Nitrogen cycling and nitrogen availability in three oak ecosystems in southeastern Michigan

Nitrogen cycling, net N mineralization and nitrification, and N availability were studied in three oak forest ecosystems located on a moisture-fertility gradient in southeastern Michigan. The three oak ecosystems were distinguished in the field by distinct combinations of physiography, soil properties, and dominant vegetation; these ecosystems were classified as dry, dry mesic and mesic. The dry ecosystem with soils low in moisture holding capacity and soil organic matter was dominated by black oak (Q. velutina L.). The mesic ecosystem with soils high in moisture holding capacity and soil organic matter was dominated by red oak (Q. rubra L.) The dry-mesic ecosystem with soils intermediate in moisture and organic matter was dominated by white oak (Q. alba L.). Basal area, biomass, and age in the dry ecosystem were approximately half of the basal area, biomass, and age of the dry-mesic and mesic ecosystems.; Despite differences in water holding capacity, other soil properties, and species composition, N contents of ecosystem components and N fluxes were similar in dry-mesic and mesic ecosystems, and larger in these two ecosystems than in the dry ecosystem.; Net N mineralization and nitrification determined by an in situ incubation indicated that although total mineralized N was not significantly different between the three ecosystems, the dry ecosystem, with lower soil pH levels, had significantly smaller nitrification than dry-mesic and mesic ecosystems. Net N mineralization was further determined by a laboratory anaerobic incubation and both methods were compared.; Nitrogen availability was assessed with soil anaerobic incubations, foliar N analysis, and growth response analysis. Soil anaerobic incubations indicated high, intermediate, and relatively low soil N supply for the mesic, dry-mesic, and dry ecosystems, respectively. Foliar N analysis of the major tree species indicated relatively high N availability and similar N availability in the three ecosystems. Growth response analysis after three growing seasons following fertilization, indicated that N was not a limiting factor in any ecosystem. Periodic, low rate additions of N fertilizer also failed to induce any significant stand responses.; This study indicates that the dry-mesic and mesic ecosystems initially distinguished according to ecological multifactor methods of land classification could not be so distinguished in terms of N cycle and N availability. It also suggests strongly that the three ecosystems were not N limited, and that N availability in permanent ecosystems must be conceived as a balance between net mineralization potential and stand demand.