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

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 N-2 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-N-2) in three pH cropland soils (acidic, near neutral and alkaline), the rates of Den +Ana-N-2 and NO3- reduction were investigated using slurry-based N-15 tracer technique. The potential rates of N-2 produced by denitrification (Denitrif-N-2) ranged from 3.78 to 5.52 nmol N g(-1) h(-1), which was 10-30 times faster than the N-2 rates (0.12-0.42 nmol N g(-1) h(-1)) produced by anammox (Anammox-N-2). The NO3- reduction rates ranged from 5.52 to 9.3 nmol N g(-1) h(-1), faster than the rates of Den+Ana-N-2 (from 3.9 to 5.94 nmol N g(-1) h-1). Both the NO3- reduction and Den+Ana-N-2 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-N-2 rates, which provided a rough estimation of proportion of NO3- reduction by Den+Ana-N-2 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-N-2 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.