Spatial variation in soil pH controls off-season N2O emission in an agricultural soil

Experiments with soils have provided ample evidence that soil pH controls the N2O/(N2O + N2) ratio of denitrification, which increases with decreasing pH, most probably because low pH interferes with the expression of N2O reductase in denitrifying bacteria. In contrast, the N2O/NO3- product ratio of nitrification appears to be unaffected by soil pH within the range relevant for agricultural soils (pH 5.5–7.0). We hypothesized that local pH variations in cultivated soil may control in situ N2O emissions during periods of active denitrification. To test this hypothesis, we identified three plots with slightly different soil pH (5.4–5.9) within an agricultural field under spring ploughed cereal cropping, and placed four frames within each plot for measuring N2O emissions throughout autumn and spring. Soil samples were taken from each frame after the experiment to characterize the kinetics of NO, N2O and N2 production by anoxic incubation. The data were used to calculate an N2O index, IN2O, which is an inverse measure of the capacity of the denitrifying community to effectively express N2O reductase under anoxia and hence a proxy for the soil’s propensity to emit N2O under denitrifying conditions. N2O emissions were greatest during spring thaw, intermediate in autumn and low in late spring. Emissions during autumn and spring thaw were inversely related to soil pH, supporting the hypothesis that soil pH influences N2O emissions when denitrification is the main source of N2O. During these periods, emissions were positively correlated with IN2O, further substantiating the idea that soil pH affects denitrification product ratios in situ. Total organic carbon and nitrate content were negatively correlated with soil pH, thus co-varying with N2O emissions. However, the relationship of N2O emission to TOC and nitrate appeared weaker than to pH. Off-season emissions dominate N2O budgets in many regions. If the pH relationship holds at greater scales, careful soil pH management by precision liming could be a viable tool to reduce N2O emissions.