Depth-discrepant impact of winter cover crops on particulate and mineral-associated organic carbon in a subtropical paddy field

Abstract

Replacing winter fallow with cover crops can improve soil organic carbon (SOC) sequestration in agroecosystems, with cover crop species potentially differing in their contributions to SOC formation and stabilization. However, how different crop species affect the accumulation of distinct SOC fractions (particulate organic C, POC; mineral-associated organic C, MAOC) and their contribution to SOC storage remain unclear, particularly in subsoils of paddy fields. Here, we investigated how POC and MAOC responded to different cover crop species (hairy vetch, Vicia villosa Roth., and winter wheat, Triticum aestivum L.) in both topsoil (0–20 cm) and subsoil (20–40 cm, 40–60 cm) in paddy soils. Our results revealed that different cover crops induced divergent responses in SOC fractions, contingently dependent upon soil depths. Both cover crops significantly stimulated POC accumulation (winter wheat: 41.3 %; hairy vetch: 46.1 %) relative to fallow in topsoil, while cover crop effects on POC gradually diminished in subsoil. Meanwhile, cover crops significantly increased contents of dissolved organic carbon and available phosphorus, particularly in deeper subsoil, which were key factors affecting SOC accumulation. POC and MAOC accumulation along soil depths differed significantly between different cover crops, suggesting a species-specific effect. Winter wheat significantly boosted MAOC in both topsoil and deeper subsoil, while hairy vetch induced a statistically nonsignificant increase in MAOC across three depths compared to fallow. These divergent responses of SOC fractions were closely related to cover crop-induced changes of microbial community composition, necromass accumulation and enzyme activity. Random forest analysis revealed that microbial necromass was the main factor defining MAOC in topsoil, whereas the Fe oxides was the main factor influencing subsoil MAOC accumulation. Overall, winter cover increased total SOC sequestration across 0–60 cm soil depths and more importantly, potential SOC stability (MAOC:POC) was enhanced in subsoil. These findings demonstrate a depth-discrepant impact of cover crops on POC and MAOC in paddy soils. Our work highlights the need to present POC and MAOC fractions into biogeochemical models to better predict responses of SOC to cover crop management practices in rice paddy ecosystems.