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

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. 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. 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. 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.