Regulatory potential of soil available carbon, nitrogen, and functional genes on N2O emissions in two upland plantation systems

Abstract

Dynamic nitrification and denitrification processes are affected by changes in soil redox conditions, and they play a vital role in regulating soil N2O emissions in rice-based cultivation. It is imperative to understand the influences of different upland crop planting systems on soil N2O emissions. In this study, we focused on two representative rotation systems in central China: rapeseed-rice (RR) and wheat-rice (WR). We examined the biotic and abiotic processes underlying the impacts of these upland plantings on soil N2O emissions. The results revealed that during the rapeseed-cultivated seasons in the RR rotation system, the average N2O emissions were 1.24±0.20 and 0.81±0.11 kg N ha-1 for the first and second seasons, respectively. These values were comparable to the N2O emissions observed during the first and second wheat-cultivated seasons in the WR rotation system (0.98±0.25 and 0.70±0.04 kg N ha-1, respectively). This suggests that upland cultivation has minimal impacts on soil N2O emissions in the two rotation systems. Strong positive correlations were found between N2O fluxes and soil ammonium (NH4+), nitrate (NO3-), microbial biomass nitrogen (MBN), and the ratio of soil dissolved organic carbon (DOC) to NO3- in both RR and WR rotation systems. Moreover, the presence of the AOA-amoA and nirK genes were positively associated with soil N2O fluxes in the RR and WR systems, respectively. This implies that these genes may have different potential roles in facilitating microbial N2O production in various upland plantation models. By using a structural equation model (SEM), we found that soil moisture, mineral N, MBN, and the AOA-amoA gene accounted for over 50% of the effects on N2O emissions in the RR rotation system. In the WR rotation system, soil moisture, mineral N, MBN, and the AOA-amoA and nirK genes had a combined impact of over 70% on N2O emissions. These findings demonstrate the interactive effects of functional genes and soil factors, including soil physical characteristics, available carbon and nitrogen, and their ratio, on soil N2O emissions during upland cultivation seasons under rice-upland rotations.