Dynamic multicrop model to characterize impacts of pesticides in food

A new dynamic plant uptake model is presented to characterize health impacts of pesticides applied to food crops, based on a flexible set of interconnected compartments. We assess six crops covering a large fraction of the worldwide consumption. Model estimates correspond well with observed pesticide residues for 12 substance-crop combinations, showing residual errors between a factor 1.5 and 19. Human intake fractions, effect and characterization factors are provided for use in life cycle impact assessment for 726 substance-crop combinations and different application times. Intake fractions typically range from 10?2 to 10?? kg(intake) kg(applied)?1. Human health impacts vary up to 9 orders of magnitude between crops and 10 orders of magnitude between pesticides, stressing the importance of considering interactions between specific crop-environments and pesticides. Time between application and harvest, degradation half-life in plants and residence time in soil are driving the evolution of pesticide masses.We demonstrate that toxicity potentials can be reduced up to 99% by defining adequate pesticide substitutions. Overall, leafy vegetables only contribute to 2% of the vegetal consumption, but due to later application times and higher intake fractions may nevertheless lead to impacts comparable or even higher than via the larger amount of ingested cereals.     

Uptake of metals during chelant-assisted phytoextraction with EDDS related to the solubilized metal concentration

The use of chelants to enhance phytoextraction is one method being tested to make phytoextraction efficient enough to be used as a remediation technique for heavy metal pollution in the field. We performed pot experiments with sunflowers in order to investigate the use of the biodegradable chelating agent SS-EDDS for this purpose. We used singly and combined contaminated soils (Cu, Zn) and multimetal contaminated field soils (Cu, Zn, Cd, Pb). EDDS (10 mmol kg(-10 soil) increased soil solution metals greatly for Cu (factor 840-4260) and Pb (factor 100-315), and to a lesser extent for Zn (factor 23-50). It was found that Zn (when present as the sole metal), Cu, and Pb uptake by sunflowers was increased by EDDS, butin multimetal contaminated soil Zn and Cd were not. EDDS was observed in the sunflower roots and shoots at concentrations equal to metal uptake. The different metal uptake in the various soils can be related to a linear relationship between Cu and Zn in soil solution in the presence of EDDS and plant uptake, indicating the great importance of measuring and reporting soil solution metal concentrations in phytoextraction studies.     

Evaluation of in Situ DGT Measurements for Predicting the Concentration of Cd in Chinese Field-Cultivated Rice: Impact of Soil Cd:Zn Ratios

DGT (diffusive gradients in thin-films) has been proposed as a tool for predicting Cd concentrations in rice grain, but there is a lack of authenticating data. To further explore the relationship between DGT measured Cd and concentrations in rice cultivated in challenging, metal degraded, field locations with different heavy metal pollutant sources, 77 paired soil and grain samples were collected in Southern China from industrial zones, a "cancer village" impacted by mining waste and an organic farm. In situ deployments of DGT in flooded paddy rice rhizospheres were compared with a laboratory DGT assay on dried and rewetted soil. Total soil concentrations were a very poor predictor of plant uptake. Laboratory and field deployed DGT assays and porewater measurements were linearly related to grain concentrations in all but the most contaminated samples where plant toxicity occurred. The laboratory DGT assay was the best predictor of grain Cd concentrations, accommodating differences in soil Cd, pollutant source, and Cd:Zn ratios. Field DGT measurements showed that Zn availability in the flooded rice rhizospheres was greatly diminished compared to that of Cd, resulting in very high Cd:Zn ratios (0.1) compared to commonly observed values (0.005). These results demonstrate the potential of the DGT technique to predict Cd concentrations in field cultivated rice and demonstrate its robustness in a range of environments. Although, field deployments provided important details about in situ element stoichiometry, due to the inherent heterogeneity of the rice rhizosphere soils, deployment of DGT in dried and homogenized soils offers the best possibility of a soil screening tool.     

Estimating the influence of forests on the overall fate of semivolatile organic compounds using a multimedia fate model

On the basis of recently reported measurements of semivolatile organic compound (SOC) uptake in forest canopies, simple expressions are derived that allow the inclusion of a canopy compartment into existing non-steady-state multimedia fate models based on the fugacity approach. One such model is used to assess how the inclusion of the canopy compartment in the model affects the calculated overall behavior of SOCs with specific physical--chemical properties. The primary effect of the forest is an increase in the net atmospheric deposition to the terrestrial environment, reducing atmospheric concentrations and accordingly the extent of deposition to the agricultural and aquatic environments. This effect was most pronounced for chemicals with log KOA around 9-10 and log KAW -2 to -3; their average air concentrations during the growing season decreased by a factor of 5 when the canopy compartment was included. Concentration levels in virtually all compartments are decreased at the expense of increased concentrations in the forest soil. The effect of the forest lies not in a large capacity for these chemicals but in the efficiency of pumping the chemicals from the atmosphere to the forest soil, a storage reservoir with high capacity from which the chemicals can return to the atmosphere only with difficulty. Because of seasonal variability of canopy size and atmospheric stability, uptake into forests is higher during spring and summer than in winter. The model suggests that this may dampen temperature-driven seasonal fluctuations of air concentrations and in regions with large deciduous forests may lead to a temporary, yet notable dip in air concentrations during leaf development in spring. A sensitivity analysis revealed a strong effect of forest cover, forest composition, and degradation half-lives. A high degradation loss on the plant surface has the effect of preventing the saturation of the small plant reservoir and can cause very significant reductions in atmospheric concentrations of those SOCs for which uptake in the canopy is limited by the size of the reservoir.     

Use of diffusive gradients in thin films (DGT) in undisturbed field soils

The technique of diffusive gradients in thin films (DGT) has been shown to be a promising tool to assess metal bioavailability in soils under laboratory conditions. In this study we used DGT to investigate the resupply kinetics of Cu and Zn under in-situ conditions in a polluted lysimeter soil and compared the results with laboratory measurements using undisturbed soil cores at defined water contents as well as homogenized soil samples. Results differed considerably between these treatments, although the same soil material was used in all experiments. A small pool of rapidly available Zn was found in the field but not in the homogenized soil. Soil solution pH and dissolved metal concentrations also varied significantly between the soil treatments. In addition, we compared the DGT-measured effective concentration with the uptake of Cu and Zn into the shoots of Lolium perenne (Ryegrass) under the same three types of conditions, i.e., field, soil cores, and homogenized soil. A close relationship was found which was not linear but could be described by a saturation-type behavior. L. perenne is a metal excluder plant, and thus, metal accumulation is limited by translocation of metals from roots to shoots. DGT predicted plant metal uptake much better than the soil solution concentration or pH. The results of this study suggest that DGT may be successfully used under field conditions to study the kinetics of metal resupply. Plant metal concentrations were not well predicted in all cases by the effective concentration CE under field conditions. Some plants took up considerably more metals than estimated by CE. Variations in metal uptake independent of their bioavailability can be caused by local variations in microsite conditions, e.g. light, temperature, water, and nutrients. To some degree, such indetermination has to be expected as an inherent feature of the system and the concept of bioavailability.     

Arsenic in soil and irrigation water affects arsenic uptake by rice: complementary insights from field and pot studies

Groundwater rich in arsenic (As) is extensively used for dry season boro rice cultivation in Bangladesh, leading to long-term As accumulation in soils. This may result in increasing levels of As in rice straw and grain, and eventually, in decreasing rice yields due to As phytotoxicity. In this study, we investigated the As contents of rice straw and grain over three consecutive harvest seasons (2005-2007) in a paddy field in Munshiganj, Bangladesh, which exhibits a documented gradient in soil As caused by annual irrigation with As-rich groundwater since the early 1990s. The field data revealed that straw and grain As concentrations were elevated in the field and highest near the irrigation water inlet, where As concentrations in both soil and irrigation water were highest. Additionally, a pot experiment with soils and rice seeds from the field site was carried out in which soil and irrigation water As were varied in a full factorial design. The results suggested that both soil As accumulated in previous years and As freshly introduced with irrigation water influence As uptake during rice growth. At similar soil As contents, plants grown in pots exhibited similar grain and straw As contents as plants grown in the field. This suggested that the results from pot experiments performed at higher soil As levels can be used to assess the effect of continuing soil As accumulation on As content and yield of rice. On the basis of a recently published scenario of long-term As accumulation at the study site, we estimate that, under unchanged irrigation practice, average grain As concentrations will increase from currently ～0.15 mg As kg(-1) to 0.25-0.58 mg As kg(-1) by the year 2050. This translates to a 1.5-3.8 times higher As intake by the local population via rice, possibly exceeding the provisional tolerable As intake value defined by FAO/WHO.     

Relationships between the critical concentrations of nitrogen,phosphorus and potassium in 17 different vegetable crops and duration of growth

Fifty-three field experiments were carried out to define the way in which the critical concentrations of nutrients in vegetable crops decline with increasing age so as to provide better criteria for diagnosing deficiencies and to provide inputs to models for fertiliser response. Each of 17 vegetable crops were grown with levels of nitrogen (N), phosphorus (P) and potassium (K) fertiliser that resulted in near maximum yields and little luxury consumption of nutrients. Between five and 20 individual plants were harvested at intervals (up to 13) during the growing period and analysed separately. A model was derived and used to summarise the data as curves of % organic-N, P and K, and sum of cations (milli-equivalents) in the dry matter with time. Mean plant dry weight-time relations were also obtained. Unexplained variation about the best fitting curves was substantial and was not appreciably reduced by taking account of mean plant weight as well as time. Much of this unexplained variation was attributed to differences between plants within a harvest and some of it to harvest to harvest deviations from the model. However, little was associated with year to year variability. The net result is that 95% confidence limits were about ±25%. The significance of these findings to detecting nutrient deficiencies and providing inputs to models are discussed.     

Simulating herbicide volatilization from bare soil affected by atmospheric conditions and limited solubility in water

A numerical model that simulates pesticide fate was developed to predictthe behavior of triallate after application to a field soil. The model has options that allow water and/ or heat transport and can limit simulated aqueous-phase concentrations to triallate solubility in water. Several methods for describing the volatilization boundary condition were tested to assess the accuracy in predicting the volatilization rate, including an approach that requires no atmospheric information and an approach that couples soil and atmospheric processes. Four scenarios were constructed and simulated, to compare with measured volatilization rates. The peak measured volatilization rate (168 g ha(-1) h(-1)) was most accurately predicted with the scenario that included the most complex model (100 g ha(-1) h(-1)). The simplest model overpredicted the peak rate (251 g ha(-1) h(-1)), and the others underpredicted the peak rate (16-67 g ha(-1) h(-1)). The simulations that limited aqueous solubility provided relatively similar values for the total emissions (21-37% of applied triallate), indicating that simplified models may compare well with measurements (31% of applied). A prospective simulation over a period of 100 days showed that applying triallate to the soil surface would ultimately lead to atmospheric emissions of 80% of the applied material with 6% remaining in soil. Incorporating triallate to a depth of 10 cm would reduce emissions to less than 5% and lead to 41% remaining in soil.     

Implications of mercury speciation in thiosulfate treated plants

Mercury uptake was induced in two cultivars of Brassica juncea under field conditions using thiosulfate. Analysis was conducted to better understand the mechanism of uptake, speciation of mercury in plants, and redistribution of mercury in the soil. Plant mercury and sulfur concentrations were increased after thiosulfate treatment, and a linear correlation between mercury and sulfur was observed. Mercury may be absorbed and transported in plants as the Hg-thiosulfate complex. The majority of mercury in treated plant tissues (two cultivars) was bound to sulfur in a form similar to β-HgS (66-94%). Remaining mercury was present in forms similar to Hg-cysteine (1-10%) and Hg-dicysteine (8-28%). The formation of β-HgS may relate to the transport and assimilation of sulfate in plant tissues. Mercury-thiosulfate complex could decompose to mercuric and sulfate ions in the presence of free protons inside the plasma membrane, while sulfide ions would be produced by the assimilation of sulfate. The concomitant presence of mercuric ions and S(2-) would precipitate β-HgS. The mercury concentration in the rhizosphere decreased in the treated relative to the nontreated soil. The iron/manganese oxide and organic-bound fractions of soil mercury were transformed to more bioavailable forms (soluble and exchangeable and specifically sorbed) and taken up by plants.     

Regulatory FOCUS Surface Water Models Fail to Predict Insecticide Concentrations in the Field

The FOrum for the Co-ordination of pesticide fate models and their USe (FOCUS) exposure models are used to predict the frequency and magnitude of pesticide surface water concentrations within the European regulatory risk assessment. The predictions are based on realistic worst-case assumptions that result in predicted environmental concentrations (PEC). Here, we compared for the first time a larger data set of 122 measured field concentrations (MFC) of agricultural insecticides extracted from 22 field studies to respective PECs by using FOCUS steps 1-4. While FOCUS step 1 and 2 PECs generally overpredicted the MFCs, 23% of step 3 and 31% of step 4 standard PECs were exceeded by surface water MFCs, which questions the protectiveness of the FOCUS exposure assessment. Using realistic input parameters, step 3 simulations underpredicted MFCs in surface water and sediment by 43% and 78%, respectively, which indicate that a higher degree of realism even reduces the protectiveness of model results. The ratios between PEC and MFC in surface water were significantly lower for pyrethroids than for organophosphorus or organochlorine insecticides, which suggests that the FOCUS predictions are less protective for hydrophobic insecticides. In conclusion, the FOCUS modeling approach is not protective for insecticide concentrations in the field.     

Rhizosphere gradients of polycyclic aromatic hydrocarbon (PAH) dissipation in two industrial soils and the impact of arbuscular mycorrhiza

Phytoremediation of organic pollutants depends on plant-microbe interactions in the rhizosphere, but the extent and intensity of such rhizosphere effects are likely to decrease with increasing distance from the root surface. We conducted a time-course pot experiment to measure dissipation of polycyclic aromatic hydrocarbons (PAHs) in the rhizosphere of clover and ryegrass grown together on two industrially polluted soils (containing 0.4 and 2 g kg(-1) of 12 PAHs). The impact of the fungal root symbiosis arbuscular mycorrhiza (AM) on PAH degradation was also assessed, as these fungi have previously improved plant establishment on PAH-polluted soils and enhanced PAH degradation in spiked soil. The two soils behaved differently with respect to the time-course of PAH dissipation. The less polluted and more highly organic soil showed low initial PAH dissipation rates, with small positive effects of plants after 13 weeks. At the final harvest (26 weeks), the amounts of PAHs extracted from nonplanted pots were higher than the initial concentrations. In parallel planted pots, PAH concentrations decreased as a function of proximity to roots. The most polluted soil showed higher initial PAH dissipation (25% during 13 weeks), but at the final harvest PAH concentrations had increased to values between the initial concentration and those at 13 weeks. An effect of root proximity was observed for the last harvest only. The presence of mycorrhiza generally enhanced plant growth and favored growth of clover at the expense of ryegrass. Mycorrhiza enhanced PAH dissipation when plant effects were observed.     

Measurement of DDT fluxes from a historically treated agricultural soil in Canada

Organocohlorine pesticide (OCP) residues in agricultural soils are of concern due to the uptake of these compounds by crops, accumulation in the foodchain, and reemission from soils to the atmosphere. Although it has been about three decades since DDT was banned for agricultural uses in Canada, residues persist in soils of some agricultural areas. Emission of DDT compounds to the atmosphere from a historically treated field in southern Ontario was determined in fall 2004 and spring 2005. The sigmaDDTs concentration in the high organic matter (71%) soil was 19 +/- 4 microg g(-1) dry weight. Concentration gradients in the air were measured at 5, 20, 72, and 200 cm above soil using glass fiber filter-polyurethane foam cartridges. Air concentrations of sigmaDDTs averaged 5.7 +/- 5.1 ng m(-3) at 5 cm and decreased to 1.3 +/- 0.8 ng m(-3) at 200 cm and were 60-300 times higher than levels measured at a background site 30 km away. Soil-air fugacity fractions, fs/(fs + fa), of p,p'-DDE, p,p'-DDD, and p,p'-DDT ranged from 0.42 to 0.91 using air concentrations measured above the soil and > or = 0.99 using background air concentrations, indicating that the soil was a net source to the background air. Fractionation of DDT compounds during volatilization was predicted using either liquid-phase vapor pressures (PL) or octanol-air partition coefficients (KOA). Relative emissions of p,p'-DDE and p,p'-DDT were better described by PL than KOA, whereas either PL or KOA successfully accounted for the fractionation of p,p'-DDT and o,p'-DDT. Soil-to-air fluxes were calculated from air concentration gradients and turbulent exchange coefficients determined from micrometeorological measurements. Average fluxes of sigmaDDTs were 90 +/- 24 ng m(-2) h(-1) in fall and 660 +/- 370 ng m(-2) h(-1) in spring. Higher soil temperatures in spring accounted for the higher fluxes. A volatilization half-life of approximately 200 y was estimated for sigmaDDT in the upper 5 cm of the soil column, assuming the average flux rate for 12 h d-(1) over 8 months of the year. Thus, in the absence of other dissipation processes, the soil will continue to be a source of atmospheric contamination for a very long time.     

Empirical models of cadmium accumulation in maize,rye grass and soya bean plants

Linear regression of published values for soil parameters and cadmium concentrations in plant tissues offers the opportunity to develop uptake coefficients that can be applied in a wide range of circumstances. A widespread literature search was performed which identified publications from the last 20 years containing information on cadmium uptake by maize and rye grass plants. After discarding experiments with inadequate data or parameters, 10 and eight papers were chosen for maize and rye grass respectively to develop linear models that related pH and cadmium concentration in the growth media (soil in pots or nutrient solution) to cadmium concentration in maize or rye grass plants (excluding roots). Cadmium concentrations in both maize and rye grass were positively correlated with soil cadmium concentration, and in the case of maize negatively correlated with soil pH. They were also negatively correlated with the product of soil cadmium concentration and soil pH, demonstrating that at high soil cadmium concentration a high soil pH reduced plant cadmium concentration. A further model of data generated from one experiment with soya beans demonstrated that other factors, such as soil temperature, can have a major influence on uptake. Copyright © 2004 Society of Chemical Industry     

烟叶中多菌灵农药残留的降解规律和影响因素

为探索有效控制烟叶中多菌灵农药残留的措施,采用液相色谱-串联质谱检测技术,研究了多菌灵及其他常见苯并咪唑类杀菌剂在烟草上的农药残留特征及环境因子、采收后处理方式和储存条件对其降解的影响,明确了合理使用条件下多菌灵农药残留水平与形成机制。结果表明:基于QuEchERS前处理与液相色谱-串联质谱检测技术可以快速、准确检测3种常用苯并咪唑类杀菌剂,标准曲线、定量限、添加回收率符合农药残留检测方法要求;多菌灵、苯菌灵、甲基硫菌灵在鲜烟叶中的理论半衰期为6.8、6.3、6.6 d;25℃、湿度60%的环境最利于多菌灵降解;3种农药残留在冻干、杀青、烘烤过程的加工因子分别为5.11~5.66、3.95~4.55、2.96~3.39;多菌灵在常温和恒温条件下储存烟叶中的半衰期为224~365 d。在植物体内活性酶的作用下,田间降解是农药残留降解的主要方式,施药时药液浓度、末次施药到采收的间隔时间对多菌灵残留量有显著影响,加工方式、储存环境及储存时间也与其有一定关系。      

Uptake of copper,nickel and zinc by crops growing in contaminated soils

A series of three replicated pot trials is reported in which various crops were grown in soils having enhanced concentrations of copper, nickel and zinc. Concentrations of these elements in the tops of plants harvested at their most sensitive stage were compared with ‘total’ and ‘extractable’ concentrations in soil and with concentrations in soil solutions. There was little difference between the relationships of ‘total’ and ‘extractable’ soil metal and concentrations in plant tissue. In general, the correlation between concentrations of metals in soils and plants was unpredictable. Plants differed in their efficiency of uptake of elements; lettuce assimilated more than the other crops tested (barley, rape and ryegrass). Similarly, soil concentrations of the elements required to achieve toxic thresholds in plant tops increased in the order lettuce, ryegrass, rape and barley. Measurements made with conventional extractants of copper, nickel and zinc in soils can be of value in predicting plant uptake and hence toxicity, only if appropriate calibration curves plotting extractable soil metal against plant uptake are at hand for the particular soils and plants under consideration. Mild extracts are more sensitive to the soil properties, especially pH value, which determine plant uptake and results with metals in soil solution were promising, especially for zinc. Nevertheless, soil analyses for copper, nickel and zinc are not always closely associated with their likely toxicity to crops.     

Integration of sensory and objective measurements of tomato quality: quantitative assessment of the effect of harvest date as compared with growth medium (soil versus rockwool), electrical conductivity,variety and maturity

Sensory characteristics as well as objective measures of texture, refractory index, colour and total acid content were measured in tomatoes (Lycopersicon esculentum Mill.) from two experiments comprising variation in harvest time, electrical conductivity (EC), growth medium (soil versus rockwool), variety and maturity. For most characteristics, simultaneous sensory evaluations were more sensitive for detection of small differences than corresponding objective measurements, while the objective measurements obtained better significances in comparisons of different harvest times. Using the correlations between parallel sensory and objective analyses of firmness, a procedure to combine objective and sensory measurements of texture was defined and used to compensate for the drift of sensory data at different evaluation times, and the calibrated measure of firmness used in a principal components analysis. For most sensory characteristics assessed, the greatest variation was due to differences in variety, followed by maturity, harvest time and EC, while the type of growth medium (soil or rockwool) had no or little effect. However, for the characteristics related to texture (crispness and firmness), the ranking was harvest time, EC, growth medium, maturity and variety, with soil‐grown tomatoes being slightly but significantly softer than the rockwool‐grown ones. Copyright © 2005 Society of Chemical Industry     

Antimony (Sb) and arsenic (As) in Sb mining impacted paddy soil from Xikuangshan, China: differences in mechanisms controlling soil sequestration and uptake in rice

Foods produced on soils impacted by antimony (Sb) mining activities are a potential health risk due to plant uptake of the contaminant metalloids (Sb) and arsenic (As). Here we report for the first time the chemical speciation of Sb in soil and porewater of flooded paddy soil, impacted by active Sb mining, and its effect on uptake and speciation in rice plants (Oryza sativa L. cv Jiahua). Results are compared with behavior and uptake of As. Pot experiments were conducted under controlled conditions in a climate chamber over a period of 50 days. In pots without rice plants, flooding increased both the concentration of dissolved Sb (up to ca. 2000 μg L(-1)) and As (up to ca. 1500 μg L(-1)). When rice was present, Fe plaque developing on rice roots acted as a scavenger for both As and Sb, whereby the concentration of As, but not Sb, in porewater decreased substantially. Dissolved Sb in porewater, which occurred mainly as Sb(V), correlated with Ca, indicating a solubility governed by Ca antimonate. No significant differences in bioaccumulation factor and translocation factor between Sb and As were observed. Greater relative concentration of Sb(V) was found in rice shoots compared to rice root and porewater, indicating either a preferred uptake of Sb(V) or possibly an oxidation of Sb(III) to Sb(V) in shoots. Adding soil amendments (olivine, hematite) to the paddy soil had no effect on Sb and As concentrations in porewater.     

Laboratory and field measurements of dry deposition of sulfur dioxide onto Chinese loess surfaces

Laboratory and field measurements were conducted to examine dry deposition of SO2 onto Chinese loess surfaces using native soil sampled in the loess plateau, China. The field tests were employed in Beijing and Lanzhou, China, by directly measuring the dry deposition of SO2 on soil, which uses soil put on a collector as an SO2 passive sampling medium. In the laboratory, a high rate of uptake to SO2 deposition for Chinese soil surfaces due to the highly alkalinity was found. The uptake of SO2 deposition was dependent on the pH soil and relative humidity. Furthermore, we evaluated some factors that affect the measurement precision: response of SO2 uptake, repeatability, recovery factor, and variability associated with the weight and the surface coverage on the collectors. As a result, it was shown that the measurement precision was primarily related to the ratio of the SO2 deposition amount relative to the sulfur content of the original soil. This result was consistent with the field observations. The laboratory and field results indicated an excellent agreement on the SO2 uptake inherent in the results from the soil surfaces in different regions.     

Short communication: Effect of corn planting population on phosphorus concentration and uptake in corn silage

Greater utilization of nutrients reduces the potential runoff of nutrients to bodies of water. The objective of this study was to determine the concentration of P in corn biomass to estimate the removal of P from the soil when planting corn at different population levels. Whole-plant corn samples were collected during an on-farm research project conducted previously. The study included 7 different growing and harvesting conditions. In each cornfield, corn was planted in plots at a theoretical seeding rate of 55,000, 70,000, 85,000, and 100,000 seeds/ha. Each seeding rate had 4 replicates within each field. At harvesting time, 5 consecutive plants from the 2 center rows and at 2 randomly selected spots within each plot were cut by hand at 15 cm above ground. Whole plants were weighed and chopped. After mixing thoroughly, a sample of the chopped material was placed in a bag, immediately placed in a cooler with dry ice, and transferred to the laboratory for storage. After thawing and drying, samples were ground and analyzed for P concentration. Single plant biomass and the number of standing plants at harvesting were used to determine dry matter yield. Total extraction of P was estimated as the product between plant biomass and P concentration. All variables were analyzed using a statistical model that included the effects of field, planting population, planting population nested within field, and random residual error. The concentration of P in the corn plant was greatest for 55,000 and 70,000 plants/ha (0.250% dry matter) and least for 85,000 and 100,000 plants/ha (0.235% dry matter), whereas the uptake of P through the harvested biomass increased when corn planting population increased. In conclusion, increasing the planting population of corn for silage can increase P uptake from the soil, therefore reducing the potential runoff of P to bodies of water.     

Solubility and toxicity of antimony trioxide (Sb2O3) in soil

Antimony trioxide (Sb2O3) is a widely used chemical that can be emitted to soil. The fate and toxicity of this poorly soluble compound in soil is insufficiently known. A silt-loam soil (pH 7.0, background 0.005 mmol Sb kg(-1)) was amended with Sb2O3 at various concentrations. More than 70% of Sb in soil solution was present as Sb(V) (antimonate) within 2 days. The soil solution Sb concentrations gradually increased between 2 and 35 days after Sb2O3 amendment but were always below that of soils amended with the more soluble SbCl3 at the lower Sb concentrations. The soil solution Sb concentrations in freshly amended SbCl3 soils (7 days equilibration) were equivalent to those in Sb2O3-amended soils equilibrated for 5 years at equivalent total soil Sb. Our data indicate that the Sb solubility in this soil was controlled by a combination of sorption on the soil surface, Sb precipitation at the higher doses, and slow dissolution of Sb2O3, the latter being modeled with a half-life ranging between 50 and 250 days. Toxicity of Sb to plant growth (root elongation of barley, shoot biomass of lettuce) or to nitrification was found in soil equilibrated with Sb2O3 (up to 82 mmol Sb kg(-1)) for 31 weeks with 10% inhibition values at soil solution Sb concentrations of 110 microM Sb or above. These concentrations are equivalent to 4.2 mmol Sb per kg soil (510 mg Sb kg(-1)) at complete dissolution of Sb2O3 in this soil. No toxicity to plant growth or nitrification was evident in toxicity tests starting one week after soil amendment with Sb2O3, whereas clear toxicity was found in a similar test using SbCl3. However, these effects were confounded by a decrease in pH and an increase in salinity. It is concluded that the Sb(V) toxicity thresholds are over 100-fold larger than background concentrations in soil and that care must be taken to interpret toxicity data of soluble Sb(III) forms due to confounding factors.