Nutrient and carbon dynamics in peat from rich fens and Sphagnum-fens during different gradations of drought

Abstract

Drought has major impacts on microbial decomposition and net N- and P-release in peat. The separate effects of aeration (oxygen intrusion) during moderate drought and desiccation (oxygen intrusion plus water deficiency) during severe drought are, however, poorly understood. This information is vital to understand the biogeochemical and ecological effects of different gradations of drought in peatlands. In addition, effects may differ between rich fen peat and Sphagnum-dominated poor fen peat. We therefore conducted a controlled incubation experiment involving both soil types to quantify the rates of decomposition, net N-mineralization, net P-release, denitrification, and the partitioning of C, N and P in soils and microbial biomass under three different incubation conditions. Soils were incubated under (1) anaerobic, waterlogged conditions, (2) aerobic, moist conditions, characteristic for moderate drought in which oxygen intrusion takes place, and (3) aerobic, desiccated conditions to simulate severe drought.Our results show that under anaerobic, waterlogged conditions, net N-mineralization rates per mass dry peat soil and per microbial C mass were much higher (on average 10 times) in the Sphagnum-peat than in peat from rich fens, probably caused by higher microbial N-demand and N-immobilization in rich fens. The response upon aeration differed greatly between rich fen peat and Sphagnum-peat. Whereas aeration led to increased carbon loss and net N-mineralization rates in the rich fen peat, these rates did not change for Sphagnum-peat. The absence of aeration effects in Sphagnum-dominated fens suggests that decomposition rates are more strongly determined by litter quality than by oxygen intrusion. Upon further desiccation, both net P-release and DOC production, which remained unchanged upon aeration, increased significantly in both fen types. This may be due to microbial mortality and/or a change in microbial composition. The low anaerobic net N-mineralization rates and the strong response to aeration in rich fens compared to Sphagnum-fens, as well as the strong increase in P-availability upon further desiccation in both fen types, have important implications for peatland management in relation to drought.