Reduced Soil Gross N<sub>2</sub>O Emission Driven by Substrates Rather Than Denitrification Gene Abundance in Cropland Agroforestry and Monoculture
Jie Luo, Lukas Beule, Guodong Shao, Edzo Veldkamp, Marife D. Corre
Abstract
Abstract Conversion of monoculture to agroforestry (integrating trees with crops) is promoted as a promising management in reducing N 2 O emissions from croplands. How agroforestry influences gross N 2 O emission (N 2 O + N 2 from N 2 O reduction) and uptake (N 2 O reduced to N 2 ) compared to monoculture is unknown. We used the 15 N 2 O 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 N 2 O emissions ( P ≤ 0.002). Although tree rows occupied only 20% in agroforestry, gross N 2 O emissions tended to decrease by 6–36% in agroforestry (0.98–1.02 kg N 2 O‐N ha −1 yr −1 ) compared to monoculture (1.04–1.59 kg N 2 O‐N ha −1 yr −1 ). Gross N 2 O emissions were influenced by soil mineral N, soil respiration, and moisture content rather than by denitrification gene abundance. Soil gross N 2 O uptake was highest in the tree row and decreased with distance into crop rows ( P = 0.012). The agroforestry tended to increase gross N 2 O uptake by 27–42% (0.38–0.44 kg N 2 O‐N ha −1 yr −1 ) compared to monoculture (0.30–0.31 kg N 2 O‐N ha −1 yr −1 ). In tree rows, soil gross N 2 O uptake correlated with nirK gene abundance which was indirectly influenced by the low mineral N‐to‐soil CO 2 ‐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 N 2 O in soils.