This study argues that several metrics are necessary to build up a picture of yield gain and nitrogen losses for ryegrass sheep pastures. Metrics of resource use efficiency, nitrous oxide emission factor, leached and emitted nitrogen per unit product are used to encompass yield gain and losses relating to nitrogen. These metrics are calculated from field system simulations using the DAYCENT model, validated from field sensor measurements and observations relating to crop yield, fertilizer applied, ammonium in soil and nitrate in soil and water, nitrous oxide and soil moisture. Three ryegrass pastures with traditional management for sheep grazing and silage are studied. As expected, the metrics between long-term ryegrass swards in this study are not very dissimilar. Slight differences between simulations of different field systems likely result from varying soil bulk density, as revealed by a sensitivity analysis applied to DAYCENT. The field with the highest resource use efficiency was also the field with the lowest leached inorganic nitrogen per unit product, and vice versa. Field system simulation using climate projections indicates an increase in nitrogen loss to water and air, with a corresponding increase in biomass. If we simulate both nitrogen loss by leaching and by gaseous emission, we obtain a fuller picture. Under climate projections, the field with the lowest determined nitrous oxide emissions factor, had a relatively high leached nitrogen per product amongst the three fields. When management differences were investigated, the amount of nitrous oxide per unit biomass was found to be significantly higher for an annual management of grazing only, than a silage harvest plus grazing, likely relating to the increased period of livestock on pasture. This work emphasizes how several metrics validated by auto-sampled data provide a measure of nitrogen loss, efficiency and best management practise.
Bibliographical noteThis study was partially supported by Soil to Nutrition, Rothamsted Research’s Institute Strategic Programme supported by the Biotechnology and Biological Sciences Research Council (BBS/E/C/000I0320).The North Wyke Farm Platform is a UK National Capability, also supported by the Biotechnology and Biological Sciences Research Council (BBS/E/C/000J0100).This study was also partially supported by the Natural Environment Research Council’s ADVENT project (NERC NE/M019691/1).Climate data were measured at the MIDAS Land Surface Station DLY3208 DEVON, UK, a weather station of the UK Meteorological Office. We would especially like to thank Dr Nadine Loick of Rothamsted Research for advice on preparation of N2O model calibration parameters, and the data team of the North Wyke Farm Platform. We owe our gratitude to the late Mr Robert Orr, grassland specialist at the North Wykesite, for his invaluable advice and information on sward growth.
- Nitrous oxide
- OXIDE EMISSIONS
- GRAZED GRASSLAND