Microbial necromass and aggregate structure jointly regulate divergent SOC sequestration pathways under straw return practices

Straw return enhances soil organic carbon (SOC) sequestration, but different practices may regulate C accumulation and stabilization through distinct pathways. A 2014 field experiment on the Loess Plateau compared conventional fertilization (CF), straw return (SR), and straw return with plastic film mulching (SRM) using SOC fractions (2016–2025), microbial necromass and enzyme activities (2016, 2019, and 2025), and aggregate characteristics (2024–2025). From 2016–2025, SOC under SR increased by 30.97%, with an early rate of 0.29 g kg −1 yr −1 and a breakpoint around the 8th experimental year. SRM showed a faster early rate of 0.39 g kg −1 yr −1 but increased SOC by 22.92% and approached an apparent equilibrium around the 6th experimental year. POC accumulated faster under SR than SRM (0.28 vs. 0.15 g kg −1 yr −1 ), whereas MAOC increased mainly after 2018 under SR but before 2019 under SRM, by 18.87% and 15.48%, respectively. PLS-PM identified significant positive paths from bacterial necromass carbon (BNC) to POC under SR and SRM, whereas MAOC was associated with fungal necromass carbon under SR and with BNC under SRM. SR increased aggregate stability by approximately 50.36%, and SOC retention was mainly associated with the increased proportion of > 2 mm macroaggregates. By contrast, SRM increased aggregate stability by 23.02%, while its SOC increase was associated with the > 2 mm macroaggregate proportion and C concentrations in < 0.25 mm fractions. Overall, SR may be more suitable for sustained SOC improvement, whereas SRM favors faster early accumulation but approaches apparent equilibrium earlier.