Edward A. Nater

Mercury budget of an upland-peatland watershed.

Inputs, outputs, and pool sizes oftotal mercury (Hg) were measured in a forested 10 hawatershed consisting of a 7 ha hardwood-dominatedupland surrounding a 3 ha conifer-dominatedpeatland. Hydrologic inputs via throughfall andstemflow, 13±0.4 μg m−2 yr−1over the entire watershed, were about doubleprecipitation inputs in the open and weresignificantly higher in the peatland than in theupland (19.6 vs. 9.8 μg m−2 yr−1). Inputs of Hg via litterfall were 12.3±0.7μg m−2 yr−1, not different in thepeatland and upland (11.7 vs. 12.5 μg m−2yr−1). Hydrologic outputs via streamflow were2.8±0.3 μg m−2 yr−1 and thecontribution from the peatland was higher despiteits smaller area. The sum of Hg inputs were lessthan that in the overstory trees, 33±3 μgm−2 above-ground, and much less than eitherthat in the upland soil, 5250±520 μgm−2, or in the peat, 3900±100 μgm−2 in the upper 50 cm. The annual flux of Hgmeasured in streamflow and the calculated annualaccumulation in the peatland are consistent withvalues reported by others. A sink for Hg of about20 μg m−2 yr−1 apparently exists inthe upland, and could be due to either or bothstorage in the soil or volatilization.

Atmospheric Inputs of Mercury and Organic Carbon into a Forested Upland/Bog Watershed

Inputs of mercury (Hg) and dissolved organic carbon (DOC) in throughfall and stemflow waters were measured for an upland/bog watershed in northern Minnesota, and were compared to the deposition in a nearby opening to determine the influence of tree canopies on Hg and DOC deposition. Twice as much Hg and seven times as much DOC was deposited in the forested watershed compared to the opening. Mass balance studies that are based on wet-only deposition in openings severely underestimate atmospheric deposition of Hg in forests. Conifer canopies are more efficient filters of airborne particulates than are deciduous canopies as indicated by much higher Hg concentrations and total deposition in throughfall and stemflow waters under conifers. Significant positive relationships existed between Hg and DOC in both throughfall (36–57% of the variation) and stemflow waters (55–88% of the variation). Hg complexation by DOC appears to be related to the contact time between precipitation and carbon sources.

Mercury Uptake by Trees: An Observational Experiment

We conducted a simple observational experiment to test whether differences in Hg in tissue of red pine (Pinus resinosa Ait.) were related to soil or to atmospheric sources of Hg. We sampled two plantations in each of three areas, and within each plantation sampled two sites with different levels of soil Hg. Woody tissue Hg concentration differed by area, and differences in foliar concentrations, though not statistically significant, were ranked in the same order. Total mass of Hg in forest floor and mineral soil also differed by area, but with ranking opposite that of tissue. On an individual-tree basis, concentrations of Hg in 1994 needles (2-year old) were about twice those in 1995 needles (1-year old) (r = 0.77). Neither woody tissue Hg nor any measure of Hg in soil or forest floor were closely related to foliar levels and some relationships were inverse. We interpret the data to indicate that Hg in plant tissue is derived directly from the atmosphere, not the soil. Tissue concentration by area was closely related to the respective growing season length (1994 needles, r = 0.88; 1995 needles, r = 0.97; wood, r = 0.97), as was total mass of Hg in forest floor and surface mineral soil (r = – 0.80). Other climatic measures, such as growing degree days and actual evapotranspiration, had similar relationships. These relationships imply that both foliar uptake of Hg0 from the atmosphere and efflux of Hg from the soil system depend on biological activity.

A new model of topographic effects on the distribution of loess

A model of topographic influence on the regional transport and accumulation of loess in sparsely vegetated landscapes is proposed, based on the critical role played by saltating eolian sand in the entrainment and long-distance transport of finer-grained suspended dust. In this model, long-term accumulation of loess occurs mainly downwind of topographic obstacles that limit eolian transport of sand, and associated re-entrainment of dust. Regional and local patterns of distribution of late Wisconsinan loess in parts of the Upper Mississippi River basin are closely related to the distribution of topographic obstacles to sand transport. These cases of topographic influence occur in a region where evidence exists for sparse tundra-like vegetation during the deposition of loess in the late Wisconsinan. Similar topographic effects may not be evident further south in the Missouri and Mississippi River basins because vegetation density was generally sufficient to prevent the movement of eolian sand out of source valleys. Thus, topographic obstacles and dense vegetation may have similar effects on loess distribution under different environmental conditions.

Characterization of laboratory weathered labradorite surfaces using X-ray photoelectron spectroscopy and transmission electron microscopy

Altered surfaces of labradorite resulting from laboratory weathering at pH 4 and 25°C were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). SEM micrographs showed nonuniform surface alteration of labradorite weathered at pH 3.7 for 415 days. TEM micrographs showed exsolution lamellae of a more calcic-rich and more sodic-rich phase, each averaging approximately 700 a thick. The more calcic phase was preferentially weathered to average depths of 1350 A in excess of the more sodic phase, producing a corrugated surface. The surface roughness caused by preferential weathering of the more calcic phase affects the relative exposure of calcic and sodic phases to the XPS detector. A three-dimensional analysis of possible surface exposures was used to predict the influence of a corrugated surface on XPS measurements. Actual XPS data showed significant Ca depletion, slight Al depletion, slight Si enrichment, and slight Na enrichment relative to unweathered labradorite. Sputter depth profiling with an Ar ion gun showed that surface alteration was significant up to depths of 500 A, similar to the depth of preferential weathering of the more calcic lamellae observed with TEM. Predicted XPS data accounting for the topographic effects of a corrugated surface showed similar trends of Ca depletion, slight Al depletion, slight Si enrichment, and moderate Na enrichment. Furthermore, predicted XPS sputter depth profiles of Ca, Al, Si, and Na were similar to actual sputter depth profiles, indicating that a significant amount of the surface alteration on labradorite can be explained by preferential weathering of the more calcic lamellae, and the subsequent surface roughness effects this has on XPS spectra. Other surface processes such as H+ or H3O+ exchange for Ca, Na, or Al and preferential weathering at sites of excess surface energy (dislocations, twin boundaries, etc.) not accounted for in the predicted XPS data may also contribute to the surface composition of weathered labradorite. Results showing preferential weathering of more calcic-rich lamellae and its effect on XPS spectra indicate the importance of understanding the micro-structure of feldspars used for laboratory weathering studies.

Forest soil mineral weathering rates: use of multiple approaches

Knowledge of rates of release of base cations from mineral dissolution (weathering) is essential to understand ecosystem elemental cycling. Although much studied, rates remain enigmatic. We compared the results of four methods to determine cation (Ca + Mg + K) release rates at five forested soils/sites in the northcentral U.S.A. Our premise was that multiple approaches, each with their own specific strengths and weaknesses, would yield a “best” overall estimate. We used (1) a cation input-output budget on a pedon scale; (2) trends in elemental and mineral depletion in silt-size particles; (3) a laboratory batch dissolution technique, with the results adjusted for field conditions; and (4) a steady-state soil chemistry model, PROFILE. The soils included a loamy sand Typic Udipsamment, a sandy loam Spodic Udipsamment, a fine sandy loam Typic Dystrochrept, a very fine sandy loam Glossic Eutroboralf, and a clayey Glossic Eutroboralf. Weathering rates varied among both soils and methods, and neither methods nor soils could easily be grouped; the data spanned a continuum with overlapping ranges of least significant differences. Although the assumptions necessary for some methods were better suited to specific soils, we rejected only one method-soil combination as being inappropriate (input-output budget for the clay). Mean release rates for the sum of cations ranged from 470 eq ha−1 yr−1 for the clayey soil, to 460 for the fine sandy loam soil, to 430 for the very fine sandy loam soil, to 375 for the sandy loam soil, to 195 for the loamy sand soil. These rates are lower than those reported for similar soils in the literature because most reported rates are based on watershed studies. Our low rates of cation release within soil pedons, the ultimate source of nutrient ions for plant growth, has implications for estimated nutrient budgets and long-term soil sustainability.

Fractal dimensions for volume and surface of interaggregate pores — scale effects

Geometrical attributes of pore systems in soil have shown fractal scaling. Scaling in natural materials is inherently statistical, i.e., fractal dimensions may change with scale. While fractal dimensions characterizing pore surface roughness, Ds, or scaling of pore sizes, Dv, have been reported, seldom are both measurements made at more than one scale. We examine a scale effect on Dv and Ds values, and relationships between fractal dimensions of both properties. Natural and artificial types of soil structure were studied in a Normania soil. Natural soil structure was sampled from experiments involving: (l) three primary tillage tools, sampled immediately after tillage; and (2) three tillage systems, sampled after consolidation. Artificial soil structure was formed in columns packed with aggregate assemblies that included two single aggregate-size fractions, and two mixtures of six aggregate-size fractions (each covering two ranges) made to obtain fractal aggregate-size distributions. Block-like samples from all sources were resin-impregnated in situ and a face was cut and polished. Images of UV-illuminated faces were obtained at three magnifications and then pooled into two groups. A box-counting technique was applied to area and outline of pores to obtain Dv-box, and Ds-box, respectively; Ds was also calculated from area-perimeter relations (Ds-AP). Box-count data showed two segments; Dv-box and Ds-box were evaluated in relation to each segment and to Ds-AP. Coefficients of determination in the relation Ds-AP vs Ds-box were relatively low, indicating discrepancies between the two methods. Fractal dimensions were not scale-invariant. Values of Ds-box for aggregate assemblies decreased with resolution, especially for single aggregate-size fractions. Values of Dv-box were more influenced by aggregate size than resolution. Both Ds-box and Dv-box varied with resolution for freshly tilled soil. For somewhat consolidated soil, variations in values of both fractal dimensions were related to tillage systems. Values of Ds-box and Dv-box were highly correlated, with linear relations depending on magnification and type of soil structure.

Prevalence and initiation of preferential flow paths in a sandy loam with argillic horizon

Numerous studies have reported on preferential solute transport, giving evidence that preferential flow is widespread. However there has been little field documentation of the relative importance of different preferential transport mechanisms. This study used a dye tracer to examine the extent and relative importance of different preferential transport mechanisms in a glacial outwash-derived soil that is used extensively for high input agriculture in central Minnesota, USA (Verndale sandy loam: coarse loamy over sandy, mixed, frigid, Udic Argiborolls). Experimental treatments included three initial soil water contents (WET, MEDIUM, and DRY) and three dye solution application rates (FLOOD, SPRINKLER-High, and SPRINKLER-Low). Thirteen cm of FD&C Blue no. 1 (also known as Brilliant Blue FCF) food dye solution (200 g l−1) were applied to replicated 1 m×1 m plots in a recently tilled 5 year old alfalfa stand and to two additional plots with no history of alfalfa. Vertical soil profile faces were exposed at 10-cm increments across each plot. Extensive and deeper preferential dye movement occurred under FLOOD conditions regardless of initial soil moisture or recent vegetation history. The two SPRINKLER rates generally resulted in relatively shorter preferential flow paths (PFPs). Within-plot variability of dye patterns—including depth and number of PFPs—was very high. Most PFPs observed were associated with roots and decayed roots, or with patterns in the abruptness and topography of the boundary between the Ap and Bt horizons. Open burrows were uncommon, but contributed to extensive preferential flow in the two NO-ALFALFA plots. Our findings indicate that preferential transport is prevalent under the variety of application rate and soil moisture conditions evaluated, and that observable soil features appear to be initiators of the majority of the PFPs. Only a few (10 of 126) of the profiles excavated had preferential flow paths that were not associated with visible soil features. The observed high variability gives support to the idea that observations of spatial variability in pesticide transport studies is due to preferential transport. Our initial goal of elucidating the relationship between rate of application and the relative number and depth of PFPs was aimed at evaluating water flow patterns. However, subsequent research found that dye retardation was increased at slower application rates, indicating that the dye patterns we observed were due to both the rate and the pattern of water movement. As a result, we caution that our findings of generally deeper and more extensive preferential dye transport under the higher velocity FLOOD application rate do not necessarily indicate more extensive preferential water (or non-adsorbing solute) transport at this rate compared to the intermittent SPRINKLER rate. It is possible that our observations may be indicative of patterns in movement of adsorbed solutes such as pesticides, however this contention would require further research.

Changes in microrelief and their effects on infiltration and erosion during simulated rainfall

Abstract The erosivity of soils under a given rainfall energy appears to vary greatly among soil orders, probably reflecting differences in clay composition and organic matter content. This study was conducted to quantify microrelief, infiltration, and sediment yield changes during three consecutive simulated rain events on a Udic Haploboroll and a Typic Hapludalf from Minnesota, and a Mollic Kandiudalf, and Typic Palehumult from Uganda. Air dry aggregates ( −1 ) for a duration of 1 h. Runoff and sediment were continuously monitored during a storm. Infiltration was measured by continued weighing of the soil and containers. An automated non-contact laser relief meter was used to measure changes in soil roughness initially and after each storm. Soil surface roughness decreased during the rain events indicating that aggregate breakdown was the dominant process in seal formation. For example, random roughness decreased form 5.9 to 4.0 mm on Barnes loam and from 9.7 to 6.9 mm on Renova silt loam with cumulative rainfall of 0 and 126 mm. These infiltration rates indicated that the Barnes Loam (Haploboroll) and Kabanyolo clay (Kandiudalf) were unstable soils while Kachwekano clay (Palehumult) and Renova silt loam (Hapludalf) were quite stable. Final infiltration rates after 3 consecutive rainfalls on Kachwekano clay (15 mm h −1 ) and Renova silt loam (13 mm h −1 ) [the stable aggregate soils] were significantly higher than those of Barnes loam (4 mm h −1 ) and Kabanyolo clay (3 mm h −1 ). For the two stable soils a high infiltration rate on a rough surface was maintained until aggregate breakdown occurred and runoff began. Sediment yield from Barnes loam (29 kg m −2 ) and Kabanyolo clay (28 kg m −2 ) was significantly greater than soil loss from Kachwekano clay (0 kg m −2 ) and Renova silt loam (6 kg m −2 ). The microrelief method to quantify aggregate stability is an improvement over wet sieving and other related measurements because of its rapidity and because the statistical quantification can be linked to physical processes.

Distribution and uptake dynamics of mercury in leaves of common deciduous tree species in Minnesota, U.S.A.

A sequential extraction technique for compartmentalizing mercury (Hg) in leaves was developed based on a water extraction of Hg from the leaf surface followed by a solvent extraction of the cuticle. The bulk of leaf Hg was found in the tissue compartment (90-96%) with lesser amounts in the surface and cuticle compartments. Total leaf concentrations of Hg varied among species and was most closely correlated with the number of stomates per sample, supporting the hypothesis that stomatal uptake of atmospheric Hg (most likely Hg(0)) is a potential uptake pathway. Mercury concentrations in leaves were monitored from emergence to senescence and showed a strong positive correlation with leaf age. Leaves accumulated Hg throughout the growing season; the highest uptake rates coincided with periods of high photosynthetic activity. Concentrations of Hg in leaf tissue increased steadily throughout the season, but no such trends were observed for surficial or cuticular accumulation. Factors affecting the variability of Hg in leaves were analyzed to improve protocols for the potential use of leaves as passive monitors of atmospheric Hg. Results show that total leaf Hg concentrations are affected by leaf age and leaf placement in the crown.