Anaerobic primed CO2 and CH4 in paddy soil are driven by Fe reduction and stimulated by biochar

Physicochemically protected organic carbon release is the rate-limiting step of rhizosphere priming in paddy soils

Iron oxides affect the stability of soil organic matter (SOM), which in turn affects greenhouse gas emissions in paddy soils. They also regulate the direction and magnitude of the rhizosphere priming effect (RPE) by restricting SOM accessibility and microbial activity. However, the controlling steps and key factors that regulate the RPE magnitude under anoxic conditions are unknown. In this study, we investigated the mechanisms through which Fe(III) reduction affects the RPE using humic acid as an electron shuttle in paddy soils and conducting continuous 13CO2 labeling of rice plants. The RPE, measured via CO2 emission, was approximately 25 % greater in soils with humic acid than in soils without. A rapid increase in the RPE of CH4 emissions after 41 days was attenuated in soils containing humic acid. Root growth and Fe(III) reduction stimulated the total primed CO2 emissions from the rhizosphere independent of the microbial biomass and enzyme activities. Humic acid accelerated Fe(III) reduction, leading to a decrease in Fe-bound organic carbon and an increase in RPE (CO2 emissions). The rhizosphere-primed CO2 emissions decreased with increasing amounts of reactive Fe(III) (oxyhydr)oxides, which protected the SOM from microbial and enzymatic attacks. Biochemical Fe(III) reduction and physical aggregate destruction controlled the abiotic transformation of inaccessible SOM into bioavailable organic carbon, thereby regulating the RPE. The results suggest that the reduction of reactive Fe(III) minerals is the rate-limiting step in the release of the physicochemically protected SOM, which in turn determines the magnitude of rhizosphere priming in paddy soils.

Maize straw increases while its biochar decreases native organic carbon mineralization in a subtropical forest soil

Organic soil amendments have been widely adopted to enhance soil organic carbon (SOC) stocks in agroforestry ecosystems. However, the contrasting impacts of pyrogenic and fresh organic matter on native SOC mineralization and the underlying mechanisms mediating those processes remain poorly understood. Here, an 80-day experiment was conducted to compare the effects of maize straw and its derived biochar on native SOC mineralization within a Moso bamboo (Phyllostachys edulis) forest soil. The quantity and quality of SOC, the expression of microbial functional genes concerning soil C cycling, and the activity of associated enzymes were determined. Maize straw enhanced while its biochar decreased the emissions of native SOC-derived CO2. The addition of maize straw (cf. control) enhanced the O-alkyl C proportion, activities of β-glucosidase (BG), cellobiohydrolase (CBH) and dehydrogenase (DH), and abundances of GH48 and cbhI genes, while lowered aromatic C proportion, RubisCO enzyme activity, and cbbL abundance; the application of biochar induced the opposite effects. In all treatments, the cumulative native SOC-derived CO2 efflux increased with enhanced O-alkyl C proportion, activities of BG, CBH, and DH, and abundances of GH48 and cbhI genes, and with decreases in aromatic C, RubisCO enzyme activity and cbbL gene abundance. The enhanced emissions of native SOC-derived CO2 by the maize straw were associated with a higher O-alkyl C proportion, activities of BG and CBH, and abundance of GH48 and cbhI genes, as well as a lower aromatic C proportion and cbbL gene abundance, while biochar induced the opposite effects. We concluded that maize straw induced positive priming, while its biochar induced negative priming within a subtropical forest soil, due to the contrasting microbial responses resulted from changes in SOC speciation and compositions. Our findings highlight that biochar application is an effective approach for enhancing soil C stocks in subtropical forests.

Carbon emissions and priming effects derived from crop residues and their responses to nitrogen inputs

Crop residue-derived carbon (C) emissions and priming effects (PE) in cropland soils can influence the global C cycle. However, their corresponding generality, driving factors, and responses to nitrogen (N) inputs are poorly understood. As a result, the total C emissions and net C balance also remain mysterious. To address the above knowledge gaps, a meta-analysis of 1123 observations, taken from 51 studies world-wide, has been completed. The results showed that within 360 days, emission ratios of crop residues C (ER) ranged from 0.22% to 61.80%, and crop residues generally induced positive PE (+71.76%). Comparatively, the contribution of crop residue-derived C emissions (52.82%) to total C emissions was generally higher than that of PE (12.08%), emphasizing the importance of reducing ER. The ER and PE differed among crop types, and both were low in the case of rice, which was attributed to its saturated water conditions. The ER and PE also varied with soil properties, as PE decreased with increasing C addition ratio in soils where soil organic carbon (SOC) was less than 10‰; in contrast, the opposite phenomenon was observed in soils with SOC exceeding 10‰. Moreover, N inputs increased ER and PE by 8.31% and 3.78%, respectively, which was predominantly attributed to (NH4 )2 SO4 . The increased PE was verified to be dominated by microbial stoichiometric decomposition. In summary, after incorporating crop residues, the total C emissions and relative net C balance in the cropland soils ranged from 0.03 to 23.47 mg C g-1 soil and 0.21 to 0.97 mg C g-1 residue-C g-1 soil, respectively, suggesting a significant impact on C cycle. These results clarify the value of incorporating crop residues into croplands to regulate global SOC dynamics and help to establish while managing site-specific crop return systems that facilitate C sequestration.

Biochar carbon sequestration potential rectification in soils: Synthesis effects of biochar on soil CO2, CH4 and N2O emissions

Biochar production and its soil sequestration are promising ways to mitigate global warming. Effects of biochar on soil CO2, CH4 and N2O release have been studied extensively. In contrast, few studies have comprehensively quantified and synthesized the effect of biochar on soil greenhouse gas (GHG) emission and coupled it to the calculation of carbon sequestration potential. This study obtained the influence coefficient of biochar on soil GHG release relative to biochar carbon storage potential in soils under different environmental conditions, by literature statistics and data transformations. Our results showed that the overall average effect of biochar on soil CO2, CH4, N2O and CO2e release observed in our databases would compensate the potential of biochar soil carbon storage by -2.1 ± 3.3 %, 13.1 ± 9.8 %, -1.6 ± 8.6 % and 5.3 ± 11.4 %, respectively. By combining biochar induced soil GHG emission reduction mechanism and results from our literature statistics, some specific application environmental scenarios (such as biochar with high pyrolysis temperature of 500-600 °C, application in flooded soils, application in straw-return scenarios, etc.) were recommended, which could increase the actual carbon sequestration potential of biochar by an average of about 43.3 ± 30.2 % relative the amount of carbon buried. Our findings provide a scientific basis for developing a precise application strategy towards large scale adoption of biochar as a soil amendment for climate change mitigation.

Hydrogel bioreactor drives Feammox and synergistically removes composite pollutants: Performance optimization, microbial communities and functional genetic differences

Mixed pollutant wastewater has been a difficult problem due to the high toxicity of water bodies and the difficulty of treatment. Rice husk biochar modified with nano-iron tetroxide (RBC-nFe3O4) by polyvinyl alcohol cross-linking internal doping was used to introduce iron-reducing bacteria Klebsiella sp. FC61 to construct a bioreactor. The results of the long-term operation of the bioreactor showed that the removal efficiency of ammonia nitrogen (NH4+-N) and chemical oxygen demand best reached 90.18 and 98.49%, respectively. In addition, in the co-presence of Ni2+, Cd2+, and ciprofloxacin, the bioreactor was still able to remove pollutants efficiently by RBC-nFe3O4 and bio-iron precipitation inside the biocarrier. During the long-term operation, Klebsiella was always the dominant species in the bioreactor. And the sequencing data for functional prediction showed that the biocarrier contained a variety of enzymes and proteins involved in Feammox-related activities to ensure the stable and efficient operation of the bioreactor.

Biochar with Inorganic Nitrogen Fertilizer Reduces Direct Greenhouse Gas Emission Flux from Soil

Agricultural waste can have a catastrophic impact on climate change, as it contributes significantly to greenhouse gas (GHG) emissions if not managed sustainably. Swine-digestate-manure-derived biochar may be one sustainable way to manage waste and tackle GHG emissions in temperate climatic conditions. The purpose of this study was to ascertain how such biochar could be used to reduce soil GHG emissions. Spring barley (Hordeum vulgare L.) and pea crops in 2020 and 2021, respectively, were treated with 25 t ha^-1 of swine-digestate-manure-derived biochar (B₁) and 120 kg ha^-1 (N₁) and 160 kg ha^-1 (N₂) of synthetic nitrogen fertilizer (ammonium nitrate). Biochar with or without nitrogen fertilizer substantially lowered GHG emissions compared to the control treatment (without any treatment) or treatments without biochar application. Carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) emissions were directly measured using static chamber technology. Cumulative emissions and global warming potential (GWP) followed the same trend and were significantly lowered in biochar-treated soils. The influences of soil and environmental parameters on GHG emissions were, therefore, investigated. A positive correlation was found between both moisture and temperature and GHG emissions. Thus, biochar made from swine digestate manure may be an effective organic amendment to reduce GHG emissions and address climate change challenges.

Microbial iron reduction compensates for phosphorus limitation in paddy soils

Limitation of rice growth by low phosphorus (P) availability is a widespread problem in tropical and subtropical soils because of the high content of iron (Fe) (oxyhydr)oxides. Ferric iron-bound P (Fe(III)-P) can serve as a P source in paddies after Fe(III) reduction to Fe(II) and corresponding H₂PO₄^- release. However, the relevance of reductive dissolution of Fe(III)-P for plant and microbial P uptake is still an open question. To quantify this, ³²P-labeled ferrihydrite (30.8 mg P kg^-1) was added to paddy soil mesocosms with rice to trace the P uptake by microorganisms and plants after Fe(III) reduction. Nearly 2% of ³²P was recovered in rice plants, contributing 12% of the total P content in rice shoots and roots after 33 days. In contrast, ³²P recovery in microbial biomass decreased from 0.5% to 0.08% of ³²P between 10 and 33 days after rice transplantation. Microbial biomass carbon (MBC) and dissolved organic C content decreased from day 10 to 33 by 8-54% and 68-77%, respectively, suggesting that the microbial-mediated Fe(III) reduction was C-limited. The much faster decrease of MBC in rooted (by 54%) vs. bulk soil (8-36%) reflects very fast microbial turnover in the rice rhizosphere (high C and oxygen inputs) resulting in the mineralization of the microbial necromass. In conclusion, Fe(III)-P can serve as small but a relevant P source for rice production and could partly compensate plant P demand. Therefore, the P fertilization strategies should consider the P mobilization from Fe (oxyhydr)oxides in flooded paddy soils during rice growth. An increase in C availability for microorganisms in the rhizosphere intensifies P mobilization, which is especially critical at early stages of rice growth.

Evaluation of the synergistic effects of biochar and biogas residue on CO2 and CH4 emission, functional genes, and enzyme activity during straw composting

This study examined the effects of biochar, biogas residue, and their combined amendments on CO₂ and CH₄ emission, enzyme activity, and related functional genes during rice straw composting. Results showed that the biogas residue increased CO₂ and CH₄ emissions by 13.07 % and 74.65 %, while biochar had more obvious inhibition. Biogas residue addition enhanced functional gene abundance more than biochar. Biogas residue raised the methanogens mcrA gene by 2.5 times. Biochar improved the Acetyl-CoA synthase and β-glucosidase activities related to carbon fixation and decreased coenzyme activities related to methanogens. Biochar and biogas residue combined amendments enhanced the acsB gene abundance for CO₂ assimilation process and decreased methyl-coenzyme M reductase α subunit activity. Pearson correlation analysis indicated that organic matter was the significant variable affecting CO₂ and CH₄ emissions (P < 0.01). These results indicated biochar played significant roles in carbon loss and greenhouse emissions caused by biogas residue incorporation during composting.