Modeling Soil Quality Thresholds to Ecosystem Recovery at Fort Benning, Georgia, USA

The objective of this research was to use a simple model of soil C and N dynamics to predict nutrient thresholds to ecosystem recovery on degraded soils at Fort Benning, Georgia, in the southeastern USA. The model calculates aboveground and belowground biomass, soil C inputs and dynamics, soil N stocks and availability, and plant N requirements. A threshold is crossed when predicted soil N supplies fall short of predicted N required to sustain biomass accrual at a specified recovery rate. Four factors were important to development of thresholds to recovery: (1) initial amounts of aboveground biomass, (2) initial soil C stocks (i.e., soil quality), (3) relative recovery rates of biomass, and (4) soil sand content. Thresholds to ecosystem recovery predicted by the model should not be interpreted independent of a specified recovery rate. Initial soil C stocks influenced the predicted patterns of recovery by both old field and forest ecosystems. Forests and old fields on soils with varying sand content had different predicted thresholds to recovery. Soil C stocks at barren sites on Fort Benning generally lie below predicted thresholds to 100% recovery of desired future ecosystem conditions defined on the basis of aboveground biomass (18000 versus 360 g m{sup -2} for forests and old fields, respectively). Calculations with the model indicated that reestablishment of vegetation on barren sites to a level below the desired future condition is possible at recovery rates used in the model, but the time to 100% recovery of desired future conditions, without crossing a nutrient threshold, is prolonged by a reduced rate of forest growth. Predicted thresholds to ecosystem recovery were less on soils with more than 70% sand content. The lower thresholds for old field and forest recovery on more sandy soils are apparently due to higher relative rates of net soil N mineralization in more sandy soils. Calculations with the model indicate that a combination of desired future conditions, initial levels of soil quality (defined by soil C stocks), and the rate of biomass accumulation determines the predicted success of ecosystem recovery on disturbed soils

Isotope studies to determine dry deposition of sulfate to deciduous and coniferous trees: Final draft

Experiments have been conducted at two locations near Oak Ridge, Tennessee, with radioactive /sup 35/S (87 day half-life) to examine the cycling behavior of sulfur in yellow poplar (Liriodendron tulipifera), red maple (Acer rubrum), and loblolly pine (Pinus taeda) trees. Some findings pertain to methods development for estimating dry deposition of sulfur to forest canopies and the magnitude of sulfur emissions from natural sources (Task II). We will determine through field studies, the internal cycling, storage, and biogenic emission of sulfur, as traced by /sup 35/SO/sub 4//sup 2 -/, in environments impacted by atmospheric sulfate deposition; and will determine through isotope dilution studies, the contribution of foliar leaching and dry deposition to net throughfall (NTF) sulfate concentrations beneath deciduous and coniferous trees in such environments. 3 refs., 2 figs., 1 tab

Evaluation of models for assessing compliance with environmental radiation regulations

The use of environmental transport and dosimetry models to predict the consequences of radionuclide releases from nuclear facilities is discussed. It is pointed out that many input parameters, and hence the predictions, of these models have a high degree of variability. The determination of the uncertainties of the predictions of these models is essential for assessing the adequacy of their use to ensure compliance with radiation protection standards. Estimation of the depletion of an airborne plume via dry deposition and the subsequent transfer of materials from air to ground were studied because values of deposition velocity as applied in assessment models are often misinterpretations of the values obtained from field studies. A sensitivity analysis revealed that at distances where most maximum individual exposures would likely occur as a result of routine releases from a nuclear installation, the plume depletion model commonly used is virtually insensitive to variations in deposition velocity. This is not true, however, for the estimation of deposition, which is a linear function of deposition velocity. Therefore, any variation in the value of the deposition velocity will bring about a like variation in the estimated deposition onto vegetation or ground. The uncertainty associated with the calculation of dose to an infant's thyroid as a consequence of the transport of elemental /sup 131/I via the grass-cow-milk pathway was studied as a function of air concentration. Probabilities were determined from a statistical analysis of reported values for deposition velocity, vegetation retention, and the grass-to-milk transfer coefficient

Sampling of technetium-99 in vegetation and soils in the vicinity of operating gaseous facilities. Environmental Sciences Division Publication No. 1550

Technetium-99 was measured in vegetation and soils collected near three operating gaseous diffusion facilities to obtain estimates of the soil-to-vegetation concentration factor. Analyses were performed primarily using radiochemical separations and low-level beta counting techniques with selected samples checked by isotope dilution mass spectroscopy. The concentration factor pooled from all three facilities appeared to be lognormally distributed with a geometric mean of 9.5 and a geometric standard deviation of 2.4. These values are comparable to calculated equilibrium concentration factors derived from other field experiments in which concentrations in soil and vegetation were observed to decrease with time subsequent to a single application of/sup 95m/TcO/sub 4//sup -/. These field values are one to two orders of magnitude less than concentration factors derived from laboratory experiments using potted plants. It is suggested that the radiologic significance of /sup 99/Tc in terrestrial food chains of importance to humans may be substantially lower than indicated by the simple difference between laboratory and field concentration factors if the time dependence of technetium activity in vegetation and soil is taken into consideration in radiological assessment models

Management and micro-scale landform determine the ground flora of secondary woodlands and their verges in the Tama Hills of Tokyo, Japan

We investigated the influence of management and micro-scale landform on the species composition and richness of ground flora in secondary woodlands and their verges next to paddy fields in the Tama Hills, Tokyo, Japan. Sites representing various micro-scale landforms and different management regimes were sampled. The results of Detrended Correspondence Analysis and Indicator Species Analysis showed that there was no significant difference between management regimes on species composition in head hollow sites. However, species composition was unique in other landform types having the same management regimes. Micro-scale landform units had less effect on species composition than management regime on crest slopes and side slopes of secondary woodlands. A large variation in species composition of verges, all on the lowermost side slopes, was found within sites, but the composition was different when compared with other sites. We recognized five habitat types associated with species composition that resulted from the combined effects of landform and management regimes. Among habitat types, the verges and the well-managed woodlands had high species richness. Management practices within woodlands have enhanced the species richness on crest slopes and side slopes. High beta diversity of ground flora could be due to the variation in micro-scale landform along soil moisture regimes. Intensive management involving clear-cutting on the lower side slopes (an ecotone of mesic and wet environments) accompanied by paddy cultivation on valley bottoms enhanced the diversity of herbaceous vegetation in the verges