested soil and bamboo charcoal

49

Soil

Bamboo charcoal

Depth (cm)

0–20

20–40

Organic C (g kg−1)

23.0

11.7

Pyrolysis temperature (°C)

600

Total N (g kg−1)

3.10

2.30

pH

Total P (g kg−1)

0.68

0.42

C (%)

pH (1:1)

8.56

8.44

H (%)

2.78

8.30

N (%)

0.87

−1

CEC (cmol kg )

9.65

8.15 68.1

Clay (%)

13.7

10.4

Density (g cm−3)

Silt (%)

56.4

53.1

Specific surface area (m2 g−1)

Sand (%)

29.9

36.5

Water content (%)

2.2 Ammonium Ion Adsorption by BC In order to study the ability of BC to adsorb ammonium ions, ammonium chloride (NH4Cl) was selected as the adsorbate to obtain the adsorption isotherm in a closed system. Thirty milliliters of NH4Cl solutions with concentrations of 5, 10, 15, 20, 30, 40, 50, 70, and 100 mg L−1 NH4+ were added to 0.2 g of BC, respectively. The solutions with BC particles were then kept in a thermostatic shaker at 300 rpm and 25°C for 48 h to achieve equilibrium.

0.75 330 6.90

in the field and the soil bulk density. The top 0– 20-cm and the bottom 20–40-cm sections of the column were filled with the mixed soil subsamples from the original layers of 0–20 cm and 20–40 cm in the field, respectively. However, the soil placed in the 0–10 cm of the column was subjected to one of the following treatments: (1) no application of fertilizer (control, designated as CK), (2) ammonium chloride application alone at a rate of 400 kg N ha−1 (designated as A), and (3) ammonium chloride application at a rate of 400 kg N ha−1 with 0.5% BC (w/w) (designated as AB), with three

2.3 Multi-Layer Column Device A schematic diagram of the multi-layer soil column device is shown in Fig. 1, according to the method of Luo et al. (2003). Each column had an inner diameter of 10 cm, a height of 52 cm, and four sampling ports (including three tube sections and one tap) at different heights from the top, experimentally representing four soil depths in a profile of 10, 20, 30, and 40 cm. The three separated sections could be well joined and sealed during an experiment. Each section had a small hole drilled in the middle on the tube sidewall for extraction of soil leachate at different layers in the profile. A simple water container was used for supplying distilled water to the column to simulate leaching.

Water container

Soil profile 0 cm 10 cm 52 cm

20 cm 30 cm

2.4 Leaching Experiment A 5-cm thick, acid-washed cobblestone layer was placed at the bottom of each column for filtration of soil leachate. The <2-mm air-dried soil subsamples were carefully placed in different column sections based on the original depth of each layer

40 cm

10 cm Fig. 1 Schematic diagram of the multi-layer soil column device used in this experiment

50

2.5 Sample Analysis The leachate samples were stored in the dark at 4°C in an icebox prior to analysis. After centrifuging at 3,000 rpm for 3 min, the supernatant liquid was collected and analyzed for concentrations of NH4+-N, NO3−-N, and NO2−-N by ultraviolet and visible spectrophotometry (Yu et al. 2007). The electrical conductivity (EC) was determined by an electrical conductivity meter (Type: DDS-EC).

2.6 Statistic Analysis The results were expressed as means and standard deviations. Statistical analysis was performed using the software of SPSS for windows. Any differences between the mean values with P>0.05 were not considered statistically significant.

3 Results and Discussion 3.1 Adsorption of Ammonium Ion on BC Since plenty of pores were formed during the pyrolysis of bamboo at high temperature, BC has achieved high adsorption capacity, which is one of its important characteristics, with a large specific surface area (as shown in Table 1, the specific surface area of

the tested BC reached 330 m2 g−1 when carbonized at 600°C).

Langmuir and Freundlich models are usually used to describe the equilibrium adsorption isotherm data.

As such, our experimental data for the adsorption of NH4+ on BC was described well by the former. The linearized form of the Langmuir formula is as follow: Ce 1 1 ¼ Ce þ q0 q0 k qe where qe is the equilibrium amount of adsorbate adsorbed (milligram) per unit mass of adsorbent (gram), Ce is the equilibrium concentration of adsorbate in solution (mg L−1), and the values of q0 (mg g−1) and k (L mg−1) are the maximum adsorption capacity of adsorbent and the adsorption energy coefficient, respectively (Zheng et al. 2008; Rocha et al. 2009).

A linear relationship of the Langmuir isotherm for NH4+ adsorption on BC is shown in Fig. 2, with a good correlation coefficient (R2 =0.9975). The maximum adsorption capacity (q0) and the adsorption energy coefficient (k) calculated from the slope and the intercept of the linear regression were 0.852 mg g−1 and 0.125 L mg−1 at 25°C, respectively.

3.2 Effect of BC on NH4+-N Concentration in Leachate The temporal changes of ammonium-N (NH4+-N) concentration in the leachate of soil columns at 10-cm depth are shown in Fig. 3a. The leaching NH4+-N concentrations under treatment CK tended to vary slightly, with values ranging from 7.7 to 12.6 mg L−1.

120 100 Ce/qe (g L-1)

replicates for each treatment. The leaching experiment was conducted at 25±2°C with a relative humidity of 65% in an artificial greenhouse in Zhejiang University.

Before starting the nitrogen leaching experiment, about 1,600 mL distilled water was added from the top over a period of 3 days in order to have a homogeneously moist column at field capacity. The soil total porosity and water holding capacity in the columns were about 60% and 40%, respectively.

During the leaching period, an amount of 70–100mL distilled water was applied every three days from the top of each soil column to reflect local daily rainfall corrected for transpiration. Leachate samples were extracted at different depths in the soil column profile through the sampling ports at an interval of 3 days in the initial stage and of 7 days in the following stage. All the sampling ports were sealed when not sampled.

Water Air Soil Pollut (2010) 213:47–55

y = 1.1731x + 9.4061 R2 = 0.9975

80 60 40 20 0 0

20

40

60

80

Ce (mg L-1)

Fig. 2 Linear plot of Langmuir isotherm of NH4+ ion adsorption on bamboo charcoal

Water Air Soil Pollut (2010) 213:47–55 300

(a)

51 60

10 cm CK A AB

200

NH4 -N (mg L )

20 cm

(d)

40 cm

50

250

40

150

30

100

20

50

10

16

(b)

(c)

30 cm

14

18 16

12 14 10 12 8 10

6 4

8 0

7

14 21 28 35 42 49 56 63 70

0

7

14 21 28 35 42 49 56 63 70

Time (d) Fig. 3 Temporal changes of NH4+-N concentration in the leachate of soil columns at different depths (treatment: CK: no-fertilizer; A: ammonium chloride at 400 kg N ha−1; AB: ammonium chloride at 400 kg N ha−1 +0.5% bamboo charcoal)

Under treatment A, the observed peak of NH4+-N appeared on day 4 with a maximum level of 260 mg L−1. Thereafter, the concentration of NH4+-N decreased gradually to 54.9 mg L−1 at the end of this experiment. Under treatment AB, the NH4+-N concentration increased dramatically in the first 4 days, then slowly rose to the maximum of 207 mg L−1 on day 28. After that, the NH4+-N concentration showed a moderate declining trend, reaching 57.2 mg L−1 on day 70. In summary, in the absence of BC, a peak in NH4+-N concentration in the leachate appeared in the first 14 days; however, the peak was substantially delayed for the BC-treated soil at 10-cm depth. The reason may lie in the porous structure of BC that reduces the NH4+-N transport due to the adsorption capability of BC. The leaching behavior of NH4+-N in the soil columns at 20-cm depth (Fig. 3b) was much different from that at 10 cm. During the initial 14 days, there was no marked difference of NH4+-N concentrations

in the leachate at 20-cm depth among the treatments CK, A, and AB. The trends for treatments A and AB were similar from day 21 to day 70, with the NH4+-N concentrations obviously higher than that under treatment CK. Furthermore, the NH4+-N concentration under treatment AB was lower than that under treatment A as leaching continued beyond 42 days, with values of 38.4 and 48.7 mg L−1 for treatments AB and A on day 70, respectively. This difference may also be related to the adsorption effect of BC on NH4+-N in the top 10-cm soil layer.

The NH4+-N concentrations in the leachate of soil columns at both 30 cm (Fig. 3c) and 40 cm (Fig. 3d) depths showed no obvious difference among the treatments CK, A, and AB within the 70-day period, and they were all at a much lower level than the shallower leachates, with values between 6.6 and 14.4 mg L−1.

Overall, these results suggest that NH4+-N can be retained in the surface soil layer (0–20 cm) for a

52

Water Air Soil Pollut (2010) 213:47–55

longer time through BC addition, a delay which is potentially beneficial to nitrogen utilization of crops.

3.3 Effect of BC on Cumulative Losses of NH4+-N Through Leaching The cumulative losses of NH4+-N through leaching at the 10-cm depth in the soil columns increased over time (Fig. 4a). The increase of NH4+-N losses took place at a very slow rate under the CK treatment, and NH4+-N losses of 6.5 mg were found on day 70. It seems that the cumulative leaching losses of soil NH4+-N could be significantly reduced by BC addition (40.4 mg column−1) compared to that with ammonium chloride application alone (44.8 mg column−1), according to the results obtained during the first 14 days in this experiment. Taken all together, the data in Fig. 4a indicated that the presence of BC reduced the leaching rate of NH4+N, although the NH4+-N losses showed no obvious difference between treatment A and AB from day 21 to 70.

100

(a)

10 cm

80

20

(b)

20 cm

(d)

40 cm

15

CK A AB

60

The cumulative leaching losses of NH4+-N at 20cm depth showed no obvious difference among CK, A, and AB treatments during the first 14 days (Fig. 4b). However, the situation was quite different during the latter stage of the experiment (from day 63 to 70). NH4+-N losses of 14.3 mg were observed under treatment AB within 70 days, which was about 15.2% lower than that (16.8 mg) under treatment A. Adding BC to surface soils, therefore, may reduce agricultural losses of NH4+-N through runoff or leaching, especially in areas with high rainfall.

The cumulative losses of soil NH4+-N through leaching at 30-cm (Fig. 4c) and 40-cm depth (Fig. 4d) in the soil columns both increased over time, but they showed no obvious difference among the CK, A, and AB treatments within 70 days.

In the present experiment, we found that the concentrations of NO3−-N and NO2−-N in the leachate (data not shown) were both at low levels and did not exceed 10 mg N L−1. This indicates that the potential of nitrate leaching from the soil profile might exist, but at an unsubstantial level.

10

NH4 -N (mg column )

40 5

20

10

(c)

30 cm

10

8

8

6

6

4

4

2

2

1

4

7

10 14 21 28 35 42 49 56 63 70

1

4

7

10 14 21 28 35 42 49 56 63 70

Time (d) Fig. 4 Cumulative losses of NH4+-N in the leachate of soil columns at different depths (treatment: CK: no-fertilizer; A: ammonium chloride at 400 kg N ha−1; AB: ammonium chloride at 400 kg N ha−1 +0.5% bamboo charcoal)

Water Air Soil Pollut (2010) 213:47–55

53

3.4 Effect of BC on EC in the Leachate Electrical conductivity (EC) estimates the amount of total dissolved salts or the total amount of dissolved ions in the water. Therefore, EC in leachate measures the risk of groundwater pollution by dissolved base ions, such as NH4+.

In the present experiment, a lower EC was found in the leachate at 10-cm depth of the soil columns under treatment AB compared to that under treatment A (Fig. 5a). The observed peaks of EC in the leachate appeared on day 7 under both treatment A and AB, with the maximum value of 5.19 and 4.78 mS cm−1, respectively. Following that, a gradual decline of EC was found under both treatments, with values of 0.75 and 0.84 mS cm−1 on day 70, respectively, approaching that under treatment CK (0.54 mS cm−1). These results indicate that leaching did most likely diminish as the BC adsorbed various ions not by simple exchange but also by physical adsorption and other 6

(a)

2.4

10 cm

5 CK A AB

4

processes, as evidenced by the temporal changes of NH4+-N concentration in the leachate at 10-cm depth.

Temporal changes of EC in the leachate of the soil columns at 20-cm depth showed very similar tendencies between treatments A and AB (Fig. 5b). The peaks of EC appeared on day 42, with the maximum values of 1.97 and 1.79 mS cm−1, respectively.

Furthermore, the EC in the leachate was markedly reduced under treatment AB compared to that under treatment A. These results indicated a significant downward movement of dissolved ions such as NH4+ in the upper soil, with BC influencing this movement through the adsorption effect.

Application of ammonium chloride led to a higher EC in the leachate of the deeper soil (20–40 cm) at different stages of the experiment, for example, from day 35 at 30-cm depth (Fig. 5c) and from day 63 at 40-cm depth (Fig. 5d). No significant difference was found in EC at this depth between the treatment with and without BC.

(b)

20 cm

(d)

40 cm

2.0 1.6

3 1.2

EC (mS cm )

2 .8

1 0 1.4

.4 1.0

(c)

30 cm .9

1.2

.8 1.0 .7 .8 .6 .6

.5

.4

.4 0

7

14 21 28 35 42 49 56 63 70

0

7

14 21 28 35 42 49 56 63 70

Time (d) Fig. 5 Temporal changes of electrical conductivity in the leachate of soil columns at different depths (treatment: CK: no-fertilizer; A: ammonium chloride at 400 kg N ha−1; AB: ammonium chloride at 400 kg N ha−1 +0.5% bamboo charcoal)

54

4 Conclusions BC could adsorb ammonium ion primarily by ion exchange, and the maximum adsorption capacity (q0) was 0.852 mg g−1 at 25°C. Addition of 0.5% BC to the surface soil layer retarded the vertical movement of NH4+-N into the deeper layers within the 70-day observation time, especially during the first 7 days at 10-cm depth and the later experimental period at 20cm depth. Application of BC reduced cumulative losses of NH4+-N via leaching at 20 cm by 15.2% over the experimental period. A lower EC was measured in the leachate above 20-cm depth of the soil columns in the presence of BC. Therefore, as a kind of biochar, BC could be used as a potential soil amendment for nutrient retention, especially in regions with a large amount of rainfall, to mitigate the vertical transport of ammonium nitrogen. However, the effect of BC addition on controlling inorganic nitrogen losses in soils through leaching needs to be further assessed before large-scale applications of BC to agricultural fields can be recommended.

Acknowledgement This research was supported by the Natural Science Foundation of China (project No. 40873059), Science and Technology Department of Zhejiang Province Project (Grant No. 2008C13022-1), and National Critical Project for Science and Technology on Water Pollution Prevention and Control (No. 2008ZX07101-006).

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