Agricultural land use impacts microbial community structure of streambed sediments

Agricultural land use is known to adversely affect freshwater ecosystems, and recent studies have shown that it can significantly alter dissolved organic matter (DOM) concentrations and quality. DOM quantity and quality are key determinants of microbial community structure in streams and have been linked to changes in community abundance and composition. Considering the vital ecological role sediment microbes play in stream ecosystems, it is important to understand how agricultural land use changes affect stream microbial community structure. This study was conducted to determine if variation in sediment microbial biomass community structure could be attributed to differences in agricultural land use. Microbial biomass, microbial community structure, and stream water parameters were compared on 2 dates from 6 sites representing a gradient of agricultural land use from 18 to 64% within the Bear Creek Watershed in northwest Alabama, USA. Total microbial biomass, determined as phospholipid phosphate, was strongly influenced by sediment surface area and, to a lesser extent, correlated with sampling date and land use. Sediment microbial community structure, determined by phospholipid fatty acid analysis, was significantly correlated to percent agricultural land use in the watershed, as well as stream water DOM humification index, and sediment δ15N values and surface area. Microbial community structure also differed between sampling dates. These findings demonstrate that agricultural land use is linked via alteration of stream water and sediment organic matter to changes in sediment microbial structure which may, at least in part, account for widely observed changes in stream ecosystem functioning associated with agricultural activities.