Effects of land use on dissolved organic matter biogeochemistry in piedmont headwater streams of the Southeastern United States

Abstract

We measured dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and dissolved organic phosphorous (DOP) concentrations over the course of a year in 15 headwater streams within a Georgia Piedmont watershed that is a mosaic of areas with intensive animal agriculture, forestry, and residential development. The watershed receives large non-point-source organic waste inputs, mostly from poultry and beef production, with lesser quantities from the rapidly growing human population. The organic wastes from animal agriculture are deposited on nearby pastures and the human wastes are mostly treated with septic systems. We selected the headwater catchments to fall along a gradient of organic waste inputs, and hypothesized that stream dissolved organic matter (DOM) concentrations would increase along this gradient. We used a set of readily obtainable landscape indicators (land use categories, soil carbon content, and soil clay content), factor scores derived from a principal components analysis (PCA) of the landscape indicators, and organic waste input estimates to develop simple empirical models that describe our measured DOM concentrations and elemental ratios (C:N, C:P, and N:P). The models show positive correlations of stream DOC, DON, and DOP concentrations with pasture land cover, and with estimated organic waste inputs, lending support to our hypothesis. Over the entire measurement period, buffer land cover better described mean DOC and DON concentrations [maximum adjusted R 2 (AdjR 2) of 0.86 and 0.90, respectively] whereas the best models for mean DOP concentration and elemental ratios were comprised of both watershed and buffer scale indicators or PCA factor scores (maximum AdjR 2 ranged from 0.54 to 0.64). Organic waste estimates were also useful DOM descriptors (maximum AdjR 2 ranged from 0.23 to 0.55), although they were less effective than the landscape indicators. Landscape indicators and organic waste estimates were useful descriptors of DOC concentration variation (maximum AdjR 2 ranged from 0.22 to 0.77) after grouping the observations by season. Watershed scale indicators were better descriptors during most of the seasons. We conclude that landscape indicators and organic waste input estimates are useful descriptors of DOM concentrations and elemental ratios in watersheds dominated by non-point-source organic matter inputs.