Nitrogen availability through a coarse-textured soil profile in the shortgrass steppe

We conducted a study of N resources in a loamy sand soil in the shortgrass steppe of northeastern Colorado, on a site dominated by both shrubs and gra sses. Our objective was to determine whether the soil N resources were more evenly distributed with depth than is typical for this environment, thereb y helping to explain the coexistence of the two plant life forms. We measur ed total soil C and N, potential net N mineralization, and in situ availabl e inorganic N (using ion exchange resin bags) through the soil profile to a depth of 150 cm, All three measures confirmed that available N was greates t in the surface soil layers (0-10 cm) and decreased substantially with dep th. A supplemental watering treatment was imposed on the soil during the sp ring of 1996 to examine N-leaching potential. The water addition increased soil water content and available NO3- to a depth of 60 cm, indicating that NO3- leaching might occur on this soil type under favorable conditions. To confirm this assertion, we used two simple models to examine the impact of increased soil moisture on in situ mineralization and solute transport proc esses. The results indicated that NO3- leaching could better account for th e observed patterns in available N, This process, by contributing to a more even distribution of resources through a coarse-textured soil profile, cou ld be important For improving the competitive status of shrubs in an otherw ise grass-dominated ecosystem.