Effects of Long-Term Fertilization on Particulate and Mineral-Associated Organic and Inorganic Carbon in Southwest China

Soil organic carbon (SOC) and soil inorganic carbon (SIC) are two key components of soil total carbon (STC) pools. However, most studies have focused excessively on SOC, while research on SIC remains limited, especially with regard to different pools of particulate (POM) and mineral-associated organic matter (MAOM) in humid regions. Here, a 13-year field experiment was conducted in the farmland of Jiangjin District, Chongqing, to explore the variations of inorganic carbon in POM (POM-IC) and MAOM (MAOM-IC) in humid subtropical soils under long-term fertilization. Four fertilization regimes were arranged in this field experiment: high-rate fertilization (1050 kg N, 480 kg P2O5, and 255 kg K2O ha−1 yr−1), conventional fertilization (480 kg N, 180 kg P2O5, and 255 kg K2O ha−1 yr−1), zero nitrogen fertilization (0 kg N, 180 kg P2O5, and 255 kg K2O ha−1 yr−1), and zero phosphorus fertilization (480 kg N, 0 kg P2O5, and 255 kg K2O ha−1 yr−1). Soil samples were collected from surface soil (0–15 cm) and subsoil (15–30 cm) to determine STC, SOC, SIC, organic carbon in POM (POM-OC) and MAOM (MAOM-OC), POM-IC, and MAOM-IC. Results showed that SOC accumulation under high-rate fertilization was primarily associated with increased POM-OC. Compared with the zero nitrogen treatment, the other three fertilization regimes significantly decreased subsoil SIC, which was primarily associated with reduced MAOM-IC. High-rate fertilization increased the contributions of POM-OC to SOC and POM-IC to SIC, respectively, yet reduced the corresponding contributions from MAOM. Linear relationship analysis revealed that POM-OC was more sensitive to fertilization regimes than MAOM-OC. However, responses of POM-IC and MAOM-IC to fertilization regimes were roughly equivalent. This is of great significance for understanding the stabilization mechanisms of SIC. This study highlights the non-negligible MAOM-IC loss in subsoil induced by nitrogen fertilization in humid subtropical soils. Given that STC was the highest under high-rate fertilization, this treatment is recommended. This study is of great significance for improving the understanding of soil organic carbon and inorganic carbon dynamics in humid regions.