Abstract. Peat decomposition in managed peatlands is responsible for a decrease of 0.52 GtC yr−1 in global carbon stock and is strongly linked to drainage to improve the agricultural bearing capacity, which increases aeration of the soil. Microbial aerobic decomposition is responsible for the bulk of the net CO2 emission from the soil and could be reduced by wetting efforts or minimizing drainage. However, the effects of rewetting efforts on microbial respiration rate are largely unknown. In this study, we aimed to obtain more process-based understanding of these rewetting effects on peat decomposition by integrating high-quality field measurements and literature relationships with an advanced hydrological modelling approach where soil moisture and temperature are centralized as the main drivers for peat decomposition. In 2020 and 2021, two dairy farming peatlands, where subsoil irrigation and drainage (SSI) was tested against a control situation, were continuously monitored for CO2 fluxes, groundwater table, soil moisture and soil temperature. After successfully representing field hydrology and carbon dynamic measurements within our process-based model, we further explored the effects of rewetting under different weather conditions, water management strategies (raising ditchwater levels and SSI) and hydrological seepage settings. To represent peat carbon dynamics we introduced a methodology to estimate potential aerobic microbial respiration rate, based on potential respiration rate curves for soil temperature and water-filled pore space (WFPS). Measurements show that rewetting with SSI resulted in higher summer groundwater levels, soil temperatures and WFPS. SSI reduced the net ecosystem carbon balance (NECB) by 1.58 ± 0.56 kg CO2 m−2 yr−1 (83 ± 25 %) and 0.66 ± 0.62 kg CO2 m−2 yr−1 (28 ± 15 %) for Assendelft and Vlist respectively in 2020. SSI had a negligible effect in 2021 for both research locations, due to more precipitation, lower temperatures and different SSI management (in Assendelft) as compared to 2020. Simulated rewetting effects were in agreement with measured rewetting effects. Model simulations indicate that raising ditchwater levels always reduces peat respiration rates. Furthermore, we found that the application of SSI (i) reduces yearly peat respiration rates in a dry year and/or with downward hydrological fluxes and (ii) increases peat respiration rates in a wet year and/or when upward groundwater seepage is present. Moreover, combining SSI with high ditchwater levels or pressurizing SSI systems will further reduce peat respiration rates. We show that our process-based approach based on temperature and WFPS soil conditions to determine NECB represents observed variance to a greater extent than empirical relationships that involve average groundwater level observations or simulations. Therefore, we recommend using this kind of approach to estimate the effectiveness of rewetting. When this is not possible, we recommend using mean summer groundwater level instead of mean annual groundwater level as a proxy to estimate NECB. Such relations between mean groundwater levels and NECB are prone to underestimating NECB for SSI parcels.
<p>Peat decomposition in managed peatlands is responsible for a decrease of 0.52 GtC yr<sup>-1</sup> in global carbon stock and is strongly linked to drainage, which increases the oxygen availability in the soil. Microbial aerobic decomposition is responsible for the bulk of the net CO<sub>2</sub> emission from the soil. This decomposition could be reduced by rewetting efforts or minimizing drainage, but the effects of rewetting on microbial respiration rate are largely unknown. Our research aims to assess the effects of rewetting measures on soil wetness, soil temperatures and CO<sub>2</sub> emissions by field data collection and simulations of peatland parcels under dairy farming. Here we present the results for two dairy farming peatlands where subsoil irrigation and drainage (SSI), which aims to increase summer groundwater tables. At both dairy farms parcels with rewetting measures were tested against a control situation for the year 2020. Furthermore, we introduce a process-based methodology to estimate potential aerobic microbial respiration rate as measure for peat decomposition in managed peatlands, based on potential respiration rate curves for soil temperature and water filled pore space (WFPS). This methodology enables us to quantify effects of rewetting under different weather conditions, water management strategies (raising ditch water levels and SSI) and hydrological settings (i.e. seepage). We present the effects of the water management strategies on CO<sub>2</sub> emissions, groundwater table height and soil moisture and discuss to what extent we can rely on commonly used groundwater table-based proxies to estimate peat decomposition. Towards improved understanding of biophysical soil processes and peatland management!</p>
<p>Following the Paris Agreement (2015) that aims to limit climate warming, the Dutch government presented a National Climate Agreement. The National Climate Agreement allocates the overall ambition of reducing the national greenhouse gas emission by 49% in 2030 (compared to 1990) to different sectors, such as industry, mobility or agriculture and land use. Within the latter sector, the peat meadow areas currently contribute ~4.6 to 7 Mton per year of CO<sub>2</sub> to the national greenhouse gas emission. In the National Climate Agreement, the aim is to reduce the net CO<sub>2</sub> emission from the peat meadow areas with 1 Mton per year by 2030. &#160;</p><p>The peat meadows of the Netherlands are drained peatlands for dairy farming. Drainage of peatlands causes land subsidence, and as a result of peat oxidation, greenhouse gas emissions (CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O). Critical factors that determine the level of greenhouse gas emissions from the peat meadows are amongst others the groundwater level, peat thickness, macrofossil composition, mineral cover-soil thickness, the level of fertiliser addition. In the National Climate Agreement, the main focus is on raising groundwater levels in the peat meadow area to reduce greenhouse gas emissions and subsidence. This can be either passively achieved by raising the ditch water levels, surface irrigation, reducing transpiration losses or actively by using submerged drainage systems that drain in winter, but infiltrate water in summer.</p><p>It is now time to produce regional spatial plans that comprise a compilation of measures that raise groundwater levels enough to reduce the greenhouse emissions with 1 Mton per year by 2030. To do so, it is imperative that the exact effects of the proposed measures on greenhouse gas emissions and subsidence are known, under different environmental conditions. In ongoing and previously executed studies, results so far show mixed outcomes. Therefore, a national research programme commenced autumn 2019, in which the greenhouse gas emission and subsidence is continuously measured in five field sites. The programme focusses on the effects of submerged drainage/irrigation on emissions in the first 2 growing seasons.</p><p>The consortium in charge of the national research programme consists of parties in the Netherlands that have ample experience in measuring greenhouse emission and subsidence. Each of the five field sites consists of one measurement plot in an area where the groundwater level is raised and one reference plot where the groundwater level dynamics remained the same. A measurement plot consists of continuously operating gas analyser chambers that rotate within the plot every two weeks. In two field sites, emissions are also measured using the eddy covariance method. In addition, subsidence is measured with extensometers and spirit levelling. Sensors, both in situ and above ground, provide information on relevant parameters such as soil moisture, soil temperature, oxygen availability, and meteorological parameters. Samples are being extracted from the field sites and tested on microbiological assemblages, and soil (mechanical) parameters. The whole programme is designed to run for at least five years, but first results that support policy development, are supposed to be reported in 2021.</p>
Abstract. Peat decomposition in managed peatlands is responsible for a decrease of 0.52 GtC yr−1 in global carbon stock and is strongly linked to drainage to improve the agricultural bearing capacity, which increases aeration of the soil. Microbial aerobic decomposition is responsible for the bulk of the net CO2 emission from the soil and could be reduced by wetting efforts or minimizing drainage. However, the effects of rewetting efforts on microbial respiration rate are largely unknown. We aimed to obtain more insight in these rewetting effects and measured them for 1 year for two dairy farming peatlands where submerged drainage subsurface irrigation (SDSI) was tested against a control situation. With a modelling approach, we explored the effects of rewetting under different weather conditions, water management strategies (raising ditch water levels and SDSI) and hydrological settings. We introduced a methodology to estimate potential aerobic microbial respiration rate as measure for peat decomposition in managed peatlands, based on potential respiration rate curves for soil temperature and water filled pore space (WFPS). Rewetting with SDSI resulted in higher summer groundwater levels, soil temperatures and WFPS. SDSI reduced net ecosystem production (NEP) with 1.27 ± 0.39 kg CO2 m−2 yr−1 (83 %) and 0.78 ± 0.37 kg CO2 m−2 yr−1 (35 %) for Assendelft and Vlist respectively. With the process based modelling approach we found that raising ditch water levels always reduces peat respiration rates. Furthermore, we found that the application of SDSI reduces yearly peat respiration rates in environments in a dry year and/or with downward hydrological fluxes, and increases peat respiration rates in a wet year and/or when upward groundwater fluxes are present. Moreover, combining SDSI with high ditch water levels or pressurizing SDSI systems will further reduce peat respiration rates. We highly recommend to use a process-based approach based on temperature and WFPS soil conditions to determine effectivities of rewetting efforts over empirical relationships between average groundwater level and NEP. Such a more process based approach allows to distinguish between groundwater levels raised by SDSI and ditch water levels. When this is not possible, we recommend using mean summer groundwater level instead of mean annual groundwater level as a proxy to estimate NEP. Such relations between mean groundwater levels and NEP need to be corrected for situations with SDSI.
<p>The Netherlands plans to cut greenhouse gas emissions by 1 megaton CO<sub>2</sub>-equivalents in 2030 by implementing measures reducing peat decomposition. In order to achieve this, a national research program on peatland pasture greenhouse gas emissions has been set up. In the program, five peatland sites with each two fields, with and without submerged tube drainage systems, are continuously monitored. Here, we present our research with the objective to understand the rate of biochemical peat decomposition by assessing electron acceptor availability from a hydrological perspective. Soil (< 100 cm depth) redox conditions are continuously measured at five depths. Preliminary data on soil electron acceptor availability distribution suggest counterintuitive behavior of the peat soils. We find reducing conditions in the topsoil (0-20 cm) and oxidative conditions in the subsoil (40-80 cm) for the sites without tube drainage. For sites with tube drainage, we find oxidative conditions in the topsoil (0-20 cm) and reducing conditions starting at 60 cm depth at the drain location and at 80 cm depth between the drains. A novel 2D groundwater model is being set up, enabling to capture saturation dynamics, water origin and solute transport in the peat soil. We will present our modelling setup and initial simulation results for water origin and travel paths. These results will indicate how electron acceptors are distributed through the soil, helping to interpret redox measurements in the field at different depths. In a later stage of the research, the effects of redox conditions on microbial soil respiration will be evaluated with greenhouse gas chamber and eddy covariance measurements.</p>
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