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Europ. J. Agronomy 34 (2011) 231–238

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Biochar as a strategy to sequester carbon and increase yield in durum wheat F.P. Vaccari a,∗ , S. Baronti a , E. Lugato a , L. Genesio a , S. Castaldi b , F. Fornasier c , F. Miglietta a,d a

Institute of Biometeorology (IBIMET), National Research Council (CNR), Via Caproni 8, 50145 Firenze, Italy Second University of Naples (SUN), Department of Environmental Sciences (DSA), Via Vivaldi 43, 81100 Caserta, Italy c Plant/Soil Interactions Research Centre (RPS), Council for Research and Experimentation in Agriculture (CRA), Via Trieste 23, 34170 Gorizia, Italy d FoxLab (Forest and Wood) E. Mach Foundation - Iasma, Via E. Mach 1, 38010 S. Michele all’Adige (TN), Italy b

a r t i c l e

i n f o

Article history: Received 11 November 2010 Received in revised form 25 January 2011 Accepted 26 January 2011 Keywords: Charcoal Grain quality Soil amendment Soil carbon sequestration Temperate climate

a b s t r a c t Carbon sequestration in agricultural soils is a climate change mitigation option since most of cultivated soils are depleted of soil organic carbon and far from saturation. The management practices, most frequently suggested to increase soil organic carbon content have variable effects depending on pedoclimatic conditions and have to be applied for a long time periods to maintain their sink capacity. Biochar (BC), a carbon rich product obtained through carbonization of biomass, can be used for carbon sequestration by applying large amounts of carbon very resistant to decomposition. The BC remains into soil for a long time and there is evidence that the BC stores atmospheric carbon from centennial, to millennial timescales. However most of the agronomic studies on BC application have been made in tropical and sub-tropical climates, while there is a substantial lack of studies at mid-latitudes and in temperate climates. This paper presents the results on an investigation of large volume application of BC (30 and 60 t ha−1 ) on durum wheat in the Mediterranean climate condition, showing the viability of BC application for carbon sequestration on this crop. BC application also has positive effects up to 30% on biomass production and yield, with no differences in grain nitrogen content. Moreover no significant differences between the two BC treatments were detected, suggesting that even very high BC application rates promote plant growth and are, certainly, not detrimental. The effect of the biochar on durum wheat was sustained for two consecutive seasons when BC application was not repeated in the second year. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Effective mitigation of greenhouse gases emission requires the exploration of a range of alternatives in the energy, transport, manufacturing, construction and agricultural sectors. Carbon sequestration in agricultural soils has been repeatedly considered an interesting option, as the amount of carbon that can be potentially stored in soils is vast. Changes in soil tillage practices, improved rotations, application of biosolids (manure, crop residue, compost), cover and deep-rooting crops (Lal, 2004; Lugato et al., 2006; Morari et al., 2006; Paustian et al., 1997) offer indeed the possibility to increase soil organic carbon content (SOC) via increased carbon input and reduced decomposition rates. Recent studies highlighted that the global technical potential for mitigation options in agriculture by 2030 is estimated to be 4500–6000 Mt CO2 equiv. yr−1 using appropriate strategies (Smith et al., 2007). Analyzing different scenarios, Smith (2005) estimated that 90–120 Mt of carbon per year could be potentially sequestered in European soils simply through affordable and non-traumatic

∗ Corresponding author. Tel.: +39 0553033711; fax: +39 055308910.

E-mail address: f.vaccari@ibimet.cnr.it (F.P. Vaccari).

1161-0301/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.eja.2011.01.006

changes in crop management strategies. However, the adoption of these practices may be limited by the uncertainty of carbon sequestration rates across different pedo-climatic conditions (Freibauer et al., 2004), making the changes in SOC difficult to be measured and verified. Moreover, in order to maintain the carbon soil sink capacity, the farmer is obliged to keep the management practice adopted for a long period.

In a theoretical framework, effective and sustainable carbon sequestration can be obtained when the fraction of carbon, which is added to soils, is hardly or not at all decomposable and when such additional carbon input does not cause yield and yield quality reductions and does not enhance the occurrence and frequency of dangerous pests and crop diseases.

Recent papers have proposed the use of biochar (BC) as soil amendment to achieve such large carbon sequestration (Lehmann and Steiner, 2009; Sohi et al., 2010) as BC application to soils apparently fulfil all those requisites. BC is a carbon-rich product obtained through carbonization of biomass as it for instance occurs during pyrolysis and pyrogasification (Antal and Grønli, 2003). Biomass cracking reactions at the temperature of 400–800 ◦ C which are used to produce renewable energy, generate in fact a solid product (BC), a viscous black liquid (tar), and gas (syngas). BC can be produced at large industrial facilities, farm and even at the domestic level using

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pyrolysis technologies that are commercially available. According to Woolf (2008) the global implementation of BC-based carbon sequestration can potentially offset 12% of current anthropogenic CO2 -C equivalent emission.

The long-term stability of the BC was demonstrated to be greater compared to non-pyrolyzed organic matter that was incorporated into soils with the same environmental conditions (Baldock and Smernik, 2002; Cheng et al., 2008; Liang et al., 2008). The BC has an approximate mean residence time (MRT) in the soil more than 1000 years (Cheng et al., 2008; Lehmann et al., 2008; Liang et al., 2008; Zimmerman, 2010) and this long-term stability is a fundamental prerequisite to consider BC as a suitable method for carbon sequestration.

Such optimistic scenario requires however more detailed and reliable assessment of direct BC effects on crops and the environment as well as an evaluation of socio-economic implications. Most of the agronomic studies on BC application were made in tropical and sub-tropical climates (Gaskin et al., 2010; Kimetu et al., 2008; Noguera et al., 2010; Sinclair et al., 2008; Van Zwieten et al., 2008) while there is a substantial lack of studies at mid-latitudes and in temperate climates.

The data presented in this paper are the first results of a field experiment where large volume of BC was applied to durum wheat crop for two consecutive seasons in a Mediterranean climate.

The focus is on durum wheat production that is a typical crop for the region and widespread in North Africa, Mediterranean Europe and the Middle East.

2. Materials and methods The field experiment was made over two consecutive seasons in 2008/2009 and 2009/2010 near Pistoia (Toscana, Lat. 43◦ 56 N, Long.

10◦ 54 E, 65 m a.s.l.), using the durum wheat (Triticum durum L.) cultivar Neolatino. Meteorological parameters were collected by an automatic weather station, installed close to the experimental field.

During the period September–July of 2008/2009 and 2009/2010, total rainfall was 1159 and 1222.8 mm respectively and the mean air temperature was 13.9 and 15.1 ◦ C (Fig. 1). The soil was a siltyloam (USDA, 2005) with a sub-acid pH of 5.2 (Table 1). In the first experimental season (2008/2009), a randomized block experiment with four replicates was set up in plots of 25 m2 , considering three treatments: Control (C0 ), biochar at a rate of 30 t ha−1 (B30 ) and 60 t ha−1 (B60 ). In order to evaluate the potential residual effect of BC application on wheat yield, the same plots (thereinafter called C0w , B30w and B60w ) were cultivated without BC application in the following growing season (2009/2010). In 2009/2010, new plots with BC application were added in the experimental site, maintaining the same layout of the previous year (C0 , B30 and B60 ).

Table 1 Soil characteristics of the experimental site at Pistoia.

Parameters −1 a

Sand (g kg ) 2 mm 0.05 mm Silt (g kg−1 ) 0.05 mm 0.002 mm Clay (g kg−1 ) < 0.002 mm Bulk density (Mg m−3 ) OC (g kg−1 )b N (g kg−1 )c CEC (mequiv./100 g)d pHe a

Pistoia site 501 433 67 1.2 21 1.2 18 5.2

It refers to fine (<2 mm) texture fraction.

Organic carbon (OC) content was determined using a CHN auto-analyzer (CHN 1500, Carlo Erba). c Nitrogen (N) content was determined using a CHN auto-analyzer (CHN 1500, Carlo Erba). d Cation exchange capacity (CEC) was determined using a NH4 OAc method. e The pH was measured in a 1:2.5 (mass/vol) soil solution. b

Fig. 1. Average air temperature (◦ C; closed symbols) and total rain (mm; gray bars) on monthly basis at Pistoia experimental site. In the upper graph (1989–2010) the rain values are monthly averages and error bars are monthly standard errors. The two lower graphs show average of air temperature and monthly rain for the two experimental years at Pistoia.

Biochar was manually applied, before sowing operation in 2009 and in 2010 and partially buried with a rotary hoeing tillage. Wheat was sown on 16th January 2009 (experiment 2008/2009) and on 14th December 2009 (experiment 2009/2010) in rows with a sowing rate of 450 germinable seeds per m2 . Nitrogen-phosphate and phosphorous fertilizer were distributed at sowing (22 kg ha−1 of N and 50 kg ha−1 of P2 O5 ) and a second fertilization was made on April using ammonium nitrate fertilizer at a rate of 100 kg N ha−1 for both experiments. During the wheat growing season, three destructive biomass samples were done at Zadoks scale of (Zadoks et al., 1974): 32 (stem elongation or jointing, 2nd node detectable), 50 (heading, first spikelet of head visible) and 91 (ripening, kernel hard difficult to separate by fingernail). The plots were manually harvested on 25th June 2009 and on 4th July 2010 by selecting

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Table 2 Chemical and physical characteristics of the biochar applied.

Chemical element −1

Total C (g kg ) Total N (g kg−1 ) Available N (g kg−1 ) P (g kg−1 ) K (g kg−1 ) Ca (g kg−1 ) S (g kg−1 ) Mg (g kg−1 ) pH (1:2.5 H2 O) Max water absorption (g g−1 of d.m.)

Bulk density (Mg m−3 ) Total porosity (mm3 g−1 ) Transmission pores (>30 ␮m) Storage pores (30–0.3 ␮m) Residual pores (<0.3 ␮m)

Value 840 12 0.03 0.5 4.3 2.6 1.1 2.8 7.2 4.5 0.42 1365 117 1008 240

three subplots of 0.25 m2 in the central part of each plot to avoid the edge effects. Total above ground biomass (AGB) was oven-dried and weighted. Wheat ears were then separated from the straw and the grains were separated from the ears using a laboratory thresher (LD 350, Wintersteiger, Ried, Austria); nitrogen (N) grain concentration was determined using a Near-Infrared Spectroscopy (NIR Analyzer, Carlo Erba, MI, Italy). During the 2008/2009 experimental season the soil temperature was monitored at five different dates (January 15th, 29th; February 20th, 24th and March 24th) at 5 cm of soil depth, using a soil temperature probe (STP-1, PPSystems, Hitchin, UK). Moreover, the number of durum wheat plants along a row (1 m length) was counted in each plot at Zadoks scale 12 (2nd leaves unfolded) and the weed productions were harvested by selecting three subplots of 0.25 m2 on each plot at durum wheat harvest (25th June) and at 16th October 2009. Soil pH was measured in 1:2.5 (soil:water) suspension adding CaCl2 (0.01 M), sampling the soil before and after (end of June) BC application, in 2009.

The BC applied in both field experiments was a commercial horticultural charcoal provided by Lakeland Coppice Products (England) obtained from coppiced woodlands (beech, hazel, oak and birch). BC has been obtained at pyrolysis temperatures of 500 ◦ C in a transportable ring kiln (2.15 m in diameter and holding around 2 t of hardwood). The BC was crushed into particles smaller than 1 cm before application into soil in order to increase the area/volume ratio and to enhance its expected effects on soil properties. C and N contents of BC were determined using a CHN Elemental Analyzer (Carlo Erba Instruments, mod 1500 series 2). Samples were screened by means of a 2 mm sieve and oven dried at 105 ◦ C for 24 h. The dry samples were acid digested with a microwave oven (CEM, MARSXpress) according to the EPA method 3052. The solutions obtained after the mineralization were filtered (0.45 ␮m PTFE) and diluted. Total contents of P, K, S, Ca, and Mg were determined by an ICP optical spectrometer (Varian Inc., Vista MPX) using scandium as internal standard. Samples of BC were analyzed by mercury porosimetry to measure the pore size distribution, within the range of 0.003–100 mm (Table 2). Maximum water absorption was derived using a vacuum pump: 5 g of each sample, previously dried (105 ◦ C for 24 h), were dip into distilled water and subjected for 5 min cycle at −900 mbar vacuum and then weighted after each cycle. The measurement was repeated for three cycles to obtain the maximum water absorption. The pH was measured in a soil/water solution at a 1:2.5 ratio and the bulk density was calculated gravimetrically. The main characteristics of BC are reported in Table 2.

2.1. Statistical analysis Analysis of variance (ANOVA) was used to compare treatment effects. Since the addition of new plots receiving BC in 2009/2010,

Fig. 2. Above ground biomass increments (ABG t ha−1 of dry matter) in BC treatments with respect to the control, sampled at three Zadoks scale values (32, 50, and 91) in 2009 and 2010. B30 and B60 are the treatments receiving 30 and 60 t ha−1 of BC respectively. Treatments receiving BC only in the 2009 are labeled with ‘w’.

a one-way ANOVA was performed separately in each growing season to compare the three treatments (C0 , B30 and B60 ). Moreover, the residual effect of BC in 2009/2010 (treatments C0w , B30w and B60w ) was evaluated including in the analysis the new treatments with BC application in the same year. Prior to ANOVA, Bartlett’s test was used on the data to test the homogeneity of variance.

Student–Newman–Keuls test at 0.05 significance level was used as means multiple comparison test.

3. Results High volume BC application was successfully made over two consecutive seasons. The mean bulk density of the soil was initially reduced by 4 and 2% in B30 and B60 treatments. The highest BC dose (60 t ha−1 ) increased the mass and volume of soil in the first 20 cm by 2.5% and 7%, respectively. Those changes did not interfere at all with ploughing operations such as rotary tillage that was made to a depth of 15 cm. Over a period of 6 months, BC was uniformly incorporated into the soil and carbonaceous particles were clearly detectable at a depth of more than 20 cm at the time of harvest, in both years. Changes in soil surface color were visible after BC application and ploughing both in B30 and in B60 and such color change persisted over time.

No appreciable negative BC effect on crops was observed but, instead, AGB increased in the BC-treated plots with respect to the

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Fig. 3. Differences in average daily soil temperature between B30 and B60 with respect to the control (C0 ). Hourly global solar radiation and air temperature measured at Pistoia experimental field (2009 campaign) at five different dates (January 15th, 29th; February 20th, 24th and March 24th). Errors bars are the variation of the soil temperatures. In the lower graph the black lines represent the measured values while the dotted gray lines represent the mean monthly air temperature and the clear global solar radiation.

Control (C0 ) with a similar behavior during the two growing seasons (2008/2009 and 2009/2010) (Fig. 2). Differences in AGB, including also B30w , B60w treatments were observed since Zadosk value of 32 (stem elongation) ranging from 0.21 to 0.31 t ha−1 . The positive anomaly was maintained also at Zadoks value of 50 (heading) but with a higher variability. At ripening (Zadoks value 91) the positive effect of BC was more evident with values exceeding 2 t ha−1 in B60 treatments including also B60w treatment in both seasons; moreover the lowest increase was observed in B30w even if this value was affected by a higher variability.

At harvest the AGB was increased in B30 and B60 with respect to C0 , in both 2009 and 2010. The differences between B30 and B60 were not significant for both years. In 2010, the residual effect of BC on AGB was clearly detectable as B30w and B60w treatments produced more biomass than the control. Such stimulation was comparable, in absolute term, to B30 and B60 (Table 3).

In the first year of application, both in 2009 and 2010 BC significantly increased grain production with respect to the control, but no differences were detectable between B30 and B60 treatments.

Yields of B30w and B60w plots were 32.1 and 23.6% larger than controls but such difference was significant only at P = 0.1. On average BC application resulted in a grain yield increase ranging between 28 and 39%.

Grain nitrogen content was not significantly affected by BC application (Table 3).

Soil temperature measurements in 2009, revealed positive anomalies up to 2 ◦ C in BC plots (Fig. 3). Highest differences were

measured on 20th February a clear day with a large daily temperature range. On the contrary BC application only slightly affected the soil temperature in the most cloudy day (24th March). No differences in soil temperature were observed between BC application rates.

Weed biomass measured at harvest (25th June) was less in BC treated plot but the differences were not significant. Three months after harvest (16th October) weed biomass in BC plots was significantly higher than control plots (Fig. 4).

BC application proportionally affected the soil pH (Fig. 5). At harvest, soil pH was increased from 5.1 (in C0 ) to 6.39 and 5.51 in B60 and B30 respectively.

4. Discussions Two years of field experiments supported the view, that the addition of large quantities of BC to sequester atmospheric CO2 is a viable option for durum wheat crops, at least for the typical conditions of Southern Europe, where this species is commonly cultivated. In the specific case of this experiment, the addition of 30–60 t ha−1 of BC was equivalent to 92–184 t of atmospheric CO2 that was taken from the atmosphere by plants and transferred into the soil. There is ample evidence that BC degradation in soils is generally a very slow process. Very old charcoal samples (>8000 years BP) originating from past wildfires have been found almost unaltered in forest soils (Marguerie and Hunot, 2007) and the existence of very old anthropogenic deposits of charcoal in soils has been

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Table 3 Above ground biomass (AGB), grain yield (t ha−1 ), nitrogen content (% of the dry matter) of the experimental treatments at harvest. y is the AGB, yield and nitrogen, increment due to biochar application with respect to the control (s.e. is the standard error).

Year

Treat.

ABG ± s.e. (t ha−1 )

Stat.a

y (%)

Grain ± s.e. (t ha−1 )

Stat.a

2009 2009 2009

C0 B30 B60

5.84 ± 0.25 7.50 ± 0.38 8.09 ± 0.44

b a a

28.5 38.5

2.28 ± 0.16 2.92 ± 0.11 2.93 ± 0.07

2010 2010 2010

C0 B30 B60

6.11 ± 0.24 7.88 ± 0.26 8.16 ± 0.62

bB aA aA

29.0 33.6

2010 2010 2010

C0w B30w B60w

6.02 ± 0.33 7.25 ± 0.41 8.18 ± 0.47

B AB A

20.6 36.0

y (%)

N ± s.e.

Stata

y (%)

b a a

28.2 28.6

2.53 ± 0.04 2.53 ± 0.06 2.43 ± 0.03

a a a

0.3 −3.7

2.40 ± 0.22 3.19 ± 0.21 3.34 ± 0.15

bB aA aA

32.7 39.1

2.27 ± 0.11 2.15 ± 0.05 2.25 ± 0.03

aA aA aA

−5.2 −0.9

2.18 ± 0.17 2.90 ± 0.18 2.71 ± 0.08

B AB AB

32.1 23.6

2.25 ± 0.04 2.26 ± 0.07 2.23 ± 0.04

A A A

0.6 −0.6

Lower case refers to statistical analysis made separately for each year. Upper case refers to ANOVA considering all treatments in Pistoia 2010. a Values followed by the same letters are not statistically different at P = 0.05 by the Student–Newman–Keuls test.

Fig. 4. Weed biomass (t ha−1 ) measured in the same plots of the 2009 experiment at durum wheat harvest (25th June) and on 16th October 2009. The data are averages of three measurements and the bars are the standard errors if the means. Values followed by the same letters are not statistically different at P = 0.05 by the Student–Newman–Keuls test.

extensively documented both in the Amazon region (Glaser et al., 2002; Liang et al., 2006; Woods and Denevan, 2009), in Germany and Italy (Cremaschi et al., 2006; Schmid et al., 2002; Schmidt et al., 1999), where charcoal deposit were associated to human colonization during the bronze age. Some recent studies have analyzed the

fate of BC after direct incorporation into the soil (Bruun et al., 2009; Lehmann et al., 2006), but the fraction of BC which is decomposed depends on the production process (Brewer et al., 2009) and the type of feedstock (Balwant et al., 2006; Sohi et al., 2010). However Major et al. (2010), using a stable isotope technique to fit a

Fig. 5. Soil pH measured before sowing (BS) and at the time of the harvest (HR) in 2009, in the experimental treatments considered. Values followed by the same letters are not statistically different at P = 0.05 by the Student–Newman–Keuls test.

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Table 4 List of the experiment results reported in the literature that have applied the biochar. In the column named Exp is indicated the type of the experimentations (P = pot; F = field; L = lysimeter; G = glasshouse); in the column named BC is indicated the type of feedstock used to get the biochar (GW = greenwaste; W = wood; C = charcoal; AC = activated charcoal); in column Treat are indicated the biochar rate applied in the experimentations.

Location

Plant

Exp

BC

Treat. (t ha−1 )a

Effectb

Ref.

Australia Australia Australia Australia Australia Australia Australia Brazil Brazil Brazil

Radish Radish Radish Pasture Sorghum Spring wheat Radish Rice/Sorghum Rice/Sorghum Rice

P P P F P P P F F –

GW GW GW GW GW GW GW W W W

10 50 100 10 10 10 10 11 11 7.9 (of carbon)

Chan et al. (2007) Chan et al. (2007) Chan et al. (2007) Sinclair et al. (2008) Van Zwieten et al. (2010) Van Zwieten et al. (2010) Van Zwieten et al. (2010) Steiner et al. (2007) Steiner et al. (2007) Nehls (2002)

Brazil Brazil Colombia Colombia Colombia Colombia

Cow pea Rice Bean Bean Bean Rice

P L F F F G

W W W W W W

Charcoal site Charcoal site 30 g kg−1 60 g kg−1 90 g kg−1 25 g kg−1

Colombia Colombia Colombia Ghana Ghana Indonesia India India India Japan Japan Japan Japan Japan Japan Kenia Laos Laos Laos New Zealand USA USA Zambia

Rice Carrot Bean Corn Corn Corn Pea Soybean Moong Soybean Soybean Soybean Sugi tree Sugi tree Sugi tree Corn Rice Rice Rice Corn Com Corn Bauhinia tree

G F F F F F P P P – – – – – – F F F F F F F P

C W W W W W – – – – – – W W AC W W W W W W W W

45 g kg−1 30 30 Charcoal site Charcoal site 15 0.5 0.5 0.5 0.5 5 15 0.5 0.5 0.5 14 4 8 16 10 11 22 –

−30% b +91% b +130% b +7.6% b nsd nsd +b +22% b +17% g + b (from 115% to 320%) +38% b +45% b ne +39% b +46% b +294% g +166% b +800% g +100% b +30% b +44% b +91% g +50% y +60% g +51% g +22% g +51% g −37% g −71% g +149% b +224% b +144% b +100% y nsd −10% g −26% g nsd −g −g +13% b

Sohi et al. (2010) Lehmann et al. (2003) Rondon et al. (2007) Rondon et al. (2007) Rondon et al. (2007) Noguera et al. (2010) Noguera et al. (2010) Rondon et al. (2004) Rondon et al. (2004) Oguntunde et al. (2004) Oguntunde et al. (2004) Yamato et al. (2006) Iswaran et al. (1980) Iswaran et al. (1980) Iswaran et al. (1980) Kishimoto and Sugiura (1985) Kishimoto and Sugiura (1985) Kishimoto and Sugiura (1985) Kishimoto and Sugiura (1985) Kishimoto and Sugiura (1985) Kishimoto and Sugiura (1985) Kimetu et al. (2008) Asai et al. (2009) Asai et al. (2009) Asai et al. (2009) Free et al. (2010) Gaskin et al. (2010) Gaskin et al. (2010) Chidumayo (1994)

If not specified the treatments are referred to t ha−1 of char.

List of abbreviations used in the effect column to describe the effects of the biochar treatments. + = increment; − = decrement; g = grain yield; b = biomass yield; y = yield; nsd = no significant differences; ne = nil effect. a

b

first order decomposition kinetics, calculated the mean residence time (MRT) of BC in soil of 600 years at a mean annual temperature of 26 ◦ C. When scaled the mean annual temperature of our experimental site (15 ◦ C) using a Q10 of 3.4 (Cheng et al., 2008) we calculated that 97–95% of the carbon contained in BC will still be found after 50 and 100 years, respectively.

Our experiment provided evidence that such important carbon sequestration potential may be realized without any negative consequence on crop yield. Instead, BC application had a strong positive effect on productivity, with important implications for future real scale BC application in agriculture. This matches the results obtained in other studies made with other crops in other parts of world (Glaser et al., 2002), as well as more recent investigations made with durum wheat in Italy (Baronti et al., 2010) where yield was increased by 10% after the addition of 10 t ha−1 of BC. When combined together, the different rates of BC application which are found in the literature (from 0.5 t ha−1 to 135 t ha−1 ) show wide range of positive plant responses, with biomass increment up to +324% (Table 4). In spite of the fact that the interactions between plant species and pedo-climatic conditions are not fully understood (Lehmann and Rondon, 2006), chemical, physical and possibly microbiological actions may explain the observed responses (Sohi et al., 2010). BC can capture

high amounts of exchange cations (Lehmann et al., 2005) due to its high porosity and surface/volume ratio and increase the pH of acid soil (due to the basic compounds present in BC), improving plant nutrient uptake and availability of P, Ca, and K while decreasing free Al in solution (Chan et al., 2007; Yamato et al., 2006). Slow oxidization in soils can produce carboxylic groups and can increase cation exchange capacity and oxygen carbon ratio thus finally leading to an enhanced capacity to retain nutrients (Brodowski et al., 2005). Steiner et al. (2007) measured both an higher soil N retention and an enhanced N cycling in fertilized plots that received BC, suggesting a reduction in N leaching (higher retention of NH4+ ; N immobilization in microbial biomass) or as a reduction of denitrification (Yanai et al., 2007). Recently Spokas et al. (2010) have demonstrated that the biochar added to the soil caused an increased in ethylene production that is an important plant hormone and an inhibitor for soil microbial processes.

In our experiment BC application significantly raised the pH, especially at the highest rate (Fig. 5) in agreement with numerous studies where BC application increased the pH in acidic soils (Lehmann et al., 2003; Matsubara et al., 2002; Mbagwu, 1989).

Increased concentration of alkaline metal (Ca2+ , Mg2+ , and K+ ) oxides, present in BC, and a reduced concentration of soluble soil Al3+ was assumed to explain such effect (Steiner et al., 2007). Values

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of pH close to neutrality favor the nutrient bio-availability, consequently increasing the crop productivity as we observed in the first experimental year. Indeed, beside indirect effect on nutrient availability through soil pH change, BC could also exert a direct action such as induced surface sorption of chelating organic molecules containing phosphorus, that is often precipitated with Al3+ and Fe3+ in acidic soils.

In the specific case of winter crops, as durum wheat, the mulching effect of BC addition to soil may have contributed to such positive yield response. In our study, soil temperature was substantially raised during the sowing-to-emergence period and during the initial phases of the crop (Fig. 3) and this likely promoted initial crop growth. Crop emergence was anticipated in the treated plots and one month after emergence, in 2009, before tillering, the average (±s.e.) number of stems was higher in BC treated plots compared to the control (C0 = 35 ± 2; B30 = 45 ± 2; B60 = 45 ± 2). Such effect is likely associated to a faster emergence rate in BC-treated plots possibly caused by higher soil temperatures during periods in which seed germination is critically affected by low temperatures (Bassu et al., 2009). Although this was not directly assessed, reduced weed competition might have also contributed to yield increase: weed biomass was apparently lower in the treated plots at harvest time (Fig. 4), likely as a consequence of an increased leaf area index and light interception which inevitably accompanied biomass growth stimulation. Soil warming effects that led to faster crop cover might have contributed to an overall decrease in weed development and growth.

One interesting aspects of our results is that biomass growth stimulation due to BC was effective and evident even in the second year, in BCw treatment (Table 3). This suggests the idea that the yield response to BC is not simply due to a rapid priming effect on soil organic matter decomposition finally leading to faster mineralization and enhanced nutrient availability to the crop. The observation of enhanced weed growth in the post harvest period, suggests that BC addition may also ameliorate soil water status and mitigate drought effect. Although no detailed measurements of plant water status were made during the summer, it is well known that growth of weed species is in fact severely limited by the availability of soil water, during drought in Central Italy. Detailed laboratory estimations showed that BC used in this study, due to its porous structure, can contain water up to 4.5 times its initial dry weight (Mulcahy et al., 2009) and, as a consequence, when a large fraction of BC is incorporated into the soil, the overall water holding capacity is expected to increase (Brockhoff et al., 2010).

In our experiment growth stimulation was accompanied by a sustained production of grain nitrogen.

No N-dilution effect was observed in the grains in response to increased yield (Table 3) as the C/N ratio of the grains was basically not affected by the treatments. As a consequence, the total amount of harvestable proteins was substantially increased. Grain protein content is a crucial characteristics defining quality in durum wheat and this observation suggests that given the same amount of fertilizers distributed in the treated and control plots, N uptake was eventually larger in BC-treated plots.

5. Conclusions The results presented and discussed in this paper provide important evidence that BC can be successfully used to sequester atmospheric CO2 in durum wheat crops. Large BC applications had no harmful effects on yield and yield quality over two consecutive years and also did not interfere with the execution of conventional agricultural management.

Lower bulk density in BC-treated plots has the potential to reduce the tensile strength of mineral soils eventually leading to reduced tillage costs.

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The occurrence of a significant and consistent positive effect on growth and yield confirms that BC has amendment properties capable of ameliorating soils and promoting fertility. In this perspective, the use of BC in durum wheat cultivation might substantially contribute to the objectives of the United Nation Framework Convention on Climate Change (UNFCCC) that explicitly consider agronomic strategies to reduce carbon losses from soils and to increase soil organic carbon. Large scale application of BC might also successfully combine carbon sequestration, renewable energy production and an overall amelioration of agricultural soils finally leading to increased durum wheat yields with unaltered grain quality.

Despite the promising results obtained in this study a full evaluation of BC as a mitigation strategy to be applied on the larger scale is still needed. Observed changes in soil color are likely to have affected surface albedo, thus raising major concerns on the occurrence of possible feedback on Earth radiative balance. Risks for the accumulation of harmful compounds in soils (i.e. polycyclic aromatic hydrocarbons, PAHs) and their possible presence in crops need to be further addressed before recommending full-scale application of BC in durum wheat cultivation.

Acknowledgements Lorenzo Albanese, Irene Criscuoli, Filippo Di Gennaro, Sara Di Lonardo, Alessandro Matese, Francesco Sabatini, Giacomo Tagliaferri and Alessandro Zaldei (IBIMET-CNR), are acknowledged for their valuable technical assistance during the field experiments and during harvest operations. The authors would like to acknowledge the contribution Marco Silvestri and Clara Berdini (Barilla Alimentare spa) for the grain nitrogen analysis. We owe our special thanks to CESPEVI (Centro Sperimentale per il Vivaismo) di Pistoia where we done our experimental trial. This study was supported by the Italian Biochar Association (ICHAR http://www.ichar.org) and contributes to the EuroCHAR project (FP7-ENV-2010 ID-265179).

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