Dynamics of Gross Nitrogen Transformations in an Old‐Growth Forest: The Carbon Connection

Abstract

We conducted a 456—d laboratory incubation of an old—growth coniferous forest soil to aid in the elucidation of C controls on N cycling processes in forest soils. Gross rates of N mineralization, immobilization, and nitrification were measured by 15 N isotope dilution, and net rates N mineralization and nitrification were calculated from changes in KCl—extractable inorganic N and NO 3—@ON pool sizes, respectively. Changes in the availability of C were assessed by monitoring rates of CO 2 evolution and the sizes of extractable organic C and microbial biomass pools. Net and gross rates of N mineralization (r 2 = 0.038, P = .676) and nitrification (r 2 = 0.403, P = .125) were not significantly correlated over the course of the incubation, suggesting that the factors controlling N consumptive and productive processes do not equally affect these processes. A significant increase in the NO 3— pool size (net nitrification) only occurred after 140 d, when the NO 3— pool size increased suddenly and massively. However, gross nitrification rates were substantial throughout the entire incubation and were poorly correlated with these changes in NO 3— pool sizes. Concurrent decreases in the microbial biomass suggest that large increases in NO 3— pool sizes after prolonged incubation of coniferous forest soil may arise from reductions in the rate of microbial immobilization of NO 3—, rather than from one of the mechanisms proposed previously (e.g., sequestering of NH 4+ by microbial heterotrophs, the deactivation of allelopathic compounds, or large increases in autotrophic nitrifier populations). Strong correlations were found between rates of CO 2 evolution and gross N mineralization (r 2 = 0.974, P < .0001) and immobilization (r 2 = 0.980, P < .0001), but not between CO 2 evolution and net N mineralization rates. Microbial growth efficiency, determined by combining estimates of gross N immobilization, CO 2 evolution, and microbial biomass C and N pool sizes, declined exponentially over the incubation. These results suggest the utilization of lower quality substrates as C availability declined during incubation. Results from this research indicate the measurement of gross rates of N transformations in soil provides a powerful tool for assessing C and N cycling relationships in forests.