Significant climate risks are associated with a positive carbon-temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO 2 and CH 4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH 4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO 2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence from natural to managed conditions to quantify the "cost" of CH 4 emissions for the benefit of net carbon sequestration. With a sustained pulseresponse radiative forcing model, we found a significant increase in atmospheric forcing due to land management, in particular for wetland converted to cropland. Our results quantify the role of human activities on the climate footprint of northern wetlands and call for development of active mitigation strategies for managed wetlands and new guidelines of the Intergovernmental Panel on Climate Change (IPCC) accounting for both sustained CH 4 emissions and cumulative CO 2 exchange.wetland conversion | methane | radiative forcing | carbon dioxide F or their ability to simultaneously sequester CO 2 and emit CH 4 , wetlands are unique ecosystems that may potentially generate large negative climate feedbacks over centuries to millennia (1) and positive feedbacks over years to several centuries (2). Wetlands are among the major biogenic sources of CH 4 , contributing to about 30% of the global CH 4 total emissions (3), and are presumed to be a primary driver of interannual variations in the atmospheric CH 4 growth rate (4, 5). Meanwhile, peatlands, the main subclass of wetland ecosystems, cover 3% of the Earth's surface and are known to store large quantities of carbon

Shallow fresh water bodies in peat areas are important contributors to greenhouse gas fluxes to the atmosphere. In this study we determined the magnitude of CH 4 and CO 2 fluxes from 12 water bodies in Dutch wetlands during the summer season and studied the factors that might regulate emissions of CH 4 and CO 2 from these lakes and ditches. The lakes and ditches acted as CO 2 and CH 4 sources of emissions to the atmosphere; the fluxes from the ditches were significantly larger than the fluxes from the lakes. The mean greenhouse gas flux from ditches and lakes amounted to 129.1 ± 8.2 (mean ± SE) and 61.5 ± 7.1 mg m -2 h -1 for CO 2 and 33.7 ± 9.3 and 3.9 ± 1.6 mg m -2 h -1 for CH 4 , respectively. In most water bodies CH 4 was the dominant greenhouse gas in terms of warming potential. Trophic status of the water and the sediment was an important factor regulating emissions. By using multiple linear regression 87% of the variation in CH 4 could be explained by PO 4 3-concentration in the sediment and Fe 2?concentration in the water, and 89% of the CO 2 flux could be explained by depth, EC and pH of the water. Decreasing the nutrient loads and input of organic substrates to ditches and lakes by for example reducing application of fertilizers and manure within the catchments and decreasing upward seepage of nutrient rich water from the surrounding area will likely reduce summer emissions of CO 2 and CH 4 from these water bodies.

A B S T R A C TFluxes of methane (CH 4 ) and carbon dioxide (CO 2 ) estimated by empirical models based on small-scale chamber measurements were compared to large-scale eddy covariance (EC) measurements for CH 4 and to a combination of EC measurements and EC-based models for CO 2 . The experimental area was a flat peat meadow in the Netherlands with heterogeneous source strengths for both greenhouse gases. Two scenarios were used to assess the importance of stratifying the landscape into landscape elements before up-scaling the fluxes measured by chambers to landscape scale: one took the main landscape elements into account (field, ditch edge ditch), the other took only the field into account. Non-linear regression models were used to up-scale the chamber measurements to field emission estimates. EC CO 2 respiration consisted of measured night time EC fluxes and modeled day time fluxes using the Arrhenius model. EC CH 4 flux estimate was based on daily averages and the remaining data gaps were filled by linear interpolation. The EC and chamber-based estimates agreed well when the three landscape elements were taken into account with 16.5% and 13.0% difference for CO 2 respiration and CH 4 , respectively. However, both methods differed 31.0% and 55.1% for CO 2 respiration and CH 4 when only field emissions were taken into account when up-scaling chamber measurements to landscape scale. This emphasizes the importance of stratifying the landscape into landscape elements. The conclusion is that small-scale chamber measurements can be used to estimate fluxes of CO 2 and CH 4 at landscape scale if fluxes are scaled by different landscape elements. ß

Methane (CH 4 ) emissions were compared for an intensively and extensively managed agricultural area on peat soils in the Netherlands to evaluate the effect of reduced management on the CH 4 balance. Chamber measurements (photoacoustic methods) for CH 4 were performed for a period of three years in the contributing landscape elements in the research sites. Various factors influencing CH 4 emissions were evaluated and temperature of water and soil was found to be the main driver in both sites. For upscaling of CH 4 fluxes to landscape scale, regression models were used which were specific for each of the contributing landforms. Ditches and bordering edges were emission hotspots and emitted together between 60% and 70% of the total terrestrial CH 4 emissions. To further evaluate the effect of agricultural activity on the CH 4 balance, the annual CH 4 fluxes of the two managed sites were also compared to the emissions of a natural peat site with no management and high ground water levels. By comparing the terrestrial and additional farm based emissions of the three sites, we finally concluded that transformation of intensively managed agricultural land to nature development will lead to an increase in terrestrial CH 4 emission, but will not by definition lead to a significant increase in CH 4 emission when farm based emissions are included.

An intercomparison is made of the Net Ecosystem Exchange of CO 2 , NEE, for eight Dutch grassland sites: four natural grasslands, two production grasslands and two meteorological stations within a rotational grassland region. At all sites the NEE was determined during at least 10 months per site, using the eddy-covariance (EC) technique, but in different years. The NEE does not include any import or export other than CO 2. The photosynthesis-light response analysis technique is used along with the respiration-temperature response technique to partition NEE into Gross Primary Production (GPP) and Ecosystem Respiration (R e) and to obtain the eco-physiological characteristics of the sites at the field scale. Annual sums of NEE, GPP and R e are then estimated using the fitted response curves with observed radiation and air temperature from a meteorological site in the centre of The Netherlands as drivers. These calculations are carried out for four years (2002-2005). Land use and management histories are not considered. The estimated annual R e for all individual sites is more or less constant per site and the average for all sites amounts to 1390±30 gC m −2 a −1. The narrow uncertainty band (±2%) reflects the small differences in the mean annual air temperature. The mean annual GPP was estimated to be 1325 g C m −2 a −1 , and displays a much higher standard deviation, of ±110 gC m −2 a −1 (8%), which reflects the relatively large variation in annual solar radiation. The mean annual NEE amounts to-65±85 gC m −2 a −1. From two sites, four-year records of CO 2 flux were available and analyzed (2002-2005). Using the weather record of 2005 with optimizations from the other years, the standard deviation of annual GPP was estimated to be 171-206 gC m −2 a −1 (8-14%), of annual R e 227-247 gC m −2 a −1 (14-16%) and of annual NEE 176-276 gC m −2 a −1. The Correspondence to: C. M. J. Jacobs (cor.jacobs@wur.nl) inter-site standard deviation was higher for GPP and R e , 534 gC m −2 a −1 (37.3%) and 486 gC m −2 a −1 (34.8%), respectively. However, the inter-site standard deviation of NEE was similar to the interannual one, amounting to 207 gC m −2 a −1. Large differences occur due to soil type. The grasslands on organic (peat) soils show a mean net release of CO 2 of 220±90 g C m −2 a −1 while the grasslands on mineral (clay and sand) soils show a mean net uptake of CO 2 of 90±90 g C m −2 a −1. If a weighing with the fraction of grassland on organic (20%) and mineral soils (80%) is applied, an average NEE of 28 ±90 g C m −2 a −1 is found. The results from the analysis illustrate the need for regionally specific and spatially explicit CO 2 emission estimates from grassland.