Landscape position, substrate quality, and nitrate deposition effects on forest soil nitrogen dynamics in the hoosier national forest

Abstract

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, Ca 2+, and molar Ca:Al ratio and the highest organic matter and soluble A1 3+. The soils from the two lower/north slopes tended to have the highest pH, Ca 2+, and Ca:Al ratio, with the upper/south soils from HR intermediate. NH 4 +-N pool sizes did not differ between bedrock types or slope positions. NO 3 −-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 NH 4 +-N subsequently nitrified did differ both between sites (DW > HR) and positions (upper/south>lower/north). To quantify the effect of heavy NO 3 − 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 NO 3 − in the ‘precipitation’ used to keep them near field capacity. Heavy NO 3 − addition had no significant effect on N mineralization; however it did result in mean reductions of 49–96% in proportional nitrification and NO 3 − accumulation. In these sites, slope position and NO 3 addition exert greater effects on soil chemical properties and organic N turnover than does bedrock type.