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

Abstract. The drainage and cultivation of fen peatlands create complex small-scale mosaics of soils with extremely variable soil organic carbon (SOC) stocks and groundwater levels (GWLs). To date, the significance of such sites as sources or 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 fail to account for this complexity, holistic approaches combining gas fluxes with the underlying processes are required to understand the carbon (C) gas exchange of drained fens. It can be assumed that the stocks of SOC and N located above the variable GWL – defined as dynamic C and N stocks – play a key role in the regulation of the plant- and microbially mediated CO₂ fluxes 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 – R_eco; net ecosystem exchange – NEE; CH₄) of maize using manual chambers for 4 years. The study sites were located near Paulinenaue, Germany, where we selected three soil types representing the full gradient of GWL and SOC stocks (0–1 m) of the landscape: (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 (SOC_dyn) and N (N_dyn) stocks.

Average annual NEE differed considerably among sites, ranging from 47 ± 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 N stocks showed no significant effect on C fluxes, SOC_dyn and N_dyn and their interaction with GWL strongly influenced the C gas exchange, particularly NEE and the GPP : R_eco ratio. Moreover, based on nonlinear regression analysis, 86% of NEE variability was explained by GWL and SOC_dyn. The observed high relevance of dynamic SOC and N stocks in the aerobic zone for plant and soil gas exchange likely originates from the effects of GWL-dependent N availability on C formation and transformation processes in the plant–soil system, which promote CO₂ input via GPP more than CO₂ emission via R_eco.

The process-oriented approach of dynamic C and N stocks is a promising, potentially generalisable method for system-oriented investigations of the C gas exchange of groundwater-influenced soils and could be expanded to other nutrients and soil characteristics. However, in order to assess the climate impact of arable sites on drained peatlands, it is always necessary to consider the entire range of groundwater-influenced mineral and organic soils and their respective areal extent within the soil landscape.