Reduced Soil Gross N2O Emission Driven by Substrates Rather Than Denitrification Gene Abundance in Cropland Agroforestry and Monoculture
Conversion of monoculture to agroforestry (integrating trees with crops) is promoted as a promising management in reducing N2O emissions from croplands. How agroforestry influences gross N2O emission (N2O + N2 from N2O reduction) and uptake (N2O reduced to N2) compared to monoculture is unknown. We used the 15N2O pool dilution technique to quantify these processes using soil cores (top 5 cm) incubated in the field with monthly measurements over two growing seasons (2018–2019) at two sites (each with paired agroforestry and monoculture) and one site with monoculture only. The unfertilized tree rows showed the lowest gross N2O emissions (P ≤ 0.002). Although tree rows occupied only 20% in agroforestry, gross N2O emissions tended to decrease by 6–36% in agroforestry (0.98–1.02 kg N2O-N ha−1 yr−1) compared to monoculture (1.04–1.59 kg N2O-N ha−1 yr−1). Gross N2O emissions were influenced by soil mineral N, soil respiration, and moisture content rather than by denitrification gene abundance. Soil gross N2O uptake was highest in the tree row and decreased with distance into crop rows (P = 0.012). The agroforestry tended to increase gross N2O uptake by 27–42% (0.38–0.44 kg N2O-N ha−1 yr−1) compared to monoculture (0.30–0.31 kg N2O-N ha−1 yr−1). In tree rows, soil gross N2O uptake correlated with nirK gene abundance which was indirectly influenced by the low mineral N-to-soil CO2-C ratio. Adjusting the tree and crop areal coverages of agroforestry and optimizing fertilization can further augment the benefits of agroforestry in reducing emission and increasing uptake of N2O in soils.