Nitrous oxide (N2O) is a greenhouse gas produced during nitrification and denitrification. Grassland conversion to row-crop agriculture increase N2O emissions from soils, but cropland to grassland restoration effect on N2O flux is understudied. Long-term reduced tillage reduces N2O emissions, likely from improved soil structure, organic matter accumulation, and less available nitrogen. This indicates land-use can decrease N2O emissions. Our objective was to ascertain whether grassland restoration from cultivated conditions reduces soil N2O emissions. We collected intact soil cores to measure N2O efflux and a composite of several soil cores to measure a suite of soil properties and processes from a continuously cultivated field, prairies restored from 1-35 y, and never-cultivated prairie. The intact soil cores were incubated for 16 d and headspace gas was sampled every 2-3 d for N2O. The composited soil from each site was measured for pH, Melich P, potentially mineralizable C, CO2 efflux, total soil C and N stock, soil C:N ratio, microbial biomass C and N, and inorganic N availability. Initial N2O efflux (3 d) and cumulative efflux rates across restoration ages were analyzed using linear regression. The relationship between N2O production and other variables were analyzed using Pearson and Spearman correlations (α= 0.05).
Cumulative N2O efflux rate decreased linearly across the 1-35 y chronosequence of restored prairies (r2 = 0.64, P = 0.034) and was positively correlated to inorganic N availability (r = 0.81, P = 0.049). Cumulative rate of N2O efflux was related similarly with inorganic N availability, but negatively related to total soil C stock (r = -0.97, P = 0.002), total soil N stock (r = -0.94, P = 0.004), soil C:N ratio (r = -0.84, P = 0.035), and potentially mineralizable C (r = -0.90, P = 0.013), microbial biomass C (r = 0.-82, P = 0.043), and microbial biomass N (r = -0.89, P = 0.033). These results demonstrate N2O emissions from soil decline during grassland restoration due to increases in total and microbial soil C and N pools, which in addition to plant uptake, contribute to limited N available for nitrification and denitrification.