Nitrogen Cycling

In this chapter, we discuss the current understanding of internal N cycling, or the flow of N through plant and soil components, in the Niwot Ridge alpine ecosystem. We consider the internal N cycle largely as the opposing processes of uptake and incorporation of N into organic form and mineralization of N from organic to inorganic form. We will outline the major organic pools in which N is stored and discuss the transfers of N into and from those pools. With a synthesis of information regarding the various N pools and relative turnover of N through them, we hope to provide greater understanding of the relative function of different components of the alpine N cycle. Because of the short growing season, cold temperatures, and water regimes tending either toward very dry or very wet extremes, the alpine tundra is not a favorable ecosystem for either production or decomposition. Water availability, temperature, and nutrient availability (N in particular) all can limit alpine plant growth (chapter 9). Cold soils also inhibit decomposition so that N remains bound in organic matter and is unavailable for plant uptake (chapter 11). Consequently, N cycling in the alpine often is presumed to be slow and conservative (Rehder 1976a, 1976b; Holzmann and Haselwandter 1988). Nonetheless, studies reveal large spatial variation in primary production and N cycling in alpine tundra across gradients of snowpack accumulation, growing season water availability, and plant species composition (May and Webber, 1982, Walker et al., 1994, Bowman, 1994, Fisk et al. 1998; chapter 9). Furthermore, evidence for relatively large N transformations under seasonal snowcover (Brooks et al., 1995a, 1998) and maintenance of high microbial biomass in frozen soils (Lipson et al. 1999a) provide a complex temporal component of N cycling on Niwot Ridge. Our discussion of N cycling on Niwot Ridge will focus on two main points: first, the spatial variation in N turnover in relation to snowpack regimes and plant community distributions; and second, the temporal variability of N transformations during both snow-free and snow-covered time periods.