EARLY SPRING NITROGEN DYNAMICS IN A TEMPERATE FOREST LANDSCAPE

Early spring nitrogen cycling in temperate forests is dynamic and impo rtant to site fertility and retention of N in these ecosystems. In thi s study, we combined short-term (2 d) N-15-based measurements of plant uptake, microbial nitrification, denitrification, and immobilization with more conventional measurements of these processes over an 8-wk pe riod from early March to early May within a temperate forest landscape unit in Michigan, USA. Measurements were made in two landscape positi ons, a summit position with a well-drained soil and a toe-slope positi on with a poorly drained soil. While soil mineral N levels showed litt le spatial and temporal variation over the 8-wk period, nitrification rates, microbial biomass (chloroform-labile) N, and denitrification we re highly variable. The poorly drained soil consistently had high leve ls of nitrification and denitrification relative to the well-drained s oil. Pools of microbial biomass N increased by a factor of 10 over the 8-wk period in both soils, but were relatively stable during April, w hen the N-15 experiment was conducted. Microbial biomass appeared to b e the key regulator of the fate of added N-15. In the well-drained soi l, the largest movement of N was into microbial biomass and total soil N. As a result of this strong immobilization, there was relatively li ttle nitrification and denitrification of N-15 in the well-drained soi l. In the poorly drained soil, there was no apparent movement of N-15 into microbial biomass. As a consequence, availability of (NH4+)-N-15 to nitrifiers was high, and rates of nitrification were very high. Acc umulation of NO3- in the poorly drained soil, along with high soil moi sture, fostered high denitrification N losses. The results suggest tha t N retention processes and the fate of either atmospherically or agri culturally derived N inputs will likely show strong temporal and spati al variation during the early spring period in temperate forest landsc apes.