Ecological controls on N₂O emission in surface litter and near-surface soil of a managed grassland: modelling and measurements

Abstract. Large variability in N₂O emissions from managed grasslands may occur because most emissions originate in surface litter or near-surface soil where variability in soil water content (θ) and temperature (T_s) is greatest. To determine whether temporal variability in θ and T_s of surface litter and near-surface soil could explain this in N₂O emissions, a simulation experiment was conducted with ecosys, a comprehensive mathematical model of terrestrial ecosystems in which processes governing N₂O emissions were represented at high temporal and spatial resolution. Model performance was verified by comparing N₂O emissions, CO₂ and energy exchange, and θ and T_s modelled by ecosys with those measured by automated chambers, eddy covariance (EC) and soil sensors on an hourly timescale during several emission events from 2004 to 2009 in an intensively managed pasture at Oensingen, Switzerland. Both modelled and measured events were induced by precipitation following harvesting and subsequent fertilizing or manuring. These events were brief (2–5 days) with maximum N₂O effluxes that varied from  <  1 mgNm⁻²h⁻¹ in early spring and autumn to  >  3 mgNm⁻²h⁻¹ in summer. Only very small emissions were modelled or measured outside these events. In the model, emissions were generated almost entirely in surface litter or near-surface (0–2 cm) soil, at rates driven by N availability with fertilization vs. N uptake with grassland regrowth and by O₂ supply controlled by litter and soil wetting relative to O₂ demand from microbial respiration. In the model, NO_x availability relative to O₂ limitation governed both the reduction of more oxidized electron acceptors to N₂O and the reduction of N₂O to N₂, so that the magnitude of N₂O emissions was not simply related to surface and near-surface θ and T_s. Modelled N₂O emissions were found to be sensitive to defoliation intensity and timing which controlled plant N uptake and soil θ and T_s prior to and during emission events. Reducing leaf area index (LAI) remaining after defoliation to half that under current practice and delaying harvesting by 5 days raised modelled N₂O emissions by as much as 80 % during subsequent events and by an average of 43 % annually. Modelled N₂O emissions were also found to be sensitive to surface soil properties. Increasing near-surface bulk density by 10 % raised N₂O emissions by as much as 100 % during emission events and by an average of 23 % annually. Relatively small spatial variation in management practices and soil surface properties could therefore cause the large spatial variation in N₂O emissions commonly found in field studies. The global warming potential from annual N₂O emissions in this intensively managed grassland largely offset those from net C uptake in both modelled and field experiments. However, model results indicated that this offset could be adversely affected by suboptimal land management and soil properties.