No abstract

The use of urea as a nitrogen source for crop production has increased in recent years. Because the hydrolysis of urea to ammonia and carbon dioxide is mediated by the enzyme urease, efficient use of urea requires an understanding of factors that influence urea hydrolysis in soil. The purpose of this research was to relate the rate of urea hydrolysis to chemical, physical, and biological properties for soils in pasture and in cultivation. For each of 22 soils in Kansas, Missouri, and Oklahoma, samples were collected from paired fields, one in pasture and one in cultivation. The samples were taken from both 0 to 2.5 cm and 0 to 15 cm, and urea hydrolysis rates were measured in field‐moist and air‐dried samples. Urea hydrolysis rates in air‐dried soils ranged from 1 to 149 µg urea g soil−1 h−1 and from 1 to 117 µg urea g soil−1 h−1 in field moist soils. For each soil, the greatest urea hydrolysis rate was in the 0 to 2.5‐cm depth of pasture samples. The urea hydrolysis rate for each soil was greater in pasture samples than in samples from cultivated fields. For samples from cultivated fields, urea hydrolysis rates in both field‐moist and air‐dry samples were positively correlated with total nitrogen (TN) and organic carbon (OC). Only in the field moist soils were urea hydrolysis rates correlated with TN and OC in the soils from pastures. Urea hydrolysis rates in the 0 to 2.5‐cm depth of pastures exhibited the greatest change from field‐moist to air‐dry conditions. Rates generally decreased with drying, but did remain constant or increased with drying in some cases. Both the greatest rates and the greatest variation in urea hydrolysis rates between field‐moist and air‐dry soils were observed in the surface 0 to 2.5 cm samples from pastures.

Abstract-2,4-Dichlorophenoxyacetic acid (2,4-D) was first registered in 1947 as an agricultural herbicide, and it is still the most widely used herbicide worldwide. End-use products, however, are generally formulated as inorganic or amine salts, or as esters. Because of the various forms available, there was concern that by testing only one form, the environmental profile generated might be insufficient to represent all forms. Therefore, as part of the 2,4-D reregistration process in the U.S., 30 soil dissipation studies were conducted with 2,4-D dimethylamine salt and 2,4-D 2-ethylhexyl ester over a 2-year period. Trials were located in seven states and included four cropping practices and bare soil. The results, averaged over all conditions, showed equivalent rates of 2,4-D dissipation in soil when applied as either the amine salt or ester forms. These results also confirm data from earlier field studies in Canada and Washington state showing equivalent 2,4-D dissipation in soil from applications of isooctyl ester, dimethylamine salt, and mixed amine salt forms. The data from the current and former studies show that ester and amine forms have little effect on the rate of dissipation of 2,4-D per se because they are converted rapidly to the same anionic form.

2,4-Dichlorophenoxyacetic acid (2,4-D) was first registered in 1947 as an agricultural herbicide, and it is still the most widely used herbicide worldwide. End-use products, however, are generally formulated as inorganic or amine salts, or as esters. Because of the various forms available, there was concern that by testing only one form, the environmental profile generated might be insufficient to represent all forms. Therefore, as part of the 2,4-D reregistration process in the U.S., 30 soil dissipation studies were conducted with 2,4-D dimethylamine salt and 2,4-D 2-ethylhexyl ester over a 2-year period. Trials were located in seven states and included four cropping practices and bare soil. The results, averaged over all conditions, showed equivalent rates of 2,4-D dissipation in soil when applied as either the amine salt or ester forms. These results also confirm data from earlier field studies in Canada and Washington state showing equivalent 2,4-D dissipation in soil from applications of isooctyl ester, dimethylamine salt, and mixed amine salt forms. The data from the current and former studies show that ester and amine forms have little effect on the rate of dissipation of 2,4-D per se because they are converted rapidly to the same anionic form.

Dustfall deposition samples were collected during the months of June, July, and August for 2 years prior (1973 and 1974) and 2 years following (1976 and 1977) the addition of an oil‐fired, 660 MW generating unit (No. 3) to two 330 MW coal‐fired units (No. 1 & 2) at Chalk Point located in Prince Georges County, Md. Triplicate collectors were placed at 12 research sites located at distances of 1.6, 4.8, and 9.6 km in the north, east, south, and west directions from the power plant and were analyzed for water‐insoluble particulate matter and soluble and water‐insoluble Zn, Cu, Mn, Pb, and Ni contents.The average rates of deposition for water‐insoluble particulates declined from 4.73 ± 3.67 g m−2 month−1 during 1973 and 1974 to 1.9 ± 1.79 g m−2 month−1 during 1976 and 1977. The reductions in particulate deposition rates coincided with a 37% reduction in power generation output from Units 1 and 2 and the onset of Unit 3 operations.Prior to Unit 3 operations, soluble and total Ni deposition rates were found to be significantly (p < 0.05) related to direction; however, the rates of Cu, Mn, and Pb deposition were observed to be significantly (p < 0.05) higher at 1.6‐km than at 4.8‐ or 9.6‐km distances from the facility. After Unit 3 began operating, rates for Cu, Mn, Zn, and Pb deposition were significantly (p < 0.05) reduced and showed no relationships to distance or directions from the source. The shifts in soluble Ni and soluble Pb deposition rates from pre‐ to post‐Unit 3 operations were particularly noteworthy. The concentrations of Cu, Mn, and Zn in water‐insoluble particulates were higher with increased distance and showed significant (p < 0.05) increases after Unit 3 began operation.The relative rankings for average rates of total deposition for the five metals during pre‐Unit 3 operations were Pb (191 µg m−2 day−1) > Cu ≅ Zn > Mn > Ni (11 µg m−2 day−1), which changed to Cu (98 µg m−2 day−1) > Pb ≅ Zn > Mn > Ni (12 µg m−2 day−1) after Unit 3 began operations.