Short-term fluxes of nitrous oxide from soil : measurement and modelling
Gaseous nitrous oxide (N₂0) undergoes physical and chemical reactions in the atmosphere, contributing to both global warming and the catalytic destruction of stratospheric ozone. This chemically reactive greenhouse gas is produced both naturally and anthropogenically. The greatest source of N₂0 is from the microbial transformation of N compounds during the processes of nitrification and denitrification in natural and cultivated soils. However, there is some uncertainty in the strength of these emission sources. Therefore one of the directives of the Terrestrial Initiative in Global Environmental Research programme, of which the following work was a part, was to elucidate the factors which influence the emission rates of N₂0 from these systems. It is essential that these factors are quantified, in order to correctly assess the effect of N₂0 as an environmental determinant. A reliable automated soil core headspace gas analyser system for the continuous measurement of N₂0 at the laboratory scale was developed. The system determined N₂0 evolution rates from reconstructed soil cores consisting of re-packed aggregates of known diameters, incubated under different environmental conditions. There was an increase in N₂0 emission rate (range = 0.5-61 x 10-7 mol N m-2 h-l) with aggregate size, soil N0₃-concentration and soil water content under unsaturated conditions. However, the extent of these trends was masked by the variability in emission rates. One source of variability in N₂0 emissions from unsaturated soil, was related to localized organic (e.g. faunal) residues. Subsequent investigations involving the incorporation of discrete faunal residues, DFRs (dead Earthworms), was found to greatly stimulate N₂0 emission from unsaturated re-packed soil cores. These N₂0 emission rates approached those attained when the soil was under saturated conditions, which were up to 3 orders of magnitude greater than emission rates from unamended, unsaturated soil. There was no apparent influence of DFR on N₂0 emissions from soil under saturated conditions suggesting that the effect of DFRs under aerobic conditions was the creation of localized anoxic zones. N₂0 emission rates increased with increasing soil water content reaching a maximum under fully saturated conditions for three different soils (range = 0.25-1.8 x 10-4 mol N m-2 h-1). The emissions of N₂0 from the three soils were different under both unsaturated and saturated conditions and appeared to be related to soil parameters, specifically organic matter content, clay content and soil pH. The contrast in rates of N₂0 emission from unsaturated and saturated soil prompted a test of the hypothesis that wetting/draining cycles increase the total emission rate. During the saturated phase, N₂0 is produced, but its egress is restricted by saturated transmission pores. Rapid drainage causes a flush of N₂0 from saturated aggregates by providing open emission channels. The rapid increase in N₂0 flux that was observed during the draining of saturated soil occurred in all three soil types (range = 1-5 x 10-3 mol N m-2 h-1). This almost instantaneous N₂0 pulse, which in some cases lasted less than 2 hours, occurred repeatedly, emitting similar rates of N₂0 during 10 cycles of flooding and draining. An attempt was made to simulate N₂0 emission using the results gained from these investigations to parameterize a reaction-diffusion model. The model successfully predicted N₂0 emission from soil undergoing a transformation from unsaturated to saturated conditions. However, model deficiencies were found during simulations involving the sequential rise and fall in water table height. The inability of the model to accurately predict the rapid increase in flux that occurred following core drainage, exposed gaps in knowledge and areas of future research regarding the short-term fluxes of N₂0 from soil.