LANDSCAPE POSITION, SUBSTRATE QUALITY, AND NITRATE DEPOSITION EFFECTS ON 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 deposition 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 bedrock (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 organic matter and soluble Al3+. The soils from the two lower/north slopes tended to have the highest pH, Ca2+, and Ca:Al ratio, with the upper/south soils from HR intermediate. NH4-N pool sizes did not differ between bedrock types or slope positions. NO3--N pools were significantly larger in soils from lower north positions than in those from upper south positions. There was no significant effect of either slope position or bedrock geology on N-mineralization in 30 day aerobic incubations done in the laboratory. The proportion of NH4+-N subsequently nitrified did differ both between sites (DW > HR) and positions (upper/south > 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 'precipitation' used to keep them near field capacity. Heavy NO3- addition had no significant effect on N mineralization; however it did result in mean reductions of 49-96% in proportional nitrification and NO; accumulation. In these sites, slope position and NO3- addition exert greater effects on soil chemical properties and organic N turnover than does bedrock type.