Nitrous oxide emission from intensively managed grasslands

The aims of this thesis are to quantify nitrous oxide (N ₂ O) emission from intensively managed grasslands in the Netherlands, to increase the insight in the factors controlling N ₂ O emission from intensively managed grasslands and to explore the possibilities to reduce N ₂ O emission from intensively managed grasslands. The study was part of the integrated project N ₂ O emissions from grassland', with laboratory experiments, field measurements, field monitoring studies, and modelling at process and field levels. The focus of the present study was predominantly on field monitoring studies using vented closed flux chambers and a photo-acoustic spectroscopic infra-red gas analyzer. Emissions of N ₂ O from unfertilized and mown grasslands ranged from 1 to 9 kg N ha ^-1yr ^-1and were larger on peat soils than on sandy and clay soils. The large emissions from peat soils were attributed to the high mineralizable organic carbon (C) and nitrogen (N) contents, in combination with the shallow groundwater levels. Application of N fertilizer increased N ₂ O emission; on an annual basis 1.0% of the N applied as calcium ammonium nitrate (CAN) was emitted from mineral soils and 1.9 to 3.9% from peat soils. Emission of N ₂ O from N fertilized and grazed grasslands ranged from 10 to 39 kg N ha ^-1yr ^-1On annual basis, 1.5 to 9.8% of the N excreted as urine and dung during grazing was emitted as N ₂ O Spatial and temporal variability of N ₂ O fluxes were large and mainly controlled by variations in contents of mineral N and moisture. During wet conditions, N ₂ O emissions from nitrate fertilizers were much larger than those from ammonium fertilizers. Total N ₂ O emissions from dairy farming systems in the Netherlands were estimated at 13.7 ± 5.1 Gg N yr ^-1. Model calculations of N ₂ O budgets for three dairy farming systems on sandy soil with different nutrient management indicate that the potentials to reduce N ₂ O emission from dairy farming systems are large. Improving the N fertilizer use efficiency and choosing the N fertilizer type depending on the soil moisture status were shown to be effective measures to reduce N ₂ O emissions.