Increases in humic dissolved organic carbon export from upland peat catchments: the role of temperature, declining sulphur deposition and changes in land management

Peat soils represent a highly significant carbon store and are usually assumed to serve as net sinks within carbon budgets for climate modelling. However, marked increases in dissolved organic carbon (DOC) concentrations in upland peat drainage waters over the past few decades are widespread in England. This suggests that the assumed sink status of some upland peats could be changing, although, as these observations are seldom quantified into actual carbon flux measurements, their overall importance is difficult to gauge. We investigated the fluvial export of the humic component of DOC (hDOC) from 3 South Pennine catchments and found that the actual carbon export via this pathway has doubled over the last 3 decades. Temperature increases to date appear poorly related to this change. Declining sulphur deposition was more strongly related (inversely), but was not a significant determinant in any of the catchments. The major driver for elevated hDOC export appears to be changes in local land management, namely the extensive use of burning for grouse moor management. Substitution of increased hDOC export into published carbon budget models for upland peat environments imply the study sites have shifted from carbon sinks to net sources over the examined period. This suggests that moorland burning, where undertaken on blanket peat, is incompatible with the ecosystem services these landscapes are presumed to provide and that ideas about the role of upland peat moorland in regional and national carbon cycling need reappraisal.