Abstract
Nitrogen-fertilized agricultural soils are major emitters of the potent greenhouse gas nitrous oxide (N2O). Although soil moisture is known to drive N2O production, the influence of moisture on the priming effects of labile N additions, which alter the N2O production from native soil organic matter (SOM), remains understudied. We investigated how moisture impacts N2O primings, nitrification kinetics, and allocation of N2O sources between nitrification and denitrification. We evaluated urea additions at five water-filled pore spaces (WFPS): 31, 41, 53, 65, and 78%. Based on δ15N–N2O site preference (SP), the denitrification source (encompassing bacterial- and nitrifier-denitrification) amply dominated across all moistures. At moistures >60% WFPS, SP exhibited a narrow range of 1.2–1.6‰, whereas at 31% WFPS, SP rose to 9.4‰ (P < 0.001). These SPs translated into denitrification sourcing consistently four-fifth of the N2O production in the wettest soils. With drier soils, denitrification gradually decreased, but it still produced three-fifth of the N2O even at the driest soil (31% WFPS). Based on time courses of ammonium and nitrate concentrations, first-order kinetics (k1) modelling showed much slower nitrification rates at 31% WFPS (0.1 day−1) than the peak nitrification witnessed consistently at both 65 and 78% WFPSs (1.0 day−1). By using isotopically-labelled urea, we further allocated N2O production into SOM-N and urea-N sources. In soils receiving urea, SOM-N contribution to N2O production changed from being 77% in the driest soils to 46% in the wettest soil. Along this aeration-to-moisture gradient, net N2O priming steadily switched from being negative (−20%) in the driest soils to become positive (+14%) at the wettest soil, and this shifting trajectory intercepted the neutral 0% priming at 60% WFPS. Results indicate that pulse N2O emissions commonly reported in recently N-fertilized fields are triggered by soil moisture surpassing this threshold, which collectively accelerates positive primings of SOM and denitrification.
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