Short-term nitrous oxide emissions from pasture soil as influenced by urea level and soil nitrate

Summary

Nitrogen excreted by cattle during grazing is a significant source of atmospheric nitrous oxide (N2O). The regulation of N2O emissions is not well understood, but may vary with urine composition and soil conditions. This laboratory study was undertaken to describe short-term effects on N2O emissions and soil conditions, including microbial dynamics, of urea amendment at two different rates (22 and 43 g N m-2). The lower urea concentration was also combined with an elevated soil NO3- concentration. Urea solutions labelled with 25 atom% 15N were added to the surface of repacked pasture soil cores and incubated for 1, 3, 6 or 9 days under constant conditions (60% WFPS, 14°C). Soil inorganic N (NH4+, NO2- and NO3-), pH, electrical conductivity and dissolved organic C were quantified. Microbial dynamics were followed by measurements of CO2 evolution, by analyses of membrane lipid (PLFA) composition, and by measurement of potential ammonium oxidation and denitrifying enzyme activity. The total recovery of 15N averaged 84%. Conversion of urea-N to NO3- was evident, but nitrification was delayed at the highest urea concentration and was accompanied by an accumulation of NO2-. Nitrous oxide emissions were also delayed at the highest urea amendment level, but accelerated towards the end of the study. The pH interacted with NH4+ to produce inhibitory concentrations of NH3(aq) at the highest urea concentration, and there was evidence for transient negative effects of urea amendment on both nitrifying and denitrifying bacteria in this treatment. However, PLFA dynamics indicated that initial inhibitory effects were replaced by increased microbial activity and net growth. It is concluded that urea-N level has qualitative, as well as quantitative effects on soil N transformations in urine patches.