Wet‐dry cycles control the emissions and sources of greenhouse gases in agricultural soil: An incubation study

This study examines the impact of wet-drying cycles and nitrogen (N) fertilization on soil greenhouse gas fluxes, specifically nitrous oxide (N₂O) and carbon dioxide (CO₂). Nine treatments were tested, combining three soil moisture regimes (55% constant, 55%-30% cycle, and 80%-55% cycle) with three N addition rates (0, 100, and 150 kg N ha^-1) using ¹⁵N-labeled urea. Soil samples from a potato (Solanum tuberosum) field in Lethbridge, Alberta, were incubated for 28 days under controlled conditions. Wet-drying cycles involved initially wetting the soil to the upper threshold (55% or 80% WFPS) and allowing it to dry to the lower threshold (30% or 55% WFPS), followed by rewetting to restore upper moisture levels. N₂O and CO₂ fluxes were measured regularly using a recirculation chamber system to quantify gas emissions and determine N₂O sources. Soil moisture significantly increased N₂O and CO₂ production (p < 0.001), with the highest emissions under wet conditions (80%-55% WFPS cycle), moderate production at 55% WFPS, and the lowest under dry conditions (30%-55% WFPS cycle). Compared to constant 55% WFPS, N₂O and CO₂ production were 33% and 403% higher, respectively, under wet conditions and 28% and 3% lower under dry conditions. Rewetting events triggered temporary increases in gas emissions due to enhanced microbial activity. Urea addition caused a stronger priming effect on N₂O production under wet conditions, with urea-derived N₂O more prominent in wetter soils.