Steven K. Schmidt

Winter forest soil respiration controlled by climate and microbial community composition.

Most terrestrial carbon sequestration at mid-latitudes in the Northern Hemisphere occurs in seasonal, montane forest ecosystems. Winter respiratory carbon dioxide losses from these ecosystems are high, and over half of the carbon assimilated by photosynthesis in the summer can be lost the following winter. The amount of winter carbon dioxide loss is potentially susceptible to changes in the depth of the snowpack; a shallower snowpack has less insulation potential, causing colder soil temperatures and potentially lower soil respiration rates. Recent climate analyses have shown widespread declines in the winter snowpack of mountain ecosystems in the western USA and Europe that are coupled to positive temperature anomalies. Here we study the effect of changes in snow cover on soil carbon cycling within the context of natural climate variation. We use a six-year record of net ecosystem carbon dioxide exchange in a subalpine forest to show that years with a reduced winter snowpack are accompanied by significantly lower rates of soil respiration. Furthermore, we show that the cause of the high sensitivity of soil respiration rate to changes in snow depth is a unique soil microbial community that exhibits exponential growth and high rates of substrate utilization at the cold temperatures that exist beneath the snow. Our observations suggest that a warmer climate may change soil carbon sequestration rates in forest ecosystems owing to changes in the depth of the insulating snow cover.

LINKS BETWEEN MICROBIAL POPULATION DYNAMICS AND NITROGEN AVAILABILITY IN AN ALPINE ECOSYSTEM

Past studies of plant-microbe interactions in the alpine nitrogen cycle have revealed a seasonal separation of N use, with plants absorbing N primarily during the summer months and microbes immobilizing N primarily during the autumn months. On the basis of these studies, it has been concluded that competition for N between plants and microbes is minimized along this seasonal gradient. In this study, we examined more deeply the links between microbial 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 hypothesized that microbial biomass would decline before the plant growing season and would release N that would become available to plants. Microbial biomass was highest when soils were cold, in autumn, winter, and early spring. During this time, N was immobilized in microbial biomass. After snow melt in spring, microbial biomass decreased. A peak in the soil protein concentration 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 concluded that the key event controlling N availability to alpine plants occurs after snow melt, when protein is released from the winter microbial biomass. This protein pulse provides substrate for soil proteases, which supply 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 numbers of amino-acid-degrading microorganisms and the max- imum amount of microbial biomass coincided with the peak in soil amino acids, when plants are most active. All bacterial strains isolated from this summer community had the ability to grow rapidly on low concentrations of amino acids and to degrade protein. This explains the previously observed result that the soil microbial biomass can compete strongly with plants for organic N, despite the seasonal offset of maximum plant and microbial N uptake.

Phosphorus Limitation of Microbial Processes in Moist Tropical Forests: Evidence from Short-term Laboratory Incubations and Field Studies

AbstractAlthough there is a widespread belief that phosphorus (P) limits basic ecosystem processes in moist tropical forests, direct tests of this supposition are rare. At the same time, it is generally believed that P does not limit soil microorganism respiration or growth in terrestrial ecosystems. We used natural gradients in P fertility created by soils of varying age underlying tropical rain forests in southwestern Costa Rica, combined with direct manipulations of carbon (C) and P supply, to test the effects of P availability on the decomposition of multiple forms of C, including dissolved organic carbon (DOC) and soil organic carbon (SOC). Results from a combination of laboratory and field experiments suggest that C decomposition in old, highly weathered oxisol soils is strongly constrained by P availability. In addition, P additions to these soils (no C added) also revealed that microbial utilization of at least labile fractions of SOC was also P limited. To our knowledge, this is the first direct evidence of P limitation of microbial processes in tropical rain forest soil. We suggest that P limitation of microbial decomposition may have profound implications for C cycling in moist tropical forests, including their potential response to increasing atmospheric carbon dioxide. Furthermore, this site is still relatively rich in P when compared to many other tropical forests on old soils; thus, we believe that P limitation of soil microorganisms throughout the humid tropics is a possibility.

Inorganic nitrogen and microbial biomass dynamics before and during spring snowmelt

AbstractRecent work in seasonally snow covered ecosystems has identifiedthawed soil and high levels of heterotrophic activity throughout the winterunder consistent snow cover. We performed measurements during the winter of1994 to determine how the depth and timing of seasonal snow cover affectsoil microbial populations, surface water NO

Seasonal changes in an alpine soil bacterial community in the colorado rocky mountains.

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Microbial Community Succession in an Unvegetated, Recently Deglaciated Soil

loss during snowmelt, and plant Navailability early in the growing season. Soil under early accumulating,consistent snow cover remained thawed during most of the winter and bothmicrobial biomass and soil inorganic N pools gradually increased under thesnowpack. At the initiation of snowmelt, microbial biomass N pools increasedfrom 3.0 to 5.9 g n m-2,concurrent with a decrease in soil inorganic N pools. During the latterstages of snowmelt, microbial biomass N pools decreased sharply without aconcurrent increase in inorganic N pools or significant leaching losses. Incontrast, soil under inconsistent snow cover remained frozen during most ofthe winter. During snowmelt, microbial biomass initially increased from 1.7to 3.1 g N m-2 and thendecreased as sites became snow-free. In contrast to smaller pool sizes,NO

Changes in Soil Microbial Community Structure and Function in an Alpine Dry Meadow Following Spring Snow Melt

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