LANDSCAPE POSITION, SUBSTRATE QUALITY, AND NITRATE DEPOSITION EFFECTSON FOREST SOIL-NITROGEN DYNAMICS IN THE HOOSIER-NATIONAL-FOREST

The purpose of this study was to examine possible interactions between landscape position, soil parent material, and the level of acidic dep osition on soil chemical properties and organic N turnover. We sampled A horizon soils from upper/south and lower/north slopes in two forest sites in the Hoosier National Forest, Indiana which differed in bedro ck (Deam Wilderness (DW), acid sandstone; Harden Ridge (HR), sandstone interbedded with limestone). The soils from the upper/south position at DW had the lowest pH, Ca2+, and molar Ca:AI ratio and the highest o rganic matter and soluble Al3+. The soils from the two lower/north slo pes tended to have the highest pH, Ca2+, and Ca:Al ratio, with the upp er/south soils from HR intermediate. NH4-N pool sizes did not differ b etween bedrock types or slope positions. NO3--N pools were significant ly larger in soils from lower north positions than in those from upper south positions. There was no significant effect of either slope posi tion or bedrock geology on N-mineralization in 30 day aerobic incubati ons done in the laboratory. The proportion of NH4+-N subsequently nitr ified did differ both between sites (DW > HR) and positions (upper/sou th > lower/north). To quantify the effect of heavy NO3- deposition in precipitation on N turnover, we also incubated paired samples with the equivalent of either 5.6 or 30.0 kg ha(-1) year(-1) of NO3- in the 'p recipitation' used to keep them near field capacity. Heavy NO3- additi on had no significant effect on N mineralization; however it did resul t in mean reductions of 49-96% in proportional nitrification and NO; a ccumulation. In these sites, slope position and NO3- addition exert gr eater effects on soil chemical properties and organic N turnover than does bedrock type.