Inefficient C sequestration with long term high-level straw return as linked to protected C pools saturation on the North China Plain

Straw return has been widely used as a key measure to increase carbon (C) sequestration and sustain crop yields on the North China Plain, however, its effects were limited under long-term application. The mechanisms underlying the transformation of residue-derived C in soil and saturation status of different protected pools following long-term different straw return levels remain unclear. Soil samples were collected from a 41-yr field experiment with varying straw return rates: 0 kg·ha⁻¹ (CK), 2250 kg·ha⁻¹ (S1), 4500 kg·ha⁻¹ (S2), and 9000 kg·ha⁻¹ (S3), and incubated with ¹³C-labeled straw for 300 days. Soil organic carbon (SOC) and its active fractions in different C pools were measured. After 300 days of incubation, SOC concentration (14.2 %-26.1 %) and active C fractions (32.8 %-220.5 %) increased with an increase in straw application rates. Residue-derived particulate organic C (POC), microbial biomass C (MBC), and dissolved organic C (DOC) concentrations were greater in S3 than in S1 and S2, and these values were greater in S2 than in S1. Furthermore, straw return increased SOC concentration in different protected pools by 4.2 %-36.0 %, but no significant differences were found between S2 and S3 for all protected pools. Residue-derived C was primarily stored in the unprotected pool, accounting for 73.9 %-80.8 %. The residue-derived C concentration in S3 was significantly higher than those in S1 and S2 (except for the microaggregate-protected organic C and occluded silt and clay fractions). All protected pools showed signs of C saturation, but the mechanisms of C saturation differed. The physically protected pool reached C sequestration limit due to microaggregate restrictions. In contrast, the chemically and biochemically protected pools reached a steady-state balance through the mineralization loss of native SOC and the fixation of residue-derived C. Overall, after long-term straw return in North China, surface soils still retain C sequestration potential, but no significant differences in C sequestration were observed between medium and high rates of maize straw return. Integrating straw return with targeted nutrient management or diversified planting is essential to enhance the C sequestration capacity of straw return in the long run.