Cropland and riparian buffer influences on nutrient dynamics

Nutrient inputs from agriculture are a leading contributor to nonpoint source pollution of surface and ground waters. Crop production while maintaining water quality requires the use of best management practices (BMPs) to minimize nutrient loss. Nutrient management is the broad term to describe the suite of practices used to control the rate, form, timing, and transport of plant nutrients. Agricultural water quality BMPs can be broadly categorized into source and transport practices. Source BMPs focus on plant nutrient applications and include the quantity of nutrients applied, manure application strategies, and agronomic and environmental soil testing. Transport BMPs generally refer to physical structures designed to mitigate nutrient losses to the environment and include sediment control structures, grass filter strips, and riparian buffers. Source and transport BMPs are widely used, but few field-based studies have been conducted to evaluate their effectiveness in relation to soil-landscape variability.; A series of three studies were undertaken to investigate site-level factors affecting nitrogen (N) and phosphorus (P) dynamics in soil and shallow ground water among cropland and riparian buffers. The first study was designed to determine the potential for inorganic N movement to shallow ground water, and the potential for mitigation in riparian buffers. The second study involved a more detailed study of soil-landscape factors influencing N leaching in cropland and transformations in buffers. The last study was undertaken to examine relationships among soil P availability, soil-landscape factors, and concentrations and forms of P in shallow ground water.; Soil-landscape factors critically influenced ground water N concentrations and fluxes in cropland and riparian buffers. Results showed that N inputs, water table depth, and ground water dissolved oxygen concentration could be used to estimate shallow ground water nitrate concentrations in cropland, and as an index of attenuation in buffers. Poorly and very poorly drained riparian soils tended to have lower ground water nitrate concentrations and fluxes, higher ammonium, and low dissolved oxygen. Soil P availability in cropland soils was strongly correlated with soil solution P concentrations, but not correlated with ground water P. Riparian soils maintained significantly lower soil solution dissolved P concentrations relative to cropland. Two-year mean buffer ground water dissolved reactive P concentrations were lower than cropland concentrations. Average total dissolved and particulate ground water P were highest in riparian forest buffers. Results showed that poorly drained riparian soils were consistently effective at reducing ground water nitrate concentrations and fluxes. However, thermodynamic conditions favoring nitrate removal in buffers were associated with greater total P in ground water. Results suggest that the integration of nutrient source factors with site-specific soil and landscape variability was a useful approach for indexing nutrient loss potentials from cropland and attenuation in buffers.