Leaf-litter production and soil organic matter dynamics along a nitrogen-availability gradient in Southern Wisconsin (U.S.A.)

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Abstract
Annual net N mineralization in the 0-10 cm mineral soil zone of 9 forest stands [dominant trees: oak, maple, birch, pine and spruce] on silt-loam soils was measured using a series of in situ soil incubations April 1980-April 1981. Differences in soil organic matter (SOM) dynamics among sites were shown with net N mineralization ranging 0.54-2.10 mg N mineralized .cntdot. g SOM-1 .cntdot. yr-1. This variation was not related to percent N in SOM. Net N mineralization varied seasonally with maximum rates in June and low rates in winter. Nitrification rates were constant from May-Sept. despite fluctuations in soil ammonium pools. Nitrification was greater than 50% of annual net N minerlization at all sites. N uptake by vegetation, as estimated by net N mineralization plus mineral N inputs via precipitation, with minor corrections for mineralization below the incubation depth and for mineral N losses to groundwater, ranged 40.3-119.2 kg N.cntdot.ha-1 .cntdot. yr-1. Annual leaf and needle litter production ranged 2.12-4.17 Mg.cntdot.ha-1 .cntdot. yr-1 and was strongly correlated with N uptake (r = 0.938, P < 0.01). N returned in leaf litter was also correlated with N uptake (r = 0.755, P < 0.05). Important feedbacks may exist between N availability and litter quality and quantity.