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).     

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.     

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.     

Effects of biochar,earthworms, and litter addition on soil microbial activity and abundance in a temperate agricultural soil

Biochar application to arable soils could be effective for soil C sequestration and mitigation of greenhouse gas (GHG) emissions. Soil microorganisms and fauna are the major contributors to GHG emissions from soil, but their interactions with biochar are poorly understood. We investigated the effects of biochar and its interaction with earthworms on soil microbial activity, abundance, and community composition in an incubation experiment with an arable soil with and without N-rich litter addition. After 37 days of incubation, biochar significantly reduced CO₂ (up to 43 %) and N₂O (up to 42 %), as well as NH₄ ⁺-N and NO₃ ^?-N concentrations, compared to the control soils. Concurrently, in the treatments with litter, biochar increased microbial biomass and the soil microbial community composition shifted to higher fungal-to-bacterial ratios. Without litter, all microbial groups were positively affected by biochar × earthworm interactions suggesting better living conditions for soil microorganisms in biochar-containing cast aggregates after the earthworm gut passage. However, assimilation of biochar-C by earthworms was negligible, indicating no direct benefit for the earthworms from biochar uptake. Biochar strongly reduced the metabolic quotient qCO₂ and suppressed the degradation of native SOC, resulting in large negative priming effects (up to 68 %). We conclude that the biochar amendment altered microbial activity, abundance, and community composition, inducing a more efficient microbial community with reduced emissions of CO₂ and N₂O. Earthworms affected soil microorganisms only in the presence of biochar, highlighting the need for further research on the interactions of biochar with soil fauna.     

Positive and negative carbon mineralization priming effects among a variety of biochar-amended soils

Pyrogenic carbon (biochar) amendment is increasingly discussed as a method to increase soil fertility while sequestering atmospheric carbon (C). However, both increased and decreased C mineralization has been observed following biochar additions to soils. In an effort to better understand the interaction of pyrogenic C and soil organic matter (OM), a range of Florida soils were incubated with a range of laboratory-produced biochars and CO₂ evolution was measured over more than one year. More C was released from biochar-amended than from non-amended soils and cumulative mineralized C generally increased with decreasing biomass combustion temperature and from hardwood to grass biochars, similar to the pattern of biochar lability previously determined from separate incubations of biochar alone.The interactive effects of biochar addition to soil on CO₂ evolution (priming) were evaluated by comparing the additive CO₂ release expected from separate incubations of soil and biochar with that actually measured from corresponding biochar and soil mixtures. Priming direction (positive or negative for C mineralization stimulation or suppression, respectively) and magnitude varied with soil and biochar type, ranging from −52 to 89% at the end of 1 year. In general, C mineralization was greater than expected (positive priming) for soils combined with biochars produced at low temperatures (250 and 400 °C) and from grasses, particularly during the early incubation stage (first 90 d) and in soils of lower organic C content. It contrast, C mineralization was generally less than expected (negative priming) for soils combined with biochars produced at high temperatures (525 and 650 °C) and from hard woods, particularly during the later incubation stage (250-500 d). Measurements of the stable isotopic signature of respired CO₂ indicated that, for grass biochars at least, it was predominantly pyrogenic C mineralization that was stimulated during early incubation and soil C mineralization that was suppressed during later incubation stages. It is hypothesized that the presence of soil OM stimulated the co-mineralization of the more labile components of biochar over the short term. The data strongly suggests, however, that over the long term, biochar-soil interaction will enhance soil C storage via the processes of OM sorption to biochar and physical protection.     

Contribution of exudates,arbuscular mycorrhizal fungi and litter depositions to the rhizosphere priming effect induced by grassland species

The presence of plants induces strong accelerations in soil organic matter (SOM) mineralization by stimulating soil microbial activity – a phenomenon known as the rhizosphere priming effect (RPE). The RPE could be induced by several mechanisms including root exudates, arbuscular mycorrhizal fungi (AMF) and root litter. However the contribution of each of these to rhizosphere priming is unknown due to the complexity involved in studying rhizospheric processes. In order to determine the role of each of these mechanisms, we incubated soils enclosed in nylon meshes that were permeable to exudates, or exudates & AMF or exudates, AMF and roots under three grassland plant species grown on sand. Plants were continuously labeled with ¹³C depleted CO₂ that allowed distinguishing plant-derived CO₂ from soil-derived CO₂. We show that root exudation was the main way by which plants induced RPE (58–96% of total RPE) followed by root litter. AMF did not contribute to rhizosphere priming under the two species that were significantly colonized by them i.e. Poa trivialis and Trifolium repens. Root exudates and root litter differed with respect to their mechanism of inducing RPE. Exudates induced RPE without increasing microbial biomass whereas root litter increased microbial biomass and raised the RPE mediating saprophytic fungi. The RPE efficiency (RPE/unit plant-C assimilated into microbes) was 3–7 times higher for exudates than for root litter. This efficiency of exudates is explained by a microbial allocation of fresh carbon to mineralization activity rather than to growth. These results suggest that root exudation is the main way by which plants stimulated mineralization of soil organic matter. Moreover, the plants through their exudates not only provide energy to soil microorganisms but also seem to control the way the energy is used in order to maximize soil organic matter mineralization and drive their own nutrient supply.     

Contrasting effects of bamboo leaf and its biochar on soil CO2 efflux and labile organic carbon in an intensively managed Chinese chestnut plantation

The effects and associated mechanisms of the application of organic residues or their derived biochar on the dynamics of soil organic C and soil CO₂ efflux in planted soils are poorly understood. This paper investigated the impact of bamboo leaf and the derived biochar applications on soil CO₂ efflux and labile organic C in an intensively managed Chinese chestnut plantation in a 12-month field study. The treatments studied included Control, application of bamboo leaf (Leaf), and application of biochar (Biochar). The Leaf treatment increased (P?2 efflux and concentrations of water-soluble organic C (WSOC) and microbial biomass C (MBC). The Biochar treatment increased soil CO₂ efflux and WSOC and MBC only in the first month after application, but such effects diminished thereafter. The annual cumulative soil CO₂ emission was increased by 16 % by the Leaf treatment as compared to the Control, but there was no difference between the Biochar and Control treatments. The soil organic C (SOC) storage was increased by biochar addition but not by bamboo leaf addition. An exponential relationship between soil temperature and soil CO₂ efflux was observed regardless of the treatment. Soil CO₂ efflux was correlated to soil WSOC (P?Q ₁₀) of soil CO₂ efflux was ranked as Leaf?>?Biochar?>?Control. In comparison with the application of fresh bamboo leaf, pyrolyzed bamboo leaf (biochar) application decreased CO₂ effluxes and increased C sequestration in the soil.     

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.     

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.     

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.     

Soil priming effects following substrates addition to biochar-treated soils after 431 days of pre-incubation

Biochar has been widely proposed to be valuable in the sequestering of carbon (C) in soil due to its chemical and biological recalcitrance. However, whether biochar could cause soil positive priming effects (PEs), which offset the effects of soil organic C sequestration, has raised a very controversial issue and debate recently. Changed soil properties, like microbial community composition, caused by biochar addition, might induce different primed CO₂ following substrate addition, compared to soil which never received biochar. However, this remains largely unknown. This study aimed to understand the substrate-induced PEs in biochar-amended soil and the microbial mechanisms involved. Using ¹³C analysis, a further 28 days of laboratory incubation was conducted after incorporation of biochar for 431 days of pre-incubation to investigate primed soil CO₂ emissions induced by the addition of sucrose and Miscanthus giganteus (Miscanthus), in both biochar free soil (L1) and biochar (produced at 350 and 700 °C)-amended soils (L2 and L3). Biochar-amended soils had larger substrate-induced PEs. Larger primed soil C losses (311 μg CO₂-C g^?1 soil) were observed following Miscanthus feedstock addition in BC700-amended soil (L3 + Miscanthus), compared to soil without BC700 (193 μg CO₂-C g^?1 soil) (L3). The changes in soil microbial community composition, indicated by PCA analysis of PLFAs, especially actinomycetes and Gramme-negative bacteria, might be responsible for the larger substrate (Miscanthus and sucrose)-induced PEs observed in biochar-amended soils after 431 days compared to biochar-free soils.     

Impact of biochar on mineralisation of C and N from soil and willow litter and its relationship with microbial community biomass and structure

Using a laboratory experiment, we investigated the effect of applying willow biochar to short rotation coppice soil on C and N dynamics and microbial biomass and community composition, in the presence and absence of willow litter. Application of biochar at a rate of 0.5 % had no effect on net CO₂ mineralisation in the presence or absence of litter. However at a rate of 2 %, net CO₂ mineralisation was reduced by 10 and 20 % over a 90-day period in the absence and presence of litter respectively. Biochar reduced N mineralisation when applied at both 0.5 and 2 % concentrations. pH was increased by application of 2 % biochar to soil. Phospholipid fatty acid analysis demonstrated that both concentrations of biochar affected microbial community composition, although the effect of biochar was not as great as the effect of time or litter application in shaping community structure. In particular, the amount of bacterial biomass was increased by biochar application to soil, and there was evidence for increased abundance of Gram-negative bacteria and actinobacteria following biochar application. The data is discussed in the context of microbial mechanisms underlying impacts of biochar on C cycling in soil, and the coupling of C and N cycles following amendment of soil with biochar.     

Effects of biochar amendment on greenhouse gas emissions,net ecosystem carbon budget and properties of an acidic soil under intensive vegetable production

Biochar addition to soils has been frequently proposed as a means to increase soil fertility and carbon (C) sequestration. However, the effect of biochar addition on greenhouse gas emissions from intensively managed soils under vegetable production at the field scale is poorly understood. The effects of wheat straw biochar amendment with mineral fertilizer or an enhanced‐efficiency fertilizer (mixture of urea and nitrapyrin) on N₂O efflux and the net ecosystem C budget were investigated for an acidic soil in southeast China over a 1‐yr period. Biochar addition did not affect the annual N₂O emissions (26–28 kg N/ha), but reduced seasonal N₂O emissions during the cold period. Biochar increased soil organic C and CO₂ efflux on average by 61 and 19%, respectively. Biochar addition greatly increased C gain in the acidic soil (average 11.1 Mg C/ha) compared with treatments without biochar addition (average ?2.2 Mg C/ha). Biochar amendment did not increase yield‐scaled N₂O emissions after application of mineral fertilizer, but it decreased yield‐scaled N₂O by 15% after nitrapyrin addition. Our results suggest that biochar amendment of acidic soil under intensive vegetable cultivation contributes to soil C sequestration, but has only small effects on both plant growth and greenhouse gas emissions.     

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.     

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.     

Contrasting effects of wheat straw and its biochar on greenhouse gas emissions and enzyme activities in a Chernozemic soil

Biochar produced from plant biomass through pyrolysis has been shown to be much more resistant to biodegradation in the soil as compared with the raw biomass, such as cereal straw that is routinely shredded and discharged on to farm fields in large amounts. Biochar application to soil has also been reported to decrease greenhouse gas (GHG) emissions, although the mechanisms are not fully understood. In this study, the emissions of three main GHGs (CO₂, CH₄, and N₂O) and enzyme activities (urease, β-glycosidase, and dehydrogenase) were measured during a 100-day laboratory incubation of a Chernozemic soil amended with either straw or its biochar at rates of 0.67 and 1.68 % (based on the amount of C added) for the low and high rates, respectively. The biochar application dramatically reduced N₂O emissions, but CO₂ or CH₄ emissions were not different, as compared with the un-amended soil. At the same C equivalent application rate, CO₂ and N₂O emission rates were greater while CH₄ emission rates were lower in straw than in biochar application treatments. The activities of both the dehydrogenase and β-glycosidase significantly declined while that of urease significantly increased with the biochar as compared with the straw treatment. We conclude that pyrolysis of cereal straw prior to land application would significantly reduce CO₂ and N₂O emissions, in association with changed enzyme activities, while increasing the soil C pool through the addition of stable C in the form of biochar.     

Short-term biochar-induced increase in soil CO2 release is both biotically and abiotically mediated

The application of biochar to soil has been shown to cause an apparent increase in soil respiration. In this study we investigated the mechanistic basis of this response. We hypothesized that increased CO₂ efflux could occur by: (1) Biochar-induced changes in soil physical properties (bulk density, porosity, moisture content); (2) The biological breakdown of organic carbon (C) released from the biochar; (3) The abiotic release of inorganic C contained in the biochar; (4) A biochar-induced stimulation of decomposition of native soil organic matter (SOM) which could occur both biotically or abiotically; (5) The intrinsic biological activity of the biochar results in the liberation of CO₂. Our results show that most of the extra CO₂ produced after biochar addition to soil came from the equal breakdown of organic C and the release of inorganic C contained in the biochar. Using long-term ¹⁴C-labelled SOM, we show that biochar repressed native SOM breakdown, counteracting the release of CO₂ from the biochar. A range of mechanisms to describe this negative priming response is presented. Although biochar-induced significant changes in the physical characteristics of the soil, overall this made no contribution to changes in soil respiration. Similarly, the evidence from our study suggests that changes in soluble polyphenols do not help explain the respiration response. In summary, biochar induced a net release of CO₂ from the soil; however, this C loss was very small relative to the amount of C stored within the biochar itself (ca. 0.1%). This short-term C release should therefore not compromise its ability to contribute to long-term C sequestration in soil environments.     

Review: Factors affecting rhizosphere priming effects

Living plants change the local environment in the rhizosphere and consequently affect the rate of soil organic matter (SOM) decomposition. The rate may increase for 3‐ to 5‐folds, or decrease by 10 % to 30 % by plant cultivation. Such short‐term changes of rate (intensity) of SOM decomposition are due to the priming effect. In the presence of plants, a priming effect occurs in the direct vicinity of the living roots, and it is called rhizosphere priming effect (RPE). Plant‐mediated and environmental factors, such as, plant species, development stage, soil organic matter content, photosynthesis intensity, and N fertilization which affect RPE are reviewed and discussed in this paper. It was concluded that root growth dynamics and photosynthesis intensity are the most important plant‐mediated factors affecting RPE. Environmental factors such as amount of decomposable C in soil and N_min content are responsible for the switch between following mechanisms of RPE: concurrence for N_min between roots and microorganisms, microbial activation or preferential substrate utilization. Succession of mechanisms of RPE along the growing root in accordance with the rhizodeposition types is suggested. Different hypotheses for mechanisms of filling up the C amount loss by RPE are suggested. The ecosystematic relevance of priming effects by rhizodeposition relates to the connection between exudation of organic substances by roots, the increase of microbial activity in the rhizosphere through utilization of additional easily available C sources, and the subsequent intensive microbial mobilization of nutrients from the soil organic matter.     

Priming effects in different soil types induced by fructose, alanine, oxalic acid and catechol additions

It is well established that certain substrate additions to soils may accelerate or retard the mineralisation of soil organic matter. But up to now, research on these so called ‘priming effects’ was almost exclusively conducted with arable soils and with plant residues or glucose as additives. In this study, the effects of the uniformly ¹⁴C-labelled substrates fructose, alanine, oxalic acid and catechol on the mineralisation of soil organic carbon (SOC) from different horizons of two forest soils (Haplic Podzol and Dystric Cambisol) and one arable soil (Haplic Phaeozem) under maize and rye cultivation were investigated in incubation experiments for 26 days. Apart from the controls, all samples received substrate additions of 13.3 μg substrate-C mg⁻¹ C_org. During the incubation, CO₂-evolution was measured hourly and the amount of ¹⁴CO₂ was determined at various time intervals. In almost all soils, priming effects were induced by one or several of the added substrates. The strongest positive priming effects were induced by fructose and alanine and occurred in the Bs horizon of the Haplic Podzol, where SOC mineralisation was nearly doubled. In the other soil samples, these substrates enhanced SOC mineralisation by +10 to +63%. Catechol additions generally reduced SOC mineralisation by −12 to −43% except in the EA horizon of the Haplic Podzol where SOC-borne CO₂-evolution increased by +46%. Oxalic acid also induced negative as well as positive priming effects ranging from −24 to +82%. The data indicate that priming effects are ubiquitously occurring in surface and subsoil horizons of forest soils as well as in arable soils. Although a broad variety of soils was used within this study, relationships between soil properties and priming effects could not be ascertained. Therefore, a prediction on occurrence and magnitude of priming effects based on relatively easily measurable chemical and physical soil properties was not possible. Nevertheless, the data suggest that positive priming effects are most pronounced in forest soils that contain SOC of low biodegradability, where the added substrates may act as an important energy source for microbial metabolism.     

Effects of biochar application on greenhouse gas emissions,carbon sequestration and crop growth in coastal saline soil

To evaluate the benefits of application of biochar to coastal saline soil for climate change mitigation, the effects on soil organic carbon (SOC), greenhouse gases (GHGs) and crop yields were investigated. Biochar was applied at 16 t ha^?1 to study its effects on crop growth (Experiment I). The effects of biochar (0, 3.2, 16 and 32 t ha^?1) and corn stalk (7.8 t ha^?1) on SOC and GHGs were studied using ¹³C stable isotope technology and a static chamber method, respectively (Experiment II). Biochar increased grain mass per plant of the wheat by 27.7% and increased SOC without influencing non‐biochar SOC. On average, 92.3% of the biochar carbon and 16.8% of corn‐stalk carbon were sequestered into the soil within 1 year. The cumulative emissions of CO₂, CH₄ and N₂O were not affected significantly by biochar but cornstalk application increased N₂O emissions by 17.5%. The global warming mitigation potential of the biochar treatments (?3.84 to ?3.17 t CO₂‐eq. ha^?1 t^?1 C) was greater than that of the corn stalk treatment (?0.11 t CO₂‐eq ha^?1 t^?1 C). These results suggest that biochar application improves saline soil productivity and soil carbon sequestration without increasing GHG emissions.