James M. Kaste

Short-term soil mixing quantified with fallout radionuclides

Soil mixing plays a significant role in contaminant transport, carbon sequestration, and landscape evolution, yet the rates and driving mechanisms are poorly constrained. Here we use depth profiles and advection-diffusion modeling of fallout nuclides to quantify differences in short-term ( 7 Be and weapons-derived isotopes, and quantify mixing with a steady-state model of vertical 210 Pb transport. On a forested landscape in the Bega Valley in southeastern Australia and on grasslands in Marin County, California, where bioturbation is documented as the dominant sediment transport mechanism, we calculate diffusion-like mixing coefficients of 1–2 cm 2 yr −1 . In montane forest soils of northern New England, we observe little field evidence of short-term mixing, and find that the traditional advection-diffusion model fails to describe 210 Pb profiles. Because nuclide profiles here can be described with a simple model of litterfall, organic matter decay, and radioactive decay, we argue that diffusion-like processes are barely active on short time scales, and that the advection-diffusion model overestimates diffusion-like transport. While animal bioturbation and soil freezing cycles have little effect on the fate of elements in New England, physical soil mixing drives transport at Bega Valley and Marin County. We suggest that the absence of soil stirring that we quantify in New England forests may explain the slow physical erosion here (∼0.2 cm/k.y.) relative to the actively bioturbated soils of Bega Valley and Marin County (5–10 cm/k.y.).

Climate-driven processes of hillslope weathering

Climate controls erosion and weathering on soil-mantled landscapes through diverse processes that have remained diffi cult to disentangle due to their complex interactions. We quantify denudation, soil and saprolite weathering, and soil transport near the base and crest of the western slope of the Sierra Nevada to examine how large differences in climate affect these processes. Depth profi les of fallout radionuclides and fi eld observations show relative differences in erosion and weathering processes at these two climatically diverse sites, and our data suggest fundamentally different patterns of soil production and transport mechanisms: biotically driven soil transport at low elevation, and surface erosion driven by overland fl ow at high elevation. Soil production rates from cosmogenic 10 Be decrease from 31.3 to 13.6 m/Ma with increasing soil depth at low elevation, but show uncertain depth dependence at the high elevation site. Our data also show a positive correlation between physical erosion and saprolite weathering at both sites. Highly weathered saprolites are overlain by weakly weathered and rapidly eroding soils, while chemically less depleted saprolites are overlain by slowly eroding, more weathered soils. Our data are among the fi rst to quantify the critical role of saprolite weathering in the evolution of actively eroding upland landscapes, and our results provide quantitative constraints on how different climates can shape hillslopes by driving processes of erosion and weathering.

Threshold increases in soil lead and mercury from tropospheric deposition across an elevational gradient.

Atmospheric deposition is the primary mechanism by which remote ecosystems are contaminated, but few data sets show how fluxes change and control soil metal burdens at the landform scale. We present mercury (Hg), lead ((210)Pb and total Pb), and cosmogenic beryllium-7 ((7)Be) measurements in organic (O) soil horizons at high-resolution elevation intervals of ∼60 m from 540 to 1160 m on Camels Hump in northern Vermont, USA. Across this gradient, average O horizon Hg ranges from 0.99 mg m(-2) in the low elevation deciduous forest zone to 7.6 mg m(-2) in the higher elevation coniferous forest at 1030 m. We measure two pronounced threshold increases in soil metal burdens above 801 and 934 m, corresponding to the two most common altitudes of cloud base, which coincide with changes in vegetation species. Lead-210, a unique tracer of tropospheric deposition, also increased from 3200 Bq m(-2) to 11500 Bq m(-2) in O horizons, exhibiting threshold responses at the same elevations as Hg and total Pb. Concentrations of (210)Pb and Hg in foliage double from 760 to 900 m elevation, indicating enhanced deposition across the transition from deciduous to coniferous forest. In contrast, (7)Be is constant across the entire elevational gradient because of its upper atmospheric source. This indicates that the effects of orographic precipitation have a smaller control on soil contaminant burdens than the coupled cloudwater deposition-vegetation scavenging effect in the presence of upwind sources. By measuring soil contaminants and unique tracers of atmospheric deposition, we show that tropospheric fluxes of Hg and Pb are higher by a factor of 2 in high-elevation coniferous forests than in adjacent lowlands. Total O horizon Hg and Pb burdens increase by over 4-fold with elevation because of the compounding effects of enhanced deposition and longer metal residence times at higher elevations (>50 years).

Spatial and vertical distribution of mercury in upland forest soils across the northeastern United States

Assessing current Hg pools in forest soils of the northeastern U.S. is important for monitoring changes in Hg cycling. The forest floor, upper and lower mineral horizons were sampled at 17 long-term upland forest sites across the northeastern U.S. in 2011. Forest floor Hg concentration was similar across the study region (274 ± 13 μg kg(-1)) while Hg amount at northern sites (39 ± 6 g ha(-1)) was significantly greater than at western sites (11 ± 4 g ha(-1)). Forest floor Hg was correlated with soil organic matter, soil pH, latitude and mean annual precipitation and these variables explained approximately 70% of the variability when multiple regressed. Mercury concentration and amount in the lower mineral soil was correlated with Fe, soil organic matter and latitude, corresponding with Bs horizons of Spodosols (Podzols). Our analysis shows the importance of regional and soil properties on Hg accumulation in forest soils.

Sulfur species behavior in soil organic matter during decomposition

Received 27 June 2007; accepted 27 August 2007; published 11 December 2007. [1] Soil organic matter (SOM) is a primary reservoir of terrestrial sulfur (S), but its role in the global S cycle remains poorly understood. We examine S speciation by X-ray absorption near-edge structure (XANES) spectroscopy to describe S species behavior during SOM decomposition. Sulfur species in SOM were best represented by organic sulfide, sulfoxide, sulfonate, and sulfate. The highest fraction of S in litter was organic sulfide, but as decomposition progressed, relative fractions of sulfonate and sulfate generally increased. Over 6-month laboratory incubations, organic sulfide was most reactive, suggesting that a fraction of this species was associated with a highly labile pool of SOM. During humification, relative concentrations of sulfoxide consistently decreased, demonstrating the importance of sulfoxide as a reactive S phase in soil. Sulfonate fractional abundance increased during humification irrespective of litter type, illustrating its relative stability in soils. The proportion of S species did not differ systematically by litter type, but organic sulfide became less abundant in conifer SOM during decomposition, while sulfate fractional abundance increased. Conversely, deciduous SOM exhibited lesser or nonexistent shifts in organic sulfide and sulfate fractions during decomposition, possibly suggesting that S reactivity in deciduous litter is coupled to rapid C mineralization and independent of S speciation. All trends were consistent in soils across study sites. We conclude that S reactivity is related to speciation in SOM, particularly in conifer forests, and S species fractions in SOM change during decomposition. Our data highlight the importance of intermediate valence species (sulfoxide and sulfonate) in the pedochemical cycling of organic bound S.

Meteoric 7Be and 10Be as Process Tracers in the Environment

7Be (T1/2 = 53 days) and 10Be (T1/2 = 1.4 Ma) form via natural cosmogenic reactions in the atmosphere and are delivered to Earth’s surface by wet and dry deposition. The distinct source term and near-constant fallout of these radionuclides onto soils, vegetation, waters, ice, and sediments makes them valuable tracers of a wide range of environmental processes operating over timescales from weeks to millions of years. Beryllium tends to form strong bonds with oxygen atoms, so 7Be and 10Be adsorb rapidly to organic and inorganic solid phases in the terrestrial and marine environment. Thus, cosmogenic isotopes of beryllium can be used to quantify surface age, sediment source, mixing rates, and particle residence and transit times in soils, streams, lakes, and the oceans. A number of caveats exist, however, for the general application of these radionuclides as tracers in the environment, as steady deposition and geochemical immobility are not guaranteed in all systems. Here we synthesize and review scientific literature documenting the deposition and behavior of these nuclides at the Earth’s surface, focusing on current and potential applications for Earth scientists working to quantify terrestrial and marine processes.

Measuring environmental change in forest ecosystems by repeated soil sampling: a North American perspective

Environmental change is monitored in North America through repeated measurements of weather, stream and river flow, air and water quality, and most recently, soil properties. Some skepticism remains, however, about whether repeated soil sampling can effectively distinguish between temporal and spatial variability, and efforts to document soil change in forest ecosystems through repeated measurements are largely nascent and uncoordinated. In eastern North America, repeated soil sampling has begun to provide valuable information on environmental problems such as air pollution. This review synthesizes the current state of the science to further the development and use of soil resampling as an integral method for recording and understanding environmental change in forested settings. The origins of soil resampling reach back to the 19th century in England and Russia. The concepts and methodologies involved in forest soil resampling are reviewed and evaluated through a discussion of how temporal and spatial variability can be addressed with a variety of sampling approaches. Key resampling studies demonstrate the type of results that can be obtained through differing approaches. Ongoing, large-scale issues such as recovery from acidification, long-term N deposition, C sequestration, effects of climate change, impacts from invasive species, and the increasing intensification of soil management all warrant the use of soil resampling as an essential tool for environmental monitoring and assessment. Furthermore, with better awareness of the value of soil resampling, studies can be designed with a long-term perspective so that information can be efficiently obtained well into the future to address problems that have not yet surfaced.

Forest floor lead, copper and zinc concentrations across the northeastern United States: synthesizing spatial and temporal responses.

Understanding how metal concentrations in soil have responded to reductions of anthropogenic emissions is essential for predicting potential ecosystem impacts and evaluating the effectiveness of pollution control legislation. The objectives of this study were to present new data and synthesize existing literature to document decreases in Pb, Cu, and Zn concentrations in forest soils across the northeastern US. From measurements at 16 sites, we observed that forest floor Pb, Cu, and Zn concentrations have decreased between 1980 and 2011 at an overall mean rate of 1.3 ± 0.5% yr(-1). E-folding times, a concentration exponential decay rate (1/k), for Pb, Cu and Zn at the 16 sites were estimated to be 46 ± 7, 76 ± 20 and 81 ± 19 yr, respectively. Mineral soil concentrations were correlated with forest floor concentrations for Pb, but not for Cu and Zn, suggesting an accumulation in one pool does not strongly influence accumulation in the other. Forest floor Pb, Cu and Zn concentrations from our sites and 17 other studies conducted from 1970-2014 in remote forests across the northeastern US were compiled into pooled data sets. Significant decreasing trends existed for pooled forest floor Pb, Cu, and Zn concentrations. The pooled forest floor Pb e-folding time was determined to be 33 ± 9 yrs, but the explanatory power of pooled Cu and Zn regressions were inadequate for calculating e-folding times (r(2)<0.25). Pooled Pb, Cu, and Zn concentrations in forest floor were multiple-regressed with latitude, longitude, elevation, and year of sampling, cumulatively explaining 55, 38, and 28% of the variation across compiled studies. Our study suggests anthropogenic Pb in the forest floor will continue to decrease, but decreases in forest floor Cu and Zn concentrations may be masked by spatial heterogeneity or are at a new steady state.

Forest Floor Lead Changes from 1980 to 2011 and Subsequent Accumulation in the Mineral Soil across the Northeastern United States.

Quantifying the transport rate of anthropogenic lead (Pb) in forest soils is essential for predicting air pollution impacts on northeastern United States soil quality. In 2011, we resampled the forest floor at 16 sites across the northeastern United States previously sampled in 1980, 1990, and 2002 and also sampled the upper two mineral soil horizons. The mean forest floor Pb concentration decreased from 151 ± 29 mg kg in 1980 to 68 ± 13 mg kg in 2011. However, the mean forest floor Pb amount per unit area remained similar (10 ± 2 kg ha in 1980 and 11 ± 4 kg ha in 2011). Study sites were divided into three geographic regions: western, central, and northern. The modeled forest floor Pb response time (1/) was longer at frigid soil temperature regime sites (61 ± 15 yr) compared with mesic sites (29 ± 4 yr). Mineral soil Pb concentration and amount were approximately four times greater at western and central sites compared with northern sites for both mineral horizons. Furthermore, mean isotope ratios of Pb/Pb (1.201 ± 0.006) and Pb/Pb (2.060 ± 0.021) indicated that Pb in the western and central forest floor and mineral soil was primarily gasoline derived. Our combined analytical approach using long-term forest floor monitoring and stable Pb isotopes suggest that the majority of anthropogenic Pb deposited on soils in the western and central sites has been transported to the mineral soil, whereas it continues to reside in the forest floor at northern sites.

Incorporation of radiometric tracers in peat and implications for estimating accumulation rates

Accurate dating of peat accumulation is essential for quantitatively reconstructing past changes in atmospheric metal deposition and carbon burial. By analyzing fallout radionuclides (210)Pb, (137)Cs, (241)Am, and (7)Be, and total Pb and Hg in 5 cores from two Swedish peatlands we addressed the consequence of estimating accumulation rates due to downwashing of atmospherically supplied elements within peat. The detection of (7)Be down to 18-20 cm for some cores, and the broad vertical distribution of (241)Am without a well-defined peak, suggest some downward transport by percolating rainwater and smearing of atmospherically deposited elements in the uppermost peat layers. Application of the CRS age-depth model leads to unrealistic peat mass accumulation rates (400-600 g m(-2) yr(-1)), and inaccurate estimates of past Pb and Hg deposition rates and trends, based on comparisons to deposition monitoring data (forest moss biomonitoring and wet deposition). After applying a newly proposed IP-CRS model that assumes a potential downward transport of (210)Pb through the uppermost peat layers, recent peat accumulation rates (200-300 g m(-2) yr(-1)) comparable to published values were obtained. Furthermore, the rates and temporal trends in Pb and Hg accumulation correspond more closely to monitoring data, although some off-set is still evident. We suggest that downwashing can be successfully traced using (7)Be, and if this information is incorporated into age-depth models, better calibration of peat records with monitoring data and better quantitative estimates of peat accumulation and past deposition are possible, although more work is needed to characterize how downwashing may vary between seasons or years.