Juncus effusus mono-stands in restored cutover peat bogs – Analysis of litter quality, controls of anaerobic decomposition, and the risk of secondary carbon loss

Abstract

Rewetting of bogs after peat extraction in eutrophied, agricultural areas often results in mono-species stands of Juncus effusus L. In contrast to typical peat forming vegetation such as Sphagnum, J. effusus is highly productive and supplies large amounts of labile carbon, but it is still unknown how this species affects biogeochemistry and the potential of restored peatlands to store carbon. To study degradation of J. effusus litter of different quality and its effect on peat decomposition, we conducted a full-factorial, anaerobic incubation with dried, 13C labelled litter (roots and shoots) of fertilized (C/N ratio 19.6, C/P ratio 55.3) and non-fertilized plants (C/N ratio 57.8, C/P ratio 304.5). We furthermore analyzed weakly (fibric) to highly (earthyfied) humified, and acidic Sphagnum-Eriophorum peat from three different rewetted and active peat extraction sites of the temperate, nemoral NW-German Plain. Materials were incubated fully water saturated at 15°C for 70 days; peat-only and litter incubations served as controls to the full-factorial re-combination of peat and litter. By this experimental design, we investigated the effect of fertilization on litter degradability, decomposition rates of litter in relation to peat, and the controls on decomposition rates to explain potential priming effects by recombination of fresh organic carbon and refractory peat. Surprisingly, fertilization of J. effusus only insignificantly affected decomposition rates between litter types. Decomposition rates of J. effusus litter were 2–18 times higher than of peat, and spectroscopic analysis of the organic matter confirmed the lability of litter and its leachates in comparison to peat. Decomposition rates were equally controlled (73% variance explained) by availability of terminal electron acceptors and the activity of hydrolyzing enzymes. However, hydrolase activity was strongly negatively correlated (R2=0.95, P<0.05) with aromaticity, suggesting an inhibitory effect. Furthermore, upon combination of litter and peat, litter was preferentially decomposed. Litter amendments in general lead to faster depletion of electron acceptors. Thus, in these anaerobic incubations, electron acceptor availability limited a priming effect on the comparably refractory peat, and importantly, input of labile carbon from J. effusus did not compromise the preservation of peat. Instead, rapid exhaustion of electron acceptors may even lead to preservation of this comparably readily decomposable plant litter.