Emissions of N2O from fertilized and grazed grassland on organic soil in relation to groundwater level

Summary

Intensively managed grasslands on organic soils are a major source of nitrous oxide (N2O) emissions. The Intergovernmental Panel on Climate Change (IPCC) therefore has set the default emission factor at 8 kg N–N2O ha-1 year-1 for cultivation and management of organic soils. Also, the Dutch national reporting methodology for greenhouse gases uses a relatively high calculated emission factor of 4.7 kg N–N2O ha-1 year-1. In addition to cultivation, the IPCC methodology and the Dutch national methodology account for N2O emissions from N inputs through fertilizer applications and animal urine and faeces deposition to estimate annual N2O emissions from cultivated and managed organic soils. However, neither approach accounts for other soil parameters that might control N2O emissions such as groundwater level. In this paper we report on the relations between N2O emissions, N inputs and groundwater level dynamics for a fertilized and grazed grassland on drained peat soil. We measured N2O emissions from fields with different target groundwater levels of 40 cm (‘wet’) and 55 cm (‘dry’) below soil surface in the years 1992, 1993, 2002, 2006 and 2007. Average emissions equalled 29.5 kg N2O–N ha-1 year-1 and 11.6 kg N–N2O ha-1 year-1 for the dry and wet conditions, respectively. Especially under dry conditions, measured N2O emissions exceeded current official estimates using the IPCC methodology and the Dutch national reporting methodology. The N2O–N emissions equalled 8.2 and 3.2% of the total N inputs through fertilizers, manure and cattle droppings for the dry and wet field, respectively and were strongly related to average groundwater level (R 2 = 0.74). We argue that this relation should be explored for other sites and could be used to derive accurate emission data for fertilized and grazed grasslands on organic soils