Nitrogen Mineralization Potentials of Shrub‐Steppe Soils with Different Disturbance Histories

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Abstract
Disturbance of shrub‐steppe soils and changes in the dominant plant cover may alter soil N‐cycling processes. The mineralization of organic N to plant‐available forms is an important component of the N cycle in shrub‐steppe soils. Therefore, the soil N‐mineralization potential (No) was determined for two arid ecosystems, an undisturbed perennial shrub‐steppe and an annual grassland, which was initially shrub‐steppe and has been an annual grassland since farming disturbance in the 1940s. The soils at both study sites are similar and are classified as coarse‐silty, mixed, mesic Xerollic Camborthids. Soils were sampled at 0‐ to 5‐, 5‐ to 10‐, and 10‐ to 15‐cm depths, before plant growth in the spring, underneath big sagebrush (Artemisia tridentata Nutt.) bluebunch wheatgrass [Elytrigia spicata (Pursh) D.R. Dewey], Sandberg bluegrass (Poa secunda J.S. Presl), and soil crust at the perennial site and underneath downy brome (Bromus tectorum L.) at the annual grassland site. The 0‐ to 5‐cm soil depth from underneath all plants had the highest No. The No at the 0‐ to 5‐cm soil depth was influenced by plant type and decreased in the order P. secunda, B. tectorum, A. tridentata, E. spicata, and soil crust. The No decreased with soil depth until the 10‐ to 15‐cm soil depth, when all soils had the same value. The production of plant‐available N will, therefore, be largely limited to the soil surface, and losses from the first 5 cm of soil will be detrimental to both ecosystems. Spatial distribution of these plant species in the arid ecosystem result in “islands” where enhanced N mineralization occurs. When plant cover was used to compute a site‐specific No for the perennial shrub‐steppe and the annual grassland, a similar value was obtained indicating similar dynamics of soil N processes at these two sites.

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