Variance and main drivers of field nitrous oxide emissions: A global synthesis

The precise estimation of global nitrous oxide (N₂O) emissions in nitrogen cycling will facilitate improved projections of future climate change. However, the geographical variations and the primary controlling factors of N₂O emissions remain elusive at the global scale. What is lacking is their specific evaluation based on field data. We compiled a new dataset of soil N₂O emission rates, including 6016 field observations from 219 articles, to synthesize N₂O emission rates for different ecosystems and to explore the key determinants of N₂O emission variations. The global mean soil N₂O emission rate was 1111.8 ± 26.6 μg N m⁻² day⁻¹, with the largest one from humid subtropical regions and the smallest one from semi-arid areas. The soil N₂O emission rates were positively correlated with the mean air annual temperature, soil pH, cation exchange capacity, soil moisture, soil organic carbon (C), total soil nitrogen (N), dissolved organic N, ammonium, nitrate, available phosphorus concentrations, microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN) at a global scale. Conversely, the soil N₂O rates were negatively correlated with soil bulk density, C:N ratio, and MBC:MBN ratio. The results of structural equation models revealed that the joint direct effects of soil nitrate, ammonium, and total N (combined standard coefficient = 0.45) accounted for most of the variability in global soil N₂O emissions (total standard coefficient = 0.84), while climate factors and other soil physicochemical properties accounted for less. This study highlights the critical roles of soil N substrates on N₂O emissions, which will facilitate the optimization of process-models for soil N₂O emissions.