The stability of biochar carbon (C) is the major determinant of its value for long-term C sequestration in soil. A long-term (5 year) laboratory experiment was conducted under controlled conditions ...using 11 biochars made from five C3 biomass feedstocks (Eucalyptus saligna wood and leaves, papermill sludge, poultry litter, cow manure) at 400 and/or 550 °C. The biochars were incubated in a vertisol containing organic C from a predominantly C4-vegetation source, and total CO2–C and associated δ13C were periodically measured. Between 0.5% and 8.9% of the biochar C was mineralized over 5 years. The C in manure-based biochars mineralized faster than that in plant-based biochars, and C in 400 °C biochars mineralized faster than that in corresponding 550 °C biochars. The estimated mean residence time (MRT) of C in biochars varied between 90 and 1600 years. These are conservative estimates because they represent MRT of relatively labile and intermediate-stability biochar C components. Furthermore, biochar C MRT is likely to be higher under field conditions of lower moisture, lower temperatures or nutrient availability constraints. Strong relationships of biochar C stability with the initial proportion of nonaromatic C and degree of aromatic C condensation in biochar support the use of these properties to predict biochar C stability in soil.
Agronomic practices such as crop residue return and additional nutrient supply are recommended to increase soil organic carbon (SOC) in arable farmlands. However, changes in the priming effect (PE) ...on native SOC mineralization in response to integrated inputs of residue and nutrients are not fully known. This knowledge gap along with a lack of understanding of microbial mechanisms hinders the ability to constrain models and to reduce the uncertainty to predict carbon (C) sequestration potential. Using a 13C‐labeled wheat residue, this 126‐day incubation study examined the dominant microbial mechanisms that underpin the PE response to inputs of wheat residue and nutrients (nitrogen, phosphorus and sulfur) in two contrasting soils. The residue input caused positive PE through “co‐metabolism,” supported by increased microbial biomass, C and nitrogen (N) extracellular enzyme activities (EEAs), and gene abundance of certain microbial taxa (Eubacteria, β‐Proteobacteria, Acidobacteria, and Fungi). The residue input could have induced nutrient limitation, causing an increase in the PE via “microbial nutrient mining” of native soil organic matter, as suggested by the low C‐to‐nutrient stoichiometry of EEAs. At the high residue, exogenous nutrient supply (cf. no‐nutrient) initially decreased positive PE by alleviating nutrient mining, which was supported by the low gene abundance of Eubacteria and Fungi. However, after an initial decrease in PE at the high residue with nutrients, the PE increased to the same magnitude as without nutrients over time. This suggests the dominance of “microbial stoichiometry decomposition,” supported by higher microbial biomass and EEAs, while Eubacteria and Fungi increased over time, at the high residue with nutrients cf. no‐nutrient in both soils. Our study provides novel evidence that different microbial mechanisms operate simultaneously depending on organic C and nutrient availability in a residue‐amended soil. Our results have consequences for SOC modeling and integrated nutrient management employed to increase SOC in arable farmlands.
This study examined the dynamics of priming effect (PE), controlled by the interaction of crop residue input and balanced supply of nutrients (N, P, and S), and the underlying mechanisms in relation to microbial community growth and extracellular enzyme activity. The results showed that the “microbial nutrient mining” and “microbial stoichiometry decomposition” mechanisms relating to nutrient availability mainly operated at high residue input. The image presents a conceptualized model based on key findings on the dominant occurrence of microbial mechanisms relating to PE, with implications to underpin soil organic carbon (SOC) modeling and guide integrated residue–nutrient management in croplands for SOC sequestration.
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).
•We measured C mineralisation in biochar amended soils for a two-year period.•Biochar increased the mineralisation of native organic carbon in a sandy soil.•In clayey soils, biochar suppressed the mineralisation of native organic carbon.•Biochar-C mineralisation decreased with increasing pyrolysis temperature of biochar.•Mean residence time of biochar decreased with increasing incubation temperature.
Biochar is considered as an attractive tool for long-term carbon (C) storage in soil. However, there is limited knowledge about the effect of labile organic matter (LOM) on biochar-C mineralization ...in soil or the vice versa. An incubation experiment (20 °C) was conducted for 120 days to quantify the interactive priming effects of biochar-C and LOM-C mineralization in a smectitic clayey soil. Sugar cane residue (source of LOM) at a rate of 0, 1, 2, and 4% (w/w) in combination with two wood biochars (450 and 550 °C) at a rate of 2% (w/w) were applied to the soil. The use of biochars (∼ −36‰) and LOM (−12.7‰) or soil (−14.3‰) with isotopically distinct δ13C values allowed the quantification of C mineralized from biochar and LOM/soil. A small fraction (0.4–1.1%) of the applied biochar-C was mineralized, and the mineralization of biochar-C increased significantly with increasing application rates of LOM, especially during the early stages of incubation. Concurrently, biochar application reduced the mineralization of LOM-C, and the magnitude of this effect increased with increasing rate of LOM addition. Over time, the interactive priming of biochar-C and LOM-C mineralization was stabilized. Biochar application possesses a considerable merit for long-term soil C-sequestration, and it has a stabilizing effect on LOM in soil.
Biochar properties can be significantly influenced by feedstock source and pyrolysis conditions; this warrants detailed characterisation of biochars for their application to improve soil fertility ...and sequester carbon. We characterised 11 biochars, made from 5 feedstocks Eucalyptus saligna wood (at 400°C and 550°C both with and without steam activation); E. saligna leaves (at 400°C and 550°C with activation); papermill sludge (at 550°C with activation); poultry litter and cow manure (each at 400°C without activation and at 550°C with activation) using standard or modified soil chemical procedures. Biochar pH values varied from near neutral to highly alkaline. In general, wood biochars had higher total C, lower ash content, lower total N, P, K, S, Ca, Mg, Al, Na, and Cu contents, and lower potential cation exchange capacity (CEC) and exchangeable cations than the manure-based biochars, and the leaf biochars were generally in-between. Papermill sludge biochar had the highest total and exchangeable Ca, CaCO₃ equivalence, total Cu, and potential CEC, and the lowest total and exchangeable K. Water-soluble salts were higher in the manure-based biochars, followed by leaf, papermill sludge, and wood biochars. Total As, Cd, Pb, and polycyclic aromatic hydrocarbons in the biochars were either very low or below detection limits. In general, increase in pyrolysis temperature increased the ash content, pH, and surface basicity and decreased surface acidity. The activation treatment had a little effect on most of the biochar properties. X-ray diffraction analysis showed the presence of whewellite in E. saligna biochars produced at 400°C, and the whewellite was converted to calcite in biochars formed at 550°C. Papermill sludge biochar contained the largest amount of calcite. Water-soluble salts and calcite interfered with surface charge measurements and should be removed before the surface charge measurements of biochar. The biochars used in the study ranged from C-rich to nutrient-rich to lime-rich soil amendment, and these properties could be optimised through feedstock formulation and pyrolysis temperature for tailored soil application.
Biochar has gained importance due to its ability to increase the long-term soil carbon pool and improve crop productivity. However, little research has been done to evaluate the influence of biochar ...application to soil on the bioavailability of trace elements to plants. A pot experiment was conducted to investigate the influence of biochar on the availability of As, Cd, Cu, Pb, and Zn to maize (Zea mays L.). An activated wood biochar, pyrolysed at 550°C, was applied at 3 rates (0, 5, and 15g/kg) in factorial combinations with 3 rates (0, 10, and 50mg/kg) each of As, Cd, Cu, Pb, and Zn separately to a sandy soil. After 10 weeks of growth, plants were harvested, shoot dry matter yield was measured, and concentration of trace elements in shoots was analysed. The soil in pots was analysed for extractable trace elements. The results showed that the addition of wood biochar to soil did not have any significant effect on the dry matter yield of maize plants, even at the highest rate of application. However, trace element application significantly reduced the dry matter yield from 10 to 93% depending on the type of trace element. Biochar application decreased the concentration of As, Cd, and Cu in maize shoots, especially at the highest rate of trace element application, whereas the effects were inconsistent on Pb and Zn concentrations in the shoots. The concentrations of extractable As and Zn in soil increased with biochar application, whereas extractable Cu did not change, Pb decreased, and Cd showed an inconsistent trend. Sorption of trace elements on biochar with initial loadings up to 200µmol at pH 7 occurred in the order: Pb>Cu>Cd>Zn>As. The results show that biochar application can significantly reduce the availability of trace elements to plants and suggest that biochar application may have potential for the management of soils contaminated by trace elements.
Despite increasing recognition of the critical role of coastal wetlands in mitigating climate change, sea‐level rise, and salinity increase, soil organic carbon (SOC) sequestration mechanisms in ...estuarine wetlands remain poorly understood. Here, we present new results on the source, decomposition, and storage of SOC in estuarine wetlands with four vegetation types, including single Phragmites australis (P, habitat I), a mixture of P. australis and Suaeda salsa (P + S, habitat II), single S. salsa (S, habitat III), and tidal flat (TF, habitat IV) across a salinity gradient. Values of δ13C increased with depth in aerobic soil layers (0–40 cm) but slightly decreased in anaerobic soil layers (40–100 cm). The δ15N was significantly enriched in soil organic matter at all depths than in the living plant tissues, indicating a preferential decomposition of 14N‐enriched organic components. Thus, the kinetic isotope fractionation during microbial degradation and the preferential substrate utilization are the dominant mechanisms in regulating isotopic compositions in aerobic and anaerobic conditions, respectively. Stable isotopic (δ13C and δ15N), elemental (C and N), and lignin composition (inherited (Ad/Al)s and C/V) were not completely consistent in reflecting the differences in SOC decomposition or accumulation among four vegetation types, possibly due to differences in litter inputs, root distributions, substrate quality, water‐table level, salinity, and microbial community composition/activity. Organic C contents and storage decreased from upstream to downstream, likely due to primarily changes in autochthonous sources (e.g., decreased onsite plant biomass input) and allochthonous materials (e.g., decreased fluvially transported upland river inputs, and increased tidally induced marine algae and phytoplankton). Our results revealed that multiple indicators are essential to unravel the degree of SOC decomposition and accumulation, and a combination of C:N ratios, δ13C, δ15N, and lignin biomarker provides a robust approach to decipher the decomposition and source of sedimentary organic matter along the river‐estuary‐ocean continuum.
Salinity displayed a successive increase from upstream to downstream, which modelled to be the most important factor influencing vegetation composition. Contents and storage of soil organic carbon and total nitrogen decreased from upstream to downstream, likely due to primary changes in autochthonous sources (e.g., decreased on‐site plant biomass input) and allochthonous materials (e.g., decreased fluvially transported upland river inputs, and increased tidally induced marine algae and phytoplankton). A combination of stable C and N isotopes, C:N ratios, and lignin biomarkers provides a comprehensive method to evaluate the distribution, sources, and stability of soil organic matter along the river–estuary–ocean continuum.
The influence of biochar on nitrogen (N) transformation processes in soil is not fully understood. This study assessed the influence of four biochars (wood and poultry manure biochars synthesized at ...400°C, nonactivated, and at 550°C, activated, abbreviated as: W400, PM400, W550, PM550, respectively) on nitrous oxide (N2O) emission and N leaching from an Alfisol and a Vertisol. Repacked soil columns were subjected to three wetting–drying (W–D) cycles to achieve a range of water‐filled pore space (WFPS) over a 5‐mo period. During the first two W–D cycles, W400 and W550 had inconsistent effects on N2O emissions and the soils amended with PM400 produced higher N2O emissions relative to the control. The initially greater N2O emission from the PM400 soils was ascribed to its higher labile intrinsic N content than the other biochars. During the third W–D cycle, all biochar treatments consistently decreased N2O emissions, cumulatively by 14 to 73% from the Alfisol and by 23 to 52% from the Vertisol, relative to their controls. In the first leaching event, higher nitrate leaching occurred from the PM400‐amended soils compared with the other treatments. In the second event, the leaching of ammonium was reduced by 55 to 93% from the W550‐ and PM550‐Alfisol and Vertisol, and by 87 to 94% from the W400‐ and PM400‐Vertisol only (cf. control). We propose that the increased effectiveness of biochars in reducing N2O emissions and ammonium leaching over time was due to increased sorption capacity of biochars through oxidative reactions on the biochar surfaces with ageing.
Biochar application can improve soil properties, such as increasing soil organic carbon content, soil pH and water content. These properties are important to soil dissolved organic carbon (DOC); ...however, the effects of biochar on DOC concentration and composition have received little research attention, especially several years after biochar application under field conditions. This study was conducted in a long‐term experimental field where the biochar was only applied once in 2009. The purpose of the study was to investigate the effect of different biochar application rates (0, 30, 60 and 90 t ha−1) on the dynamics of soil water content, DOC concentration and DOC compositions (reducing sugar, soluble phenol and aromatics) over nine samplings during a 12‐month period in 2014. Our results showed that soil water content and DOC concentration varied from 7.1% to 14.5% and 59 to 230 mg C kg−1 soil during the 12 months, respectively. However, the biochar application rates did not significantly (p > 0.05) affect soil water content, DOC concentration and DOC composition at the same sampling period. The DOC concentration across the biochar treatments was positively correlated to soil water content. Moreover, the DOC composition (reducing sugar, soluble phenol or aromatics) and their concentrations were positively correlated to the total DOC concentration. In addition, biochar did not affect soil bulk density, pH, saturated hydraulic conductivity and crop yields. The results indicated that some benefits of biochar to soil may not persist 5 years after the application of biochar under a field condition.
Whilst largely considered an inert material, biochar has been documented to contain a small yet significant fraction of microbially available labile organic carbon (C). Biochar addition to soil has ...also been reported to alter soil microbial community structure, and to both stimulate and retard the decomposition of native soil organic matter (SOM). We conducted a short-term incubation experiment using two 13C-labelled biochars produced from wheat or eucalypt shoots, which were incorporated in an aridic arenosol to examine the fate of the labile fraction of biochar-C through the microbial community. This was achieved using compound specific isotopic analysis (CSIA) of phospholipid fatty acids (PLFAs). A proportion of the biologically-available fraction of both biochars was rapidly (within three days) utilised by gram positive bacteria. There was a sharp peak in CO2 evolution shortly after biochar addition, resulting from rapid turnover of labile C components in biochars and through positive priming of native SOM. Our results demonstrate that this CO2 evolution was at least partially microbially mediated, and that biochar application to soil can cause significant and rapid changes in the soil microbial community; likely due to addition of labile C and increases in soil pH.
•Biochars produced from wheat and eucalypt shoots was rapidly utilised by soil microorganisms.•A small proportion of biochar-C was rapidly evolved upon addition to soil.•Biochars produced a positive priming effect on the soil organic matter.
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