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In Québec, most pig slurry is applied to agricultural soils in the spring and fall. A study was initiated to compare the impact of the contrasting spring and fall weather conditions on CO 2 and N 2 O emissions, and on the transformation of pig slurry C and N in a loamy soil cropped to maize ( Zea mays L.). Treatments were approximately 200 kg total N ha −1 either as a spring (SPRING) or fall (FALL) application of pig slurry, and 150 kg N ha −1 as NH 4 NO 3 (control). Fluxes of CO 2 and N 2 O, and soil O 2 , CO 2 , N 2 O, NH 4 + , NO 3 − , extractable C and microbial biomass C (MBC) contents were measured 50 times over a 1‐yr period. Fluxes of N 2 O were generally low during the experiment but were greatly increased in recently manured soils when soil O 2 concentration fell below 0.20 mol mol −1 Soil was warm and well‐aerated following spring slurry application. Under these conditions, slurry NH 4 –N was rapidly nitrified and high N 2 O emissions attributed to denitrification occurred when soil was rewetted by abundant rainfall. For the fall applied slurry, wet and cool conditions limited net nitrification and resulted in little accumulation of NO 3 –N, thus limiting potential for subsequent denitrification and N 2 O emissions. Cumulated N 2 O emissions during the experiment represented 1.74, 2.73, and 1.14% of added N in the FALL, SPRING, and NH 4 NO 3 plots, respectively. Fluxes of CO 2 and cumulated CO 2 –C losses were also greater for SPRING than for FALL application. Our results clearly show that the impacts of the timing of animal manure application on N 2 O emissions cannot be generalized, but will vary between years in response to interactions between crop, climatic, and soil factors.
Rochette et al. (Thu,) studied this question.