N2O and CO2 emissions from a dryland wheat cropping system with long-term N fertilization and their relationships with soil C, N, and bacterial community.

In dryland cropping ecosystems, nitrogen (N) fertilization causes changes in soil biochemical cycling and influences the crop biomass and grain yield. Based on a long-term field experiment involving three N rates (0 (N0), 160 (N160), and 320 (N320) kg N ha-1 year-1), we measured nitrous oxide (N2O) and carbon dioxide (CO2) emissions from wheat fields and determined the soil organic carbon (SOC), total N, microbial biomass C (MBC), and N (MBN), mineralized C (Cmin) and N (Nmin) concentrations and the bacterial community composition. The results showed that N fertilization significantly (P < 0.05) increased N2O and CO2 emissions. N fertilization also significantly (P < 0.05) increased SOC by 13.2%, total N by 7.4%, MBN by 34.3%, Cmin by 15.5%, and Nmin by 27.8%, as well as the soil bacterial diversity and abundances of N cycling functional groups (by 149%). Additionally, N2O and CO2 emissions were positively and linearly correlated with NH4-N and NO3-N concentrations and positively correlated with SOC, total N, MBN, and the number of bacterial species, as well as most of the bacterial phyla and functional groups. These results suggest that N fertilizer applied to the dryland soils improved soil fertility and changed the soil bacterial community composition and function, thus increased N2O and CO2 emissions. Therefore, to mitigate greenhouse gas emissions and maintain soil fertility for wheat production, an optimized N fertilizer rate should be applied in the southern region of the Loess Plateau in China.