Long-term residue returning increased subsoil carbon quality in a rice-wheat cropping system.

Residue returning (RR) was widely implemented to increase soil organic carbon (SOC) in farmland. Extensive studies concentrated on the effects of RR on SOC quantity instead of SOC fractions at aggregate scales. This study investigated the effects of 20-year RR on the distribution of labile (e.g., dissolved, microbial biomass, and permanganate oxidizable organic) and stable (e.g., microbial necromass) carbon fractions at aggregate scales, as well as their contribution to SOC accumulation and mineralization. The findings indicated a synchronized variation in the carbon content of bacterial and fungal necromass. Residue retention (RR) notably elevated the concentration of bacterial and fungal necromass carbon, while it did not amplify the microbial necromass carbon (MNC) contribution to SOC when compared to residue removal (R0) in the topsoil (0-5 cm). In the subsoil (5-15 cm), RR increased the MNC contribution to SOC concentration by 21.2%-33.4% and mitigated SOC mineralization by 12.6% in micro-aggregates (P < 0.05). Besides, RR increased soil ß-glucosidase and peroxidase activities but decreased soil phenol oxidase activity in micro-aggregates (P < 0.05). These indicated that RR might accelerate cellulose degradation and conversion to stable microbial necromass C, and thus RR improved SOC stability because SOC occluded in micro-aggregates were more stable. Interestingly, SOC concentration was mainly regulated by MNC, while SOC mineralization was by dissolved organic carbon under RR, both of which were affected by soil carbon, nitrogen, and phosphorus associated nutrients and enzyme activities. The findings of this study emphasize that the paths of RR-induced SOC accumulation and mineralization were different, and depended on stable and labile C, respectively. Overall, long-term RR increased topsoil carbon quantity and subsoil carbon quality.

The impact of crop residue return on the food-carbon-water-energy nexus in a rice-wheat rotation system under climate warming.

Rice-wheat rotation (RWR) is one of the major cropping systems in China and plays a crucial role in the country's food security. With the promotion of "burn ban" and "straw return" policies, the "straw return + rice-wheat crop rotation system" has been developed in China's RWR area. However, the effect of promotion of straw return on production and ecological benefits of RWR areas is unclear. In this study, the main planting zones of RWR were examined, and ecological footprints and scenario simulation were applied to explore the effect of straw return on the food-carbon-water-energy nexus under conditions of a warming world. The results indicate that with rising temperatures and the promotion of straw return policies, the study area was in a "carbon sink" state during 2000-2019. The study area's total yield climbed by 48 % and the carbon (CF), water (WF) and energy (EF) footprints decreased by 163 %, 20 % and 11 %, respectively. Compared to 2000-2009, the temperature increase for 2010-2019 was negatively correlated with the increase in CF and WF and positively correlated with the increase of yield and EF. A 16 % reduction in chemical fertilizers, increasing the straw return rate to 80 % and utilizing tillage techniques such as furrow-buried straw return would contribute to sustainable agriculture in the RWR area under a projection of 1.5 °C increase in air temperature. The promotion of straw return has contributed to improved production and the maintenance and reduction of CF, WF, and EF in the RWR, but further optimization measures are required to reduce the footprint of agriculture in a warmer world.

Soil organic carbon regulates CH4 production through methanogenic evenness and available phosphorus under different straw managements.

Methane (CH4) is the main greenhouse gas emitted from rice paddy fields driven by methanogens, for which methanogenic abundance on CH4 production has been intensively investigated. However, information is limited about the relationship between methanogenic diversity (e.g., richness and evenness) and CH4 production. Three independent field experiments with different straw managements including returning method, burial depth, and burial amount were used to identify the effects of methanogenic diversity on CH4 production, and its regulating factors from soil properties in a rice-wheat cropping system. The results showed that methanogenic evenness (dominance) can explain 23% of variations in CH4 production potential. CH4 production potential was positively related to methanogenic evenness (R2 = 0.310, p < 0.001), which is driven by soil organic carbon (SOC), available phosphorus (AP), and nitrate (NO3-) through structure equation model (SEM). These findings indicate that methanogenic evenness has a critical role in evaluating the responses of CH4 production to agricultural practices following changes in soil properties. The SEM also revealed that SOC concentration influenced CH4 production potential indirectly via complementarity of methanogenic evenness (dominance) and available phosphorus (AP). Increasing SOC accumulation improved AP release and stimulated CH4 production when SOC was at a low level, whereas decreased evenness and suppressed CH4 production when SOC was at a high level. A nonlinear relationship was detected between SOC and CH4 production potential, and CH4 production potential decreased when SOC was ≥14.16 g kg-1. Our results indicated that the higher SOC sequestration can not only mitigate CO2 emissions directly but CH4 emissions indirectly, highlighting the importance to enhance SOC sequestration using optimum agricultural practices in a rice-wheat cropping system.

Changes in cropland soil carbon through improved management practices in China: A meta-analysis.

Recommended management practices (RMPs, e.g., manuring, no-tillage, crop residue return) can increase soil organic carbon (SOC), reduce greenhouse gas emissions, and maintain soil health in croplands. However, there is no consensus on how RMPs affect the SOC storage potential of cropland soils for climate change mitigation. Here, based on 2301 comparisons from 158 peer-reviewed papers, a meta-analysis was conducted to explore management-induced SOC stock changes and their variations under different conditions. The results show that SOC stocks in the 0-20 cm layer were increased by 31.8% when chemical fertilization combined with manure application was compared with no fertilizer; 9.98% when no-tillage was compared with plow tillage; and 10.84% when straw return was compared with removal. The RMPs favorably increased SOC stock in arid areas, and in alkaline and fine-textured soils. Initial SOC, carbon-nitrogen ratio, and experimental duration could also affect SOC storage. Compared with the initial SOC stock, RMPs increased the SOC sequestration potential by 2.6-4.5% in the 0-20 cm soil depth, indicating that these practices can help China achieve targets to increase SOC by 4.0‰. Hence, it is essential to implement RMPs for climate change mitigation and soil fertility improvement.

Straw incorporation interacting with earthworms mitigates N2O emissions from upland soil in a rice-wheat rotation system.

Intensive attentions have been paid to the positive effects on nitrous oxide (N2O) production under straw return or the presence of earthworms. Straw return as a sustainable practice can promote earthworm growth, how the interactions between straw and earthworms affect N2O production is still not well known. A split-plot field experiment (straw return as main plot and earthworm addition as subplot) was performed to quantify the interactive effects of straw and earthworm on N2O emissions from a wheat field and to determine the underlying mechanisms from nitrification and denitrification processes. The results showed that straw return significantly increased N2O emissions by 41.0 % under no earthworm addition but decreased it by 19.0 % under earthworm addition compared with straw removal (P < 0.05). The significant interaction between straw and earthworm benefits the mitigation of N2O emissions. Random forest model showed that denitrification and nitrification were dominant processes to affect N2O emissions at the jointing and booting growth stages of wheat, respectively. The interaction between straw and earthworm significantly decreased the abundances of N2O-producing bacterial genes such as nirS and nirK at the jointing stages, and AOB at the booting stages. The contrasting mechanisms in regulating N2O emissions at different growth stages should be considered in nitrogen recycling models to accurately predict available N and N2O dynamics. Our findings suggest that N2O emissions under straw return can be weakened with the increasing earthworm populations under the scenario of widely used conservation practices (e.g., straw return and no-till) due to significant interaction between straw and earthworms.

A 40 % paddy surface soil organic carbon increase after 5-year no-tillage is linked with shifts in soil bacterial composition and functions.

Soil organic carbon (SOC) is related to soil fertility, crop yield, and climate change mitigation. Paddy soil is a significant carbon (C) sink, but its C sequestration potential has not been realized as the various driving factors are still not fully understood. We performed a 5-year paddy field experiment in southern China to estimate tillage effects on SOC accumulation and its relation with soil bacteria. The C input from rice residue, SOC content, CO2 flux, soil bacterial community composition, and predicted functions were analyzed. No-tillage (NT) increased (p < 0.05) rice residue C inputs (by 12.6 %-15.9 %), SOC (by 40 % at the surface soil layer compared with conventional tillage, CT), and CO2 fluxes compared with reduced tillage (RT) and CT. Also, NT significantly altered the soil bacterial community. The random forest model showed that the predicted bacterial functions of "Degradation/Utilization/Assimilation Other", "C1 Compound Assimilation", and "Amin and Polyamine Degradation" were the most important functions associated with SOC accumulation. Analysis of metabolic pathway differences indicated that NT significantly decreased the BENZCOA-PWY (anaerobic aromatic compound degradation) and the AST-PWY (L-arginine degradation II). Therefore, the rapid paddy SOC increase is associated with both residue C input (from higher rice yields) and the degradation functions regulated by soil bacteria.

Sieving soil before incubation experiments overestimates carbon mineralization but underestimates temperature sensitivity.

The sensitivity of soil organic carbon (SOC) mineralization to temperature could affect the future atmospheric CO2 levels under global warming. Sieved soils are widely used to assess SOC mineralization and its temperature sensitivity (Q10) via laboratory incubation. However, sieved soils cause a temporary increase in mineralization due to the destruction of soil structure, which can affect estimates of SOC mineralization, especially in soils managed with no-till (NT). To identify the effects of soil sieving on SOC mineralization and Q10, soil was collected from an 11-year field experiment under a wheat-maize cropping system managed with a combination of tillage [NT and plow tillage (PT)] and residue [residue returning (RR) and residue removal (R0)]. Soil was either sieved or left in an undisturbed state and incubated at 15 °C and 25 °C. SOC mineralization in sieved soils at 25 °C was 47.28 g C kg-1 SOC, 160.1% higher than SOC mineralization in undisturbed soils (P < 0.05). Interestingly, Q10 values in sieved soils were 1.29, 77.6% lower than Q10 in undisturbed soils (P < 0.05). Highly significant correlations (P < 0.01) were observed between sieved and undisturbed soils for SOC mineralization (r = 0.85-0.98) and Q10 (r = 0.78-0.87). Soil macro-aggregates had lower SOC mineralization by 6.1-21.9%, but higher Q10 values by 4.7-6.5% compared with micro-aggregates, contributing to lower mineralization and higher Q10 under NT and RR. Furthermore, structure equation and random forest modelling showed that increased SOC contents in NT and RR could not only reduce SOC mineralization, but also constrained the improvement of Q10 in NT and RR. Overall, these results indicated that although sieved soils overestimated SOC mineralization and underestimated Q10 due to the destruction of macro-aggregates, the patterns between treatments were similar and sieving soil for incubation is considered as a suitable approach to evaluate the relative impacts of NT and RR on SOC mineralization and Q10.

Mechanisms of soil organic carbon stability and its response to no-till: A global synthesis and perspective.

Mechanisms of soil organic carbon (SOC) stabilization have been widely studied due to their relevance in the global carbon cycle. No-till (NT) has been frequently adopted to sequester SOC; however, limited information is available regarding whether sequestered SOC will be stabilized for long term. Thus, we reviewed the mechanisms affecting SOC stability in NT systems, including the priming effects (PE), molecular structure of SOC, aggregate protection, association with soil minerals, microbial properties, and environmental effects. Although a more steady-state molecular structure of SOC is observed in NT compared with conventional tillage (CT), SOC stability may depend more on physical and chemical protection. On average, NT improves macro-aggregation by 32.7%, and lowers SOC mineralization in macro-aggregates compared with CT. Chemical protection is also important due to the direct adsorption of organic molecules and the enhancement of aggregation by soil minerals. Higher microbial activity in NT could also produce binding agents to promote aggregation and the formation of metal-oxidant organic complexes. Thus, microbial residues could be stabilized in soils over the long term through their attachment to mineral surfaces and entrapment of aggregates under NT. On average, NT reduces SOC mineralization by 18.8% and PE intensities after fresh carbon inputs by 21.0% compared with CT (p < .05). Although higher temperature sensitivity (Q10 ) is observed in NT due to greater Q10 in macro-aggregates, an increase of soil moisture regime in NT could potentially constrain the improvement of Q10 . This review improves process-based understanding of the physical and chemical mechanism of protection that can act, independently or interactively, to enhance SOC preservation. It is concluded that SOC sequestered in NT systems is likely to be stabilized over the long term.

Strategic tillage achieves lower carbon footprints with higher carbon accumulation and grain yield in a wheat-maize cropping system.

Continuous single tillage has the potential to increase greenhouse gas (GHG) emissions and decrease the accumulation of soil organic carbon (SOC), thus increasing carbon footprints (CFs). However, in a wheat-maize cropping system, limited information was available about the effects of strategic tillage on CFs. Thus, a four-year field experiment was conducted, including continuous rotary tillage (RT), continuous no-till (NT), RT + subsoiling (RS), and NT + subsoiling (NS), to investigate the effects of NS (strategic tillage) on the unit area and unit yield. The results showed that CO2 emission was the highest contributor to CFs (73.92%) in a winter wheat-summer maize cropping system, following the order of NS < NT < RS < RT. The direct N2O emissions from fertilizers and residues were 4.43-4.51 t CO2-eq ha-1 yr-1 during the wheat and maize seasons, and indirect N2O emissions from irrigation and fertilizer inputs had a proportion of >80% from total agricultural inputs. The differences in SOC storage significantly affected the CFs. Although the NS treatment increased the amount of GHG emissions from the residues returned and consumption of diesel, the enhancement of SOC storage by deeper SOC increased. Thus, lower area-scaled CFs were observed in the NS treatment. Furthermore, a higher grain yield and an annual change of SOC storage compared with other treatments were observed under the NS system, which helped to reduce the CFs. The yield-scaled CFs followed the order of RT > RS > NT > NS when considering the changes in SOC storage. Therefore, the NS treatment resulted in a higher grain yield and SOC sequestration with lower CFs, and thus, it could be recommended as the best tillage method to achieve sustainable production and environmental balance in a wheat-maize cropping system.

Effects of tillage management on soil carbon decomposition and its relationship with soil chemistry properties in rice paddy fields.

Decreasing the soil organic carbon (SOC) decomposition is critical to improve the quality of the soil and mitigate atmospheric CO2 emissions. To improve the ability to protect the SOC by optimizing tillage management, this study investigated the laboratory-based SOC mineralization (decomposition) and soil chemical properties under different tillage practices, including no tillage with straw mulch (NTS), rotary tillage with straw incorporated (RTS), moldboard plow tillage with straw incorporated (CTS) and moldboard plow tillage with straw removal (CT). Soil samples of six sampling dates from April 2017 to October 2018 were incubated at 25 °C and 70% water holding capacity for 60 d. Repeated Variance Analyses were conducted to compare the means of different treatments. The results showed that the average cumulative SOC mineralization (Cm) at the 0-5 cm soil depth was 7.09 g CO2 kg-1 soil under NTS, which was higher (P < 0.05) than that of the other treatments. However, the C mineralizability at both the 0-5 and 5-10 cm soil depths were lower (P < 0.05) under the NTS (0.16 and 0.15 g CO2 g-1 SOC) compared with the CTS and CT. Non-microbial CO2 emissions (CO2 emissions in sterilized soil) contributed to the lower C mineralizability under NTS, due to the lower mineralizability (0.041-0.089 g CO2 g-1 SOC) of sterilized soil under this treatment. Furthermore, some of the abiotic factors (e.g., C/N ratio and SOC content) significantly correlated with the Cm and C mineralizability. These factors might be critical for the ability to protect SOC under NTS. In summary, conservation tillage is an optimal management due to its protection on SOC, and part of this protection appeared to have been contributed by the soil abiotic factors, which were formed by long-term tillage management.

Dynamics in soil organic carbon of wheat-maize dominant cropping system in the North China Plain under tillage and residue management.

A site experiment was conducted to assess temporal dynamics of soil organic carbon (SOC) and the drivers under no-tillage (NT) and residue retention (RR) in the North China Plain (NCP). The results indicated that NT and RR can significantly increase SOC up to a depth of 30 cm. On average, NT increased SOC by 8.1-34.5% compared with PT, and RR increased SOC by 3.5-14.4% compared with R0 at 0-10 cm. Increases in SOC under NT or RR could be increased by 4-10 percentage points through the significantly positive interactions of NT and RR. Among the sources of SOC variations, tillage-induced variations accounted for 74.4 and 44.3% of the total variations in SOC at 0-5 cm for wheat and maize season, respectively. Experimental duration was also a significant source of variation. Stepwise regression indicated dynamics in SOC at 0-5 cm mainly due to the positive effects of precipitation, the negative effects of soil bulk density for the wheat season, the negative effects of radiation for the maize season, and antagonistic effects of temperature between wheat and maize season. Generally, positive effects of NT and RR on SOC were both confirmed, but fluctuations and variations induced by interactions of practices and seasonal climatic conditions were also significant in the NCP.