Evaluation of three insitu soil nitrogen availability assays

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Abstract
Three indices of nitrogen (N) availability were compared in the field over a 1-year period in an old-growth and a young-growth mixed-conifer forest. The indices utilized were ion exchange resin (IER) bags, buried bags, and a core-IER method employing intact soil cores enclosed in tubes capped at both ends by IER bags. The results from all three methods indicated that in the surface mineral soil, N availability was higher in the young-growth stand than in the old-growth stand. However, seasonal patterns of N availability were generally not well correlated among the methods (correlation coefficients ranged from 0.32 to 0.62). For a given amount of net N mineralized in buried bags, more N accumulated on IER bags placed in the young-growth stand than in those placed in the old-growth stand. This was the result of greater net nitrification in the young-growth stand coupled with the greater mobility of NO3- relative to NH4+ in soil. Ten-month estimates of net N mineralization measured by the core-IER and buried-bag methods were similar in the young-growth stand (about 42 mg .cntdot. kg soil-1), but the core-IER estimate was almost twice that of the buried-bag estimate in the old-growth stand (31.7 and 16.8 mg .cntdot. kg soil-1, respectively). The different sensitivities of the core-IER and buried-bag methods to changes in soil moisture and leaching probably account for much of the difference in their N availabiity estimates. Results from the core-IER method did reflect the effects of leaching; however, soil water content within the core did not follow changes in soil water content effectively. Because of the greater labor involved in using the core-IER method, its use may be most efficacious in high-precipitation environments, or when in-field soil incubations must be conducted by extended periods of time.