N2O flux dynamics and production pathways modulated by soil organic matter and litter turnover

Addition of organic material, such as plant litter, can strongly alter C and N turnover in soils and promote CO2 and N2O formation. This study aimed to elucidate the role of C availability on N turnover processes leading to N2O formation under moderately dry conditions. We measured CO2, NO, and N2O fluxes and contributing processes from an arable and a grassland soil with four different treatments: 50% water-filled pore space (WFPS), 50% WFPS + Maize litter, 60% WFPS, 60% WFPS + Maize litter. CO2 fluxes were partitioned into SOM- and litter-derived, and N2O-forming processes were estimated applying a Bayesian mixing and fractionation model to infer source contributions and N2O reduction rates. Further, we quantified bacterial and fungal population size, as well as genes involved in nitrification and denitrification at the end of the experiment. N2O flux dynamics and contributing processes strongly differed between the C-limited arable soil and the C-rich grassland soil. In the arable soil, bacterial denitrification was consistently the main source of N2O and strongly increased with addition of maize litter. In contrast, nitrification accounted for 51% of N2O formation in the grassland soil without litter, while litter addition promoted fungal denitrification. We found higher N2O emissions with higher total and SOM-derived CO2 emissions confirming that availability of organic C from SOM and litter are important controls of N2O losses. However, neither soil organic C content, nor litter addition, nor soil moisture alone were appropriate to predict the main N2O-forming process. Therefore, our study emphasizes the need to disentangle and quantify N2O-forming processes to develop successful mitigation strategies.