Coupling field observations, soil modeling, and air dispersion algorithms to estimate 1,3-dichloropropene and chloropicrin flux and exposure

Soil fumigants are volatile compounds applied to agricultural land to control nematode populations, weeds, and crop diseases. Field trials used for measuring fumigant loss from soil to the atmosphere encompass only a small proportion of the near semi-infinite parameter combinations of environmental, agronomic, and meteorological conditions. One approach to supplement field observations uses a soil physics model for fumigant emission predictions. A model is first validated against existing field study observations and then used to extrapolate results to a wider range of edaphic and climatic conditions. This work compares field observations of 1,3-dichloropropene and chloropicrin emissions to predictions from the USDA soil model CHAIN_2D. Comparison between model predictions and field observations for a Florida and California study had values between 0.62 to 0.81 and 0.99 to 1.0 for discrete and cumulative emission flux, respectively. CHAIN_2D emission rates were then coupled to several USEPA air dispersion models (ISCST3, CALPUFF6) to extend emission estimates to near field air concentrations. CALPUFF6 predicted slightly higher 1-h maximum air concentrations than ISCST3 for the same source strength (26.2-36.0% for setbacks between 1 and 250 m from the field edge, respectively). A sensitivity analysis for the CHAIN_2D/ISCST3 coupled numerical system is provided, with several soil and irrigation parameters consistently the most sensitive. Changes in the depth of incorporation, tarp material, and initial soil water content illustrate the predicted impact to emission strength and resulting near-field air concentrations with reductions of cumulative emission loss from 8.1 to 71% and average 1-h maximum air concentration reductions between 6.2 and 41% depending on the mitigation strategy chosen. Additionally, a stochastic framework based on the published SOFEA system that couples variability in experiment, model sensitivity, and site specific attributes is outlined should regional air concentration estimates resulting from fumigant use be sought.     

Phosphorus concentrations in soil and subsurface water: a field study among cropland and riparian buffers

Riparian buffers can be effective at removing phosphorus (P) in overland flow, but their influence on subsurface P loading is not well known. Phosphorus concentrations in the soil, soil solution, and shallow ground water of 16 paired cropland-buffer plots were characterized during 2004 and 2005. The sites were located at two private dairy farms in Central New York on silt and gravelly silt loams (Aeric Endoaqualfs, Fluvaquentic Endoaquepts, Fluvaquentic Eutrudepts, Glossaquic Hapludalfs, and Glossic Hapludalfs). It was hypothesized that P availability (sodium acetate extractable-P) and soil-landscape variability would affect P release to the soil solution and shallow ground water. Results showed that P availability tended to be greater in crop fields relative to paired buffer plots. Soil P was a good indicator of soil solution dissolved (<0.45 microm) molybdate-reactive P (DRP) concentrations among plots, but was not independently effective at predicting ground water DRP concentrations. Mean ground water DRP in corn fields ranged from < or =20 to 80 microg L(-1), with lower concentrations in hay and buffer plots. More imperfectly drained crop fields and buffers tended to have greater average DRP, particulate (> or =0.45 microm) reactive P (PRP), and dissolved unreactive P (DUP) concentrations in ground water. Soil organic matter and 50-cm depth soil solution DRP in buffers jointly explained 75% of the average buffer ground water DRP variability. Results suggest that buffers were relatively effective at reducing soil solution and shallow ground water DRP concentrations, but their impact on particulate and organic P in ground water was less clear.     

Rototillage,disking, and subsequent irrigation: effects on soil nitrogen dynamics,microbial biomass,and carbon dioxide efflux

Spring and summer tillage are usually followed by irrigation before planting crops in California's summer-dry Mediterranean-type climate. Tillage treatments such as rototillage or disking are known to disturb the soil structure to different extents, but little is known about how the intensity of a tillage event and subsequent irrigation affect the microbial biomass, respiration, CO2 efflux, and mineral N of agricultural soils. We carried out an experiment with a Yolo silt loam (fine-silty, mixed, superactive, nonacid, thermic Mollic Xerofluvent) with two tilled treatments (rototillage and disked and rolled) and a nontilled control. The soil was subsequently sampled throughout a 17-d period. Nine days after tillage, all treatments were lightly sprinkler-irrigated to bring the soil water potential above -10 kPa. After tillage, the soil ammonium and nitrate content increased rapidly relative to the control with highest increases in the disked soil. Mineral N remained higher in the tilled treatments after irrigation. Rototillage and disking increased the CO2 efflux of the soil within 24 h of the disturbance. The increase was higher in the disked soil, which was more than three times the CO2 efflux of the control soil at 0.25 h after tillage. This effect may be due to degassing of dissolved CO2 since microbial respiration did not increase in tilled soils. Irrigation increased the CO2 efflux of all treatments but this was most pronounced in the control soil, which had an order of magnitude increase in CO2 efflux after irrigation. An ancillary experiment carried out under similar conditions but with more frequent sampling showed that increases in CO2 efflux after irrigation were accompanied by increases in soil respiration. This study shows that different tillage implements affect CO2 efflux, nitrate accumulation, and microbial activity, and thus have different effects on soil and atmospheric environmental quality.     

Enhancing phytoextraction: the effect of chemical soil manipulation on mobility, plant accumulation, and leaching of heavy metals

For heavy metal-contaminated agricultural land, low-cost, plant-based phytoextraction measures can be a key element for a new land management strategy. When agents are applied into the soil, the solubility of heavy metals and their subsequent accumulation by plants can be increased, and, therefore, phytoextraction enhanced. An overview is given of the state of the art of enhancing heavy metal solubility in soils, increasing the heavy metal accumulation of several high-biomass-yielding and metal-tolerant plants, and the effect of these measures on the risk of heavy metal leaching. Several organic as well as inorganic agents can effectively and specifically increase solubility and, therefore, accumulation of heavy metals by several plant species. Crops like willow (Salix viminalis L.), Indian mustard [Brassica juncea (L.) Czern.], corn (Zea mays L.), and sunflower (Helianthus annuus L.) show high tolerance to heavy metals and are, therefore, to a certain extent able to use the surpluses that originate from soil manipulation. More than 100-fold increases of lead concentrations in the biomass of crops were reported, when ethylenediaminetetraacetic acid (EDTA) was applied to contaminated soils. Uranium concentrations could be strongly increased when citric acid was applied. Cadmium and zinc concentrations could be enhanced by inorganic agents like elemental sulfur or ammonium sulfate. However, leaching of heavy metals due to increased mobility in soils cannot be excluded. Thus, implementation on the field scale must consider measures to minimize leaching. So, the application of more than 1 g EDTA kg(-1) becomes inefficient as lead concentration in crops is not enhanced and leaching rate increases. Moreover, for large-scale applications, agricultural measures as placement of agents, dosage splitting, the kind and amount of agents applied, and the soil properties are important factors governing plant growth, heavy metal concentrations, and leaching rates. Effective prevention of leaching, breeding of new plant material, and use of the contaminated biomass (e.g., as biofuels) will be crucial for the acceptance and the economic breakthrough of enhanced phytoextraction.     

Stray field nuclear magnetic resonance of soil water: development of a new, large probe and preliminary results

Development, characterization, and preliminary results of a recent technique capable of local measurements of pore-size distribution by a spatially resolved low resolution nuclear magnetic resonance (NMR) technique are described. Potential environmental uses include studying the change in pore-size distribution caused by surface compaction, which influences surface runoff, and obtaining information on the physical state of non-aqueous compounds in porous materials, which should aid the selection of appropriate soil remediation methods. Stray field (STRAFI) imaging is an NMR technique that allows distortion-free imaging of materials with short NMR relaxation times. The sample is placed in the strong axial fringe field gradient of a superconducting NMR magnet. We report on a new, unique, large 5-cm-diameter STRAFI probe, and its use for three preliminary test cases: water in ceramics of known pore size, paraffin wax and oil in sandstone rock, and water in soil at different matric potentials. The imaging is confined to one dimension with a spatial resolution of the order of 100 microm for protons. The optimum position for imaging occurs at 2.62 T and a gradient of 12.1 T/m. Water relaxation decay curves can be measured at any position in the 8-cm-long sample. These curves are decomposed into a series of terms each corresponding to a different pore size. Preliminary results show continuum fits to decay curves for a soil drained to three different matric potentials. Such information will be useful for interpreting water retention curves and will lead to understanding of the behavior of fluids in the vadose zone.