Intensively managed grasslands on organic soils are a major source of nitrous oxide (N 2 O) emissions. The Intergovernmental Panel on Climate Change (IPCC) therefore has set the default emission factor at 8 kg N-N 2 O 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-N 2 O ha -1 year -1. In addition to cultivation, the IPCC methodology and the Dutch national methodology account for N 2 O emissions from N inputs through fertilizer applications and animal urine and faeces deposition to estimate annual N 2 O emissions from cultivated and managed organic soils. However, neither approach accounts for other soil parameters that might control N 2 O emissions such as groundwater level. In this paper we report on the relations between N 2 O emissions, N inputs and groundwater level dynamics for a fertilized and grazed grassland on drained peat soil. We measured N 2 O 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 N 2 O-N ha -1 year -1 and 11.6 kg N-N 2 O ha -1 year -1 for the dry and wet conditions, respectively. Especially under dry conditions, measured N 2 O emissions exceeded current official estimates using the IPCC methodology and the Dutch national reporting methodology. The N 2 O-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.

Emissions of nitrous oxide (N 2 O) from managed and grazed grasslands on peat soils are amongst the highest emissions in the world per unit of surface of agriculturally managed soil. According to the IPCC methodology, the direct N 2 O emissions from managed organic soils is the sum of N 2 O emissions derived from N input, including fertilizers, urine and dung of grazing cattle, and a constant 'background' N 2 O emission from decomposition of organic matter that depends on agro-climatic zone. In this paper we questioned the constant nature of this background emission from peat soils by monitoring N 2 O emissions, groundwater levels, N inputs and soil NO 3 --N contents from 4 grazed and fertilized grassland fields on managed organic peat soil. Two fields had a relatively low groundwater level ('dry' fields) and two fields had a relatively high groundwater level ('wet' fields). To measure the background N 2 O emission, unfertilized sub-plots were installed in each field. Measurements were performed monthly and after selected management events for 2 years (2008)(2009)). On the managed fields average cumulative emission equaled 21 ± 2 kg N ha -1 y -1 for the 'dry' fields and 14 ± 3 kg N ha -1 y -1 for the 'wet' fields. On the unfertilized sub-plots emissions equaled 4 ± 0.6 kg N ha -1 y -1 for the 'dry' fields and 1 ± 0.7 kg N ha -1 y -1 for the 'wet' fields, which is below the currently used estimates. Background emissions were closely correlated with groundwater level (R 2 = 0.73) and accounted for approximately 22% of the cumulative N 2 O emission for the dry fields and for approximately 10% of the cumulative N 2 O emissions from the wet fields. The results of this study demonstrate that the accuracy of estimating direct N 2 O emissions from peat soils can be improved by approximately 20% by applying a background emission of N 2 O that depends on annual average groundwater level rather than applying a constant value.

About 9% of the area of the Netherlands is covered by peat soils (about 290,000 ha), mainly drained and in use for dairy farming (about 223,000 ha). Decomposition (oxidation) of peat soils used in dairy farming causes subsidence rates of 12 mm.y -1 The objective of the research was to develop a method to calculate from subsidence the CO 2 emissions of peat soils in agricultural use and to test the possibilities of submerged drains to raise groundwater levels and diminish subsidence and CO 2 emissions. One mm subsidence by oxidation equals a CO 2 emission of about 2.3 tons of CO 2 per year per hectare. We calculated that about 3% of the annual anthropological CO 2 emission in the Netherlands can be accounted to the oxidation of peat soils. This is about 4.2 Mton CO 2 per year. In dry summers the groundwater level lowers well below ditchwater levels, exposing easily biological degradable peat to oxidation. Raising groundwater levels up to ditchwater levels by subsurface irrigation by submerged drains with a spacing from drain to drain of 4 to 6 meters is tested as a possibility to reduce subsidence and CO 2 emissions. The experiments started in 2003. Subsidence and so CO 2 emissions proved to be reduced by more than 80%. A disadvantage of the use of submerged irrigation might be the increased water usage. Model calculations showed that the amount of inlet water increased on average up to 30%, however, intelligent water management can be a possibility to reduce the extra water usage to about 5%. The modeled reduction in subsidence (and so the CO 2 emission) was about 40% of the subsidence in the situation without submerged drains. We concluded that the use of submerged drains can reduce subsidence and CO 2 emissions with at least 50%.

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