An empirical model of amino acid transformations in an alpine soil

Amino acids are potentially important nitrogen (N) sources for plants in many ecosystems. However, a quantitative understanding of organic N availability is lacking for most ecosystems. This study estimates seasonal amino acid fluxes in an alpine tundra soil using three independent data sets. In previous work in an alpine dry meadow ecosystem in the Front Range of the Colorado Rocky Mountains, we measured significant rates of amino acid production from soil peptides during the plant growing season. This suggested that proteolysis of native soil peptides could serve as a measure of amino acid availability to plants. Here we use a simple seasonal model to test the hypothesis that soil amino acid concentrations can be predicted from rates of soil proteolysis and amino acid degradation by microbes. We present seasonal data for turnover rates of the amino acid, glutamate, in soil, and experiments on the effects of temperature and moisture on amino acid fluxes in soil. We relate these studies to previously published values of proteolysis and 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 that most changes in the soil amino acid pool can be explained by protease and microbial uptake rates, after adjusting for environmental conditions. Immediately after snow melts in the spring and again in late summer after the onset of monsoonal rains, measured soil amino acid concentrations were slightly lower than model predictions from measured protease and microbial uptake 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 season at our study site to be 103 g amino acid-N m-2 growing-season-1. This 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 alpine. This flux could provide Kobresia myosuroides, the dominant plant in this ecosystem, with 50-100% of its annual N requirement, based on previous plant-microbe competition experiments.