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

Abstract

Annual net N mineralization in the 0–10 cm mineral soil zone of nine forest stands on silt–loam soils was measured using a series of insitu soil incubations from April 1980 through April 1981. Differences in soil organic matter (SOM) dynamics among sites were shown with net N mineralization ranging from 0.54 to 2.10 mg N mineralized•g SOM−1•year−1. This variation was not related to percent N in SOM. Net N mineralization varied seasonally with maximum rates in June and very low rates in winter. Nitrification rates were constant from May through September despite fluctuations in soil ammonium pools. Nitrification was greater than 50% of annual net N mineralization 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 from 40.3 to 119.2 kg N•ha−1•year−1. Annual leaf and needle litter production ranged from 2.12 to 4.17 Mg•ha−1•year−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.