Subsoil SOC increased by high C:N ratio straw application with optimized nitrogen supplementation

The application of straw and nutrients to agricultural subsoils is of significant interest to enhance carbon (C) sequestration and soil fertility. However, little research has explored the effect straw application on microbial stoichiometry, soil organic carbon (SOC) mineralization and accumulation and their relationships, in subsoil. In order to address these knowledge gaps, we examined the soil characteristics (organic carbon mineralization, available nutrient contents and microbial stoichiometric ratio) under straw addition (maize leaf and stem) with and without nitrogen (N) supplementation (no addition, 30 mg N kg⁻¹ dry soil⁻¹, 60 mg N kg⁻¹ dry soil⁻¹) during an 80-day incubation experiment. The microbial stoichiometric ratio imbalance (C:N and C:Phosphorus (P)) and extracellular enzyme stoichiometry were measured as indicators of the systematic relationship between soil resource availability and the mineralization of organic carbon. Our study demonstrated that the addition of straw significantly enhanced CO₂ emissions and led to an increase in the C:N imbalance, while simultaneously decreasing microbial carbon use efficiency (CUE). In addition, stem addition showed 5.6% lower CUE, but 8.2% higher SOC compared with leaf addition. We also found that nitrogen addition to subsoil alleviated microbial nitrogen limitation. 60 mg N kg⁻¹ dry soil rates of nitrogen application had a positive effect on reducing C:N imbalance and promoting the accumulation of SOC. Extracellular enzyme activity and microbial stoichiometric ratio were the most important controlling factors of SOC mineralization and microbial CUE, respectively. In conclusion, the application of straw alongside N to balance stoichiometric ratios can significantly increase SOC content, indicating the potential for carbon sequestration in agricultural subsoils.