Nitrogen cycling at treeline: Latitudinal and elevational patterns across a boreal landscape

Abstract

:We studied spatial and temporal patterns of nitrogen pools and fluxes in soils at treeline and forested sites within three mountain ranges across a 785-km transect in Alaska during 2001–2002. We measured pools of soil mineral (ammonium and nitrate) and organic (amino acid and microbial biomass) nitrogen, in situ rates of net mineralization, net nitrification, net amino acid production, and decomposition, as well as soil carbon turnover in a laboratory incubation experiment. Soils at treeline were mostly colder than forested soils, particularly during fall and over winter, and had reduced rates of nitrogen cycling and litter decomposition relative to forested stands. Treeline soils also had lower rates of potential respiration per unit carbon, suggesting reduced soil organic matter quality relative to forest soils. Therefore, effects of both colder temperatures and poorer substrate quality appeared to suppress rates of nitrogen turnover at treeline. Seasonal patterns of nitrogen turnover were similar across latitudes (i.e., mountain ranges). On average, 70% of total annual net nitrogen mineralization occurred from August through May, suggesting that fall and winter are critical periods for soil nitrogen transformations in both forested and treeline ecosystems. Among mountain ranges, pool sizes and fluxes of nitrogen were similar despite significant variation in growing season length and mean annual temperatures. Soil moisture and soil organic matter quality may have stronger effects on variation in nitrogen cycling than temperature at our sites.