Land-use effects on resource net flux rates and oxygen demand in stream sediments

Human land use in stream catchments modifies biogeochemical cycles. Here, 19 streams across a land use and cover gradient were investigated to determine how the landscape and associated conditions influenced sediment oxygen demand (SOD) and resource (carbon, nitrogen and phosphorus) net flux rates across the sediment-water interface of each stream. Resource net flux rates and SOD were determined from intact sediment cores collected in each stream and incubated in the dark. Stream water was then passed over the top of sealed cores, and net flux rates were determined by measuring the difference in nitrate (NO3 -), soluble reactive phosphorus (SRP), quality and quantity of dissolved organic carbon (DOC) and dissolved oxygen between inflowing and outflowing water. Streams with more agricultural land in their catchments had higher ambient NO3 -, SRP and protein-like dissolved organic matter (DOM) and a larger fraction of DOM in a reduced oxidative state (as inferred using an optical DOM redox index). Sediment oxygen demand was best predicted by sediment organic content and was negatively related to wetland area and positively with row crop (arable) agriculture. These patterns indicate that streams affected by agriculture have relatively more nutrients and labile DOM than streams with greater wetland cover. Most (75%) of the variation in NO3 - net flux into the stream sediments could be explained by ambient NO3 - and protein-like DOM. All but one stream had a net release of dissolved organic carbon (DOC) into the water column; however, streams with higher NO3 - uptake rates lost carbon to the water column more slowly. Among streams, SRP net flux was low, varied between uptake and release and did not relate to landscape and ambient conditions. Phosphorus cycling was not affected by processes captured in this study. Net flux rates suggest that benthic net NO3 - uptake increased with increased NO3 - concentration and that more DOC was retained when the labile carbon pool increased with agricultural land use in the catchment. These links suggest that strategies put forward to increase the efficiency of NO3 - removal should consider the composition of the carbon pool in addition to hydrological alterations.