Effects of years of straw return to soil on greenhouse gas emission in rice/wheat rotation systems

Crop straw is the largest renewable resource on earth but it is often burned after crop harvest, resulting in loss of nutrients and environmental pollution. With current advocate for ecological agriculture, straw return to soil is the most important mode of reutilization of agricultural by-products. This mode is vigorously applied and promoted in rice/wheat rotation system. Research has shown that straw return to soil as a key mode of ecological agriculture affects many aspects of agricultural systems, including soil physical and chemical properties, carbon sequestration in soils, greenhouse gas emissions, etc. Despite this, less research has focused on the effects of different years of straw return to soil on the contributions of agricultural systems to global warming. In this study, field experiment was conducted to determine the effects of different years of straw return to soil (with CK as no straw return without fertilizer; and NT, SR1 and SR5 as 0, 1 and 5 years of straw return with normal fertilization) on greenhouse gases (CH₄, CO₂ and N₂O) emissions and soil carbon sequestration in rice/wheat rotation system. Combined with the effects of straw burning on greenhouse gas emissions, the contributions of the four treatments to global warming were comprehensively estimated. Greenhouse gases (CH₄, CO₂ and N₂O) emission fluxes from the rice/wheat rotation systems were analyzed using the static chamber gas chromatography method. Greenhouse gases emission fluxes from straw burning were estimated based on published lists of greenhouse gases by IPCC 2006. The results of the study showed that straw return to soil (SR1 and SR5) significantly increased CH₄ and CO₂ emission fluxes, which were higher than those of NT by 73.52% and 13.29%, than those of CK by 309.49% and 13.06%, respectively. Straw return to soil also decreased N₂O emission flux, which was lower than those of NT (29.68%) and CK (42.55%). However, no significant difference was noted between greenhouse gases emissions under SR1 and SR5. Compared with NT, SR1 and SR5 increased one year soil carbon sequestrations respectively by 517.90% and 709.03%. Also soil carbon sequestration under SR5 was higher than that under SR1 by 30.93%. The contribution of straw burning to soil carbon sequestration under NT was 9 698.49 kg(CO₂-eqv)·hm^-2, which was higher than that of CK by 126.98%. The combined effects of the three factors (greenhouse gas emission, carbon sequestration and straw burning), the contribution to global warming of SR1 was lowest, significantly lower than that of NT by 4.72%. While full short-term straw return to soil was useful in reducing overall greenhouse gas emission, the range of reduction would decrease with time of full application of long-term straw return to soil.