Gross vs net rates of N mineralization and nitrification as indicators of functional differences between forest types

Floristic species composition and differences in litter quality between species are the primary factors controlling N mineralization in forest ecosystems. Generalizations about species effects on N cycling are based on measurements of net rates of mineralization and nitrification. However, there have been few tests on the ability of these measurements to reflect the mechanistic complexity underlying the species effects. The objectives of this study are to: (1) determine whether differences in net mineralization and net nitrification rates between stands of different species composition are due to differences in gross rates of mineralization, nitrification, and microbial consumption; (2) determine whether field and laboratory assays of net mineralization and nitrification are useful indicators of internal N dynamics; and (3) test the hypothesis that microbial immobilization increases with rates of mineralization and nitrification. We measured net rates of mineralization and nitrification in the field and in the laboratory, and gross rates of mineralization, nitrification and microbial consumption in different stands at two sites in eastern New York State. The results indicated that vegetation type was not always a robust indicator of N cycling differences between ecosystems. At one site there was no difference in net mineralization (P<0.05) between oak and maple stands, and no nitrification in either forest type. We attributed this lack of conformity to expected patterns to either differences in soil moisture regimes resulting from landscape position, forest floor disturbance by earthworms, or influences of previous land-use. At the second site, both beech and maple stands showed significantly greater rates of net nitrification than oak stands (P<0.05) and beech had significantly greater (P<0.05) rates of net mineralization than both maple and oak. Gross rates of mineralization, nitrification and microbial consumption were very high and often exceeded net rates by an order of magnitude. Gross rates were not good indicators of differences between forest types and in most cases we did not find differences in gross rates between stands where we found differences in net rates. We found a strong relationship between microbial consumption of NH4+ or NO3− and gross rates of mineralization or nitrification (R2=0.83 and R2=0.52, respectively).