Mulch decomposition impedes recovery of net carbon sink function in a restored peatland

With the increase in the drainage and extraction of peat for horticultural, agricultural and energy purposes worldwide, there is an increased pressure to restore hydrological conditions essential to the re-establishment of Sphagnum mosses and, therefore, carbon sequestration. Measurements of ecosystem CO2 were conducted at a recently restored vacuum harvested bog in Eastern Québec, Canada. The peatland was restored by blocking the drainage networks to return optimal water table positions for the growth of Sphagnum mosses and the spreading of a dense straw mulch layer to reduce evaporative losses and stabilize the surface for the moss re-growth. However, the return of the moss species is a gradual process and previous studies have suggested that initially the carbon loss from the system actually increase as the mulch decomposes. We examined old and new mulch CO2 production in lab incubations and scaled these results to the ecosystem level in order to determine if mulch decomposition is impeding the net carbon sink function of this peatland. At mean growing season temperatures fresh mulch was observed to release 151.5 μmol CO2 g−1 per day, while older mulch released 145 μmol CO2 g−1 per day. Furthermore, under moist conditions the CO2 release increased for all mulches. However, contrary to most assumptions, Q10 values for all mulches decreased with increasing temperatures and were greater for older mulch than for fresh. Thus, in the short term, the application of a surface straw mulch layer can have significant effects on the carbon balance of a restored peatland. Due to the decomposition of this mulch layer over the first several seasons of restoration, the peatland became a larger source of CO2, despite the increase in production of a re-emerging vegetation layer.