Effects of nitrogen addition on carbonate-derived CO2 emission after biochar addition

We studied the effect of nitrogen and biochar on CO2 emission from SOC and SIC.Nitrogen increased SIC-derived CO2 by 41% but decreased SOC-derived CO2 by 20%.Biochar reduced total soil-derived CO2 by neutralizing nitrogen-induced acidity.We proposed a method for 3- or 4-source partitioning CO2 emission from calcareous soils.Biochar addition generally increases the alkalinity regeneration to resist soil acidification driven by nitrogen (N) fertilization. Calcareous soils contain soil organic carbon (SOC) and inorganic C (SIC). Owing to technical limitations in three-source partitioning CO2, how biochar addition affects SOC- and SIC-derived CO2 emission has not been clarified yet. Therefore, we conducted a 70-day incubation experiment of ammonium-N and maize-straw-derived biochar additions to investigate the N plus biochar impacts on SOC- and SIC-derived CO2 emission. Over the 70-day incubation, we found that the N-only addition increased the SIC-derived CO2 emission by approximately 41% compared with the control, but decreased the SOC-derived CO2 emission by approximately 20%. This suggests that the distinct responses of SIC- and SOC-derived CO2 emission to N-only addition come from N-induced acidification and preferential substrate (N) utilization of soil microorganisms, respectively. Compared with N-only addition, N plus biochar addition decreased the SIC-derived CO2 emission by 17%-20% during the first 20 days of incubation, but increased it by 54% during the next 50 days. This result suggested that biochar addition reduced the SIC-derived CO2 emission likely due to the alkalization capacity of biochar exceeding the acidification capacity of ammonium-N in the short term, but it may increase the SIC-derived CO2 emission induced by the weak acidity produced from biochar mineralization in the long term. This study is helpful to improve the quantification of CO2 emission from calcareous soils.