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

Abstract

It has been previously hypothesised that the treading of pastures by grazing animals can increase nitrous oxide (N 2 O) emissions as the result of reduced plant N uptake. In addition, grazing animals urinate and defecate on to soils which can also increase the N 2 O 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 N 2 O emissions were measured for 47 and 30 days afterwards, respectively. The soil nitrate (NO 3 -N) pool was 15 N labelled prior to treading in Experiment 2. Treading induced greater N 2 O emissions and reduced herbage DM yields (by 31%-41%) and soil NO 3 -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 N 2 O emissions. Rather, the 15 N labelling of the NO 3 -N pool indicated this pool contributed to the N 2 O emissions under treading. In addition, 15 N 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 N 2 O emissions and effects on N and C cycling in pasture soils.

Nitrous oxide emissions from pastures during wet and cold seasons

“…The physical disturbance to soils resulting from animal treading may also induce mineralization of soil organic matter‐N and the release of plant root‐N (Pal et al . ). Thus, avoiding soil physical damage during the autumn‐winter period, by restricting grazing or improving soil drainage, are key factors when trying to mitigate winter N 2 O emissions, particularly under year‐round grazed pastures (de Klein et al .…”

“…Carbon may be less available as a consequence of reduced root exudation, while soil inorganic-N levels may increase as a result of reduced plant N uptake (Menneer et al 2005) and removal (NO À 3 ). The physical disturbance to soils resulting from animal treading may also induce mineralization of soil organic matter-N and the release of plant root-N (Pal et al 2014). Thus, avoiding soil physical damage during the autumn-winter period, by restricting grazing or improving soil drainage, are key factors when trying to mitigate winter N 2 O emissions, particularly under year-round grazed pastures (de Klein et al 2006;Luo et al 2010).…”

Combining two complementary micrometeorological methods to measure CH<sub>4</sub> and N<sub>2</sub>O fluxes over pasture

“…There had also been a winter grazing in June 2012, before our recording began. Some of the observed emissions early in the budget year would have originated from the N deposited during that event, possibly amplified by the effects of trampling (Pal et al, 2014). It is therefore likely that the annual emissions at the UUW site, of 0.34 g N m −2 , were also greater than natural background emissions.…”