Response of soil organic carbon fractions and CO2 emissions to exogenous composted manure and calcium carbonate

PURPOSE: The addition of exogenous substances has an important influence on soil organic matter mineralization. The aim of the present study was to investigate the effect of calcium carbonate (CaCO₃) and composted manure (CM) on soil organic C (SOC) fractions and soil CO₂ emission in calcareous soil. MATERIALS AND METHODS: We conducted a 105-day incubation experiment involving six treatments of two soils from Yangling (Y) and Tongchuan (T) on the Loess plateau, China, which had different CaCO₃ and total organic C (TOC) content. Treatments consisted of 0, 30, and 50 g kg⁻¹ CaCO₃ (Y0, Y30, Y50, T0, T30, and T50) and 0, 30, and 50 g kg⁻¹ CaCO₃ plus 20 g kg⁻¹ CM (Y0-M, Y30-M, Y50-M, T0-M, T30-M and T50-M). Soil pH, CaCO₃, CO₂ emissions, TOC, microbial biomass C (MBC), dissolved organic C (DOC), readily oxidizable organic C (ROC), and bound humus (B-Hs) C were determined for each of the treatments. RESULTS AND DISCUSSION: The application of CM increased MBC, DOC, and ROC, promoting the mineralization of SOC. However, the effects of CaCO₃ addition on SOC fractions differed based on soil type and CM amendment. The addition of CaCO₃ stimulated soil microbial population in the soils of Y and T. When soil was amended with CaCO₃ alone, CaCO₃ decreased DOC and ROC but increased B-Hs C, resulting in a reduction in soil CO₂ emissions. When soil was amended with both CM and CaCO₃, positive and negative net increments in soil CO₂ emissions are possible, resulting in different soil cumulative CO₂ emissions. These differences in interaction resulted from the priming effect of CaCO₃ and CM on soil microbial community activity and the protective effect of CaCO₃ on SOC. CONCLUSIONS: Soil CO₂ emissions are determined by the combined effect of the protective effect of CaCO₃ on SOM and the priming effect of soil microbes on SOC mineralization. The effect of the application of CM plus CaCO₃ on SOC cycle in calcareous soils is affected by the degree of protection of SOC from SIC on the Loess plateau, China.