Cover crops drive soil organic carbon sequestration in newly reclaimed saline-alkali land: Altitude and species specificity of improvement effects.

Land-use changes significantly affect soil organic carbon (SOC) turnover, determining whether soils act as a carbon source or sink. To evaluate the role of cover crops (CCs) in improving newly reclaimed saline-alkali land and their impact on carbon storage capacity (indicated by CO2/SOC), we measured carbon source-sink indicators after a five-year remediation using maize, oil sunflower, and wheat as CCs across different altitudes (1700-2300 m above sea level [m a.s.l.]). A field experiment with three altitude gradients, three crop types, and bare land controls was conducted in triplicate. The results showed that CCs enhanced the functional recovery of saline-alkaline soils and increased SOC storage by regulating soil physicochemical properties, enzyme activities, and microbial community structure, with the extent of improvement varying by cover crop species and altitude. Compared with bare land, SOC content increased significantly under the optimal crop at each altitude (up to 84.94% increase under wheat at 2100-2300 m a.s.l.), while CO2/SOC decreased (maximum reduction of 67.45% under maize at 1700-1900 m a.s.l.). Multiple stepwise regression identified key soil factors influencing SOC storage and turnover. The lowest CO2/SOC was observed for maize at 1700-1900 m a.s.l., oil sunflower at 1900-2100 m a.s.l., and wheat at 2100-2300 m a.s.l., indicating the most favorable SOC storage conditions at these respective altitudes. This study demonstrates that CCs effectively restore saline-alkaline soil function and promote SOC sequestration, with altitude-specific crop selection significantly enhancing remediation outcomes and ecological benefits.