Connecting organic carbon in stream water and soils in a peatland catchment

Abstract

We investigated the spatial and temporal connectivity between concentrations of organic carbon in stream water and the soil carbon pool in a small (1.3 km2) upland catchment in NE Scotland by comparing downstream changes in dissolved organic carbon (DOC) with spatial changes in the soil carbon pool. Stream water DOC concentrations increased downstream to reach a maximum in the center of the catchment and then decreased in the lower part of the catchment, a pattern that was linked to spatial changes in the soil carbon pool. Subcatchment analysis of the soil carbon pool and DOC export showed that the strong relationship between percent of peat coverage and stream water DOC in the upper part of the catchment did not hold farther downstream, where freely draining mineral soils become spatially more important. Temporal variations in stream water and soil water DOC concentrations in the central part of the catchment were studied using samples collected from two depths (0.20–0.45 m and 0.65–0.95 m) in the two most important soil types there (histosols and fluvisols). Stream water DOC concentrations were only related to soil water DOC in shallow peat. DOC concentration in shallow peat and stream water were both positively correlated with temperature. Multivariate regression analysis showed that 55% of the variation in stream water DOC concentrations could be explained by a model including stream temperature, discharge, and soil solution DOC concentrations in the shallow histosols. We conclude that the linkage between stream water DOC concentrations and the soil C pool in the upper 1.5 km of the stream reach are likely to be driven by temperature‐related DOC production in near‐surface histosols. Farther downstream, this relationship between organic carbon in the soil and stream becomes weaker as other processes (e.g., lower inputs from minerals soils and allochthonous within‐stream processing of DOC) become more important.