Dinitrogen emissions from agricultural soils increase over the growing season
Nitrogen gas losses from ecosystems are difficult to quantify because a large but unknown fraction is lost as N2, which is difficult to detect against a high atmospheric background. It is commonly assumed that N2 and N2O emissions are roughly correlated across the growing season, but there is very little empirical evidence to support this assumption. We used the Nitrogen Free Air Recirculation Method to directly measure N2 and N2O emissions from intact soil cores (0-10cm) collected from experimental farms (in Delaware and Pennsylvania, USA). At each farm, five replicated treatments (n=3) were imposed on a cornfield: no fertilizer, urea, manure, composted-manure, and biocharred-manure (225kgN/ha in Delaware and 150kgN/ha in Pennsylvania). We collected 3 replicate soil cores per treatment after planting/fertilization (May/June), in mid-season (July) and pre-harvest (October/November). We measured soil moisture over the growing season, and pre-harvest N mineralization and C respiration. We hypothesized: 1) N2 and N2O emissions would be highest following fertilization (May/June), 2) N2 and N2O emissions would be correlated throughout the growing season, and 3) N2 and N2O emissions would be highest from manure fertilizer.
Across all treatments, N2:N2O was lowest in the spring and highest in the fall (p=0.002), ranging from 9.2 ± 62 in spring to 280 ± 43 in fall. Soil moisture at the time of collection explained more variance (r2=0.36, p<0.0001) in N2 production than did C availability (r2=0.13, p=0.005). N2O emissions were positively correlated with N2 following planting (r2=0.37, p=0.0006) and mid-season (r2=0.21, p=0.01), but not in fall (p=0.27) when N2 emissions were greatest. All fertilizer treatment effects on N2 emissions were small relative to seasonal differences, but we found significantly higher N2 emissions from the biocharred-manure treatment in the spring (p=0.02) than from the other treatments. In contrast, N2O emissions did not differ by treatment, despite field based chamber data that suggest higher emissions from manure fertilizer (p<0.0001). The high seasonal variation in N2:N2O emissions, and the unobserved correlation in the timing of N2 and N2O emissions, suggest that our understanding of how and when N gasses are lost from ecosystems needs to be refined by direct quantification of N2 fluxes.