Effects of nitrogen input and grazing on methane fluxes of extensively and intensively managed grasslands in the Netherlands

 Generally, grasslands are considered as sinks for atmospheric CH4, and N input as a factor which reduces CH4 uptake by soils. We aimed to assess the short- and long-term effects of a wide range of N inputs, and of grazing versus mowing, on net CH4 emissions of grasslands in the Netherlands. These grasslands are mostly intensively managed with a total N input via fertilisation and atmospheric deposition in the range of 300–500 kg N ha–1 year–1. Net CH4 emissions were measured with vented, closed flux chambers at four contrasting sites, which were chosen to represent a range of N inputs. There were no significant effects of grazing versus mowing, stocking density, and withholding N fertilisation for 3–9 years, on net CH4 emissions. When the ground-water level was close to the soil surface, the injection of cattle slurry resulted in a significant net CH4 production. The highest atmospheric CH4 uptake was found at the site with the lowest N input and the lowest ground-water level, with an annual CH4 uptake of 1.1 kg CH4 ha–1 year–1. This is assumed to be the upper limit of CH4 uptake by grasslands in the Netherlands. We conclude that grasslands in the Netherlands are a net sink of CH4, with an estimated CH4 uptake of 0.5 Gg CH4 year–1. At the current rates of total N input, the overall effect of N fertilisation on net CH4 emissions from grasslands is thought to be small or negligible.