Response of organic carbon in black soils with different degradation levels to litter addition.

Litter plays a crucial role in the formation and turnover of soil organic carbon (SOC). However, its impact on SOC in black soils with varying degradation levels remains unclear. With the typical black soils in northeastern China with different simulated degradation levels (slight, moderate, and severe), we comparatively analyzed the changes in total organic carbon (TOC), easily oxidizable organic carbon (EOC), particulate organic carbon (POC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC) under silver poplar litter addition (LP) and maize straw addition (MS) versus a treatment without litter addition (CK) at 45 days (early stage), 90 days (mid stage), and 135 days (late stage) after litter addition in a field in-situ experiment. The results showed that: 1) For black soils at different degradation levels, TOC content generally followed the order of LP>MS>CK. The increase in TOC was relatively greater in moderately degraded black soils. Compared with the early stage, TOC content in both LP and MS treatments were significantly increased during the mid and late stages. 2) At the early stage of litter decomposition, in slightly degraded black soils, the LP treatment significantly reduced EOC and DOC contents by 35.8% and 7.7%, respectively. In moderately and severely degraded soils, LP treatment significantly increased both POC and DOC contents. MBC was significantly increased across black soils with different degradation levels. Under MS treatment, EOC increased by 32.7% only in moderately degraded soils, while POC and MBC increased significantly only in severely degraded soils respectively. At the mid stage of litter decomposition, DOC content significantly decreased while MBC significantly increased in slightly degraded black soils, with POC significantly reduced by 36.7% in MS treatment. In moderately degraded soils, LP significantly increased DOC by 41.9%, and MS significantly increased MBC by 29.2%. In severely degraded soils, LP and MS significantly increased MBC by 17.3% and 16.0%, respectively. EOC content increased significantly under both LP and MS only in moderately degraded soils. At the late stage of litter addition, POC content decreased by 13.7%-21.4% in slightly degraded black soils, increased by 25.0%-25.3% in moderately degraded black soils, while EOC in LP treatment significantly increased only in slightly degraded soils. In moderately degraded soils, DOC significantly decreased and MBC increased. In severely degraded soils, LP significantly increased DOC by 6.0%, and MS significantly increased MBC by 28.6%. 3) Total nitrogen was a key factor influencing SOC and its active components in black soils with different degradation levels. In conclusion, the response of soil organic carbon and its active components to litter addition varied across soils with different degradation levels. High-quality litter (LP) was more conducive to the formation of TOC. The transport and accumulation of SOC and its active components were jointly regulated by both litter type and soil quality.