Gross rates of soil N2O emission and uptake and denitrification gene abundance in temperate cropland agroforestry and monoculture systems

Monoculture croplands are considered as major sources of the greenhouse gas, nitrous oxide (N₂O). The conversion of monoculture croplands to agroforestry systems, e.g., integrating trees within croplands, is an essential climate-smart management system through extra C sequestration and can potentially mitigate N₂O emissions. So far, no study has systematically compared gross rates of N₂O emission and uptake between cropland agroforestry and monoculture. In this study, we used an in-situ ¹⁵N₂O pool dilution technique to simultaneously measure gross N₂O emission and uptake over two consecutive growing seasons (2018 - 2019) at three sites in Germany: two sites were on Phaeozem and Cambisol soils with each site having a pair of cropland agroforestry and monoculture systems, and an additional site with only monoculture on an Arenosol soil prone to high nitrate leaching. Our results showed that cropland agroforestry had lower gross N₂O emissions and higher gross N₂O uptake than in monoculture at the site with Phaeozem soil (P ≤ 0.018 – 0.025) and did not differ in gross N₂O emissions and uptake with cropland monoculture at the site with Cambisol soil (P ≥ 0.36). Gross N₂O emissions were positively correlated with soil mineral N and heterotrophic respiration which, in turn, were correlated with soil temperature, and with water-filled pore space (WFPS) (r = 0.24 ‒ 0.54, P < 0.01). Gross N₂O emissions were also negatively correlated with nosZ clade I gene abundance (involved in N₂O-to-N₂ reduction, r = -0.20, P < 0.05). These findings showed that across sites and management systems changes in gross N₂O emissions were driven by changes in substrate availability and aeration condition (i.e., soil mineral N, C availability, and WFPS), which also influenced denitrification gene abundance. The strong regression values between gross N₂O emissions and net N₂O emissions (R² ≥ 0.96, P < 0.001) indicated that gross N₂O emissions largely drove net soil N₂O emissions. Across sites and management systems, annual soil gross N₂O emissions and uptake were controlled by clay contents which, in turn, correlated with indices of soil fertility (i.e., effective cation exchange capacity, total N, and C/N ratio) (Spearman rank’s rho = -0.76 – 0.86, P ≤ 0.05). The lower gross N₂O emissions from the agroforestry tree rows at two sites indicated the potential of agroforestry in reducing soil N₂O emissions, supporting the need for temperate cropland agroforestry to be considered in greenhouse gas mitigation policies.