Darlene Zabowski

Phytoremediation of soil contaminated with low concentrations of radionuclides

Ecosystems throughout the world have been contaminated with radionuclides by above-ground nuclear testing, nuclear reactor accidents and nuclear power generation. Radioisotopes characteristic of nuclear fission, such as 137Cs and 90Sr, that are released into the environment can become more concentrated as they move up the food chain often becoming human health hazards. Natural environmental processes will redistribute long lived radionuclides that are released into the environment among soil, plants and wildlife. Numerous studies have shown that 137 Cs and 90Sr are not removed from the top 0.4 meters of soil even under high rainfall, and migration rate from the top few centimeters of soil is slow. The top 0.4 meters of the soil is where plant roots actively accumulate elements. Since plants are known to take up and accumulate 137 Cs and 90Sr removal of these radionuclides from contaminated soils by plants could provide a reliable and economical method of remediation. One approach is to use fast growing plants inoculated with mycorrhizal fungi combined with soil organic amendments to maximize the plant accumulation and removal of radionuclides from contaminated soils, followed by harvest of above-ground portion of the plants. High temperature combustion would be used to oxidize plant material concentrating 137 Cs and 90Sr, in ash for disposal. When areas of land have been contaminated with radionuclides are large, using energy intensive engineering solutions to remediate huge volumes of soil is not feasible or economical. Plants are proposed as a viable and cost effective method to remove radionuclides from the soils that have been contaminated by nuclear testing and nuclear reactor accidents.

Wildfire effects on carbon and nitrogen in inland coniferous forests

A ponderosa pine/Douglas-fir forest (Pinus ponderosa Dougl., Pseudotsuga menziesii (Mirb.) Franco; PP/DF) and a lodgepole pine/Engelmann spruce forest (Pinus contorta Loud., Picea engelmannii Parry ex Engelm.; LP/ES) located on the eastern slopes of the Cascade Mountains in Washington state, USA, were examined following severe wildfire to compare total soil carbon and nitrogen capitals with unburned (control) forests. One year after fire, the average C content (60 cm depth) of PP/DF and LP/ES soil was 30% (25 Mg ha-1) and 10% (7 Mg ha-1) lower than control soil. Average N content on the burned PP/DF and LP/ES plots was 46% (3.0 Mg ha-1) and 13% (0.4 Mg ha-1) lower than control soil. The reduction in C and N in the PP/DF soil was largely the result of lower nutrient capitals in the burned Bw horizons (12–60 cm depth) relative to control plots. It is unlikely that the 1994 fire substantially affected nutrient capitals in the Bw horizons; however, natural variability or past fire history could be responsible for the varied nutrient capitals observed in the subsurface soils. Surface erosion (sheet plus rill) removed between 15 and 18 Mg ha-1 of soil from the burned plots. Nutrient losses through surface erosion were 280 kg C ha-1 and 14 kg N ha-1 in the PP/DF, whereas LP/ES losses were 640 and 22 kg ha-1 for C and N, respectively. In both forests, surface erosion of C and N was ∼1% to 2% of the A-horizon capital of these elements in unburned soil. A bioassay (with lettuce as an indicator plant) was used to compare soils from low-, moderate- and high-severity burn areas relative to control soil. In both forests, low-severity fire increased lettuce yield by 70–100% of controls. With more severe fire, yield decreased in the LP/ES relative to the low-intensity burn soil; however, only in the high-severity treatment was yield reduced (14%) from the control. Moderate- and high-severity burn areas in the PP/DF were fertilized with ∼56 kg ha-1 of N four months prior to soil sampling. In these soils, yield was 70–80% greater than the control. These results suggest that short-term site productivity can be stimulated by low-severity fire, but unaffected or reduced by more severe fire in the types of forests studied. Post-fire fertilization with N could increase soil productivity where other environmental factors do not limit growth.

Nutrient composition of Douglas-fir rhizosphere and bulk soil solutions

Rhizosphere soil solution is the direct source of nutrients for plant uptake. The nutrient composition of rhizosphere soil solution can be very different from that of bulk soil solution due to root exudation, nutrient uptake and rhizosphere microorganism activity. This study examined the nutrient composition of Douglas-fir rhizosphere soil solution in two soils belonging to the Nisqually and Pitcher soil series and compared rhizosphere solution with that of bulk soil solution. Fertilized and unfertilized Nisqually soils were also compared. Soil solutions were collected using centrifugation. Results indicated that nutrient concentrations in the rhizosphere solutions were typically higher than that of bulk soil solutions when no fertilizer was applied. Differences in the concentrations of nutrients between the rhizosphere and bulk soil solutions were masked by the addition of fertilizers. Rhizosphere solution pH also appeared to be affected by the concentration of NH4 and NO3 in the solution. With a higher concentration of NH4 relative to NO3 in the rhizosphere soil solution, the solution pH of the rhizosphere was lower than that of the bulk soil, but with a lower concentration of NH4 relative to NO3, the solution pH of the rhizosphere was higher than that of the bulk soil solution.

Measuring the cation exchange capacity of forest soils

Various methods have been proposed for assessing the cation exchange capacity (CEC) of soils. A pH 7 buffered ammonium acetate solution is the most widely used method but its use for forest soils has been questioned as it may result in an overestimation of CEC in soils with pH dependent charges. The objective of this study was to determine the most appropriate method for measuring the natural CEC of forest soils using variations of 3 common methods. These methods included: (1) pH 7 buffered ammonium acetate (2) unbuffered 1.0, 0.5, and 0.1 M ammonium chloride, and (3) the compulsive exchange method using the original method and with magnesium chloride. The CEC of three reference materials (kaolinite, vermiculite and humified organic matter) was predetermined and used for comparison. In addition, samples from the major genetic horizons of 8 soils from Washington State, USA, the North Island, New Zealand, and Indonesia were analyzed for CEC. Results showed that the compulsive exchange method resulted in consistently low CEC values, and did not reflect changes in soil organic matter or pH. The pH 7 buffered ammonium acetate resulted in artificially high CECs in soils high in organic matter and low pH, undoubtedly due to increases in pH dependent charges. Although the 0.5 and 0.1 M NH4Cl would work well for many forest soils, there may be insufficient NH4 + to completely displace all cations and fill all exchange sites in soils with high CEC. The unbuffered 1.0 M NH4Cl saturation solution is recommended for forest soils.

Timber harvesting residue treatment: Part II. Understory vegetation response.

Foresters must have knowledge of understory vegetation responses to harvesting rehabilitation operations to manage competition effects on tree seedlings, protect species diversity, and reduce invasive weeds. Understory vegetation response to six post-harvest slash treatments, ranging from a high-intensity burn to mechanically chopped slash and untreated slash left in place, was documented for four eastern Washington mixed forest sites. We use species abundance graphs to show the relative effects of the six slash-treatment disturbances on vegetation dominance and diversity. Harvest substantially reduced understory cover, particularly among forbs. Shrub species persisted following harvest and in some treatments increased in cover. Slash treatments increased the abundance of weedy species that are not normally present in these forests. Treatments such as broadcast burning and pile/burn showed greater dominance by invader species. Overall, harvesting reduced species diversity but the response among slash treatments varied. After three growing seasons, species cover, richness, and diversity had no clear effect on seedling growth in slash-treatment plots.

Hyperspectral Analysis of Soil Nitrogen, Carbon, Carbonate, and Organic Matter Using Regression Trees

The characterization of soil attributes using hyperspectral sensors has revealed patterns in soil spectra that are known to respond to mineral composition, organic matter, soil moisture and particle size distribution. Soil samples from different soil horizons of replicated soil series from sites located within Washington and Oregon were analyzed with the FieldSpec Spectroradiometer to measure their spectral signatures across the electromagnetic range of 400 to 1,000 nm. Similarity rankings of individual soil samples reveal differences between replicate series as well as samples within the same replicate series. Using classification and regression tree statistical methods, regression trees were fitted to each spectral response using concentrations of nitrogen, carbon, carbonate and organic matter as the response variables. Statistics resulting from fitted trees were: nitrogen R2 0.91 (p < 0.01) at 403, 470, 687, and 846 nm spectral band widths, carbonate R2 0.95 (p < 0.01) at 531 and 898 nm band widths, total carbon R2 0.93 (p < 0.01) at 400, 409, 441 and 907 nm band widths, and organic matter R2 0.98 (p < 0.01) at 300, 400, 441, 832 and 907 nm band widths. Use of the 400 to 1,000 nm electromagnetic range utilizing regression trees provided a powerful, rapid and inexpensive method for assessing nitrogen, carbon, carbonate and organic matter for upper soil horizons in a nondestructive method.

Fire severity effects on soil organic matter from a ponderosa pine forest: a laboratory study

This study investigated the changes in soil organic matter composition by controlling fire severity of laboratory burns on reconstructed surface soil profiles (O, A1 (0–1 cm), and A2 (1–2 cm)). Laboratory burning simulated prescribed burns that would be typical in the understorey of a ponderosa pine forest at low, moderate, and high–moderate severity levels. Soils were analysed for C, N and soil organic matter composition. Soil organic matter was fractionated into humin, humic acid, fulvic acid, soluble non‐humic materials and other hydrophobic compounds. In the O horizon, low‐, moderate‐, and high‐severity treatments consumed an increasing proportion of C and N. Carbon content of the mineral soil was unaffected by burning; however, N content of the A2 horizon decreased after the moderate‐ and high‐severity treatments, likely as a result of N volatilisation. The proportion of non‐soluble material in the O horizon increased with fire severity, whereas the proportion of humin C as total C of the A horizon decreased with fire severity. The decrease in humin was followed by an increase in the other hydrophobic compounds. The higher fire intensity experienced by the burning O horizon created recalcitrant materials while an increase in labile soil organic matter was observed in mineral soil. An increase in labile soil organic matter may cause elevated C and N mineralisation rates often seen after fire.

Mining Impacts on Trace Metal Content of Water, Soil, and Stream Sediments in the Hei River Basin, China

The impacts of mining to watersheds are highly variabledepending on the type of mining, processing of ores, andenvironmental factors. This study examined the Hei River incentral China, for impacts of gold and iron mining onconcentrations of metals in river water, river sediments andstream-channel soils. No production processing of ores occurson-site at either mine. Total metal content and extractablemetals using DTPA were determined. Total concentrations of Cd,Cu, Pb and Zn were high in some stream sediments and soils nearthe mine sites; metal concentrations ranged from 4–24, 11–100,11–380, and 33–1600 μg g-1 for Cd, Cu, Pb, and Zn,respectively, in soil. Total cadmium was high in all soilsand sediments. Extractable metals were low, with the exceptionof Pb and Cu. At the gold mine, extractable Pb ranged from 8 to33%; extractable Cu ranged from 3 to 21% of total metalconcentration. Chromium and Ni were not above typicalconcentrations in either soils or river sediments. An abundance of carbonates, high river water pH, and high water flow rates all appear to contribute to limiting quantities of metals in the river water. If mining activities are not changed, impacts of mining on downstream metal concentrations in river water should be nominal.

Cadmium, copper, and zinc adsorption by a forest soil in the presence of sludge leachate

Samples from three genetic horizons of an acidic forest soil were equilibrated with solutions containing Cd, Cu, and Zn in the presence and absence of a municipal sewage sludge leachate. Copper adsorption was greater than Cd and Zn in all three horizons, while Cd and Zn adsorption was quite similar. Relative to a NaN03 background solution, sludge leachate reduced Cu adsorption in all horizons; however, in the presence of leachate Zn adsorption increased in the B2 and C horizons, while Cd adsorption decreased in the Al and was unaffected in the B2 and C horizons. Distribution coefficients generally increased as solution concentration and adsorption increased. In all horizons additions of Cd and Zn were able to reduce apparent surface charge. Copper not only reduced surface charge in the Al horizon, but caused a charge reversal in the 132 and C horizons. Adsorption data were best fit by linear or Freundlich equations. Differences in adsorption between sludge leachate and NaN03 solutions could not be explained simply by differences in metal activities as calculated using the GEOCHEM program. Competition from cations and organics present m the sludge leachate appears to contribute to lower metal adsorption.

Site disturbance effects on a clay soil under radiata pine. I. Soil solutions and clay mineral stability

Timber harvesting of forested lands can cause impacts which reduce the long-term productivity of the soil. This study examined long-term effects of timber harvesting on soil morphology, soil solutions and clay mineral stability. A disturbance study established in 1981 an Ultisol located in the North Island of New Zealand was examined in 1990. Disturbance treatments were installed following cable logging of radiata pine (Pinus radiata D. Don); treatments consisted of no disturbance (UN), O horizon removed (OR), and O and A horizons removed with compaction of the Bt1/A horizon (OARHC). The morphology of the A and Bt1/A horizons of the OR treatment showed little difference from the UN treatment. Soil solutions were collected using centrifugation and soil mineralogy determined. Soil solutions of the O horizon had nutrient concentrations that were approximately 10 times greater than that of the mineral horizons, indicating that nutrient availability would be reduced by reduced by removal of this horizon during harvesting. Soil solutions of the Bt1/A horizon showed substantially lower nutrient concentrations in the OARHC treatment compared to the UN and OR treatment, at 9-years after treatment. Stability diagrams of soil solutions for clay minerals of the soil showed that smectite was unstable and weatherable with the highest disturbance treatment, but was stable in the no and low disturbance treatment. No disturbance effects were evident in the stability of iron minerals. Results suggest that this soil is capable of returning to pre-disturbance conditions well-within a rotation period when disturbance is limited, but that recovery with the highest disturbance treatment could take substantially longer.