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The drainage and cultivation of fen peatlands create complex small-scale mosaicsof soils with extremely variable soil organic carbon (SOC) stocks andgroundwater levels (GWLs). To date, the significance of such sites as sourcesor sinks for greenhouse gases such as CO2 and CH4 is still unclear,especially if the sites are used for cropland. As individual control factors such as GWL failto account for this complexity, holistic approaches combining gas fluxes withthe underlying processes are required to understand the carbon (C) gasexchange of drained fens. It can be assumed that the stocks of SOC and Nlocated above the variable GWL – defined as dynamic C and N stocks – playa key role in the regulation of the plant- and microbially mediated CO2fluxes in these soils and, inversely, for CH4. To test this assumption,the present study analysed the C gas exchange (gross primary production –GPP; ecosystem respiration – Reco; net ecosystem exchange – NEE;CH4) of maize using manual chambers for 4 years. The study sites werelocated near Paulinenaue, Germany, where we selected three soil typesrepresenting the full gradient of GWL and SOC stocks (0–1 m) of thelandscape: (a) Haplic Arenosol (AR; 8 kg C m−2); (b) Mollic Gleysol(GL; 38 kg C m−2); and (c) Hemic Histosol (HS; 87 kg C m−2).Daily GWL data were used to calculate dynamic SOC (SOCdyn) and N(Ndyn) stocks. Average annual NEE differed considerably among sites, ranging from47 ± 30 g C m−2 yr−1 in AR to−305 ± 123 g C m−2 yr−1 in GL and−127 ± 212 g C m−2 yr−1 in HS. While static SOC and Nstocks showed no significant effect on C fluxes, SOCdyn andNdyn and their interaction with GWL strongly influenced the C gasexchange, particularly NEE and the GPP : Reco ratio. Moreover,based on nonlinear regression analysis, 86% of NEE variability wasexplained by GWL and SOCdyn. The observed high relevance of dynamicSOC and N stocks in the aerobic zone for plant and soil gas exchange likelyoriginates from the effects of GWL-dependent N availability on C formationand transformation processes in the plant–soil system, which promote CO2input via GPP more than CO2 emission via Reco. The process-oriented approach of dynamic C and N stocks is a promising,potentially generalisable method for system-oriented investigations of the Cgas exchange of groundwater-influenced soils and could be expanded to othernutrients and soil characteristics. However, in order to assess the climateimpact of arable sites on drained peatlands, it is always necessary toconsider the entire range of groundwater-influenced mineral and organicsoils and their respective areal extent within the soil landscape

Dynamic C and N stocks – key factors controlling the C gas exchange of maize in heterogenous peatland