Core Ideas Straw returned twice each year (WC‐MC and WH‐MC) significantly enhanced SOC and DOC. Return of both wheat and maize chopped straw had highest β‐1,4‐xylosidase, invertase, and enhanced Alphaproteobacteria and Gammaproteobacteria. Retention of high wheat stubble and return of chopped straw had highest β‐1,4‐glucosidase, cellobiohydrolase, and enhanced Acidobacteria and Planctomycetes. Amount and complexity of straw added led to differences in C, enzyme activity, and microbial composition. High wheat stubble and crushed straw improved yields and bacterial richness, but had less C sequestration. Little is known about the responses of organic carbon fractions, enzymes activity, and soil bacterial communities and diversity to different straw return modes in intensive maize‐wheat double‐cropping systems. An 8‐yr field experiment was conducted to evaluate the effects of different straw return modes. Four treatments were compared: (i) return of both wheat and maize chopped straw (WC‐MC); (ii) retention of high wheat stubble and return of chopped maize straw (WH‐MC); (iii) retention of high wheat stubble and no return of maize straw (WH‐MN); and (iv) no return of straw of both wheat and maize (WN‐MN). Soil organic carbon, dissolved organic carbon, and particulate organic carbon content increased significantly in the WC‐MC and WH‐MC. Beta‐1‐4‐xylosidase and invertase were higher in the WC‐MC treatment than in WH‐MC, while the highest β‐1,4‐glucosidase activities were observed in WH‐MC. However, the activity of polyphenol oxidase was low in all three straw return treatments. Alphaproteobacteria and Gammaproteobacteria dominated in WC‐MC, while the Acidobacteria and Planctomycetes phyla dominated WH‐MC. The two high wheat stubble retention modes caused significant increases in bacterial richness. Redundancy analysis showed that C input amount and organic carbon content were the most influential factors determining bacterial community structure. The observed effects were due to straw amount and the complex composition of WH‐MC added during the wheat growing season compared to WC‐MC. In conclusion, the straw return mode that employs high wheat stubble retention and chopped maize straw return is sufficient to maintain organic carbon levels and improve the soil microenvironment

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Increasing organic carbon sequestration in agricultural soils is important for improving soil fertility and mitigating climate change. Wood ash is generally applied as a potassium fertilizer, but the effects of simultaneous incorporation of wood ash and crop straw on the turnover of soil organic carbon (SOC) and inorganic carbon (SIC) are not well understood. In this study, a 118-day lab incubation experiment was conducted using a calcareous soil (with 10 years of continuous maize cropping history) to study the effects of adding wheat straw, wood ash and lime. Our study showed that straw addition led to an increase in both SOC (19%) and SIC (3%). Wood ash and lime addition decreased CO2 emission by 182 and 1210 mg kg-1 and increased SIC by 125 and 1001 mg kg-1 during the incubation, respectively, which was due to supply of CaO from wood ash and lime. The increase of SOC content was 2.4% due to the addition of lime. In addition to straw addition enhanced straw-derived OC content, the addition of lime also increased straw-derived OC content by 34.5%. This study demonstrated that lime was more effective in reducing CO2 emission and and enhancing SOC than wood ash. In conclusion, adding lime to calcareous soil might be an effective method of enhancing carbon sequestration and slowing climate change.

We quantified the effects of different straw return modes on soil organic carbon (SOC), total nitrogen content (TN) and C:N ratios in a wheat/maize double-cropping agricultural system by analysing their content in different soil aggregate sizes and density fractions under four modes of straw return: (a) no return/retention of wheat and maize straw (Control); (b) retention of long wheat stubble only (Wheat Stubble); (c) retention of long wheat stubble and return of chopped maize straw (Mixed); and (d) return of chopped wheat and maize straw (Both Chopped). The Mixed and Both Chopped straw return modes produced the highest crop yields. Relative to the Control, SOC stock was 9.6% greater with the Mixed treatment and 14.5% greater with the Both Chopped treatment, whereas the Wheat Stubble treatment had no effect on SOC. Mixed and Both Chopped significantly enhanced TN stock relative to the Wheat Stubble and Control treatments. Compared with the Control, the Mixed and Both Chopped treatments increased the mass proportions of large macroaggregates and reduced the silt plus clay fraction; Mixed and Both Chopped caused a significant increase in SOC and TN in large and small macroaggregates; the Mixed treatment significantly increased SOC content in the coarse and fine intra-aggregate particulate organic matter (iPOM) density fractions of large macroaggregates, whereas Both Chopped increased SOC in the coarse iPOM, fine iPOM and mineral-associated organic matter (mSOM) density fractions of both large and small macroaggregates;and Mixed and Both Chopped enhanced TN content in coarse iPOM and fine iPOM within small macroaggregates. Although the Mixed treatment was slightly less effective at improving C sequestration in agricultural fields than the Both Chopped treatment, the Mixed treatment may nonetheless be the optimal plant residue management mode in terms of minimizing time and labour due to its ability to improve soil structure, maintain organic carbon levels and provide a means of sustainable crop production in intensive wheat/maize double-cropping systems. K E Y W O R D SC sequestration, crop yield, SOC fractions, straw return, wheat-maize double cropping 512 | ZHAO et Al.

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