Distinct biophysical and chemical mechanisms governing sucrose mineralization and soil organic carbon priming in biochar amended soils: evidence from 10 years of field studies.

While many studies have examined the role of biochar in carbon (C) accrual in short-term scale, few have explored the decadal scale influences of biochar on non-biochar C, e.g., native soil organic C (SOC) and added substrate. To address this knowledge gap, soils were collected from decade-old biochar field trials located in the United Kingdom (Cambisol) and China (Fluvisol), with each site having had three application rates (25-30, 50-60 and 75-100 Mg ha-1) of biochar plus an unamended Control, applied once in 2009. We assessed physicochemical and microbial properties associated with sucrose (representing the rhizodeposits) mineralization and the priming effect (PE) on native SOC. Here, we showed both soils amended with biochar at the middle application rate (50 Mg ha-1 biochar in Cambisol and 60 Mg ha-1 biochar in Fluvisol) resulted in greater substrate mineralization. The enhanced accessibility and availability of sucrose to microorganisms, particularly fast-growing bacterial genera like Arenimonas, Spingomonas, and Paenibacillus (r-strategists belonging to the Proteobacteria and Firmicutes phyla, respectively), can be attributed to the improved physicochemical properties of the soil, including pH, porosity, and pore connectivity, as revealed by synchrotron-based micro-CT. Random forest analysis also confirmed the contribution of the microbial diversity and physical properties such as porosity on sucrose mineralization. Biochar at the middle application rate, however, resulted in the lowest PE (0.3 and 0.4 mg of CO2-C g soil-1 in Cambisol and Fluvisol, respectively) after 53 days of incubation. This result might be associated with the fact that the biochar promoted large aggregates formation, which enclosed native SOC in soil macro-aggregates (2-0.25 mm). Our study revealed a diverging pattern between substrate mineralization and SOC priming linked to the biochar application rate. This suggests distinct mechanisms, biophysical and physicochemical, driving the mineralization of non-biochar carbon in a field where biochar was applied a decade before. Supplementary Information: The online version contains supplementary material available at 10.1007/s42773-024-00327-0.

Identification of possible sources for potentially toxic elements and polycyclic aromatic hydrocarbons and their spatially varying relationships in urban soils of Dublin, Ireland.

Potentially toxic elements (PTEs) and polycyclic aromatic hydrocarbons (PAHs) harm the ecosystem and human health, especially in urban areas. Identifying and understanding their potential sources and underlying interactions in urban soils are critical for informed management and risk assessment. This study investigated the potential sources and the spatially varying relationships between 9 PTEs and PAHs in the topsoil of Dublin by combining positive matrix factorisation (PMF) and geographically weighted regression (GWR). The PMF model allocated four possible sources based on species concentrations and uncertainties. The factor profiles indicated the associations with high-temperature combustion (PAHs), natural lithologic factors (As, Cd, Co, Cr, Ni), mineralisation and mining (Zn), as well as anthropogenic inputs (Cu, Hg, Pb), respectively. In addition, selected representative elements Cr, Zn, and Pb showed distinct spatial interactions with PAHs in the GWR model. Negative relationships between PAHs and Cr were observed in all samples, suggesting the control of Cr concentrations by natural factors. Negative relationships between PAHs and Zn in the eastern and north-eastern regions were related to mineralisation and anthropogenic Zn-Pb mining. In contrast, the surrounding regions exhibited a natural relationship between these two variables with positive coefficients. Increasing positive coefficients from west to east were observed between PAHs and Pb in the study area. This special pattern was consistent with prevailing south-westerly wind direction in Dublin, highlighting the predominant influences on PAHs and Pb concentrations from vehicle and coal combustion through atmospheric deposition. Our results provided a better understanding of geochemical features for PTEs and PAHs in the topsoil of Dublin, demonstrating the efficiency of combined approaches of receptor models and spatial analysis in environmental studies.

An overview of the direct and indirect effects of acid rain on plants: Relationships among acid rain, soil, microorganisms, and plants.

Acid rain (AR) causes numerous environmental problems and complex negative effects on plants globally. Many studies have previously reported on direct effects of AR or its depositional substances on plant injury and performance. However, few studies have addressed the indirect effects of AR on plants as mediated by soil microorganisms and the abiotic environment of the soil rhizosphere. The indirect effects (e.g., AR â†’ soil microorganisms→plants) need greater attention, because acidic deposition not only affects the distribution, composition, abundance, function, and activity of plant-associated microorganisms, but also influences the dynamics of some substances in the soil in a way that may be harmful to plants. Therefore, this review not only focused on the direct effects of AR on plant performance, growth, and biomass allocations from a whole-plant perspective, but also addressed the pathway of AR-soil chemical characteristics-plants, which explains how soil solute leaching and acidification by AR will reduce the availability of essential nutrients and increase the availability of heavy metals for plants, affecting carbon and nitrogen cycles. Mainly, we evaluated the AR-soil microorganisms-plants pathway by: 1) synthesizing the potential roles of soil microbes in alleviating soil acidic stress on plants and the adverse effects of AR on plant-associated soil microorganisms; 2) exploring how plant mycorrhizal types affect the detection of AR effect on plants. The meta-analysis showed that the effects of AR-induced pH on leaf chlorophyll content, plant height, and plant root biomass were dependent on plant mycorrhizal types. Some possible reasons for different synergy between mycorrhizal symbiotic types and plants were discussed. Future research relating to the effects of AR on plants should focus on the combined direct and indirect effects to evaluate how AR affects plant performance comprehensively.

Soil acidification and the liming potential of biochar.

Soil acidification in managed ecosystems such as agricultural lands principally results from the increased releasing of protons (H+) from the transformation reactions of carbon (C), nitrogen (N) and sulphur (S) containing compounds. The incorporation of liming materials can neutralize the protons released, hence reducing soil acidity and its adverse impacts to the soil environment, food security, and human health. Biochar derived from organic residues is becoming a source of carbon input to soil and provides multifunctional values. Biochar can be alkaline in nature, with the level of alkalinity dependent upon the feedstock and processing conditions. This review covers the fundamental aspects of soil acidification and of the use of biochar to address constraints related to acidic soil. Biochar is increasingly considered as an effective soil amendment for reducing soil acidity owing to its liming potential, thereby enhancing soil fertility and productivity in acid soils. The ameliorant effect on acid soils is mainly because of the dissolution of carbonates, (hydro)-oxides of the ash fraction of biochar and potential use by microorganisms.

Specific PhytOC fractions in rice straw and consequent implications for potential of phytolith carbon sequestration in global paddy fields.

Phytoliths are silica biomineralization products within plants and have been considered as a promising material to sequester carbon (C). However, there is considerable uncertainty and controversy regarding the C content in phytoliths due to the lack of detailed information on variation of C under different extraction procedures. Herein, we established a series of batch digestion experimental procedures coupled with analyses of phytoliths using Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy to divide phytoliths into three fractions. We then reported an approach for standardizing across hundreds of values found in the literature. Combining this standardized approach with C contents in phytoliths extracted from different digestion degrees, we revaluated the potential production rates of phytolith-occluded carbon (PhytOC) input globally in rice paddy fields. The results showed that the C content in recovered phytoliths exhibited a significantly fitting exponential relationship (p < 0.01) with digestion degrees and decreased from 30 to 75 g kg-1 under moderate digestion to <5 g kg-1 under over digestion. On a global scale, the production of total PhytOC in the world paddy fields reached up to (2.71 ± 0.85) × 106 t year-1. Therein, the contribution of sub-stable PhytOC fraction, stable PhytOC fraction, and recalcitrant PhytOC fraction was 63 %, 28 %, and 9 %, respectively. Our results imply that the estimation of phytolith C sequestration potential across the global paddy fields is associated with specific PhytOC fractions. Therefore, further determining the storage time limits of these specific PhytOC fractions after returning to soil will be vital for predicting terrestrial biogeochemical C sequestration potentials of phytoliths.

Microspectroscopic visualization of how biochar lifts the soil organic carbon ceiling.

The soil carbon (C) saturation concept suggests an upper limit to the storage of soil organic carbon (SOC). It is set by the mechanisms that protect soil organic matter from mineralization. Biochar has the capacity to protect new C, including rhizodeposits and microbial necromass. However, the decadal-scale mechanisms by which biochar influences the molecular diversity, spatial heterogeneity, and temporal changes in SOC persistence, remain unresolved. Here we show that the soil C storage ceiling of a Ferralsol under subtropical pasture was raised by a second application of Eucalyptus saligna biochar 8.2 years after the first application-the first application raised the soil C storage ceiling by 9.3 Mg new C ha-1 and the second application raised this by another 2.3 Mg new C ha-1. Linking direct visual evidence from one-, two-, and three-dimensional analyses with SOC quantification, we found high spatial heterogeneity of C functional groups that resulted in the retention of rhizodeposits and microbial necromass in microaggregates (53-250 µm) and the mineral fraction (<53 µm). Microbial C-use efficiency was concomitantly increased by lowering specific enzyme activities, contributing to the decreased mineralization of native SOC by 18%. We suggest that the SOC ceiling can be lifted using biochar in (sub)tropical grasslands globally.

Interactions between lead(II) ions and dissolved organic matter derived from organic fertilizers incubated in the field.

This work was to study composition characteristics and the subsequent effect on the lead (Pb) binding properties of dissolved organic matter (DOM) derived from seaweed-based (SWOF) and chicken manure organic fertilizers (CMOF) during a one-year field incubation experiment using the excitation-emission matrix-parallel factor (EEM-PARAFAC) and two-dimensional correlation spectroscopy (2DCOS) analysis. Results showed that high aromatic and hydrophobic fluorescent substances were enriched in CMOF-derived DOM and SWOF-derived DOM and enhanced over time. And phenolic groups in the fulvic-like substances for SWOF-derived DOM and carboxyl groups in the humic-like substances for CMOF-derived DOM had the fastest responses over time, respectively. Moreover, both non-fluorescent polysaccharides and fluorescent humic-like substances or fulvic-like substances with aromatic (C=C) groups first participated in the binding process of Pb to SWOF-derived DOM on day 0 and 180 during the lead binding process. In contrast, humic-like substances associated with aromatic (C=C) and phenolic groups gave a faster response to Pb binding on day 360. Regarding CMOF-derived DOM, the fulvic-like substances associated with aromatic (C=C) and carboxylic groups displayed a faster response to Pb ions on day 0. Nonetheless, polysaccharides and humic-like associated with phenolic groups had a faster response on days 180 and 360. It is noteworthy that the polysaccharides, which participated in Pb binding to CMOF-derived DOM, posed a higher risk of Pb in the environment after 360 days. Therefore, these findings gave new insights into the long-term applications of commercial organic fertilizers for the amendment of soil.

Effects of slag and biochar amendments on microorganisms and fractions of soil organic carbon during flooding in a paddy field after two years in southeastern China.

Incorporating amendments of industrial waste such as biochar and steel slag in cropland has been used to enhance the storage of soil organic carbon (SOC) while sustaining crop production. Short-term laboratory and field studies have identified important influences of biochar on active SOC fractions associated with soil microbial activity in paddy soils, but the long-term effects remain poorly understood. To address these knowledge gaps, we examined the effects of slag, biochar, and slag+biochar treatments on total SOC concentration, active SOC fractions and soil microbial communities in a paddy field two years after incorporation. Across both two seasons, the addition of slag, biochar, slag+biochar increased soil salinity by 26-80%, 1.3-37% and 42-79%, and also increased soil pH by 0.8-5.7%, 2.1-2.4% and 4.0-6.3%, respectively, relative to the control. SOC concentration was higher in the slag, biochar, and slag+biochar treatments across both rice seasons by 4.3-5%, 0.5-17% and 4.3-7%, respectively. Soil C-pool activity and C-pool management indices in the late paddy season were significantly lower in the slag+biochar treatment than the control by 26.3 and 21.3%, respectively, indicating that the amendments contributed to the stability of SOC. The C concentrations of the biochar and slag amendments affected bacterial abundance more than fungal abundance and affected C cycling. Our study suggests that combined slag and biochar amendments may increase bacterial abundance that may maintain SOC storage and reduce the abundances of potential SOC decomposers in key functional genera, indicating strong coupling relationships with changes of soil properties such as salinity, pH, and SOC concentration. These outcomes due to the amendments (e.g. slag+biochar) may increase microbial C-use efficiency and support the stability of active SOC fractions, with opportunities for long-term C sequestration.

Characterization of halophyte biochar and its effects on water and salt contents in saline soil.

Biochar is a beneficial soil amendment; however, biochar-based properties are mainly determined by the feedstocks and the pyrolysis temperature. Nevertheless, considering the vast biomass of halophyte, little is known about how the halophyte-derived biochar improves saline soils. In this study, we firstly produced biochars by using three different halophytes, including Tamarix chinensis (recretohalophyte), Suaeda salsa (euhalophyte), and Phragmites australis (pseudo-halophyte) at 300, 500, and 700 °C, and compared their chemical and physical properties. We applied halophyte (Tamarix chinensis and Phragmites australis) biochars (pyrolysis at 500 °C) into 0-20 cm saline soil at 2% and 4% (w/w) rates to investigate the saline soil water, salt, and pH dynamics in a 12-month column experiment. The results showed that as the pyrolytic temperature increase, biochar yield and pore diameter decreased by 37.5-44.0% and 34.6-89.7%, respectively; in contrast, biochar pH, specific surface area, and total volume increased by 24.8-47.8%, 3-37 times and 1-9 times, respectively. The halophyte types significantly controlled biochar carbon and dissolved salt content and electrical conductivity. Halophyte biochar application can increase soil water and salt content, and application of 4% of Tamarix chinensis-derived biochar can increase more soil moisture than the soil salinity, and it can maintain soil pH at a stable level, which would be a potential way to improve saline soil properties. The results are valuable for choosing halophyte types and optimizing pyrolytic temperatures for halophyte biochar production through specific environmental usage.

Biostimulants decreased nitrogen leaching and NH3 volatilization but increased N2O emission from plastic-shed greenhouse vegetable soil.

Biostimulant application is an effective strategy to enhance soil fertility and plant growth. However, its comprehensive impacts on nitrogen (N) uptake and reactive N (Nr) losses via leaching, ammonia (NH3) volatilization, and nitrous oxide (N2O) emission from plastic-shed greenhouse vegetable system are still little known. Therefore, a field experiment was conducted with cauliflower-tomato growth rotation (from September 6, 2018, to July 17, 2019) receiving three biostimulants, i.e., humic acid (HA), algae extract (AE), and chitosan (CT), as well as a control without stimulant. The cumulative Nr losses over the cauliflower-tomato growth cycle via leaching, NH3 volatilization, and N2O emission were 104-175 kg N ha-1, 2.32-3.85 kg N ha-1, and 0.70-0.85 kg N ha-1, respectively. Biostimulant application significantly (P < 0.05) retarded the total N leaching by 17-44% in tomato season, while suppressed the NH3 volatilization by 18-38% in cauliflower season. Overall, AE showed the best inhibition efficiency on Nr losses by significantly (P < 0.05) decreasing total N leaching and NH3 volatilization by 36-44% and 38-52% in both vegetable seasons, compare to the control. However, all three biostimulants stimulated the N2O emission under both vegetable cycles. Interestingly, all biostimulant-added treatments promote the cauliflower and tomato yield, particularly following the HA and AE amendments, which bring local farmers approximately 4,384-10,035 yuan RMB ha-1 more income. Enhanced yield under biostimulant treatments was due to higher N uptake capacity and enhanced root morphology. In summary, biostimulants have a contrasting influence on three major Nr lost pathways in greenhouse vegetable production. We recommend that AE is the most optimal biostimulant as it increases vegetable yield and decreases total N leaching and NH3 volatilization while not dramatically increase the N2O emission.

Biochar protects hydrophilic dissolved organic matter against mineralization and enhances its microbial carbon use efficiency.

A combination of biochar with exogenous organic material in soils is often used in practical farmland management. The objective of this study was to determine how biochar affects organic matter decomposition by studying the decomposition of 13C-labelled hydrophilic (Hi-) and hydrophobic (Ho-) dissolved organic matter (DOM) in acid and neutral soils during a 60-day incubation experiment. The proportions of carbon (C) mineralization in Hi-DOM with or without biochar addition were 32.6% or 34.5% in acid soil (P > 0.05) and 15.4% or 22.3% in neutral soil (P < 0.05), respectively. In contrast, those proportions of Ho-DOM-C mineralization with or without biochar addition were 20.0% or 21.4% in acid soil and 19.0% or 20.5% in neutral soil (P > 0.05), respectively. These results showed that biochar could protect Hi-DOM against mineralization in neutral soil but exhibited less effect on Ho-DOM mineralization in both acid and neutral soils. Additionally, biochar did not affect microbial incorporation of Hi- or Ho-DOM in acid and neutral soils. However, biochar notably improved the microbial carbon use efficiency (CUE) of Hi-DOM while it significantly reduced the CUE of Ho-DOM in neutral soil (P < 0.05), indicating that the effect of biochar on microbial CUE was related to organic matter type and soil pH. This study suggests that Hi-DOM can outperform Ho-DOM to decrease C loss and improve microbial CUE in neutral soil with biochar addition. This phenomenon could be due mainly to the different chemical compositions of Hi-DOM and Ho-DOM and their distinct microbial preference. These findings can provide references for biochar's ability to regulate the decomposition of organic matter.

Effects of nitrogen-enriched biochar on rice growth and yield, iron dynamics, and soil carbon storage and emissions: A tool to improve sustainable rice cultivation.

Biochar is often applied to paddy soils as a soil improver, as it retains nutrients and increases C sequestration; as such, it is a tool in the move towards C-neutral agriculture. Nitrogen (N) fertilizers have been excessively applied to rice paddies, particularly in small farms in China, because N is the major limiting factor for rice production. In paddy soils, dynamic changes in iron (Fe) continuously affect soil emissions of methane (CH4) and carbon dioxide (CO2); however, the links between Fe dynamics and greenhouse gas emissions, dissolved organic carbon (DOC), and rice yields following application of biochar remain unclear. The aims of this study were to examine the effects of two rates of nitrogen (N)-enriched biochar (4 and 8 t ha-1 y-1) on paddy soil C emissions and storage, rice yields, and Fe dynamics in subtropical early and late rice growing seasons. Field application of N-enriched biochar at 4 and 8 t ha-1 increased C emissions in early and late rice, whereas application at 4 t ha-1 significantly increased rice yields. The results of a culture experiment and a field experiment showed that the application of N-enriched biochar increased soil Fe2+concentration. There were positive correlations between Fe2+concentrations and soil CO2, CH4, and total C emissions, and with soil DOC concentrations. On the other way around, these correlations were negative for soil Fe3+concentrations. In the soil culture experiment, under the exclusion of plant growth, N-enriched biochar reduced cumulative soil emissions of CH4 and CO2. We conclude that moderate inputs of N-rich biochar (4 t ha-1) increase rice crop yield and biomass, and soil DOC concentrations, while moderating soil cumulative C emissions, in part, by the impacts of biochar on soil Fe dynamics. We suggest that water management strategies, such as dry-wet cycles, should be employed in rice cultivation to increase Fe2+ oxidation for the inhibition of soil CH4 and CO2 production. Overall, we showed that application of 4 t ha-1 of N-enriched biochar may represent a potential tool to improve sustainable food production and security, while minimizing negative environmental impacts.

Distribution, sources, and decomposition of soil organic matter along a salinity gradient in estuarine wetlands characterized by C:N ratio, δ13 C-δ15 N, and lignin biomarker.

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 δ13 C increased with depth in aerobic soil layers (0-40 cm) but slightly decreased in anaerobic soil layers (40-100 cm). The δ15 N was significantly enriched in soil organic matter at all depths than in the living plant tissues, indicating a preferential decomposition of 14 N-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 (δ13 C and δ15 N), 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, δ13 C, δ15 N, and lignin biomarker provides a robust approach to decipher the decomposition and source of sedimentary organic matter along the river-estuary-ocean continuum.

Nitrous oxide emissions from cow urine patches in an intensively managed grassland: Influence of nitrogen loading under contrasting soil moisture.

In dairy grazing systems, livestock urine patches are hotspots that contribute to global warming, both directly through nitrous oxide (N2O) emissions, and indirectly, through nitrate leaching. However, under warm-dry temperate environments, N2O emission factors (EFs) have not been thoroughly evaluated, accounting for the influence of urinary nitrogen (N) concentration and urine volume, and emissions measurement approach through different urine application methods. Here we quantified and compared N2O emissions and EFs on a moderately well-drained sandy loam soil from urine patches established in naturally expanding effective area (NEEA), representing urine volumes of 2, 3 and 4 L m-2 (equivalent to urine -N loadings of 141, 211 and 282 kg N ha-1), and using the uniformly wetted area (UWA) with urine applied at 10 L m-2 (709 kg N ha-1), under two different soil moistures (below field capacity, BFC; field capacity, FC). The results showed that cumulative N2O emissions in the NEEA urine patches were 0.36-0.52 kg N2O-N ha-1 over 146 days (early-winter to late-spring). In the UWA urine patches, cumulative N2O emissions were 2.3 times higher at FC (1.96 kg N2O-N ha-1) than BFC (0.87 kg N2O-N ha-1). The EFs were similar between UWA (0.09%) and NEEA (0.07-0.10%) at BFC but were significantly higher (P < 0.05-0.1) in UWA (0.26%) than NEEA (0.09-0.16%) at FC. The EFs in NEEA were not affected by urine-N loadings under BFC and FC, ranging between 0.07 and 0.16%. The relatively high versus low urine-N loadings in NEEA enhanced pasture herbage and N-uptake responses under both soil moistures. However, there were no differences in apparent N-use efficiency (ranging from 27 to 39%) across the treatments. The EFs observed in this study are much lower than the existing Australian cattle urine annual EF of 0.4%, and further examination to determine a more accurate EF for the industry is required.

Steel slag and biochar amendments decreased CO2 emissions by altering soil chemical properties and bacterial community structure over two-year in a subtropical paddy field.

Waste amendments, such as steel slag and biochar, have been reported as a strategy for improving soil fertility, crop productivity, and carbon (C) sequestration in agricultural lands. However, information regarding the subsequent effects of steel slag and biochar on C cycling and the underlying microbial mechanisms in paddy soils remains limited. Hence, this study aimed to examine the effect of these waste amendments (applied in 2015-2017) on total soil CO2 emissions, total and active soil organic C (SOC) contents, and microbial communities in the early and late seasons in a subtropical paddy field. The results showed that despite the exogenous C input from these waste amendments (steel slag, biochar and slag + biochar), they significantly (P < 0.05) decreased total CO2 emissions (e.g., by 41.9-59.6% at the early season), compared to the control soil. These amendments also significantly (P < 0.001) increased soil salinity and pH. The increased soil pH had a negative effect (r = -0.37, P < 0.05) on microbial biomass C (MBC). The biochar and slag + biochar treatments (cf. control) significantly (P < 0.001) increased SOC contents in the both seasons. The amendments altered the soil microbial community structure that associated with soil C cycling: (1) all three amendments increased the relative abundance of Agromyces and Streptomyces, which was associated with higher soil pH (cf. control); and (2) biochar and slag + biochar treatments caused a higher relative abundance of Sphingomonas, which was supported by high SOC contents under those amendments. Overall, this study demonstrated that the steel slag and biochar amendments altered microbial community composition due to changes in key soil properties, such as salinity, pH and SOC contents, with implications for increasing soil C stocks while mitigating CO2 emissions in the paddy field.

Gain in carbon: Deciphering the abiotic and biotic mechanisms of biochar-induced negative priming effects in contrasting soils.

The biochar-induced priming effects (PEs) were investigated by applying maize straw (C4) derived biochar to eight C3 soils, with a gradient of pH and a sub-gradient of soil organic carbon (SOC). To decipher the physicochemical and microbial mechanisms, we adopted C-isotopic analysis, high-throughput sequencing and multivariate statistical analyses such as random forest (RF) and structure equation modeling (SEM). Negative and neutral PEs were observed up to -48.5% of relative PEs during 28 days of incubation. All the acidic soils exhibited negative PEs, so as the neutral Alfisol and alkaline Aridisol, which had a suppression effect on SOC mineralization accounted for -29.4 and -32.0% of relative PEs. Among all abiotic factors, soil silt-clay fraction and the initial pH values play the most important roles in PEs determination through directly inhibiting PEs by protection SOC and indirectly shaping bacterial communities respectively. On the whole community level, biochar treatments defined much less microbiome (0.6% and 1.2% for variance of bacterial and fungal community) than soil types (93.5% and 83.3% respectively) across soils. Thus, the initial community (i.e., bacteria alpha-diversity and copiotrophic bacteria as revealed by SEM) of different soils might be more critical for PE prediction. Furthermore, co-occurrence network analysis indicated out-competition of fungi by bacteria with increase of mutual exclusion and decrease of fungal occupancy. This could exacerbate negative PEs in soils with lower bacterial alpha-diversity and dominance by copiotrophys due to less functional complementary for recalcitrant SOC decomposition.

Silicon Effects on Biomass Carbon and Phytolith-Occluded Carbon in Grasslands Under High-Salinity Conditions.

Changes in climate and land use are causing grasslands to suffer increasingly from abiotic stresses, including soil salinization. Silicon (Si) amendment has been frequently proposed to improve plant resistance to multiple biotic and abiotic stresses and increase ecosystem productivity while controlling the biogeochemical carbon (C) cycle. However, the effects of Si on plant C distribution and accumulation in salt-suffering grasslands are still unclear. In this study, we investigated how salt ions affected major elemental composition in plants and whether Si enhanced biomass C accumulation in grassland species in situ. In samples from the margins of salt lakes, our results showed that the differing distance away from the shore resulted in distinctive phytocoenosis, including halophytes and moderately salt-tolerant grasses, which are closely related to changing soil properties. Different salinity (Na+/K+, ranging from 0.02 to 11.8) in plants caused negative effects on plant C content that decreased from 53.9 to 29.2% with the increase in salinity. Plant Si storage [0.02-2.29 g Si m-2 dry weight (dw)] and plant Si content (0.53 to 2.58%) were positively correlated with bioavailable Si in soils (ranging from 94.4 to 192 mg kg-1). Although C contents in plants and phytoliths were negatively correlated with plant Si content, biomass C accumulation (1.90-83.5 g C m-2 dw) increased due to the increase of Si storage in plants. Plant phytolith-occluded carbon (PhytOC) increased from 0.07 to 0.28‰ of dry mass with the increase of Si content in moderately salt-tolerant grasses. This study demonstrates the potential of Si in mediating plant salinity and C assimilation, providing a reference for potential manipulation of long-term C sequestration via PhytOC production and biomass C accumulation in Si-accumulator dominated grasslands.

CpGDB : A Comprehensive Database of Chloroplast Genomes.

Chloroplast Genome Database (CpGDB) is user friendly, web-based, freely available and dynamic relational database which provides a platform for researchers to search and download complete chloroplast genome sequences, individual gene sequences and feature records of plant species belonging to same or different families of spermatophytes. Presently, the database consists of genome sequences, individual gene sequences and feature records of chloroplast genomes of 3823 plant species belonging to 1527 genera from 256 families, which will be updated regularly with the availability of new sequences at NCBI. Extensive data mining of feature records from GenBank files, uniform nomenclature for majority of genes, enriched intron/exon feature records makes CpGDB a valuable resource for studies in chloroplast genomics while complementing existing chloroplast databases.

Unexpected increases in soil carbon eventually fell in low rainfall farming systems.

Understanding the drivers of soil organic carbon (SOC) change over time and confidence to predict changes in SOC are essential to the development and long-term viability of SOC trading schemes. This study investigated temporal changes in total SOC, total nitrogen (N), and carbon (C) fractions (particulate organic carbon - POC, resistant organic carbon - ROC and humus organic carbon - HOC) over a 16-year period for four contrasting farming systems in a low rainfall environment (424 mm) at Condobolin, Australia. The farming systems were 1) conventional tillage mixed farming (CT); 2) reduced tillage mixed farming (RT); 3) continuous cropping (CC); and 4) perennial pasture (PP). The SOC dynamics were also modelled using APSIM C and N modules, to determine the accuracy of this model. Results are presented in the context of land managers participating in Australian climate change mitigation schemes. There was an increase in SOC for all farming systems over the first 12 years (total organic C, TOC% at 0-10 cm increased from 1.33% to 1.77%), which was predominately in the POC% fraction (POC% at 0-10 cm increased from 0.14% to 0.5%). Between 2012 and 2015, there was a decrease in SOC back to starting levels (TOC = 1.22% POC = 0.12% at 0-10 cm) in all systems. The PP system had higher TOC%, POC% and HOC% levels on average and higher SOC stocks to 30 cm depth at the final measurement in 2015 (PP = 30.43 t C ha-1; cropping systems = 23.71 t C ha-1), compared to the other farming systems. There was a decrease in TN% over time in all farming systems except PP. The average C:N increased from 14.1 in 1999 to 19.7 in 2012, after which time the SOC levels decreased and C:N dropped back to 15.8. The temporal change in SOC was not able to be represented by the AusFarm model. There are three important conclusions for policy development: 1) monitoring temporal changes in SOC over 12 years did not indicate long-term sequestration, required to assure "permanence" in SOC trading (i.e. 25-100 years) due to the susceptibility of POC to degradation; 2) without monitoring SOC in reference land uses (e.g. CT cropping system as a control in this experiment) it is not possible to determine the net carbon sequestration, and therefore the true climate change mitigation value; and 3) modelling SOC using AusFarm/APSIM, does not fully represent the temporal dynamics of SOC in this low rainfall environment.