Identifying soil N2O sources by combining laboratory experiments with process-based models

Abstract

Nitrification and denitrification are two important biological processes producing N₂O in soils, but their contributions to N₂O emissions are not well understood, hindering precise mitigation measures. Here, we developed process-based models (PBM) with and without transport (T) to partition N₂O sources by tracking nitrogen flows (NF) through different reaction pathways. The model with transport (PBM-T-NF) well predicted N₂O production from nitrification and denitrification in two different repacked soils with a shallow depth of 8 mm under moisture conditions ranging from 40 to 100% water-filled pore space (WFPS), demonstrating its robustness and reliability. In comparison, the model without transport (PBM-NF) failed to capture the N₂O dynamics and the relative contribution of denitrification to N₂O production (\({C}_{D}\)), highlighting the need of including mass transport in predicting N₂O dynamics. The PBM-T-NF model was further employed to investigate the effects of soil properties on N₂O emissions and sources. Increased NH₄⁺ concentration significantly decreased \({C}_{D}\) under relatively low moisture conditions, while increased NO₃⁻ slightly promoted \({C}_{D}\) over different moisture contents, emphasizing the importance of substrate availability and moisture conditions in controlling \({C}_{D}\). Furthermore, the PBM-T-NF model was used to quantify N₂O sources from an artificial soil core of 80 mm depth. Soil depth was shown to be important in mediating \({C}_{D}\) by controlling O₂ diffusivity, which is highly dependent on moisture content. Given the long-standing challenge in experimental quantification of N₂O sources from soils, our developed model provides a novel way to estimate N₂O production from different nitrogen processes, which is key for accurately targeting mitigation of N₂O emissions from soils.