Mineral and Dissolved Organic Nitrogen Dynamics along a Soil Acidity-Fertility Gradient

Mineral (NH+4 + NO-3) and dissolved organic nitrogen (DON) dynamics were investigated along a soil chronosequence in northern California that ranges in age from about 100 000 to 500 000 BP. Younger soils are slightly acidic and fertile supporting highly productive grasslands and mixed-conifer forests. Older soils are highly acidic and infertile supporting forests of dwarf (<3 m) conifers and Ericaceous species. This edaphic gradient provides an ideal opportunity to examine changes in N dynamics on soils that are progressively older and less fertile. We examined in situ net mineralization rates using closed-top tubes and we examined mineralization and nitrification rates using 15NH+4 and 15NO-3 pool dilution techniques. Net N mineralization rates per unit of organic C decreased as soil age increased. Net mineralization rates (per unit C) were more strongly related to differences in rates of immobilization than gross mineralization. However, gross mineralization results normalized to soil N levels (N activity basis) were similar across soil ages. A similar rate of N turnover across this edaphic gradient indicates that the size of the total N pool is an important factor regulating N mineralization. It further suggests that litter quality does not appreciably hinder N mineralization. Pool dilution of added 15NO-3 indicated that nitrification is active across all sites and that microbial assimilation consumed the majority of the NO-3 produced. Dissolved organic N makes a larger relative contribution to dissolved N in older soils indicating a shift in the dominant N cycling pathway from mineral to organic forms in older less fertile soils.