A new robust diagnostic polymerase chain reaction for determining the mating status of female Anopheles gambiae mosquitoes.

The principal malaria vector in Africa, Anopheles gambiae, contains two pairs of autosomes and one pair of sex chromosomes. The Y chromosome is only associated with males and other Y chromosome-specific DNA sequences, which are transferred to women during mating. A reliable tool to determine the mating status of dried wild An. gambiae females is currently lacking. DNA was extracted from dried virgin and mated females and used to test whether Y chromosome-specific polymerase chain reaction (PCR) markers can be successfully amplified and used as a predictor of mating. Here we report a new PCR-based method to determine the mating status among successfully inseminated and virgin wild An. gambiae females, using three male-specific primers. This dissection-free method has the potential to facilitate studies of both population demographics and gene flow from dried mosquito samples routinely collected in epidemiologic monitoring and aid existing and new malaria-vector control approaches

Sensible heat measurements indicating depth and magnitude of subsurface soil water evaporation

Most measurement approaches for determining evaporation assume that the latent heat flux originates from the soil surface. Here, a new method is described for determining in situ soil water evaporation dynamics from fine-scale measurements of soil temperature and thermal properties with heat pulse sensors. A sensible heat balance is computed using soil heat flux density at two depths and change in sensible heat storage in between; the sensible heat balance residual is attributed to latent heat from evaporation of soil water. Comparisons between near-surface soil heat flux density and Bowen ratio energy balance measurements suggest that evaporation originates below the soil surface several days after rainfall. The sensible heat balance accounts for this evaporation dynamic in millimeter-scale depth increments within the soil. Comparisons of sensible heat balance daily evaporation estimates to Bowen ratio and mass balance estimates indicate strong agreement (r2 = 0.96, root-mean-square error = 0.20 mm). Potential applications of this technique include location of the depth and magnitude of subsurface evaporation fluxes and estimation of stage 2–3 daily evaporation without requirements for large fetch. These applications represent new contributions to vadose zone hydrology

Temperature Dependence of Water Retention Curves for Wettable and Water-Repellent Soils

The capillary pressure (ψ) in unsaturated porous media is known to be a function of temperature (T). Temperature affects the surface tension (σ) of the pore water, but possibly also the angle of contact (γ). Because information on the temperature dependence of γ in porous media is rare, we conducted experiments with three wettable soils and their hydrophobic counterparts. The objectives were (i) to determine the temperature dependence of the water retention curve (WRC) for wettable and water-repellent soils, (ii) to assess temperature effects on the apparent contact angle γA derived from those WRCs, and (iii) to evaluate two models (Philip-de Vries and Grant-Salehzadeh) that describe temperature effects on ψ. Columns packed with natural or hydrophobized soil materials were first water saturated, then drained at 5, 20, and 38°C, and rewetted again to saturation. Capillary pressure and water content, θ, at five depths in the columns were measured continuously. The observations were used to determine the change in γA with T, as well as a parameter β0 that describes the change in ψ with T It was found that the Philip-de Vries model did not adequately describe the observed relation between ψ and T A mean value for β0 of −457 K was measured, whereas the Philip-de Vries model predicts a value of −766 K. Our results seem to confirm the Grant-Salezahdeh model that predicts a temperature effect on γA For the sand and the silt we studied, we found a decrease in γA between 1.0 to 8.5°, when the temperature was increased from 5 to 38°C. Both β0 and γA were only weak functions of θ. Furthermore, it seemed that for the humic soil under study, surfactants, i.e., the dissolution of soil organic matter, may compound the contact angle effect of the soil solids

Subsurface Flow Barriers to Reduce Nitrate Leaching

Groundwater is a very important natural resource which directly affects many human lives. In the United States, groundwater is the source of about 22 percent of the freshwater used. About 53 percent of the total population and 97 percent of the rural population use groundwater supplies for their drinking water (Moody, 1990). Although contamination of groundwater can occur naturally, agriculture is considered to be one of the most widespread nonprofit sources of groundwater contamination. Among agricultural chemicals, nitrogen-fertilizer has been used most extensively, especially by com producers. About one million tons of nitrogen-fertilizer are used annually in Iowa. In some studies, more than 50 percent of the applied fertilizer nitrogen is not removed by the crop or stored in the soil, and leaching as a form of nitrate is thought to be a major reason for the losses (Blackmer, 1987). Leached nitrate may enter groundwater supplies. Nitrate-nitrogen concentrations found in unsaturated soil below the rootzone of agricultural fields are in the range of 5 to 100 mg!L (Bouwer, 1990). Nitrate-nitrogen concentrations in tile drainage below row crops often exceed 10 mg/L, the U.S.A. drinking water standard (Gast et al., 1978; Baker and Johnson, 1981; Timmons and Dylla, 1981; Baker et al., 1985)

Localized compaction and doming to increase N-use efficiency and reduce leaching

Nitrate-nitrogen leaching from agricultural lands results in inefficient use of nitrogen-fertilizer as well as degradation of groundwater or surface water if leachate returns to the surf ace through artificial drainage or baseflow. Subsurface barriers placed above a fertilizer band have been shown to reduce anion leaching. Laboratory data suggest that compacted soil works well as a subsurface water-flow barrier (Kiuchi et al., 1992; Kiuchi et al., 1994). A field-scale implement has been designed and constructed to inject nitrate-nitrogen fertilizer below the soil surface and create a thin compacted strip of soil above the fertilizer band covered by a small dome of soil. Data from a field study indicate that nitrate-nitrogen placed beneath such a domed, compacted strip is less susceptible to leaching than nitrate-nitrogen placed below the soil surface without such a cover. In 1993, nitrate-nitrogen remaining in the upper soil profile (32 inches deep) after three months of the growing season was 56% of the total amount applied compared with 37% remaining where there was only the typical knife injection band. Grain weight and plant weight at black layer development were not significantly different between the two application methods. Overall grain yields at harvest were different, the conventional knife application technique yielding slightly more than the localized compaction and doming application technique

Sensible Heat Observations Reveal Soil-Water Evaporation Dynamics

Soil-water evaporation is important at scales ranging from microbial ecology to large-scale climate. Yet routine measurements are unable to capture rapidly shifting near-surface soil heat and water processes involved in soil-water evaporation. The objective of this study was to determine the depth and location of the evaporation zone within soil. Three-needle heat-pulse sensors were used to monitor soil heat capacity, thermal conductivity, and temperature below a bare soil surface in central Iowa during natural wetting/drying cycles. Soil heat flux and changes in heat storage were calculated from these data to obtain a balance of sensible heat components. The residual from this balance, attributed to latent heat from water vaporization, provides an estimate of in situ soil-water evaporation. As the soil dried following rainfall, results show divergence in the soil sensible heat flux with depth. Divergence in the heat flux indicates the location of a heat sink associated with soil-water evaporation. Evaporation estimates from the sensible heat balance provide depth and time patterns consistent with observed soil-water depletion patterns. Immediately after rainfall, evaporation occurred near the soil surface. Within 6 days after rainfall, the evaporation zone proceeded \u3e 13 mm into the soil profile. Evaporation rates at the 3-mm depth reached peak values \u3e 0.25 mm h−1. Evaporation occurred simultaneously at multiple measured depth increments, but with time lag between peak evaporation rates for depths deeper below the soil surface. Implementation of finescale measurement techniques for the soil sensible heat balance provides a new opportunity to improve understanding of soil-water evaporation

Crop Residue Effects on Surface Radiation and Energy Balance - Review

Crop residues alter the surface properties of soils. Both shortwave albedo and longwave emissivity are affected. These are linked to an effect of residue on surface evaporation and water content. Water content influences soil physical properties and surface energy partitioning. In summary, crop residue acts to soil as clothing acts to skin. Compared to bare soil, crop residues can reduce extremes of heat and mass fluxes at the soil surface. Managing crop residues can result in more favorable agronomic soil conditions. This paper reviews research results of the quantity, quality, architecture, and surface distribution of crop residues on soil surface radiation and energy balances, soil water content, and soil temperature