Denitrification in a pasture soil

Abstract

The effect of multiple applications of cow slurry or inorganic nitrogen fertiliser (calcium nitrate) on the composition of the soil atmosphere was studied over three years, in an imperfectly drained pasture soil. Irrespective of fertiliser treatments concentrations of oxygen (‎O2) decreased and carbon dioxide (CC^) increased during the winter months, but remained near ambient levels during the summer. Slurry decreased 02 concentrations temporarily, especially following applications in the autumn and in the spring. Concentrations of nitrous oxide (N20) remained close to ambient during the summer under all treatments but increased over the autumn, winter and spring,especially in the few weeks following fertiliser applications. Enhanced N20 concentrations were significantly related to decreases in O2.

The relative diffusivity of soil cores from the field site was determined from the rate of diffusion of 85Kr through the cores. From this, the diffusion coefficient for N2O was calculated and used to estimate the flux of N2O from the field plot. Losses from plots receiving inorganic fertiliser were up to 6kg N ha-1 a-1 but lower than this from control plots and plots receiving slurry.

In the 3rd year acetylene (C2H2) was used to inhibit the reduction of N2O to N2 in microplots enclosed by plastic cylinders driven into the soil, so that total gaseous losses could be measured. These were low during the summer and in the winter in all treatments, but increased during the autumn and spring. Losses of N were highest in the inorganic fertiliser treatment: 10 and 21% of the N applied in the autumn and spring, respectively. Fluxes of up to 115g N ha-1 h-1 were recorded. Losses from the slurried plots were only slightly higher than from the control plots. When fluxes were low, most loss of N was as N2O, but the proportion lost as N2 increased as the total flux increased. The highest ratio of (N2 + N20) :N20 recorded was 25.

In laboratory experiments C2H2 was found to inhibit nitrification totally at a concentration of 0.04ml ml-1 but the effect was found to be reversible. Acetylene also inhibited N2O reduction. In aerobic conditions C2H2 increased soil respiration, some of the C2H2 being consumed in the process. Several colonies of bacteria were isolated from the soil able to use C2H2 as their sole carbon source.