¹⁵N-tracer approach to assess nitrogen cycling processes: Nitrate reduction, anammox and denitrification in different pH cropland soils

Abstract

© 2020 Elsevier B.V. Denitrification and anaerobic ammonium oxidation (anammox) are two key processes in the biogeochemical cycle of nitrogen (N) and are dominant pathways of nitrate (NO3−) losses as N2 and N2O from soils. To verify the mechanisms of the NO3− reduction comparable with the intensity of denitrification (Den) and/or anammox (Ana) processes (Den+Ana-N2) in three pH cropland soils (acidic, near neutral and alkaline), the rates of Den+Ana-N2 and NO3− reduction were investigated using slurry-based 15N tracer technique. The potential rates of N2 produced by denitrification (Denitrif-N2) ranged from 3.78 to 5.52 nmol N g−1 h−1, which was 10–30 times faster than the N2 rates (0.12–0.42 nmol N g−1 h−1) produced by anammox (Anammox-N2). The NO3− reduction rates ranged from 5.52 to 9.3 nmol N g−1 h−1, faster than the rates of Den+Ana-N2 (from 3.9 to 5.94 nmol N g−1 h−1). Both the NO3− reduction and Den+Ana-N2 rates were consistent with the fastest rates common in soils with near neutral pH, while both processes were much slower in acidic soils. The NO3− reduction closely correlated with the Den+Ana-N2 rates, which provided a rough estimation of proportion of NO3− reduction by Den+Ana-N2 process (81%) as the dominant and 19% by extra N2O or NO production by denitrification or other processes consuming NO3−. A constant ratio of 4.3 between Den+Ana-N2 and all other NO3− consumption processes within a wide pH range (from 5.2 to 8.0) of cropland soils strongly simplify the assessment of gaseous N losses and of NO3− transformations.