EARLY SPRING NITROGEN DYNAMICS IN A TEMPERATE FOREST LANDSCAPE

Early spring nitrogen cycling in temperate forests is dynamic and important to site fertility and retention of N in these ecosystems. In this 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 period from early March to early May within a temperate forest landscape unit in Michigan, USA. Measurements were made in two landscape positions, a summit position with a well-drained soil and a toe-slope position with a poorly drained soil. While soil mineral N levels showed little spatial and temporal variation over the 8-wk period, nitrification rates, microbial biomass (chloroform-labile) N, and denitrification were highly variable. The poorly drained soil consistently had high levels of nitrification and denitrification relative to the well-drained soil. 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, when the N-15 experiment was conducted. Microbial biomass appeared to be the key regulator of the fate of added N-15. In the well-drained soil, the largest movement of N was into microbial biomass and total soil N. As a result of this strong immobilization, there was relatively little nitrification and denitrification of N-15 in the well-drained soil. 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. Accumulation of NO3- in the poorly drained soil, along with high soil moisture, fostered high denitrification N losses. The results suggest that N retention processes and the fate of either atmospherically or agriculturally derived N inputs will likely show strong temporal and spatial variation during the early spring period in temperate forest landscapes.