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

Abstract

 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.