Decomposition in peatlands: Reconciling seemingly contrasting results on the impacts of lowered water levels

Northern peatlands represent about 30% of the global soil C pools. The C pool in peat is a result of a relatively small imbalance between production and decay. High water levels and the consequent anoxia are considered the major causes for the imbalance. As such, the C sink of a peatland is labile, and sensitive to disturbances in environmental conditions.Changes in peatland ecosystem functions may be mediated through land-use change, and/or climatic warming. In both cases, lowering of the water level may be the key factor. Logically, lowered water levels with the consequent increase in oxygen availability in the surface soil may be assumed to result in accelerated rates of organic matter decomposition. Yet, earlier research has given highly contrasting results concerning the effects of lowered water levels on the rates of decomposition and the C sink/source behaviour of peatlands. The mechanisms controlling this variation remain unresolved.This paper summarizes the changes observed in the biotic and abiotic controls of decomposition following natural or artificial lowering of peatland water levels and show that they are complex and their interactions have not been previously explored. Long-term changes in the C cycle may differ from short-term changes. Short-term changes represent a disturbance in the ecosystem adapted to the pre-water-level-lowering conditions, while long-term changes result from several adaptive mechanisms of the ecosystem to the new hydrological regime. While in a short term, the disturbed system will always lose C, the long-term changes inherently vary among peatland types, climates, and extents of change in the water level. The paper closes by identifying the gaps in our knowledge that need to be addressed when proceeding towards a causal and unifying explanation for the C sink/source behaviour of peatlands following persistent lowering of the water level.