Carbon and nitrogen effects on nitrification in Shaver Hollow Watershed, Shenandoah National Park, Virginia

Laboratory and field experiments were performed to (1) establish the capacity of soils in an upland forested watershed to nitrify and (2) show whether changes in carbon (C) and nitrogen (N) availability affect nitrate dynamics in watershed soils, and (3) demonstrate whether short-term changes in C and N availability alter fundamental characteristics of soil nitrification capacity in the field. Soils from a high-elevation red oak site, a tulip poplar cove site, and a saturated seep area, all in Shaver Hollow Watershed (SHW), an upland forested Blue Ridge watershed, were amended with combinations of C (fresh leaves or glucose {dollar}rm(Csb6Hsb{lcub}12{rcub}Osb6)){dollar} and N (uric acid, {dollar}rm Csb5Hsb4Nsb4Osb3,{dollar} or ammonium sulfate, {dollar}rm(NHsb4)sb2SOsb4).{dollar} Nitrification potential in these samples was determined using 21d laboratory incubations. A hillslope-scale field experiment was also conducted over one growing season; experimental plots were amended with 1000 kg C ha{dollar}sp{lcub}-1{rcub}{dollar} (sawdust + glucose) and 50 kg N ha{dollar}sp{lcub}-1{rcub}{dollar} {dollar}rm((NHsb4)sb2SOsb4){dollar} and nitrification variables were monitored for response to treatment.; Nitrification potential was widespread and variable (0.5-2.5 ug Ng{dollar}sp{lcub}-1{rcub}{dollar} d{dollar}sp{lcub}-1{rcub},{dollar} based on in situ assays of net nitrification). Carbon and N amendments significantly affected net nitrate production but the effect depended on the chemical form of N and on soil properties. N inputs generally promoted, while high C:N inputs significantly reduced, NO{dollar}sb{lcub}3sp-{rcub}{dollar} accumulation in the incubations. In the field experiment, N treatments significantly increased soil nitrification capacity by {dollar}sim{dollar}4 ug g{dollar}sp{lcub}-1{rcub}{dollar} d{dollar}sp{lcub}-1{rcub}{dollar} and soil nitrate content by {dollar}sim{dollar}4 ug g{dollar}sp{lcub}-1{rcub}{dollar} for several weeks following treatment. Carbon inputs did not fundamentally affect nitrification capacity, although the experimental design offered limited statistical power for detecting treatment effects. Autotrophic bacteria were primarily responsible for soil NO{dollar}sb{lcub}3sp-{rcub}{dollar} production at the field experiment site. Nitrification is potentially a major watershed N-cycle flux (16-216 kg N ha{dollar}sp{lcub}-1{rcub}{dollar} yr{dollar}sp{lcub}-1{rcub}).{dollar} Analysis of soil characteristics measured during the field experiment suggested that coarse-textured, low pH soils exhibit lower nitrification capacities and are less sensitive to C and N inputs. This and other research also suggest that soil nitrification responses to changes in C and N availability may be magnified in regions experiencing chronically elevated atmospheric N deposition.