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Title: HETEROGENEITY IN SOIL C AND N IN LATE- AND MID-SERAL SAGEBRUSH-GRASS RANGE LAND

Author
item Reeder, S
item Grant, Douglas
item JEZILE, G - COLORADO STATE UNIVERSITY
item CHILD, R - COLORADO STATE UNIVERSITY

Submitted to: Seed and Soil Dynamics in Shrubland Ecosystems Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/4/2003
Publication Date: 2/1/2004
Citation: Reeder, S.J., Grant, D.W., Jezile, G.G., Child, R.D. 2004. Heterogeneity in soil c and n in late- and mid-seral sagebrush-grass range land. p. 69-76. In: Proceedings of the Seed and Soil Dynamics in Shrubland Ecosystems.

Interpretive Summary: Nutrient cycling between soils and plants is necessary for sustaining healthy rangelands. Because changes in nutrient cycling can occur before changes in plant community structure and health are observed, indicators to measure the integrity of nutrient cycling might serve as an early warning of management-induced changes in rangeland health. These indicators would be valuable in developing rangeland management practices that promote production while maintaining ecological health. We are testing the amount of nitrate-nitrogen in the soil as an indicator of range health in sagebrush-grass plant communities. Whereas most of the nitrogen (N) in soil is bound in organic matter in forms that plants can't use directly, nitrate is a form of N that plants can take up directly. Nitrate is produced in the soil by microbial decomposition of organic matter. In a healthy sagebrush-grass plant community, N cycling between plants and the soil is tightly coupled. That is, the amount of N released by soil microorganisms is about equal to the amount of N needed by the plant community for growth, so excess nitrate does not build up and levels remain low. In an unhealthy sagebrush-grass plant community, N cycling between the plants and soil is "leaky". The plant community is sparse between the shrubs, and soil microorganisms release more N than the plants need for growth, so nitrate builds up in the soil. We found more nitrate in the soil of sagebrush-grass sites that were in poor-to-fair condition compared to sites in good condition, indicating that the amount of nitrate in the soil can serve as an indicator of range health or condition. Current studies are investigating whether the amount of nitrate in the soil can serve as an early indicator of change in range health as the result of changes in grazing management.

Technical Abstract: Development of criteria and indicators that can be used to evaluate nutrient cycling among plants, animals and the soil is essential for evaluating rangeland health and sustainability, as well as for developing proper rangeland management practices. The distribution of nutrients in space and time has been identified as a useful criterion for evaluating the integrity of rangeland nutrient cycles and energy flow in rangelands. Spatial distributions of soil organic carbon (C), and organic and mineral nitrogen (N) were measured in both mid- and late-seral stages of a sagebrush-grass association. Vegetation cover differed between the two seral stages, with grass cover significantly higher (15% vs 3%), and bare ground area significantly lower (56% vs 69%) for the late-seral compared to mid-seral plant communities. The spatial distributions of soil organic C and N differed between the two seral stages and corresponded to differences in vegetation cover. The proportion of total soil N in the nitrate form was significantly higher in mid-seral soil than in late-seral soil, suggesting less efficient capture of available N by the fragmented mid-seral plant community and an uncoupling of the N cycle. Soil nitrate-N shows promise as a sensitive indicator of seral stage in sagebrush-grass plant communities.