Ecological controls on Nsub2/subO emission in surface litter and near-surface soil of a managed grassland: modelling and measurements

摘要

Large variability in Nsub2/subO emissions from managed grasslands may occur because most emissions originate in surface litter or near-surface soil where variability in soil water content (iθ/i) and temperature (iT/isubs/sub) is greatest. To determine whether temporal variability in iθ/i and iT/isubs/sub of surface litter and near-surface soil could explain this in Nsub2/subO emissions, a simulation experiment was conducted with iecosys/i, a comprehensive mathematical model of terrestrial ecosystems in which processes governing Nsub2/subO emissions were represented at high temporal and spatial resolution. Model performance was verified by comparing Nsub2/subO emissions, COsub2/sub and energy exchange, and iθ/i and iT/isubs/sub modelled by iecosys/i 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 Nsub2/subO effluxes that varied from ?&??1?mgmspace linebreak="nobreak" width="0.125em"/Nmspace linebreak="nobreak" width="0.125em"/msup?2/supmspace linebreak="nobreak" width="0.125em"/hsup?1/sup in early spring and autumn to ?&??3?mgmspace linebreak="nobreak" width="0.125em"/Nmspace width="0.125em" linebreak="nobreak"/msup?2/supmspace linebreak="nobreak" width="0.125em"/hsup?1/sup 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 Osub2/sub supply controlled by litter and soil wetting relative to Osub2/sub demand from microbial respiration. In the model, NOsubix/i/sub availability relative to Osub2/sub limitation governed both the reduction of more oxidized electron acceptors to Nsub2/subO and the reduction of Nsub2/subO to Nsub2/sub, so that the magnitude of Nsub2/subO emissions was not simply related to surface and near-surface iθ/i and iT/isubs/sub. Modelled Nsub2/subO emissions were found to be sensitive to defoliation intensity and timing which controlled plant N?uptake and soil iθ/i and iT/isubs/sub 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 Nsub2/subO emissions by as much as 80?% during subsequent events and by an average of 43?% annually. Modelled Nsub2/subO emissions were also found to be sensitive to surface soil properties. Increasing near-surface bulk density by 10?% raised Nsub2/subO 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 Nsub2/subO emissions commonly found in field studies. The global warming potential from annual