Links between microbial population dynamics and nitrogen availability in an alpine ecosystem

Past studies of plant-microbe interactions in the alpine nitrogen cycle hav e revealed a seasonal separation of N use, with plants absorbing N primaril y during the summer months and microbes immobilizing N primarily during the autumn months. On the basis of these studies, it has been concluded that c ompetition for N between plants and microbes is minimized along this season al gradient. In this study, we examined more deeply the links between micro bial population dynamics and plant N availability in an alpine dry meadow. We conducted a year-round field study and performed experiments on isolated soil microorganisms. Based on previous work in this ecosystem, we hypothes ized that microbial biomass would decline before the plant growing season a nd would release N that would become available to plants. Microbial biomass was highest when soils were cold, in autumn, winter, and early spring. Dur ing this time, N was immobilized in microbial biomass. After snow melt in s pring, microbial biomass decreased. A peak in the soil protein concentratio n was seen at this time, followed by peaks in soil amino acid and ammonium concentrations in late June. Soil protease rates were initially high after snow melt, decreased to below detection limits by midsummer, and partially recovered by late summer. Proteolytic activity in soil was saturated early in the growing season and became protein limited later in the: summer. We c oncluded that the key event controlling N availability to alpine plants occ urs after snow melt, when protein is released from the: winter microbial bi omass. This protein pulse provides substrate for soil proteases, which supp ly plants with amino acids during the growing season. On average, microbial biomass was lower in the summer than at other times, although the biomass fluctuated widely during the summer. Within the summer months, maximum numb ers of amino-acid-degrading microorganisms and the maximum amount of microb ial biomass coincided with the peak in soil amino acids. when plants are mo st active. All bacterial strains isolated from this summer community had th e ability to grow rapidly on low concentrations of amino acids and to degra de protein. This explains the previously observed result that the soil micr obial biomass can compete strongly with plants for organic N, despite the s easonal offset of maximum plant and microbial N uptake.