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DIVISION S-7-FOREST & RANGE SOILS

Nitrogen Availability through a Coarse-Textured Soil Profile in the Shortgrass Steppe

^a Dep. of Rangeland Ecosystem Sci., Colorado State Univ., Fort Collins, CO 80523 USA
^b New Zealand Pastoral Agriculture Research Inst. Ltd (AgResearch), Ruakura Research Centre, Private Bag 3123, Hamilton 2001, New Zealand
^c Dept. of Forest Sci., Colorado State Univ., Fort Collins, CO 80523 USA
^d Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523 USA

doddm{at}agresearch.cri.nz

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 grasses. Our objective was to determine whether the soil N resources were more evenly distributed with depth than is typical for this environment, thereby helping to explain the coexistence of the two plant life forms. We measured total soil C and N, potential net N mineralization, and in situ available 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 greatest in the surface soil layers (0–10 cm) and decreased substantially with depth. A supplemental watering treatment was imposed on the soil during the spring of 1996 to examine N-leaching potential. The water addition increased soil water content and available NO^-₃ to a depth of 60 cm, indicating that NO^-₃ 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 processes. The results indicated that NO^-₃ leaching could better account for the observed patterns in available N. This process, by contributing to a more even distribution of resources through a coarse-textured soil profile, could be important for improving the competitive status of shrubs in an otherwise grass-dominated ecosystem.

Abbreviations: CPER, Central Plains Experimental Range • GLM, general linear model • PMA, phenyl mercuric acid • TDR, time domain reflectometry • WFPS, water-filled pore space

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