Seasonal patterns of soil nitrogen availability in moist acidic tundra

Abstract

Our ability to predict effects of changing soil nitrogen (N) in Arctic tundra has been limited by our poor understanding of the intra-annual variability of soil N in this strongly seasonal ecosystem. Studies have shown that microbial biomass declines in spring accompanied by peaks in inorganic nutrients. However, subsequent to this early pulse, there are few high temporal resolution measurements during the growing season. We hypothesized that (1) low N would be maintained throughout the growing season, (2) peaks of total free primary amines (TFPA), ammonium (NH4+), and nitrate (NO3−) would follow a sequential pattern driven by mineralization and nitrification, and (3) a peak in soil N would occur as plants senesce. We conducted weekly measurements of TFPA, NH4+, and NO3− in two tundra sites, from soil thaw in spring to freeze in fall. At each site, NH4+ peaks were followed by smaller peaks in NO3−, supporting the hypothesis that excess NH4+ would be nitrified. Furthermore, peaks in NH4+ were observed both shortly after leaf expansion and at plant senescence. The variation in timing between sites and the peaks in NH4+ subsequent to thaw indicates that nutrient limitation in these ecosystems is more dynamic and spatially variable than previously thought.