Micrometeorological observations of CH4 and N2O at a managed fen meadow in the Netherlands

Global atmospheric concentrations of CO2, CH4 and N2O have increased markedly as a result of human activities since 1750 (30%, 150% and 17% respectively). The global increases in carbon dioxide concentration are primarily due to fossil fuel and land-use change, while those of methane and nitrous oxide are primarily due to agriculture (IPCC, 2007). Our research is part of the BSIK ME1 project that has its main focus on fen meadow ecosystems in the western part of the Netherlands. Since the 19th century these areas have been a strong net source of carbon dioxide as a result of increased peat oxidation caused by drainage. To understand the effect of natural changes and management on the green house gas emission and uptake in the areas an integral assessment of the greenhouse gases balance in these areas is required. Three experimental sites have been selected for this purpose: a restored nature reserve with relatively high ground water level and no agricultural activities (Horstermeer) a site under extensive agricultural exploitation where the ground water level will be raised in the coming years (Stein) and a site under intensive agricultural exploitation with low a ground water level (Oukoop). However, there are significant uncertainties in the estimated CH4 and N2O fluxes, mainly due to a combination of complexity of the source (i.e. spatial and temporal variation) and instrument limitations. High-frequency micrometeorological methods are an option to obtain integrated emission estimates on a hectare scale that also has continuous coverage in time. In this study, a quantum cascade laser spectrometer (Aerodyne Research Inc.) is used for eddy covariance measurements of CH4 and N2O. The system has been running continuously in Oukoop since August 2006. An automatic liquid nitrogen filling system is employed for unattended operation of the system. A sampling frequency of 10 Hz is obtained using a 1.0 GHz PC system. A precision of 2.6 and 0.3 ppb Hz-1/2 is obtained for CH4 and N2O, respectively. However, it proved to be important to calibrate the equipment frequently using a low and a high standard. Drift in the system is removed using a 120 s running mean filter. Average fluxes and standard deviations in these averages of 461 ± 485 ngC m-2s-1 (2.21 ± 2.33 mg m-2hr-1) and 38 ± 64 ngN m-2s-1 (0.22 ± 0.36 mg m-2hr-1) were observed over the period August 17th to November 6th 2006. About 40% of this total N2O emission was due to a fertilizing event. Besides these observed emissions, uptake of CH4 and N2O occurred in short events lasting at most a few hours.