Short term soil priming effects and the mineralisation of biochar following its incorporation to soils of different pH

The aim of this work was to determine the magnitude of the priming effect, i.e. short-term changes in the rate (negative or positive) of mineralisation of native soil organic carbon (C), following addition of biochars. The biochars were made from Miscanthus giganteus, a C4 plant, naturally enriched with ¹³C. The biochars were produced at 350 °C (biochar350) and 700 °C (biochar700) and applied with and without ryegrass as a substrate to a clay-loam soil at pH 3.7 and 7.6. A secondary aim was to determine the effect of ryegrass addition on the mineralisation of the two biochars.After 87 days, biochar350 addition caused priming effects equivalent to 250 and 319 μg CO₂-C g⁻¹ soil, in the low and high pH soil, respectively. The largest priming effects occurred at the start of the incubations. The size of the priming effect was decreased at higher biochar pyrolysis temperatures, which may be a way of controlling priming effects following biochar incorporation to soil, if desired. The priming effect was probably induced by the water soluble components of the biochar. At 87 days of incubation, 0.14% and 0.18% of biochar700 and 0.61% and 0.84% of biochar350 were mineralized in the low and high pH soil, respectively. Ryegrass addition gave an increased biochar350 mineralisation of 33% and 40%, and increased biochar700 at 137% and 70%, in the low and high pH soils, respectively. Certainly, on the basis of our results, if biochar is used to sequester carbon a priming effect may occur, increasing CO₂-C evolved from soil and decreasing soil organic C. However, this will be more than compensated for by the increased soil C caused by biochar incorporation. A similar conclusion holds for accelerated mineralisation of biochar due to incorporation of fresh labile substrates. We consider that our results are the first to unequivocally demonstrate the initiation, progress and termination of a true positive priming effect by biochar on native soil organic C.     

Effect of temperature on biochar priming effects and its stability in soils

Recent studies have shown both increased (positive priming) and decreased (negative priming) mineralisation of native soil organic carbon (SOC) with biochar addition. However, there is only limited understanding of biochar priming effects and its C mineralisation in contrasting soils at different temperatures, particularly over a longer period. To address this knowledge gap, two wood biochars (450 and 550 °C; δ¹³C −36.4‰) were incubated in four soils (Inceptisol, Entisol, Oxisol and Vertisol; δ¹³C −17.3 to −28.2‰) at 20, 40 and 60 °C in the laboratory. The proportions of biochar- and soil-derived CO₂–C were quantified using a two-pool C-isotopic model.Both biochars caused mainly positive priming of native SOC (up to +47 mg CO₂–C g⁻¹ SOC) in the Inceptisol and negative priming (up to −22 mg CO₂–C g⁻¹ SOC) in the other soils, which increased with increasing temperature from 20 to 40 °C. In general, positive or no priming occurred during the first few months, which remained positive in the Inceptisol, but shifted to negative priming with time in the other soils. The 550 °C biochar (cf. 450 °C) caused smaller positive priming in the Inceptisol or greater negative priming in the Entisol, Oxisol and Vertisol at 20 and 40 °C. At 60 °C, biochar caused positive priming of native SOC only in the first 6 months in the Inceptisol. Whereas, in the other soils, the native SOC mineralisation was increased (Entisol and Oxisol) and decreased (Vertisol) only after 6 months, relative to the control. At 20 °C, the mean residence time (MRT) of 450 °C and 550 °C biochars in the four soils ranged from 341 to 454 and 732−1061 years, respectively. At 40 and 60 °C, the MRT of both 450 °C biochar (25−134 years) and 550 °C biochar (93−451 years) decreased substantially across the four soils. Our results show that biochar causes positive priming in the clay-poor soil (Inceptisol) and negative priming in the clay-rich soils, particularly with biochar ageing at a higher incubation temperature (e.g. 40 °C) and for a high-temperature (550 °C) biochar. Furthermore, the 550 °C wood biochar has been shown to persist in soil over a century or more even at elevated temperatures (40 or 60 °C).     

Negative priming of native soil organic carbon mineralization by oilseed biochars of contrasting quality

Oilseed‐derived biochar, a by‐product of pyrolysis for biodiesel production, is richer in aliphatic compounds than the commonly studied wood‐derived biochar, affecting both its mineralization in soil and its interaction with native soil organic carbon (nSOC). Here, we investigated the soil C sequestration potential of three different oilseed biochars derived from C₃ plant material: soyabean, castor bean and jatropha cake. The chemical composition of these biochars was determined by elemental analysis (CHN) and ¹³C NMR spectroscopy. The cumulative CO₂ efflux from 30‐day laboratory incubations of biochar mixed with a sandy soil containing nSOC from C₄ plants was measured as a proxy for mineralization rate. The relative contribution of each source to CO₂ production was calculated based on the ¹³C‐signatures of total CO₂ efflux and the source materials (soil and biochars). Our results showed that: (i) castor bean biochar contained relatively large amounts of aliphatic compounds, resulting in a greater mineralization rate than soyabean and jatropha biochars; (ii) CO₂ efflux from the soil‐biochar mixtures originated mostly from the biochars, suggesting that these biochars contain rapidly decomposable compounds; and (iii) all three oilseed biochars decelerated nSOC mineralization. This negative priming effect appeared to be caused by different factors. We conclude that oilseed biochars have the potential to increase soil C stocks directly and increase soil C sequestration indirectly in the short term through negative priming of nSOC mineralization.     

Carbon potentials of different biochars derived from municipal solid waste in a saline soil

There are numerous studies conducted on biochar for its carbon (C) sequestration potential;however,there are limited studies available on the behavior of salt-affected soils related to biochar application.Therefore,more studies are needed to elucidate the mechanisms through which biochar affects saline soil properties.In this study,biochars were produced from solid waste at pyrolysis temperatures of 300,500,and 700?C (BC300,BC500,and BC700,respectively)and applied to a saline soil to evaluate their impacts on soil carbon dioxide (CO₂) efflux,C sequestration,and soil quality.A soil incubation experiment lasting for 107 d was conducted.The results showed that soil CO₂ efflux rate,cumulative CO₂ emission,active organic C (AOC),and organic matter (OM)significantly increased with BC300 application to a greater extent than those with BC500 and BC700 as compared to those in the no-biochar control (CK).However,soil C non-lability did not significantly increase in the treatments with biochars,except BC700,as compared to that in CK.Besides improving the soil quality by increasing the soil AOC and OM,BC300 showed positive impacts in terms of increasing CO₂ emission from the saline soil,while BC500 and BC700 showed greater potentials of sequestering C in the saline soil by increasing the soil non-labile C fraction.The recalcitrance index (R50) values of BC500 and BC700 were>0.8,indicating their high stability in the saline soil.It could be concluded that biochars pyrolyzed at high temperatures (?500?C)could be suitable in terms of C sequestration,while biochars pyrolyzed at low temperatures (?300?C) could be suitable for improving saline soil quality.     

Carbon mineralization in subtropical alluvial arable soils amended with sugarcane bagasse and rice husk biochars

Subtropical recent alluvial soils are low in organic carbon (C). Thus, increasing organic C is a major challenge to sustain soil fertility. Biochar amendment could be an option as biochar is a C-rich pyrolyzed material, which is slowly decomposed in soil. We investigated C mineralization (CO₂-C evolution) in two types of soils (recent and old alluvial soils) amended with two feedstocks (sugarcane bagasse and rice husk) (1%, weight/weight), as well as their biochars and aged biochars under a controlled environment (25 ±2 ℃) over 85 d. For the recent alluvial soil (charland soil), the highest absolute cumulative CO₂-C evolution was observed in the sugarcane bagasse treatment (1 140 mg CO₂-C kg^-1 soil) followed by the rice husk treatment (1 090 mg CO₂-C kg^-1 soil); the lowest amount (150 mg CO₂-C kg^-1 soil) was observed in the aged rice husk biochar treatment. Similarly, for the old alluvial soil (farmland soil), the highest absolute cumulative CO₂-C evolution (1 290 mg CO₂-C kg^-1 soil) was observed in the sugarcane bagasse treatment and then in the rice husk treatment (1 270 mg CO₂-C kg^-1 soil); the lowest amount (200 mg CO₂-C kg^-1 soil) was in the aged rice husk biochar treatment. Aged sugarcane bagasse and rice husk biochar treatments reduced absolute cumulative CO₂-C evolution by 10% and 36%, respectively, compared with unamended recent alluvial soil, and by 10% and 18%, respectively, compared with unamended old alluvial soil. Both absolute and normalized C mineralization were similar between the sugarcane bagasse and rice husk treatments, between the biochar treatments, and between the aged biochar treatments. In both soils, the feedstock treatments resulted in the highest cumulative CO₂-C evolution, followed by the biochar treatments and then the aged biochar treatments. The absolute and normalized CO₂-C evolution and the mineralization rate constant of the stable C pool (K_s) were lower in the recent alluvial soil compared with those in the old alluvial soil. The biochars and aged biochars had a negative priming effect in both soils, but the effect was more prominent in the recent alluvial soil. These results would have good implications for improving organic matter content in organic C-poor alluvial soils.     

Microbial biomass growth,following incorporation of biochars produced at 350 °C or 700 °C,in a silty-clay loam soil of high and low pH

Biochar has been widely proposed as a soil amendment, with reports of benefits to soil physical, chemical and biological properties. To quantify the changes in soil microbial biomass and to understand the mechanisms involved, two biochars were prepared at 350 °C (BC350) and 700 °C (BC700) from Miscanthus giganteus, a C4 plant, naturally enriched with ¹³C. The biochars were added to soils of about pH 4 and 8, which were both sampled from a soil pH gradient of the same soil type. Isotopic (¹³C) techniques were used to investigate biochar C availability to the biomass. Scanning Electron Microscopy (SEM) was used to observe the microbial colonization, and Attenuated Total Reflectance (ATR) to highlight structural changes at the surface of the biochars. After 90 days incubation, BC350 significantly increased the biomass C concentration relative to the controls in both the low (p < 0.05) and high pH soil (p < 0.01). It declined between day 90 and 180. The same trend occurred with soil microbial ATP. Overall, biomass C and ATP concentrations were closely correlated over all treatments (R² = 0.87). This indicates that neither the biomass C, nor ATP analyses were affected by the biochars, unless, of course, they were both affected in the same way, which is highly unlikely. About 20% of microbial biomass ¹³C was derived from BC350 after 90 days of incubation in both low and high pH soils. However, less than 2% of biomass ¹³C was derived from BC700 in the high pH soil, showing very low biological availability of BC700. After 90 days of incubation, microbial colonization in the charsphere (defined here as the interface between soil and biochar) was more pronounced with the BC350 in the low pH soil. This was consistent with the biomass C and ATP results. The microbial colonization following biochar addition in our study was mainly attributed to biochar C availability and its large surface area. There was a close linear relationship between ¹³CO₂ evolved and biomass ¹³C, suggesting that biochar mineralization is essentially a biological process. The interactions between non-living and living organic C forms, which are vital in terms of soil fertility and the global C cycle, may be favoured in the charsphere, which has unique properties, distinct from both the internal biochar and the bulk soil.     

Priming effect of bamboo (Phyllostanchys edulis Carrière) biochar application in a soil amended with legume

Abstract

Biochar application to soils can mitigate carbon dioxide (CO₂) by increasing soil carbon (C) sink, but also causes increased CO₂ released from soils through priming effects of soil organic carbon (SOC). However, priming effects of biochar application on SOC are complex, showing inconsistent results, and further complicated when applied with other substrates such as organic amendment (OA). Incubation experiments were conducted using Typic Durudand with bamboo (Phyllostanchys edulis Carrière) biochar (400°C) and OA (crotalaria) applied individually, simultaneously or with biochar applied 5 weeks prior to OA application. After 56 d of incubation, cumulative CO₂ released from soils with no amendments (control), biochar only (BC), OA only (OA), simultaneous (BC+OA), and differently timed (BCP+OA) applications reached 313, 326, 1270, 1535 and 1311 mg CO₂ kg^?1, respectively. The OA application distinctly increased CO₂ released from the soils due to its decomposition. The OA decomposition rates were comparable with OA and BC+OA, while those with BCP+OA were lower than those with other treatments during early incubation. Net CO₂ (CO_{2-(treatment)} ? CO_2-control) from soils with BC, OA, BC+OA and BCP+OA yielded 13, 957, 1222 and 998 mg CO₂ kg^?1, respectively. Primed CO_2-BC of 13 mg CO₂ kg^?1 was equivalent to 4.2% of priming effect relative to CO_2-control. Primed CO_2-BC+OA [net CO_2-BC+OA ? (net CO_2-BC + net CO_2-OA)] and primed CO_2-BCP+OA were 252 and 28 mg CO₂ kg^?1, equivalent to 26% and 2.9% of priming effects relative to sum of net CO_2-BC + net CO_2-OA, respectively. The priming effect with BC was negligible likely because of limited amounts of biochar labile C to induce co-metabolism, while BC+OA showed a modest priming effect most likely as a result of co-metabolism induced by additional mineralization of presumably SOC and/or biochar, because the OA decomposition rates were not affected by biochar application. The priming effect with BCP+OA was comparable to that with BC likely due to changes in soil properties caused by biochar application prior to OA, likely from slowed decomposition rates of OA.     

秸秆及其生物炭对土壤碳库管理指数及有机碳矿化的影响

以河南省粮食主产区壤质潮土和砂土为研究对象,通过盆栽试验和室内恒温培养试验,研究了生物炭与不同腐殖化程度的传统有机物料(秸秆和腐熟鸡粪)单施及配施对壤质潮土和砂土有机碳储量、活性及碳库管理指数的影响,并进一步比较了小麦秸秆直接还田和制炭还田对土壤有机碳矿化的影响,以及生物炭对土壤原有有机碳矿化的调控作用。结果表明:相同添加量下,生物炭对土壤有机碳含量的提升效果优于秸秆和腐熟鸡粪,在壤质潮土和砂土上分别较对照提升了63.15%和115.62%。另外,生物炭显著增加了土壤稳态碳含量和土壤碳库指数(CPI),但降低了土壤碳素有效率(SC)和碳库活度指数(AI),对土壤易氧化有机碳(POXC)和碳库管理指数(CMPI)无显著影响,添加秸秆显著增加了2种土壤POXC含量、基础呼吸和CPMI。进一步通过室内恒温培养试验发现,秸秆可在培养前期(0～37天)大幅度提升2种类型土壤有机碳矿化速率和累积矿化量,秸秆制炭还田对土壤有机碳矿化无显著影响。此外生物炭对土壤原有有机碳矿化的调控作用受其施用量、外源活性有机碳输入和土壤类型的影响,高量生物炭(2%)对非秸秆还田土壤有机碳矿化表现出较强的负激发效应,而低量生物炭(0.55%)对秸秆还田土壤有机碳矿化表现出较明显的负激发效应。因此,从"固碳减排"角度考虑,秸秆制炭还田是更合理的利用方式,且应根据土壤施肥管理措施和土壤类型考虑生物炭的施用量,添加质量比为2%的生物炭可显著抑制土壤原有有机碳矿化,降低CO_2排放,但应避开秸秆快速腐解期施用。     

Biochar reduces the rhizosphere priming effect on soil organic carbon

Biochar has the potential to store carbon (C) in soils on a millennial time scale and hence it is proposed as a tool to aid in the mitigation of climate change. However, the presence of biochar in soil can induce either a positive or negative priming effect on native soil C, or the converse, which may either reduce or enhance the C storage potential of biochar. Thus far, priming effects between soil and biochar have been predominately assessed in the exclusion of plants. Therefore, this study set out with the aim to assess the priming effect of plants, i.e., rhizosphere priming effect (RPE) in the presence and absence of biochar and within different soil types. Three soils (Arenosol, Cambisol and Ferralsol) were used in full factorial combination with or without soybean plants and with or without 2% blue mallee biochar that was produced at 500 °C by slow pyrolysis. Plants were labelled with an isotopically depleted δ¹³C signature to that of the soil and biochar to allow the separation of plant-derived CO₂–C from the total CO₂–C. Carbon dioxide was trapped three times over a period of 13 days. Subsequent titration of the CO₂ trap samples followed by IRMS analysis was used to quantify the CO₂–C captured and its source. Biochar was found to have no effect on plant or microbial biomass. Plant treatments had significantly higher overall respiration rates than those without plants. Plants induced a negative priming in the Arenosol which was similar in the absence and presence of biochar. In the Cambisol, biochar induced a significant negative RPE in comparison to the positive RPE in the control. The RPE in the Ferralsol was positive and substantially decreased in the presence of biochar. Our results suggest that blue mallee biochar amendments may partially offset the positive RPE, or reduce it further where it is already negative.     

Short‐term releases of CO2 from newly mixed biochar and calcareous soil

The work aimed to quantify native organic C mobilized in one calcareous soil in the 21 days after addition of biochar at a range of large to very large applications. The experiment was carried out in unplanted microcosms, and CO₂ flux was used as a measure of net mineralization. A rapid methodological approach, which does not require ¹³C as a tracer, was used to assess any priming effects induced by the biochar. The amount of CO₂‐C mobilized was small relative to the amount of biochar C and proportional to the amount of the biochar added. The additional CO₂‐C was similar to the content of the water‐soluble organic carbon in the biochar added with each application. No interaction with native soil C, that is priming effect, was observed.     

Application of mixed straw and biochar meets plant demand of carbon dioxide and increases soil carbon storage in sunken solar greenhouse vegetable production

Solar vegetable greenhouse soils show low soil organic carbon content and thus also low rates of soil respiration. Processing vegetable residues to biochar and mixing biochar with maize straw might improve soil respiration and increase soil organic carbon stocks, while preventing the spread of soil-borne diseases carried by vegetable residues. In an incubation experiment, we tested how additions of maize straw (S) and biochar (B) added in varying ratios (100S, 75S25B, 50S50B, 25S75B, 100B and 0S0B (control)) affect soil respiration and fraction of added C remaining in soil. Daily CO₂ emissions were measured over 60 days incubation, the natural abundance of ¹³C in soil and in the added biochar and maize straw were analysed. Our result shows that (a) soil CO₂ emissions were significantly increased compared to soil without the straw additions, while addition of biochar only decreased soil respiration; (b) cumulative CO₂ emissions decreased with increasing ratio of added biochar to maize straw; (c) the abundance of soil ¹³C was significant positively correlated with cumulative CO₂ emissions, and thus with the ratio of straw addition. Our results indicate that incorporation of maize straw in greenhouse soils is a meaningful measure to increase soil respiration and to facilitate greenhouse atmosphere CO₂ limitation while producing vegetables. On the other hand, additions of biochar from vegetable residues will increase soil organic carbon concentration. Therefore, the simultaneous application of maize straw and biochar obtained from vegetable residues is an effective option to maintain essential soil functions for vegetable production in sunken solar greenhouses.     

Responses of Soil Microbial Community Structure and Activity to Incorporation of Straws and Straw Biochars and Their Effects on Soil Respiration and Soil Organic Carbon Turnover

Like straw, biochar incorporation can influence soil microorganisms and enzyme activities and soil carbon(C) responses; however,few studies have compared the various effects of straw and biochar and the underlying mechanisms. An experiment was performed to study the changes in soil respiration(SR) and soil organic C(SOC) fluxes in response to the incorporation of three kinds of straw(reed, smooth cordgrass, and rice) and their pyrolyzed products(biochars) at Chongming Island, China. In addition, the microbial activity and community structure of some amended soils were also analyzed to clarify the mechanisms of these responses. The results showed that all biochar incorporation(BC) induced lower SR than the corresponding unpyrolyzed straw incorporation(ST), and the average SR in the soils following BC and ST during the experimental periods was 21.69 and 65.32 μmol CO_2 m~(-2)s~(-1), respectively.Furthermore, the average SOC content was 16.97 g kg~(-1) following BC, which was higher than that(13.71 g kg~(-1)) following ST,indicating that compared to ST, BC was a low-C strategy, even after accounting for the C loss during biochar production. Among the BC treatments, reed-BC induced the lowest SR(17.04 μmol CO_2 m~(-2)s~(-1)), whereas smooth cordgrass-BC induced the highest SR(27.02 μmol CO_2 m~(-2)s~(-1)). Furthermore, in contrast with ST, BC significantly increased the abundance of some bacteria with poorer mineralization or better humification ability, which led to lower SR. The lower easily oxidizable C(EOC) and higher total C contents of biochars induced lower SR and higher SOC in the soil following BC compared to that following ST. Among the BC treatments,the higher total nitrogen content of rice biochar led to significantly higher soil microbial biomass, and the lower EOC content of reed biochar led to lower soil microbial activity and SR.     

Interactions between biochar stability and soil organisms: review and research needs

The stability of biochar in soils is the cornerstone of the burgeoning worldwide interest in the potential of the pyrolysis/biochar platform for carbon (C) sequestration. While biochar is more recalcitrant in soil than the original organic feedstock, an increasing number of studies report greater C‐mineralization in soils amended with biochar than in unamended soils. Soil organisms are believed to play a central role in this process. In this review, the variety of interactions that occur between soil micro‐, meso‐ and macroorganisms and biochar stability are assessed. In addition, different factors reported to influence biochar stability, such as biochar physico‐chemical characteristics, soil type, soil organic carbon (SOC) content and agricultural management practices are evaluated. A meta‐analysis of data in the literature revealed that biochar‐C mineralization rates decreased with increasing pyrolysis temperature, biochar‐C content and time. Enhanced release of CO₂ after biochar addition to soil may result from (i) priming of native SOC pools, (ii) biodegradation of biochar components from direct or indirect stimulation of soil organisms by biochar or (iii) abiotic release of biochar‐C (from carbonates or chemi‐sorbed CO₂). Observed biphasic mineralization rates suggest rapid mineralization of labile biochar compounds by microorganisms, with stable aromatic components decomposed at a slower rate. Comparatively little information is available on the impact of soil fauna on biochar stability in soil, although they may decrease biochar particle size and enhance its dispersion in the soil. Elucidating the impacts of soil fauna directly and indirectly on biochar stability is a top research priority.     

四种农业土壤上生物炭-土壤的交互效应

Soils in south-western Australia are highly weathered and deficient in nutrients for agricultural production. Addition of biochar has been suggested as a mean of improving soil C storage, texture and nutrient retention of these soils.~Clay amendment in sandy soils in this region is a management practice used to improve soil conditions, including water repellence.~In this study a woody biochar (Simcoa biochar) was characterised using scanning electron microscopy before, and four weeks after, it was incorporated into each of four soils differing in clay content and organic matter. Scanning electron microscopy of Simcoa biochar after incubation in soil showed different degrees of attachment of soil particles to the biochar surfaces after 28 d. In addition, the effects of three biochars, Simcoa biochar, activated biochar and Wundowie biochar, on soil microbial biomass C and soil respiration were investigated in a short-term incubation experiment. It was hypothesised that all three biochars would have greater potential to increase soil microbial activity in the soil that had higher organic matter and clay. After 28-d incubation in soil, all three biochars had led to a higher microbial biomass C in the clayey soil, but prior to this time, less marked differences were observed in microbial biomass C among the four soils following biochar application.     

Kinetics of native and added carbon mineralization on incubating at different soil and moisture conditions in Typic Ustochrepts and Typic Halustalf

The carbon dynamics in soils is of great importance due to its links to the global carbon cycle. The prediction of the behavior of native soil organic carbon (SOC) and organic amendments via incubation studies and mathematical modeling may bridge the knowledge gap in understanding complex soil ecosystems. Three alkaline Typic Ustochrepts and one Typic Halustalf with sandy, loamy sand, and clay loam texture, varying in percent SOC of 0.2; S₁, 0.42; S₂, 0.67; S₃ and 0.82; S₄ soils, were amended with wheat straw (WS), WS + P, sesbania green manure (GM), and poultry manure (PM) on 0.5% C rate at field capacity (FC) and ponding (P) moisture levels and incubated at 35 °C for 1, 15, 30 and 45 d. Carbon mineralization was determined via the alkali titration method after 1, 5, 7 14, 21, and 28 d. The SOC and inorganic carbon contents were determined from dried up (50 °C) soil samples after 1, 15, 30, and 45 d of incubation. Carbon from residue mineralization was determined by subtracting the amount of CO₂-C evolved from control soils. The kinetic models; monocomponent first order, two-component first order, and modified Gompertz equations were fitted to the carbon mineralization data from native and added carbon. The SOC decomposition was dependent upon soil properties, and moisture, however, added C was relatively independent. The carbon from PM was immobilized in S₄. All the models fitted to the data predicted carbon mineralization in a similar range with few exceptions. The residues lead to the OC build-up in fine-textured soils having relatively high OC and cation exchange capacities. Whereas, fast degradation of applied OC in coarse-textured soils leads to faster mineralization and lower build-up from residues. The decline in CaCO₃ after incubation was higher at FC than in the P moisture regime.     

The effect of young biochar on soil respiration

The low temperature pyrolysis of organic material produces biochar, a charcoal like substance. Biochar is being promoted as a soil amendment to enhance soil quality, it is also seen as a mechanism of long-term sequestration of carbon. Our experiments tested the hypothesis that biochar is inert in soil. However, we measured an increase in CO₂ production from soils after biochar amendment which increased with increasing rates of biochar. The ∂¹³C signature of the CO₂ evolved in the first several days of the incubation was the same as the ∂¹³C signature of the biochar, confirming that biochar contributed to the CO₂ flux. This effect diminished by day 6 of the incubation suggesting that most of the biochar C is slowly decomposing. Thus, aside from this short-term mineralization increasing soil C with young biochar may indeed be a long-term C storage mechanism.     

Microbial mineralization of biochar and wheat straw mixture in soil: A short-term study

A short-term incubation study was carried out to investigate the effect of biochar addition to soil on CO₂ emissions, microbial biomass, soil soluble carbon (C) nitrogen (N) and nitrate–nitrogen (NO₃–N). Four soil treatments were investigated: soil only (control); soil + 5% biochar; soil + 0.5% wheat straw; soil + 5% biochar + 0.5% wheat straw. The biochar used was obtained from hardwood by pyrolysis at 500 °C. Periodic measurements of soil respiration, microbial biomass, soluble organic C, N and NO₃–N were performed throughout the experiment (84 days). Only 2.8% of the added biochar C was respired, whereas 56% of the added wheat straw C was decomposed. Total net CO₂ emitted by soil respiration suggested that wheat straw had no priming effect on biochar C decomposition. Moreover, wheat straw significantly increased microbial C and N and at the same time decreased soluble organic N. On the other hand, biochar did not influence microbial biomass nor soluble organic N. Thus it is possible to conclude that biochar was a very stable C source and could be an efficient, long-term strategy to sequester C in soils. Moreover, the addition of crop residues together with biochar could actively reduce the soil N leaching potential by means of N immobilization.     

Plant-biochar interactions drive the negative priming of soil organic carbon in an annual ryegrass field system

There is a knowledge gap on biochar carbon (C) longevity and its priming effects on soil organic carbon (SOC) and recent root-derived C under field conditions. This knowledge would allow the potential of biochar in long-term soil C sequestration to be established. However, most studies on biochar C longevity and its priming effect have been undertaken in plant-free laboratory incubations.A 388 d field study was carried out in the presence of an annual ryegrass (C₃) growing on a rhodic ferralsol with established C₃/C₄ plant-derived SOC (δ¹³C: −20.2‰) in a subtropical climate. A ¹³C-depleted hardwood biochar (δ¹³C: −35.7‰, produced at 450 °C) was applied at 0 and 30 dry t ha⁻¹ and mixed into the top 100-mm soil profile (equivalent to 3% w/w). We report on the differentiation and quantification of root respiration and mineralisation of soil-C and biochar-C in the field. Periodic ¹³CO₂ pulse labelling was applied to enrich δ¹³C of root respiration during two separate winter campaigns (δ¹³C: 151.5–184.6‰) and one summer campaign (δ¹³C: 19.8–31.5‰). Combined soil plus root respiration was separated from leaf respiration using a novel in-field respiration collar. A two-pool isotope mixing model was applied to partition three C sources (i.e. root, biochar and soil). Three scenarios were used to assess the sensitivity associated with the C source partitioning in the planted systems: 1) extreme positive priming of recent SOC derived from the current ryegrass (C₃) pasture; 2) equivalent magnitude of priming of SOC and labile root C; and 3) extreme positive priming of the native C4-dominant SOC.We showed that biochar induced a significant negative priming of SOC in the presence of growing plants but no net priming was observed in the unplanted soil. We also demonstrated the importance of experimental timeframe in capturing the transient nature of biochar-induced priming, from positive (day 0–62) to negative (day 62–388). The presence/absence of plants had no impact on biochar-C mineralisation in this ferralsol during the measurement period. Based on a two-pool exponential model, the mean residence time (MRT) of biochar varied from 351 to 449 years in the intensive pasture system to 415–484 years in the unplanted soils.     

The priming potential of biochar products in relation to labile carbon contents and soil organic matter status

Recognition of biochar as a potential tool for long-term carbon sequestration with additional agronomic benefits is growing. However, the functionality of biochar in soil and the response of soils to biochar inputs are poorly understood. It has been suggested, for example, that biochar additions to soils could prime for the loss of native organic carbon, undermining its sequestration potential. This work examines the priming potential of biochar in the context of its own labile fraction and procedures for their assessment. A systematic set of biochar samples produced from C4 plant biomass under a range of pyrolysis process conditions were incubated in a C3 soil at three discrete levels of organic matter status (a result of contrasting long-term land management on a single site). The biochar samples were characterised for labile carbon content ex-situ and then added to each soil. Priming potential was determined by a comparison of CO₂ flux rates and its isotopic analysis for attribution of source. The results conclusively showed that while carbon mineralisation was often higher in biochar amended soil, this was due to rapid utilisation of a small labile component of biochar and that biochar did not prime for the loss of native organic soil organic matter. Furthermore, in some cases negative priming occurred, with lower carbon mineralisation in biochar amended soil, probably as a result of the stabilisation of labile soil carbon.     

Taxa-specific changes in soil microbial community composition induced by pyrogenic carbon amendments

The effects of pyrogenic carbon on the microbial diversity of forest soils were examined by comparing two soil types, fire-impacted and non-impacted, that were incubated with laboratory-generated biochars. Molecular and culture-dependent analyses of the biochar-treated forest soils revealed shifts in the relative abundance and diversity of key taxa upon the addition of biochars, which were dependent on biochar and soil type. Specifically, there was an overall loss of microbial diversity in all soils treated with oak and grass-derived biochar as detected by automated ribosomal intergenic spacer analysis. Although the overall diversity decreased upon biochar amendments, there were increases in specific taxa during biochar-amended incubation. DNA sequencing of these taxa revealed an increase in the relative abundance of bacteria within the phyla Actinobacteria and Gemmatimonadetes in biochar-treated soils. Together, these results reveal a pronounced impact of pyrogenic carbon on soil microbial community composition and an enrichment of key taxa within the parent soil microbial community.