For two years, we quantified the exchange of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) at two different large-scale Sphagnum farming sites. At both, peat extraction left a shallow layer of highly decomposed peat and low hydraulic conductivities. One site was characterized by preceding multi-annual inundation and irrigated by ditches, while the other one was inoculated directly after peat extraction and irrigated by ditches and drip irrigation. Further, GHG emissions from an irrigation polder and the effect of harvesting Sphagnum donor material at a near-natural reference site were determined. GHG mitigation potentials lag behind the results of less decomposed sites, although our results were also affected by the extraordinary hot and dry summer 2018. CO2 exchanges ranged between -0.6 and 2.2 t CO2-C ha−1 y−1 and were mainly influenced by low water table depths. CH4 emissions were low with the exception of plots with higher Eriophorum covers, while fluctuating water tables and poorly developing plant covers led to considerable N2O emissions at the ditch irrigation site. The removal of the upper vegetation at the near-natural site resulted in increased CH4 emissions and, on average, lowered CO2 emissions. Overall, best plant growth and lowest GHG emissions were measured at the previously inundated site. At the other site, drip irrigation provided more favourable conditions than ditch irrigation. The size of the area needed for water management (ditches, polders) strongly affected the areal GHG balances. We conclude that Sphagnum farming on highly decomposed peat is possible but requires elaborate water management.

Aims Drained peatlands are a major source of greenhouse gases (GHG). Paludiculture is the production of biomass under wet and peat preserving conditions. Despite the growing recognition as GHG mitigation measure, the potential influence of climate warming on paludiculture is still unknown. Methods For two years, we quantified the exchange of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) using manual chambers and surveyed the vegetation composition of warmed and control sites at a near-natural bog and two Sphagnum farming areas in North-Western Germany. Passive warming was achieved using Open Top Chambers (OTC). Results OTCs significantly increased air and soil temperatures, while soil moisture, humidity and light availability differed only marginally. The latter was considered when calculating gross primary production. Warming tended to increase vascular plant cover, but differences to the control plots were still small after two years. Emissions of CO2 and CH4 increased with warming, dominated by CH4 at the near-natural bog and by CO2 at the paludiculture areas, where vegetation was in a successional stage and topsoils temporarily dried out during summer. N2O emissions were negligible at the near-natural bog and ceased with increasing biomass at the paludiculture sites. Interannual variability was high due to a heatwave in the second measurement year. Conclusions Climate warming could increase GHG emissions from near-natural bogs and Sphagnum farming. In the latter case, this puts even more emphasis on water management systems ensuring high water table depths during dry periods. Further, control of vascular plants might both reduce CH4 emissions and improve biomass quality.

<p>Undisturbed raised bogs are characterised by permanent water saturation which prevents decomposition of peat, limits the spread of vascular plants and makes the ecosystem a peat moss dominated sink of carbon. Lower water levels e.g. due to climate change or drainage endanger ecosystem functions and lead to an altered vegetation composition. In particular, encroaching vascular plant species are a growing thread to natural or restored peatlands in Central Europe. While previous work often focussed on how greenhouse gas emissions or ecosystem functions of raised bogs respond to changing environmental conditions, the impact of encroaching vascular plants is only sparsely covered. As process-based SVAT models are able to simulate different vegetation compositions and their respective water and carbon fluxes, they are an optimal tool to answer this relevant question. However, this requires the relevant processes of a vegetation shift impacting the system’s water and carbon relations to be correctly implemented.</p> <p>Models capable of simulating bryophyte processes are sparse. We use the process-based SVAT model 'pyAPES' including a bryophyte layer, developed and tested for boreal peatlands, at a bog site in northern Germany. The overall objective of this study is to identify whether pyAPES is able to simulate carbon fluxes of a temperate raised bog under a changing vegetation composition. We addressed four research questions: (i) Does model parametrization need to be changed when using pyAPES for temperate conditions? (ii) How do these changes affect modelled gross primary production (GPP) and ecosystem respiration (R<sub>eco</sub>)? (iii) Which photosynthetic and respiratory parameters are most crucial for model performance? (iv) Does the order of crucial parameters depend on vegetation composition and on environmental conditions?</p> <p>We answer these questions by calibrating pyAPES to measured soil temperature (T<sub>s</sub>), water table depth (WTD), seasonal dynamics of vascular plant leaf area index (LAI) and GPP and R<sub>eco </sub>fluxes. Morris sensitivity analysis (MSA) is conducted with the calibrated model to investigate parameter impacts on modelled GPP and R<sub>eco</sub>.</p> <p>Preliminary results show that pyAPES performs well for a temperate raised bog after adaptation of the model parameters. Most important parameters for calibrating pyAPES were parameters of the unimodal Van-Genuchten-Mualem water retention model for both moss and peat and Farquhar parameters, which are sparse in literature.</p> <p>MSA is conducted for GPP and R<sub>eco</sub> using annual sums. Boundaries for MSA are set to ± 20% around initial parametrisation in order to derive a standardized rank of parameter importance as well as to observed boundaries from literature to cover the whole range of possible site conditions. Subsequent analysis will give evidence whether meaningful parameter inference with respect to ecosystem carbon fluxes is possible under different site conditions.</p> <p>Further, we investigate the impact of changing vascular plant LAI and moss biomass due to encroachment on ecosystem carbon fluxes by applying full factorial parameter combinations inferring possible shifts in model sensitivities. In a last step, we investigate intraannual shifts of sensitivities in half-hourly resolution to assess the impact of dynamic environmental conditions like WTD and moss surface temperature.</p>

<p>Drained organic soils are large sources of anthropogenic greenhouse gases (GHG) in many European and Asian countries. In Germany, they account for more than 7% of the national GHG emissions. Carbon dioxide (CO<sub>2</sub>) forms the vast majority of emissions from these soils and is thus the main target for mitigation measures. Bog peatlands are mainly found in North-Western Germany and frequently used for high-intensity grassland use. Further, former peat extraction areas are restored for nature protection. While restoration has decades of tradition, paludiculture and active water management in agriculture are comparatively new.</p> <p>Here, we will compile data on GHG exchange of bog peatlands and highlight recent results on water management by ditch blocking and subsurface irrigation, on <em>Sphagnum</em> paludiculture and on restored bog peatlands. Groundwater levels are usually considered as the major control for both CO<sub>2</sub> and methane (CH<sub>4</sub>) emissions. The effects of water management on CO<sub>2</sub> emissions are strongly depending on the site. Surprisingly, raising the groundwater level by subsurface irrigation in a grassland under bog peat to levels considered as acceptable even in restoration projects did not only fail to reduce CO<sub>2</sub> emissions, but raised them compared to deeply drained control parcel. These results might be explained by an interaction of increased soil moisture in the topsoil and improved nutrient retention during phases of high soil temperatures and, at the same time, by limitations of microbial activity due to low soil moisture at the control parcels. However, at a second grassland site with subsurface irrigation, this did not occur, but a combination with grassland renewal caused extremely high nitrous oxide emissions. In contrast, both re-wetting for restoration purposes and <em>Sphagnum</em> farming reliably reduce GHG emission or may even lead to a carbon sink. Here, the effects of the groundwater level on CO<sub>2</sub> and, even more, on CH<sub>4</sub> emissions in a <em>Sphagnum</em> farming experiment were partially overridden by vegetation development dynamics.</p>

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