Effects of agricultural runoff dispersion on nitrate reduction in forestedfilter zone soils

Forested filter zones (FFZ) are being used more frequently for remediation of agricultural non-point source pollution. The objective of this study was to determine the effects of short-term dispersal (1-2 yr) of agricultural runoff on the denitrification potential of the soil microbial population an d denitrification rates, to a depth of 1 m, in forest soils in two small wa tersheds (W1 and W2) in the Piedmont of North Carolina. Each watershed cons isted of a field and a FFZ, Denitrification potential was measured in a ser ies of soil slurry incubations of soils from inside the FFZ that received a gricultural runoff and from soils immediately adjacent to the FFZ that rece ived no runoff (control), Soils were amended with both glucose and nitrate (G + NO3) to ensure adequate supply of substrate and energy source, Denitri fication rates were measured at ambient C conditions in a similar incubatio n with only NO3-N amendment (NO3). We measured NO3-N disappearance in both incubations and reported loss as a percentage of initial concentrations. Fo r the FFZ soils, >80% of the added NO3-N was lost in the G + NO3 incubation from soils from the upper 50 cm in W1 and from the upper 30 cm in W2. In c ontrol soils, high levels of NO3-N loss were observed in only the upper 20 cm of the profile in W1, and in W2 surface soils had significantly lower de nitrification potential than FFZ soils at all depths, Denitrification poten tial was greatly enhanced (P = 0.05) throughout the entire first 100 cm in the first FFZ and in the surface 40 cm in the second FFZ, Denitrification r ates under ambient C conditions were higher (>40%) in the surface 20 cm of the profile of the FFZ in W1, compared with the unexposed control (similar to 20%), but no enhancement was observed on W2, Exposure of soil to agricul tural runoff had a significant impact on the soil microbial community. Deni trification potential in subsoil was Limited by the absence of denitrifiers in unexposed soils, but subsoils exposed to agricultural runoff had a sign ificant denitrifier population. The fact that higher denitrification potent ial did not translate to higher denitrification rates in these incubations indicates that C availability limited the denitrification process at all de pths in these Piedmont forest soils.