Contrasting effects of different straw return modes on net ecosystem carbon budget and carbon footprint in saline-alkali arid farmland

Saline-alkali soils seriously restrict agricultural productivity, reduce carbon (C) sequestration and increase global warming potential (GWP) of farmland systems. Straw return is usually used to improve soil physicochemical properties, increase soil organic carbon (SOC) and crop yield in saline-alkali arid farmland. The performance of straw return is influenced by varieties of factors including incorporation mode, soil type, crop and climate. Therefore, from 2017 to 2019, a three-year short-term study was conducted to investigate the impact of different straw return modes on soil greenhouse gas (GHG) emissions, net ecosystem C budget (NECB), net global warming potential (NGWP) and C footprint in a saline-alkali farmland under arid conditions. Four straw return modes, namely straw return with rotary tillage, straw return with plowing, and straw-derived biochar treatment and a control without straw return, were conducted in saline-alkali arid farmland. Results indicated that compared with the control, straw return with rotary tillage increased direct GHG emissions (soil borne CO2 and N2O emissions), while straw-derived biochar decreased direct GHG emissions. Straw return with plowing had no significant impact on direct GHG emissions. Straw return with plowing, straw return with rotary tillage, and straw-derived biochar significantly increased indirect CO2 emissions due to more consumption of energy, fossil fuels and diesel than the control. For C sequestration, straw return significantly increased crop C fixation and soil C sequestration compared with the control. The C sequestration in straw-derived biochar treatment was significantly higher than that of the return with rotary tillage and plowing treatments, thus achieving the highest NECB and the lowest NGWP and yield-scaled C footprint. Compared to the control, straw-derived biochar mode increased NECB by 2618.4−11379.4 kg ha−1, and decreased NGWP and yield-scaled C footprint by 12660.4−43431.9 kg ha−1 and 0.03−1.9 kg kg−1, respectively. Overall, the short-term results suggested that crop C fixation and soil C sequestration are crucial determinants of the NECB and C footprint, respectively. Return in the form of biochar could be recommended as an appropriate straw return practice for enhancing C sequestration and mitigating the GWP of the saline-alkali farmland.