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Robert Stottlemyer

Publications and source records attributed to Robert Stottlemyer.

5 recordsLinked to original sources
USGS2015

Soil nitrogen accretion along a floodplain terrace chronosequence in northwest Alaska: Influence of the nitrogen-fixing shrub Shepherdia canadensis

Nitrogen enters terrestrial ecosystems through multiple pathways during primary succession. We measured accumulation of total soil nitrogen and changes in inorganic nitrogen (N) pools across a 300-y sequence of river terraces in northwest Alaska and assessed the contribution of the nitrogen-fixing shrub Shepherdia canadensis . Our work compared 5 stages of floodplain succession, progressing from a sparsely vegetated silt cap to dense shrubby vegetation, balsam poplar-dominated ( Populus balsamifera ) and white spruce-dominated ( Picea glauca ) mixed forests, and old-growth white spruce forest. Total soil N (0–30 cm depth) increased throughout the age sequence, initially by 2.4 g N·m −2 ·y −1 during the first 120 y of terrace development, then by 1.6 g N·m −2 ·y −1 during the subsequent 2 centuries. Labile soil N, measured by anaerobic incubation, increased most rapidly during the first 85 y of terrace formation, then remained relatively constant during further terrace development. On recently formed terraces, Shepherdia shrubs enriched soil N pools several-fold compared to soil beneath Salix spp. shrubs or intercanopy sites. Total and labile soil N accretion was proportional to Shepherdia cover during the first century of terrace development, and mineral soil δ 15 N content indicated that newly formed river terraces receive substantial N through N-fixation. About half the 600 g total N·m −2 accumulated across the river terrace chronosequence occurred during the 120 y when S. canadensis was dominant. Sediment deposited by periodic flooding continued to add N to terrace soils after the decline in Shepherdia abundance and may have contributed 25% of the total N found in the floodplain terrace soils.

Alaska↗
USGS1999

Seasonal relationships between precipitation, forest floor, and streamwater nitrogen, Isle Royale, Michigan

The Upper Great Lakes receive large amounts of precipitation-NH + 4 and moderate NO - 3 inputs. Increased atmospheric inorganic N input has led to concern about ecosystem capacity to utilize excess N. This paper summarizes a 5-yr study of seasonal N content and flux in precipitation, snowpack, forest floor, and streamwater in order to assess the source of inorganic N outputs in streamflow from a small boreal watershed. Average precipitation N input was 3 kg ha -1 yr -1 . The peak snowpack N content averaged 0.55 kg ha -1 . The forest floor inorganic N pool was ≈ 2 kg ha -1 , eight times larger than monthly precipitation N input. The inorganic N pool size peaked in spring and early summer. Ninety percent of the forest floor inorganic N pool was made up of NH + 4 -N. Forest floor inorganic N pools generally increased with temperature. Net N mineralization was 15 kg ha -1 yr -1 , and monthly rates peaked in early summer. During winter, the mean monthly net N mineralization rate was twice the peak snowpack N content. Streamwater NO - 3 concentration peaked in winter, and inorganic N output peaked in late fall. Beneath the dominant boreal forest species, net N mineralization rates were positively correlated ( P < 0.05) with streamwater NO - 3 concentrations. Forest floor NO - 3 pools beneath alder [ Alnus rugosa (Du Roi) Spreng] were positively correlated ( P < 0.01) to streamwater NO - 3 output. At the watershed mouth, streamwater NO - 3 concentrations were positively correlated ( P < 0.05) with precipitation NO - 3 input and precipitation amount. The relatively small snowpack N content and seasonal precipitation N input compared to forest floor inorganic N pools and net N mineralization rates, the strong ecosystem retention of precipitation N inputs, and the seasonal streamwater NO - 3 concentration and output pattern all indicated that little streamwater NO - 3 came directly from precipitation or snowmelt.

Michigan↗
USGS1998

Nitrogen excess in North American ecosystems: Predisposing factors, ecosystem responses, and management strategies

Most forests in North America remain nitrogen limited, although recent studies have identified forested areas that exhibit symptoms of N excess, analogous to overfertilization of arable land. Nitrogen excess in watersheds is detrimental because of disruptions in plant/soil nutrient relations, increased soil acidification and aluminum mobility, increased emissions of nitrogenous greenhouse gases from soil, reduced methane consumption in soil, decreased water quality, toxic effects on freshwater biota, and eutrophication of coastal marine waters. Elevated nitrate ( NO 3 − ) loss to groundwater or surface waters is the primary symptom of N excess. Additional symptoms include increasing N concentrations and higher N:nutrient ratios in foliage (i.e., N:Mg, N:P), foliar accumulation of amino acids or NO 3 − , and low soil C:N ratios. Recent nitrogen-fertilization studies in New England and Europe provide preliminary evidence that some forests receiving chronic N inputs may decline in productivity and experience greater mortality. Long-term fertilization at Mount Ascutney, Vermont, suggests that declining and slow N-cycling coniferous stands may be replaced by fast-growing and fast N-cycling deciduous forests. Symptoms of N saturation are particularly severe in high-elevation, nonaggrading spruce–fir ecosystems in the Appalachian Mountains and in eastern hardwood watersheds at the Fernow Experimental Forest near Parsons, West Virginia. In the Los Angeles Air Basin, mixed conifer forests and chaparral watersheds with high smog exposure are N saturated and exhibit the highest streamwater NO 3 − concentrations for wildlands in North America. High-elevation alpine watersheds in the Colorado Front Range and a deciduous forest in Ontario, Canada, are N saturated, although N deposition is moderate (∼8 kg·ha −1 ·yr −1 ). In contrast, the Harvard Forest hardwood stand in Massachusetts has absorbed >900 kg N/ha during 8 yr of N amendment studies without significant NO 3 − leaching, illustrating that ecosystems vary widely in the capacity to retain N inputs. Overly mature forests with high N deposition, high soil N stores, and low soil C:N ratios are prone to N saturation and NO 3 − leaching. Additional characteristics favoring low N retention capacity include a short growing season (reduced plant N demand) and reduced contact time between drainage water and soil (i.e., porous coarse-textured soils, exposed bedrock or talus). Temporal patterns of hydrologic fluxes interact with biotic uptake and internal cycling patterns in determining ecosystem N retention. Soils are the largest storage pool for N inputs, although vegetation uptake is also important. Recent studies indicate that nitrification may be widespread in undisturbed ecosystems, and that microbial assimilation of NO 3 − may be a significant N retention mechanism, contrary to previous assumptions. Further studies are needed to elucidate the sites, forms, and mechanisms of N retention and incorporation into soil organic matter, and to test potential management options for mitigating N losses from forests. Implementation of intensive management practices in N-saturated ecosystems may only be feasible in high-priority areas and on a limited scale. Reduction of N emissions would be a preferable solution, although major reductions in the near future are unlikely in many areas due to economic, energy-use, policy, and demographic considerations.

Ecological Applications↗
USGS1987

Snowpack ion accumulation and loss in a basin draining to Lake Superior

The objective of this study was to relate winter precipitation ionic inputs, snowpack retention, and change in first-order stream chemistry with spring snowpack melt. During winter 1982–83, measurement of precipitation inputs, snowpack concentration and loading, and streamwater concentration and discharge of Ca2+, K+, H+, NO3−, and SO42− from a 176-ha watershed reveals that only H+ might be lost from the snowpack before first thaw. Above-freezing soil temperature beneath the snowpack may be a factor in H+ loss. An initial 1-d thaw resulted in loss of over one third (6 eq∙ha−1) of the snowpack Ca2+. Over one half the snowpack load of K+, H+, NO3−, and SO42−, was lost in a subsequent midwinter freeze–thaw period. Snowpack loading of ionic species was reduced by 70–90% before peak spring melting and stream discharge. Ecosystem H+ retention and biological uptake of NO3− further mitigate ionic "pulses" in streamwater. Sulfate discharge exceeds bulk inputs, which suggests significant dry deposition input and little forest soil retention of this anion. The snowpack was relatively small, which limits wider application of these results to the region.

Lake Superior↗