An empirical model of amino acid transformations in an alpine soil

Amino acids are potentially important nitrogen (N) sources for plants in ma ny ecosystems. However, a quantitative understanding of organic N availabil ity is lacking for most ecosystems. This study estimates seasonal amino aci d fluxes in an alpine tundra soil using three independent data sets. In pre vious work in an alpine dry meadow ecosystem in the Front Range of the Colo rado Rocky Mountains, we measured significant rates of amino acid productio n from soil peptides during the plant growing season. This suggested that p roteolysis of native soil peptides could serve as a measure of amino acid a vailability to plants. Here we use a simple seasonal model to test the hypo thesis that soil amino acid concentrations can be predicted from rates of s oil proteolysis and amino acid degradation by microbes. We present seasonal data for turnover rates of the amino acid, glutamate, in soil, and experim ents on the effects of temperature and moisture on amino acid fluxes in soi l. We relate these studies to previously published values of proteolysis an d soil amino acid concentrations in this alpine soil. Our model shows that independent measurements of proteolytic rates, microbial amino acid uptake, and soil amino acid concentrations are consistent with each other, and tha t most changes in the soil amino acid pool can be explained by protease and microbial uptake rates, after adjusting for environmental conditions. Imme diately after snow melts in the spring and again in late summer after the o nset of monsoonal rains, measured soil amino acid concentrations were sligh tly lower than model predictions from measured protease and microbial uptak e rates. This could indicate that, at certain times, an additional sink for amino acids exists, such as rapid plant uptake or loss from the ecosystem due to leaching. We estimate amino acid production during the snow-free sea son at our study site to be 103 g amino acid-N m(-2) growing season(-1). Th is value is not only consistent with the three data sets used in this study . but also with previous studies of N transformations in the Colorado alpin e, This Aux could provide Kobresia myosuroides, the dominant plant in this ecosystem, with 50-100% of its annual N requirement, based on previous plan t-microbe competition experiments. (C) 2001 Elsevier Science Ltd. All right s reserved.