Methane emissions from grasslands

IntroductionMethane (CH 4 ) is an important greenhouse gas. The concentration of greenhouse gases in the atmosphere has been increasing since pre-industrial times, mainly due to human activities. This increase gives concern, because it may cause global warming due to an enhanced greenhouse effect.In the soil, CH 4 may be produced under anaerobic conditions, and consumed under aerobic conditions. Net CH 4 emissions, i.e. the resultant of CH 4 exchanges between soil and atmosphere, encompasses the processes CH 4 production, CH 4 consumption, and CH 4 transport. If CH 4 production exceeds CH 4 consumption, the soil is a source of CH 4 ; if CH 4 consumption exceeds CH 4 production, the soil is a sink of CH 4 . The contribution of soils to the global CH 4 balance is significant: 14-47% of the total source and 3-9% of the total sink (IPCC, 1995a).The major aims of this study were to provide insight into the major factors that contribute to net CH 4 emissions from grasslands, and to provide quantitative data on net CH 4 emissions from typical grasslands with a range of soil wetness and N input in the Netherlands. CH 4 emissions from grasslands were measured with flux chambers at a number of sites in the period 1994-1997. Furthermore, several incubation experiments were carried out. Since large variability of net CH 4 emissions is a common phenomenon, special attention was paid to temporal and spatial variability.Wet grasslands with low N input on peat soilsThe area of wet grasslands on peat soil in the Netherlands is small, but slowly increasing at the expense of drained, agriculturally used grasslands. Net CH 4 emissions were measured at wet grasslands on peat soil in the nature preserve "Nieuwkoopse Plassen", which is a former peat mining and agricultural area with narrow grassland and reed fields surrounded by ditches. Ground water level is kept near the surface via the water level of the ditches. Mean ground water level is 10-20 cm below the surface. N input is 30-50 kg N ha -1yr -1via atmospheric deposition. Measurements were carried out at three sites, Drie Berken Zudde, Koole and Brampjesgat, during three years. The sites were considerable sources of CH 4 with average CH 4 emissions of 79, 133 and 204 kg CH 4 ha -1yr -1, respectively. Ditches near the sites emitted 42-225 kg CH 4 ha -1yr -1. The time course of CH 4 emissions for all experimental sites and years was fit with a multiple linear regression model with ground water level and soil temperature as independent variables. Lowering or raising the ground water level by 5 cm could decrease or increase CH 4 emissions by 30-50%. Therefore, ground water level management of these grasslands should be done with care (Chapter 2). Spatial variability of CH 4 emissions was high. Most important determinants of spatial variability were CH 4 production capacity and aboveground biomass of sedges ( Carex spp.). Sedges and other plants may affect CH 4 emissions by stimulating CH 4 transport from anaerobic layers in the soil to the surface and by serving as substrate for methanogens (Chapter 3).In order to improve our understanding of CH 4 emissions, wet peat soils were fractionated into different size and density fractions. Incubation experiments showed that the individual fractions were rather similar with respect to C mineralisation capacity and C/N ratio, but not with respect to CH 4 production capacity. Significant CH 4 production only occurred for fractions with a large particle size. Furthermore, CH 4 production capacity strongly decreased with depth. This indicates that in these wet peat soils recently died plant material is a major substrate for methanogens (Chapter 4).Intensively managed grasslands with a range of N input on drained peat soilsPeat soils are often considered to have a high CH 4 emitting potential, because they are anoxic at shallow depth and have high organic matter contents. In the Netherlands, the majority of the peat soils is drained. Before this study, it was not known whether drained peat soils would be a source or a sink of CH 4 . Net CH 4 emissions from drained peat soils were measured at grasslands at the experimental farm Zegveld with mean ground water levels of 20-40 cm below the surface and an annual N input via fertilisation and atmospheric deposition ranging from 35-460 kg N ha -1yr -1. Net CH 4 emissions from these grasslands were low; they consumed 0.1 to 0.3 kg CH 4 ha -1yr -1. Effect of mean ground water level, in the range of 20-40 cm below the surface, was significant, but small. There were no significant effects of grazing versus mowing and withholding N fertilisation for three years on net CH 4 emissions (Chapter 5).In order to assess the spatial variability and spatial dependence of greenhouse gas emissions (CH 4 , N 2 O, and CO 2 ), and their underlying soil processes and properties, a field campaign was carried out at grasslands on drained peat soil in Fallköping, Sweden. Emissions and potential factors controlling CH 4 emissions were measured on two adjacent sites on two successive days for each site. Spatial variability was analysed using geostatistics. Both sites were small sinks of CH 4 . Spatial variability of emissions was high with coefficients of variation of 50 to 1400%. Emissions either showed a spatial trend or were spatially dependent. However, spatial dependence of emissions showed differences between sites and also between succeeding days. This implies that emissions can not realistically be estimated by the use of geostatistics (Chapter 6).Extensively managed grasslands with low N input on relatively dry soilsTo explore the maximum uptake of atmospheric CH 4 by grasslands in the Netherlands, net CH 4 emissions were measured at extensively managed heather grasslands on sandy soil in Wolfheze with a mean ground water level more than 3 m below the surface and an annual N input via atmospheric deposition of 40 kg N ha -1yr -1. These grasslands consumed 1.1 kg CH 4 ha -1yr -1. Temporal variability of net CH 4 emissions at Wolfheze was related to differences in soil temperature and soil moisture content. CH 4 uptake was highest at high soil temperatures and intermediate soil moisture contents. Incubation experiments showed that at low soil moisture contents, CH 4 consumption was completely inhibited, probably due to physiological water stress of methanotrophs. At high soil moisture contents, CH 4 consumption was greatly reduced, probably due to the slow down of diffusive CH 4 and O 2 transport in the soil. Optimum soil moisture contents were in the same range as prevailing in the field (Chapter 7).Determining factors for CH 4 emissionsImportant environmental factors determining CH 4 emissions are soil organic matter, ground water level, soil moisture content, temperature, and vegetation characteristics. Ground water level exerts a primary control over CH 4 emissions, since ground water level forms the transition zone between anaerobic (potential CH 4 producing) and aerobic (potential CH 4 consuming) layers in the soil. Therefore, drainage of wet grasslands is an important management factor determining CH 4 emissions. At the current rates of N input via fertilisation and atmospheric deposition in the Netherlands, the overall net effect of N fertilisation on net CH 4 emissions from grasslands is small or negligible. Furthermore, the effects of grazing versus mowing and stocking density on net CH 4 emissions are negligible (Chapter 8). In conclusion, grassland management, other than drainage, is not an option to mitigate net CH 4 emissions from grasslands in the Netherlands.Quantification of net CH 4 emissions from grasslands in the NetherlandsMost grasslands in the Netherlands are intensively managed with a total N input via fertilisation and atmospheric deposition in the range of 300-500 kg N ha -1yr -1. Thus far, CH 4 emissions from grasslands in the Netherlands were not well-documented. However, in this study, CH 4 emissions from a number of grasslands with a range of soil wetness and N input have been quantified. Total net CH 4 uptake by grasslands in the Netherlands (excluding wet grasslands) is estimated at 0.5 Gg CH 4 yr -1. Wet soils, which occupy only 0.5% of the total surface area, emit 5-10 Gg CH 4 yr -1. Estimates of CH 4 emissions in the Netherlands should be adjusted to put straight the role of grasslands in the national CH 4 budget.