Influence of soil pH on NO x and N2O emissions from bovine urine applied to soil columns

Abstract

The nitrous oxide (N2O) molecule is both a greenhouse gas and a precursor to the formation of nitric oxide (NO) in the stratosphere, which is subsequently involved in catalytic destruction of ozone (O3). Tropospheric NO can form NO2 with both gases (NOx) able to react with volatile organic compounds to produce the detrimental O3 molecule. Information on how NO and N2O emissions respond to agricultural management is lacking. Limited data suggest N2O emissions following bovine urine deposition may vary with soil pH. This work hypothesised that soil liming would also reduce NOx emissions due to enhanced nitrification and reduced opportunity for HNO2 formation as soil pH increased. Bovine urine was applied (575 kg N/ha) to repacked soil cores (34% water-filled pore space) that ranged in pH from4.4 to 7.6, and these were maintained at 21°C. With the exception of the pH 4.4 treatment, increases in NO-N fluxes occurred during the first 11 days, when nitrification was proceeding. Increasing the initial soil pH enhanced the net ammonium depletion rate. The loss of NO-N, expressed as a percentage of the net ammonium depletion rate, decreased with increasing initial pH during this first 11 days with NO-N fluxes reduced due to competition between abiotic and biological demands for NO2-. After 21 days, the optimal pH for the net NO-N flux was observed to be 5.7 to 6.0. However, cumulative NO-N fluxes after 35 days showed little variation due to initial soil pH and ranged from 0.02 to 0.05% of urine-N applied. The cumulative N2O-N flux ranged from 0.04 to 0.09% of urine applied with a linear increase frompH 5.7 to pH 7.6. Based on these results, where soils are relatively dry, liming acid soils from pH 5.5 to pH 6.5 will not affect cumulative NO fluxes fromurine patches. However, cumulative N2O fluxes may be enhanced. © 2011 The Royal Society of New Zealand.