Evaluation of Three in Site Soil Nitrogen Availability Assays

1989

S. 185-191

2 Abb., 4 Tab., 24 Lit. Ang.

Unselbständiges Werk

200038937

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, 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.kg soil-1), but the core-IER estimatewas almost twice that of the buried-bag estimate in the old-growth stand (31.7 and 16.8 mg.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 availability 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 for extended periods of time.