Processes controlling nitrogen release and turnover in Arctic tundra

Abstract

This thesis provides data on nitrogen cycling among communities representative of the major vegetation types in arctic Alaska. Through field studies, I examined the pattern of nitrogen dynamics in four tundra ecosystems (dry lichen heath, wet meadow, tussock tundra, and deciduous shrub tundra) of contrasting structure and productivity near Toolik Lake, Alaska. In addition, through field and laboratory experiments, I sought to identify the major controls over nitrogen release and turnover in these nitrogen-limited systems. These ecosystems, representing extremes of productivity in arctic Alaska, show order-of-magnitude differences in biomass and net primary productivity, and likewise, exhibit order-of-magnitude differences in net nitrogen mineralization and nitrogen turnover. Decomposition, soil respiration, net nitrogen mineralization, and the turnover of soil inorganic nitrogen were all highly correlated with net primary production. These results show that nutrient availability, in particular nitrogen availability, is a major control over tundra ecosystem function. Soil pools of organic nitrogen are large, whereas the pools of inorganic nitrogen are small, and the net rate of nitrogen mineralization in situ is low. Thus, nitrogen mineralization represents a major control point in the nitrogen cycle. Net nitrogen mineralization is relatively insensitive to changes in soil temperature, but highly responsive to changes in available soil carbon and nitrogen. Thus, the effect of organic matter quality on microbial activity is a more important control of nitrogen release than is the direct effect of temperature. Free amino acids constitute a larger proportion of extractable soil nitrogen than do ammonium and nitrate. Tundra species have the capacity to absorb some amino acids directly at rates comparable to ammonium absorption. These experimental results contrast with the widely held assumption that mineral nitrogen is the only form of nitrogen available to plants. I conclude that we must examine the behavior of both inorganic and organic soil nitrogen in order to adequately understand nitrogen cycling in tundra soils and the functioning of arctic ecosystems.