Soil gross N2O emission and uptake under two contrasting agroforestry systems: riparian tree buffer versus alley-cropping tree row

Abstract

In addition to the removal of excess mineral nitrogen (N) via root uptake, trees in agroforestry systems may mitigate negative effects of high N fertilization of adjacent crops by enhancing complete denitrification of excess mineral N aside from root uptake. Presently, little is known about the potential for NO₃⁻ reduction through denitrification (conversion to greenhouse gas N₂O and subsequently to non-reactive N₂) in contrasting agroforestry systems: riparian tree buffer versus tree row of an upland alley-cropping system. Our study aimed to (1) quantify gross N₂O emissions (both N₂O + N₂ emissions) and gross N₂O uptake (N₂O reduction to N₂), and (2) determine their controlling factors. We employed the ¹⁵N₂O pool dilution technique to quantify gross N₂O fluxes from 0 to 5 cm (topsoil) and 40 to 60 cm (subsoil) depths with seasonal field measurements in 2019. The riparian tree buffer exhibited higher topsoil gross N₂O emissions and uptake than the alley-cropping tree row (P < 0.03). Gross N₂O emissions were regulated by N and carbon (C) availabilities and aeration status rather than denitrification gene abundance. Gross N₂O uptake was directly linked to available C and nirK gene abundance. In the subsoil, gross N₂O emission and uptake were low in both agroforestry systems, resulting from low mineral N contents possibly due to N uptake by deep tree roots. Nonetheless, the larger available C and soil moisture in the subsoil of riparian tree buffer than in alley-cropping tree row (P < 0.05) suggest its large potential for N₂O uptake whenever NO₃⁻ is transported to the subsoil.