Sources of N2O-N following simulated animal treading of ungrazed pastures

It has been previously hypothesised that the treading of pastures by grazing animals can increase nitrous oxide (N2O) emissions as the result of reduced plant N uptake. In addition, grazing animals urinate and defecate on to soils which can also increase the N2O emissions. To avoid these additional N inputs, a treading machine was used in two field experiments where the pre-grazing dry matter (DM) was present or removed to test the above hypothesis. The N2O emissions were measured for 47 and 30 days afterwards, respectively. The soil nitrate (NO3−–N) pool was 15N labelled prior to treading in Experiment 2. Treading induced greater N2O emissions and reduced herbage DM yields (by 31%–41%) and soil NO3−–N concentrations when soil gravimetric water contents ranged from 0.45 to 0.63 g water/g soil. A comparison of treading in the presence and absence of pre-grazing DM showed that reduced plant N uptake did not induce greater N2O emissions. Rather, the 15N labelling of the NO3−–N pool indicated this pool contributed to the N2O emissions under treading. In addition, 15N labelling showed other soil–N pools became available as a consequence of either, (1) treading inducing soil perturbation and releasing N from the soil organic matter–N and/or plant root–N pools, or (2) as a result of simulated grazing causing the release of plant root–N. Soil water soluble carbon (C) also increased under treading in the absence of pasture, supporting the theory that organic–N was released from either the soil organic matter (OM) or plant root pools. Further research should investigate the effect of treading on the turnover and contribution of organic–N pools (roots, OM) in pasture soils in order to more fully understand the contributions these make to background N2O emissions and effects on N and C cycling in pasture soils.