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Effect of physical weathering on the carbon sequestration potential of biochars and hydrochars in soil

Authors


Abstract

Physical weathering can modify the stability of biochar after field exposure. The aim of our study was to determine the potential carbon sequestration of the two chars at different timescales. We investigated the modification in composition and stability resulting from physical weathering of two different chars produced (i) at low temperature (250 °C) by hydrothermal carbonization (HTC); and (ii) at high temperature (1200 °C) by gasification (GS) using contrasting feedstocks. Physical weathering of HTC and GS placed on a water permeable canvas was performed through successive wetting/drying and freezing/thawing cycles. Carbon loss was assessed by mass balance. Chemical stability of the remaining material was evaluated as resistance to acid dichromate oxidation, and biological stability was assessed during laboratory incubation. Moreover, we assessed modification in potential priming effects due to physical weathering. Physical weathering induced a carbon loss ranging between 10 and 40% of the total C mass depending on the feedstock. This C loss is most probably related to leaching of small particulate and dissolved compounds. GS produced from maize silage showed the highest C loss. The chemical stability of HTC and GS was unaffected by physical weathering. In contrast, physical weathering strongly increased the biological stability of HTC and GS char produced from maize silage. After physical weathering, the half-life (t1/2) of GS was doubled but only slight increase was noted for those of HTC. During the first weeks of incubation, HTC addition to soil stimulated native soil organic matter (SOM) mineralization (positive priming effect), while the GS addition led to protection of the native SOM against biologic degradation (negative priming effect). Physical weathering led to reduction in these priming effects. Model extrapolations based on our data showed that decadal C sequestration potential of GS and HTC is globally equivalent when all losses including those due to priming and physical weathering were taken into account. However, at century scale only GS may have the potential to increase soil C storage.

Introduction

Soil plays an important role in the carbon cycle at global scale, both as sink or source of carbon dioxide and the soil organic carbon (SOC) content is often taken as an indicator of soil fertility (Schlesinger, 1984; Lal, 2006). Land use as well as climate change may lead to decreasing SOC contents (Grønlund et al., 2008; Bellamy et al., 2005; Saby et al., 2008; Heikkinen et al., 2013).

Recently, pyrolysis and hydrothermal carbonization were suggested as novel techniques to produce soil amendments capable of increasing carbon sequestration as well as fertility (Lehmann et al., 2006; Sohi et al., 2010). Commercial biochar production from agro-industrial biomass by thermal conversion at high temperature (pyrolysis, gasification) simplifies structure of the resulting material by inducing loss of functional groups and formation of aromatic material (Wiedner et al., 2013a). Biochar produced from plant biomass retains the structure of cell walls (Haas et al., 2009), which can provide habitat for soil microorganisms (Lehmann et al., 2011). These porous structures of biochar are composed mainly of aromatic C, with possible linkage to N and P atoms (Brennan et al., 2001).

Biomass transformation by hydrothermal carbonization carried out at low temperature (180–250 °C) and under pressure, leads to the formation of carbon spheres in liquid phase (Libra et al., 2011). The advantage of this process is the use of a wide range of biomass feedstocks with high moisture including sewage sludge or animal wastes. During the hydrothermal carbonization process, coupling hydrolysis and carbonization reactions various carbonaceous materials are synthesized dominated by aliphatic compounds (Hu et al., 2010; Fuertes et al., 2010). Wiedner et al. (2013b) showed that the chemical composition of hydrochar is more influenced by temperature than feedstocks properties. Biochar and hydrochar are thus characterized by contrasting chemical composition (Wiedner et al., 2013a,b), pH and chemical stability (Naisse et al., 2013). Busch et al. (2012) showed that hydrochar presented significant short-term phytotoxic effects.

To evaluate the soil carbon sequestration potential of both, biochar and hydrochar, the integrated C balance of these products must be established by taking into account: their proper potential mineralization and their proper C loss due to leaching as well as their potential effect on native soil organic matter mineralization (‘positive or negative priming effect’). When exposed in the field, biochar as well as hydrochar may be subject to physical weathering, which could lead to fragmentation affecting their nature and reactivity (Rumpel et al., 2007; Nocentini et al., 2010). Aging has also been shown to increase the soluble fraction of biochar (Abiven et al., 2011), which could be susceptible to export from soil (Jaffé et al., 2013). Moreover, physical weathering could alter significantly the interaction between biochar and soil microorganisms (Spokas, 2013).

In this study, we performed artificial weathering on biochar produced by gasification (GS) and hydrochar produced by hydrothermal carbonization (HTC) from different feedstocks. The weathering protocol was hypothesized to reproduce physical processes operating after field exposure. We estimated carbon loss and evaluated the effect of physical weathering on the biological and chemical stability of the different materials. To assess the biological stability and priming effects on native soil organic matter (SOM), we used laboratory incubation and monitored C mineralization as well as the stable isotope composition of the evolved CO2. Moreover, we investigated the relationship between biological stability and chemical stability. The aim of our study was to quantitatively evaluate weathering effects on the chemical and biological stability of biochar and hydrochar and to assess their C sequestration potential at different timescales.

Materials and methods

GS Biochar and HTC production

The GS samples were produced at commercial scale by gasification (Advanced Gasification Technology, Cremona, Italy) from poplar wood (Populus spp. L.) and maize silage (Zea mays L.). Samples were heated at 1200 °C during 30–45 min under atmospheric pressure with a biochar production of 10% w/w.

The HTC sample was prepared from maize silage in a lab scale batch reactor with a total volume of 5 l, with similar temperature, residence time and pressure profiles as in the commercial scale system (CS CarbonSolutions Deutschland GmbH, Kleinmachnow, Brandenburg, Germany). The system consists of two reaction stages with a temperature of 230 °C in stage 1 and a temperature of 180 °C in stage 2 respectively. The HTC feedstock total solid content was adjusted by addition of distilled water for better performance of lab scale HTC reactor system. Citric acid was added until a final pH value of 4 was reached. The HTC process produces slurry consisting of hydrochar particles suspended in a liquid phase.

While from the technological perspective, maize silage is an entirely suitable input material for the HTC and GS process, it is a high grade biomass compared to the organic waste materials usually considered the main potential application for biochar and hydrochar production. In this study, maize silage was used solely because of its C4 isotope signature, allowing to specifically follow char degradation.

Artificial weathering

The weathering protocol was performed with 3–5 grams of GS or HTC. Pretreatment included heating at 150 °C during 10 min to degas volatile organic carbon (Spokas et al., 2011). These compounds were shown to be lost during initial phases of biochar degradation in soil. Thereafter, GS or HTC were placed on a polyamide canvas with 20 μm mesh size (SEFAR-Nitex, Sefar AG, Haiden, Switzerland), which was pasted on the bottom of a PVC cylinder (5 cm ∅). Simulation of climate effects was performed by three successive cycles including three steps of wetting/drying and three steps of freezing/thawing. These experiments were replicated 4 times. The device was placed in a sampling pot to collect particles leached.

For drying and rewetting, the samples were saturated with distilled water and then dried overnight at 60 °C in an oven. This procedure was repeated three times. Thereafter, the samples were again saturated with water and frozen at −20 °C for 1 day, and then thawed at room temperature. This procedure was also repeated three times. Both treatments (3 × wetting/drying and 3 × freezing/thawing) were again repeated three times. At the end, the samples were frieze-dried, weighted and a fraction was grinded for the determination of elemental contents and chemical oxidation. Carbon loss after artificial weathering was assessed by mass balance. The size of particles lost from weathered GS and HTC were determined using a laser particle size analyzer (Beckman Coulter LS 13 320 MW; Beckman Coulter, Inc., Fullerton, CA, USA).

Chemical oxidation

Acid dichromate oxidation was used to assess the chemical stability of GS and HTC as described by Naisse et al. (2013). Briefly, 0.3 g of each sample was mixed with 0.1 m K2Cr2O7/2 m H2SO4 (5 ml). The reaction was carried out during 12 h at 78 °C in an ultrasonic bath. Several times during the reaction, color change indicated that the reagent was consumed. In this case, the solution was renewed. After 12 h, the samples were washed four times with 5 ml of deionized water, centrifuged, and then dried for 3 days in an extractor hood.

Laboratory incubation

Fresh (GS and HTC) and weathered GS and HTC (WGS and WHTC) were added to soil and incubated at 20 °C and optimum water content (i.e. pF = 2.5) for 222 days. The soil, a Cambisol, was sampled at the experimental research unit of Lusignan (INRA) in western France from the top 20 cm under temporary grassland. This soil was characterized by a carbon content of 1.14% and a pH of 5.89. After sampling, the soil was passed through a 5 mm sieve. The experimental unit consisted of 1 l-jars containing a mix of 30 g of soil with 0.3 g C of either fresh or weathered GS and HTC. Correspondingly, we added 0.57 g and 0.65 g for HTC and WHTC and 0.43 g and 0.47 g for GS and WGS. The sample moisture was then adjusted to pF = 2.5 with deionized water taking into account the water contents of the different chars. It was maintained throughout the experiment. The atmosphere was filled with CO2-free gas and the jars were sealed and pre-incubated for 2 weeks at 20 °C. All treatments were replicated three times. Soil without GS and HTC was used as a control. The specific mineralization of soil organic C induced by GS and HTC addition (priming effect), was determined by stable isotope tracing, thanks to the C4 isotope signature of the GS and HTC chars produced from maize silage and the C3 isotope signature of soil.

Carbon mineralization and stable C isotope signature of CO2

Soil and char mineralization was monitored by measuring the CO2 concentration (ppm) and δ13C-CO2 (%) of the C respired after 1, 2, 4, 7, 9, 15, 22, 29, 36, 61, 80, 101, 147, 181 and 222 days of incubation in the headspace of the incubation jar. The CO2 concentration was measured with a micro-GC (Agilent 490, Santa Clara, CA, USA) at constant temperature (60 °C) and pressure (150 kPa). The isotopic composition (δ13C, %) of the CO2-C was determined using a Gas Chromatograph GC (5890 Hewlett Packard, Avondale, PA, USA) coupled to an isotope ratio mass spectrometer (Isochrom III, Micromass-GVI Optima) and equipped with a 3-m full column filled with a Prorapak QS stationary phase (80–100 mesh). Temperature was increased from 80 °C to 160 °C at a rate of 50 °C min−1. After CO2 measurements, the headspace of the jars was flushed with synthetic air to avoid CO2 saturation.

Calculations

The stable carbon isotope signature of GS and HTC produced from C4 plants is similar to those of their feedstock (Naisse et al., 2013) and higher than that of soil organic C from C3 plants. Therefore, mineralization of GS and HTC as well as priming effects (PE) were monitored by recording δ13C of CO2 evolved from soil incubated with GS and HTC produced from maize silage. The proportion of CO2 evolved from C4 GS and HTC was calculated by isotopic mass balance using Eqn (1):

display math(1)

where δ13CO2 treatment is the δ13C (%) of evolved CO2 from soil-biochar mixtures, δ13CO2 control is the δ13C (%) of CO2 evolved from control soil without chars. δ13C of CO2 evolved from chars was shown to be stable during incubation (Malghani et al., 2013) Therefore, we used δ13C of bulk HTC and GS instead of δ13CO2 evolved from chars. The priming effect (PE) induced by GS and HTC addition on native SOC was calculated as the difference between the mineralization of soil organic C in treatments with HTC and GS (Csoil) and the control treatment without HTC and GS addition (Ccontrol):

display math(2)

Carbon sequestration potential (% of C input) was calculated as the sum of C losses due to biochar mineralization, priming and losses due to weathering.

Statistics

Data obtained after the incubation experiments were expressed as means of the three replicate incubations ± SE. Differences between soil treatments were determined using one way analysis of variance (anova) and Tukey's multiple comparison test.

Char mineralization was modeled by a two component first order exponential model using the following equation:

display math(3)

Curve fitting was carried out using a Bayesian curve fitting method and the R software for statistical computing version 2.15.1 (R Development Core Team, Vienna, Austria).

Results

Selected characteristics of fresh biochar (GS) and hydrochar (HTC) as well as weathered biochar (W/GS) and hydrochar (W/HTC) are given in Table 1. As shown before, GS showed higher C content compared to HTC (Naisse et al., 2013). Use of different feedstocks did not significantly influence the C content of GS. Artificial weathering led to decrease in GS and HTC C content (Table 1). C loss during artificial weathering ranged between 16.2 ± 0.4% and 38.3 ± 2.4% for GS from poplar and was intermediate for HTC with 23.9 ± 2.8% (Fig. 1). Feedstock significantly influenced the resistance of GS to physical weathering, with lower losses when produced from wood compared to maize feedstock. No effects were observed for N content. The C/N ratio thus decreased after physical weathering (Table 1). The H/C and O/C atomic ratios were higher for HTC than both GS, and after weathering these ratios increased for GS from poplar and HTC and decreased for GS from maize (Table 1). During weathering, size of particles leached from HTC had a broad range (Fig. 1), while the size of particles leached from GS showed low standard deviations. The size of particles leached from GS depended on feedstock with smaller particles leached from poplar GS (6.1 μm) than maize GS (18.3 μm).

Table 1. Elemental composition and pH of fresh and weathered GS and HTC (mean ± SD, n = 4)
 pHChemical composition (wt%)
CΔ13CNC/NH/CO/C
  1. Different letters within the same column are related to significant differences (n = 3).

GS Poplar9.6 ± 0.1d70.5 ± 1.8b−28.4 ± 0.1c1.7 ± 0.0b40.4 ± 1.8ab0.330.25
W/GS Poplar7.9 ± 0.1c66.4 ± 3.0b−28.4 ± 0.0c1.7 ± 0.0b40.0 ± 1.3ab0.410.31
GS Maize10.1 ± 0.0e69.6 ± 1.3b−13.7 ± 0.2a1.7 ± 0.1ab41.7 ± 1.2a0.400.33
W/GS Maize9.4 ± 0.2d63.7 ± 1.2b−13.8 ± 0.1a1.8 ± 0.1ab35.6 ± 1.4bc0.300.24
HTC3.6 ± 0.0a52.2 ± 4.9a−14.6 ± 0.4b1.5 ± 0.1a36.0 ± 2.8bc1.220.63
W/HTC4.5 ± 0.1b46.1 ± 6.1a−14.5 ± 0.2b1.4 ± 0.2a32.5 ± 1.8c1.490.67
Figure 1.

Carbon and nitrogen loss during artificial weathering as well as size of weathered fraction (mean ± SD, n = 4).

Figure 2.

Carbon resistant to acid dichromate oxidation for fresh and weathered GS and HTC. (mean ± SD, n = 4).

The pH values recorded for GS were strongly alkaline, close to 10, while HTC was acid (3.6). The artificial weathering affected the pH values of both char types and the pH values tended towards neutrality, (i.e. GS less basic and HTC less acidic). After weathering, the δ13C signature of GS Maize and HTC were decreased, by +0.5 and +1.3.

Chemical stability

The amounts of C remaining after 12 h of K2Cr2O7 oxidation for fresh and weathered GS and HTC are reported in Fig. 2. The data ranged from 27% for HTC to 88.6% for GS. As observed before, GS were more resistant to the oxidation than HTC (Naisse et al., 2013). Weathering did not significantly change the chemical reactivity of GS and HTC (Fig. 2).

Figure 3.

Cumulative total CO2 mineralization from soil after addition of fresh and weathered GS and HTC (mean ± SD, n = 3).

Biological stability

Carbon mineralization

The GS input did not lead to a significant increase in the total C mineralization (P > 0.05), while the HTC input increased significantly the total C mineralization (P < 0.001) (Fig. 3). After 222 days of incubation, HTC and W/HTC mineralization represented with 77% and 56% the majority of total CO2 evolved. The GS and W/GS mineralization was a magnitude lower than that of HTC and W/HTC (Table 2; Fig. 4). For both GS and W/GS, mineralization rates decreased rapidly during the early stages of the incubation to reach nearly constant mineralization rates after 36 days. Moreover, differences in cumulated mineralization at the end of the incubation between GS and W/GS were due to differences in mineralization rates during early stages of incubation. Artificial weathering had no significant effect (P < 0.05) on the total CO2 evolved for poplar GS, while it decreased C mineralization of GS and HTC from maize silage (Fig. 3). However, the weathering led to significantly lower GS and HTC mineralization after 222 days of incubation compared to unweathered chars (Table 2),

Table 2. Summary data of cumulative char mineralization and cumulative priming effects (PE) at the end of the incubation period and extrapolated from a double exponential model after 10 and 100 years, (mean ± SD, n = 3)
 Cumulative chars mineralization (% of C input)Cumulative PE (% of C input) at 222 days
at 222 daysat 10 yearsat 100 years
GS Maize1.4 ± 0.1a9.861.4−0.85 ± 0.11a
W/GS Maize0.6 ± 0.1b4.938.2−0.38 ± 0.04b
HTC16.7 ± 0.7c61.799.90.83 ± 0.03c
W/HTC6.5 ± 0.2d50.199.91.25 ± 0.11d
Figure 4.

Char mineralization in soil of fresh and weathered GS and HTC produced from maize feedstock. Marks with error bars indicate standard deviation of the mean (n = 3) and curves are the result of two pool model calculations.

Priming effects

The addition of GS induced a negative priming effect (PE) on SOM, while HTC led to a positive PE (Table 2; Fig. 5). After 3 weeks of incubation, the priming effect quickly decreased to zero. After 222 days of incubation, the decrease in SOM mineralization for the GS treatment corresponded to 0.097 ± 0.016 mg C-CO2 g−1 dry soil, and for HTC to an increase in SOM mineralization of 0.084 ± 0.014 mg C-CO2 g−1 dry soil. At the end of incubation, weathering reduced significantly the positive PE (P < 0.001) for GS as well as the negative PE for HTC (P < 0.001).

Figure 5.

Priming effect rates (PE) (a) and cumulative PE (b) of soil organic matter induced by hydrochar (HTC), biochar (GS) and weathered hydrochar (W/HTC) and weathered biochar (W/GS) produced from maize feedstock (n = 3, mean ± SD).

Modeling data

The evaluation of model performance showed that the ‘Biexponential’ model was the most adequate for both GS and HTC, and providing the best fit for GS and W/GS (lower AIC values) to the experimental data (Fig. 4). Nevertheless, the model ‘Biexponential + constant’ was the most appropriate model for HTC and W/HTC. The residence time of labile and stable pools and stable pool size are given in Table 3. The proportion of stable pool carbon was higher for GS than HTC, with respectively 99.1% and 87.6%. The residence time of stable C pool, was about 10 times higher for GS than HTC, with respectively a half-life of 73.6 ± 5.6 and 8.3 ± 0.9 years. The artificial aging increased the size and residence time of the stable pool size of both chars (Table 3).

Table 3. Estimation with biexponential model of turnover time (t1/2) for labile and stable pool and proportion of stable C pool of biochar (k values) (mean ± SD, n = 3)
 Labile pool CStable C poolStable pool size (%)
k (days)t1/2 (days)k (days)t1/2 (years)
GS Maize(6.4.10−1)1.1 ± 0.1(2.6.10−5)73.6 ± 5.699.1
W/GS Maize(8.2.10−2)8.4 ± 2.0(1.3.10−5)145.0 ± 24.099.7
HTC(6.4.10−2)10.8 ± 0.6(2.3.10−4)8.4 ± 0.987.6
W/HTC(1.3.10−1)5.4 ± 0.4(1.8.10−4)10.4 ± 0.397.6

Discussion

Elemental content

The aim of the artificial weathering process was to simulate the exposure of biochar (GS) and hydrochar (HTC) to environmental conditions. This includes physical fractionation leaching of soluble compounds, erosion of fine particles and desorption of volatile organic compounds. We assume that the protocol we established was a good proxy to simulate these effects, as we observed significantly lower C contents and pH modifications after the artificial weathering cycle (Table 1). Loss of dissolved compounds and fine particles from the system due to weathering was significant and has been observed by other authors (Abiven et al., 2011). Our data are in agreement with the suggestion that dissolution could be an important factor of pyrogenic C export from soil (Jaffé et al., 2013; Norwood et al., 2013), although these compounds could also be susceptible to sorption and stabilization by mineral interactions. Feedstock had an influence on C loss after artificial weathering of GS, with GS produced from maize feedstock showing greater modifications of chemical parameters due to weathering than GS produced from woody feedstock. This could be related to differences in physical structure of the two feedstock materials with most likely bigger particles of wood GS than maize silage GS, which is in turn more susceptible to erosion. It is interesting to note that despite the strong differences in C loss between GS from different feedstocks (Fig. 1), their chemical stability was similar (Fig. 2). C losses from HTC were intermediate between both GS. These results are in contrast to a recent study showing that biochar produced at lower temperature showed the highest mobility in soil (Wang et al., 2012). The discrepancy may be explained by the completely different production procedures of both chars. Hydrothermal carbonization products and gasification chars may not be directly comparable in terms of environmental reactivity although the evolution of their elemental composition seems to follow the general temperature trend recorded for chars (Table 1; Wiedner et al., 2013b).

Furthermore, the increased of O/C atomic ratio for popular GS and HTC after artificial weathering (Table 1), due to an increased O content, seems to be the result of the oxidation process (Baldock & Smernik, 2002). Indeed, Yao et al. (2010) showed for laboratory aging that the C content of biochar decreased by 5.7% due to increasing O and H contents by formation of carbonyl and carboxylic groups, indicative of biochar oxidation. However, preferential loss of highly aromatic material may also have led to the observed changes in elemental content. Many studies observed an increase in O and H content of weathered biochar sampled in the field, associated with a decrease in biochar pH (Joseph et al., 2010; Jones et al., 2012; Spokas, 2013). Our study showed that weathering leads to decreasing pH for GS and increasing pH for HTC. These physicochemical changes most probably occurring during the first year of field exposure might influence the chemical and biological stability of both chars.

Chemical and biological stability

Biological stability of both chars was evaluated after incubation in soil. Total C mineralization increased after GS addition during 222 days of incubation, by 7.28 ± 1.59% and 14.9 ± 1.8% for poplar GS and maize GS. For HTC, much higher increase in total C mineralization was noted (Fig. 3). The smallest increase in total CO2 mineralization was noted after poplar GS addition and may be attributed to the complex organization of woody tissues and large size of poplar GS particles which most likely exposes the lowest surface available for microbial activity.

After 222 incubation days, 1.4% and 16% of the added GS and HTC were mineralized (Table 2, Fig. 4.). These differences are in accordance with studies by Malghani et al. (2013), and Gajić et al. (2012), indicating that HTC has limited C storage potential compared to GS. GS mineralization rates are in between those observed for biochar in the literature, i.e. Luo et al. (2011), who observed mineralization of 0.14–0.18% of biochar input after 87 days and Kuzyakov et al. (2009) who observed a biochar mineralization of 1.84 to 2.10% after 60 days. Differences in mineralization rates may probably be explained by different soil types used in different studies.

Weathering reduced the mineralization of GS and HTC in comparison to fresh HTC and GS. This could be due to removal of volatile compounds and nutrients during the weathering procedure (Yao et al., 2010). It was shown that K, S, Ca, Mg and P are particularly susceptible to leaching (Spokas, 2013; Novak et al., 2009). In agreement with these studies, the N content of GS and HTC did not change after weathering (Table 1). Weathering solutions of biochar were found to have very low concentration of ammonium-N and nitrate-N (Yao et al., 2010), most probably related to the fact that biochar N is present within heterocyclic structures (Knicker, 2007). Moreover, we can assume that the N loss was related to the C loss in the form of fine particles, instead of soluble compounds. Our results indicate that physical weathering during the early exposure of HTC in soil would increase significantly its C storage capacity, and should be taken into account for the assessment of its C storage potential.

Evaluation of chemical stability by use of chemical reagents may be a promising technique to rapidly predict the long-term fate of exogenous organic matter additions in soil (Naisse et al., 2013; Ngo et al., 2013). While GS was much more resistant to dichromate oxidation than HTC, artificial weathering had little effect on the chemical stability of GS and HTC. This may be explained by the contrasting chemical composition of both chars (Wiedner et al., 2013a) and the relatively small changes introduced by artificial weathering (Table 1). This suggests that both chars have contrasting long-term stability, and that artificial weathering does not influence the long-term stability of these chars. The time required for obtaining carbon loss equivalent to chemical oxidation during laboratory incubation under optimum conditions is 15.0 years and 23.3 years, respectively, for the GS and the weathered GS and 10.7 years and 15.6 years, respectively, for HTC and weathered HTC. Thus, chemical oxidation with acid dichromate would be a fast and robust experience allowing determining the biochar fraction lost within a decade.

Our results indicate that changing physicochemical properties due to weathering after exposure in soil have to be considered in addition to initial chemical recalcitrance for the evaluation of biochar residence time in soil. In the following, we assessed the C sequestration potential of the chars by integrating all C losses due to weathering, C mineralization from biochar itself and from soil organic matter.

Carbon sequestration potential

Comparisons of four different models to fit the mineralization data showed that the ‘biexponential’ model with or without constant were most suitable. This is in accordance with many studies on soil organic matter mineralization, showing the presence of two pools with contrasting turnover times (Paul & Clarck, 1996). Our data showed that the half-life of the stable C pool, which represents a large majority of the total C in HTC and GS, was of 8.4 ± 0.9 years for HTC and 73.6 ± 5.6 years for GS (Table 3). Bai et al. (2013) by incubating biochar produced at 575 °C and HTC produced at 200 °C, obtained in three different soils, different half-lives from 19.7 up to 44.5 years for biochar and from 0.7 up to 2.1 years for HTC. Lower half-lives obtained by Bai et al. (2013), could indicate that soil intrinsic properties may be crucial for the stability of biochar in soil.

Artificial aging tends to reduce the size of the labile pool C, and increased the stability of stable pool C, especially for GS (Table 3). Thus, we can hypothesize that similar to compost (Peltre et al., 2010), easily mobilizable and decomposable C could drive the degradation of the most stable C of biochar.

In addition to carbon mineralization from biochar and hydrochar themselves, we also need to consider the modification in mineralization rates of the native SOM after addition to soil (positive priming effect or negative priming effect) to estimate the C sequestration potential of GS and HTC. Generally, it has been observed that biochar produced at lower temperature induces a positive priming effect (PE), while biochar produced at higher temperature induces a negative PE (Zimmemann et al., 2011). Our results were consistent with this observation, since up to 35 days of incubation, we observed a negative PE for GS and a positive PE for HTC. Weathering led to a reduction in both priming effects (Fig. 5). Our data are in accordance with Keith et al. (2011) showing that the interaction between native SOM and biochar would be a short-term reaction with little impact on long-term carbon sequestration (i.e. at the time scale of decades and centuries).

The C sequestration potential was calculated as % of chars input in soil, considering chars mineralization, priming effect and C losses during weathering (Fig. 6). At the end of incubation (222 days), the C sequestration potential was higher for GS than HTC (P < 0.05) representing 99.4% and 82.5% of initial C input. The C sequestration close to 100% for GS was the result of the combination of the high biological stability and the negative priming effect. Taking into account leaching and the results of the model, we can expect 91.1% and 36.8% from the initial GS and HTC to be remaining in soils after 10 years. At the century scale, the C sequestration potential would still be important for GS (39.0%), but slightly negative for HTC (−0.7%). The negative C sequestration potential of HTC is the result of the positive priming effect and the almost complete mineralization after several decades (Table 2). The high sensitivity to carbon loss during physical weathering of maize GS induced a lower C sequestration potential at the end of the incubation period (P < 0.05). In this case, weathering reduced the annual C sequestration potential by 37.6%. However, due to the high stability of weathered GS, the C sequestration potential at the century scale would likely be close to the fresh GS. The C sequestration potential at 222 days was lower by 13% for weathered HTC compared to fresh HTC (P < 0.05), and 12% higher compared to weathered GS (P < 0.05).

Figure 6.

Carbon sequestration potential (% of C input) of HTC and GS produced from maize feedstock represented as sum of GS or HTC remaining in soil considering C losses due to mineralization, priming effect (GS, HTC), as well as additional weathering effect (W/GS, W/HTC).

Thus, a user who aims to store massive amounts of C in soil at the decade scale could consider the use of either GS or HTC depending on feedstock properties. The GS input could store a large amount C at the century scale, while the HTC input would likely be incompatible with this objective. However, these results were obtained in an artificial laboratory context. Experiments under field conditions involving complex interactions between plants, microorganism, earthworms, minerals and soil organic matters need to be carried out to assess the C sequestration potential of this technique before to be widely used in a temperate European context.

Conclusion

The thermochemical transformations of biomass by gasification or hydrothermal carbonization are two attractive techniques being developed on an industrial scale to produce energy and soil amendments. Both processes yield char materials intended to increase C storage when applied to soil. In this study, we assessed the carbon sequestration potential of GS and HTC from different feedstocks at contrasting timescales and quantified effects due to physical weathering. Short-term biological stability was evaluated during 222 days of laboratory incubation, while chemical oxidation was used to assess char stability at longer timescale. Comparison of the two methods showed that 10.3–23.3 years will be necessary for an equivalent C loss during biological oxidation compared to the C loss after 12 h of chemical oxidation with acid dichromate. Thus, chemical methods might be good means for the medium-term evaluation of biological stability of chars.

Our results confirmed that the chemical and biological stability of GS is more important than that of HTC. During the early stage of soil incubation, the HTC input induced a positive priming effect, indicating a stimulation of SOM mineralization by addition of easily degradable carbon. In contrast, the GS input induced a negative priming effect showing a rapid SOM protection. However, for both chars, no priming effect after the first weeks of incubation was observed, indicating that priming is a short-term phenomenon in pasture soil.

Physical weathering of chars led to increasing O/C atomic ratios, due to the surface oxidation, and a loss of fine particles. A feedstock effect was observed for GS with a massive C loss during weathering from maize stover compared to wood. Thus, the wide range of feedstock potentially used for the production of GS and HTC requires a detailed investigation of main feedstock types. Physical weathering effects on biological and chemical stability as well as priming suggest that it has consequences for the C sequestration potential at decadal timescale.

Half-lives of fresh and weathered HTC were estimated between 8.4 ± 0.9–10.4 ± 0.3 years, and between 73.6 ± 5.6–145.0 ± 24.0 years for fresh and weathered GS. Carbon sequestration, potential including char mineralization, priming effect and physical weathering assessed for both chars, indicate that at the decadal scale, HTC and GS would lead to similar increase in soil C storage. At the century scale, only GS would have the potential to increase soil carbon sequestration.

Acknowledgements

The research conducted by the authors was financed by European Community within the EuroChar project (FP7-ENV-478 2010ID-265179). Andréa Rigaud and Daniel Billiou are acknowledged for their help with the laboratory work. We thank Pierre Barré of the ENS Paris for the access to the laser for particle size determination, as well as for helpful discussions.

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