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 ¹⁵N isotope dilution, and net rates N mineralization and nitrification were calculated from changes in KCl—extractable inorganic N and NO₃^—_@ON pool sizes, respectively. Changes in the availability of C were assessed by monitoring rates of CO₂ evolution and the sizes of extractable organic C and microbial biomass pools. Net and gross rates of N mineralization (r² = 0.038, P = .676) and nitrification (r² = 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 NO₃^— pool size (net nitrification) only occurred after 140 d, when the NO₃^— pool size increased suddenly and massively. However, gross nitrification rates were substantial throughout the entire incubation and were poorly correlated with these changes in NO₃^— pool sizes. Concurrent decreases in the microbial biomass suggest that large increases in NO₃^— pool sizes after prolonged incubation of coniferous forest soil may arise from reductions in the rate of microbial immobilization of NO₃^—, rather than from one of the mechanisms proposed previously (e.g., sequestering of NH₄⁺ by microbial heterotrophs, the deactivation of allelopathic compounds, or large increases in autotrophic nitrifier populations). Strong correlations were found between rates of CO₂ evolution and gross N mineralization (r² = 0.974, P < .0001) and immobilization (r² = 0.980, P < .0001), but not between CO₂ evolution and net N mineralization rates. Microbial growth efficiency, determined by combining estimates of gross N immobilization, CO₂ 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.