Geology topics

S.K. Schmidt

Publications and source records attributed to S.K. Schmidt.

4 recordsLinked to original sources

USGS2011

Estimating phosphorus availability for microbial growth in an emerging landscape

Estimating phosphorus (P) availability is difficult—particularly in infertile soils such as those exposed after glacial recession—because standard P extraction methods may not mimic biological acquisition pathways. We developed an approach, based on microbial CO 2 production kinetics and conserved carbon:phosphorus (C:P) ratios, to estimate the amount of P available for microbial growth in soils and compared this method to traditional, operationally-defined indicators of P availability. Along a primary succession gradient in the High Andes of Perú, P additions stimulated the growth-related (logistic) kinetics of glutamate mineralization in soils that had been deglaciated from 0 to 5 years suggesting that microbial growth was limited by soil P availability. We then used a logistic model to estimate the amount of C incorporated into biomass in P-limited soils, allowing us to estimate total microbial P uptake based on a conservative C:P ratio of 28:1 (mass:mass). Using this approach, we estimated that there was < 1 μg/g of microbial-available P in recently de-glaciated soils in both years of this study. These estimates fell well below estimates of available soil P obtained using traditional extraction procedures. Our results give both theoretical and practical insights into the kinetics of C and P utilization in young soils, as well as show changes in microbial P availability during early stages of soil development.

Geoderma↗

USGS1999

Landscape patterns of CH4 fluxes in an alpine tundra ecosystem

We measured CH4 fluxes from three major plant communities characteristic of alpine tundra in the Colorado Front Range. Plant communities in this ecosystem are determined by soil moisture regimes induced by winter snowpack distribution. Spatial patterns of CH4 flux during the snow-free season corresponded roughly with these plant communities. In Carex-dominated meadows, which receive the most moisture from snowmelt, net CH4 production occurred. However, CH4 production in one Carex site (seasonal mean = +8.45 mg CH4 m-2 d-1) was significantly larger than in the other Carex sites (seasonal means = -0.06 and +0.05 mg CH4 m-2 d-1). This high CH4 flux may have resulted from shallower snowpack during the winter. In Acomastylis meadows, which have an intermediate moisture regime, CH4 oxidation dominated (seasonal mean = -0.43 mg CH4 m-2 d-1). In the windswept Kobresia meadow plant community, which receive the least amount of moisture from snowmelt, only CH4 oxidation was observed (seasonal mean = -0.77 mg CH4 m-2 d-1). Methane fluxes correlated with a different set of environmental factors within each plant community. In the Carex plant community, CH4 emission was limited by soil temperature. In the Acomastylis meadows, CH4 oxidation rates correlated positively with soil temperature and negatively with soil moisture. In the Kobresia community, CH4 oxidation was stimulated by precipitation. Thus, both snow-free season CH4 fluxes and the controls on those CH4 fluxes were related to the plant communities determined by winter snowpack.

Biogeochemistry↗

USGS1999

Methane flux in subalpine wetland and unsaturated soils in the southern Rocky Mountains

Methane exchange between the atmosphere and subalpine wetland and unsaturated soils was evaluated over a 15-month period during 1995-1996. Four vegetation community types along a moisture gradient (wetland, moist-grassy, moist-mossy, and dry) were included in a 100 m sampling transect situated at 3200 m elevation in Rocky Mountain National Park, Colorado. Methane fluxes and soil temperature were measured during snow-free and snow-covered periods, and soil moisture content was measured during snow-free periods. The range of mean measured fluxes through all seasons (a positive value represents CH4 efflux to the atmosphere) were: 0.3 to 29.2 mmol CH4 m-2 d-1 wetland area; 0.1 to 1.8 mmol CH4 m-2 d-1, moist-grassy area; -0.04 to 0.7 mmol CH4 m-2 d-1, moist-mossy area; and -0.6 to 0 mmol CH4 m-2 d-1, dry area. Methane efflux was significantly correlated with soil temperature (5 cm) at the continuously saturated wetland area during snow-free periods. Consumption of atmospheric methane was significantly correlated with moisture content in the upper 5 cm of soil at the dry area. A model based on the wetland flux-temperature relationship estimated an annual methane emission of 2.53 mol CH4 m-2 from the wetland. Estimates of annual methane flux based on field measurements at the other sites were 0.12 mol CH4 m-2, moist-grassy area; 0.03 mol CH4 m-2, moist-mossy area; and -0.04 mol CH4 m-2, dry area. Methane fluxes during snow-covered periods were responsible for 25, 73, 23, and 43% of the annual fluxes at the wetland, moist-grassy, moist-mossy, and dry sites, respectively.

Global Biogeochemical Cycles↗

USGS1998

Inorganic nitrogen and microbial biomass dynamics before and during spring snowmelt

Recent work in seasonally snow covered ecosystems has identified thawed soil and high levels of heterotrophic activity throughout the winter under consistent snow cover. We performed measurements during the winter of 1994 to determine how the depth and timing of seasonal snow cover affect soil microbial populations, surface water NO3/- loss during snowmelt, and plant N availability early in the growing season. Soil under early accumulating, consistent snow cover remained thawed during most of the winter and both microbial biomass and soil inorganic N pools gradually increased under the snowpack. At the initiation of snowmelt, microbial biomass N pools increased from 3.0 to 5.9 g N m-2, concurrent with a decrease in soil inorganic N pools. During the latter stages of snowmelt, microbial biomass N pools decreased sharply without a concurrent increase in inorganic N pools or significant leaching losses. In contrast, soil under inconsistent snow cover remained frozen during most of the winter. During snowmelt, microbial biomass initially increased from 1.7 to 3.1 g N m-2 and then decreased as sites became snow-free. In contrast to smaller pool sizes, NO3/-export during snowmelt from the inconsistent snow cover sites of 1.14 (??0.511) g N m-2 was significantly greater (p < 0.001) than the 0.27 (??0.16) g N m-2 exported from sites with consistent snow cover. These data suggest that microbial biomass in consistently snow-covered soil provides a significant buffer limiting the export of inorganic N to surface water during snowmelt. However, this buffer is very sensitive to changes in snowpack regime. Therefore, interannual variability in the timing and depth of snowpack accumulation may explain the year to year variability in inorganic N concentrations in surface water these ecosystems.

Biogeochemistry↗