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) ¹ ⁵N—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 temperature 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 ¹ ⁵N experiment was conducted. Microbial biomass appeared to be the key regulator of the fate of added ¹ ⁵N. 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 ¹ ⁵N in the well—drained soil. In the poorly drained soil, there was no apparent movement of ¹ ⁵N into microbial biomass. As a consequence, availability of ¹ ⁵NH₄ ⁺ to nitrifiers was high, and rates of nitrification were very high. Accumulation of NO₃ — in the poorly drained soil, along with high soil moisture, forested 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.