Wondi Mersie

SOIL REDOX, pH, TEMPERATURE, AND WATER-TABLE PATTERNS OF A FRESHWATER TIDAL WETLAND

Knowledge of wetland hydrology, soil redox potential, pH, and temperature dynamics are key components required to understand the capacity of tidal wetlands to function, in particular to attenuate agrichemicals. In a freshwater tidal wetland along the James River in Virginia, USA, redox potential, pH, water-table level, and soil temperature were monitored continuously at two depths (20 and 50 cm) at three sites during a 12-month period from September 1, 1997 to August 31, 1998. Redox potentials were at or below − 150 mV (methanogenic or sulfate reducing conditions) at the 50-cm depth during the entire monitoring period. At the 20-cm depth, redox potentials remained highly reducing 95% of the time. The soil is continuously wet throughout the year, with the water-table level above the 20-cm soil depth for 95% of the time. Water-table level or hydrology was the primary factor controlling fluctuations in the redox state. Soil pH values were generally between 6 and 8, and they dropped 1 pH unit upon an oxidation event, which was reversible. Soil temperature at the 50-cm depth never dropped below 5° C, indicating a year-round biological activity season. This wetland supports a large diversity of plant species. Permanently reduced sub-surfaces, year-round biological activity, and large organic matter accumulations are characteristic features of this freshwater tidal wetland.

Atrazine and Metolachlor Sorption to Switchgrass Residues

Abstract Herbicide retention by residue mulch in vegetative filter strips could be an effective attenuation mechanism for removing herbicides from runoff. Adsorption studies were conducted to quantify the amount of atrazine and metolachlor that can be adsorbed and removed from runoff by thatch or fresh switchgrass residue. Adsorption of C14‐atrazine and metolachlor on thatch or fresh switchgrass residue was conducted by using concentrations of 2.5, 7.5, 13.2, and 26.4 µmol L−1 and a 24‐h equilibration period. Adsorption coefficients (Kd) decreased in the order, atrazine sorption on fresh switchgrass residue (81.1 L−1 kg−1), metolachlor sorption on fresh residue (32.9 L−1 kg−1), atrazine sorption on thatch residue (21.4 L−1 kg−1), and metolachlor sorption on thatch switchgrass residue (15.1 L−1 kg−1). On a volumetric basis (Kd-vol), only atrazine showed a significantly greater Kd-vol value on fresh residue than on the thatch residue. Absorption through cut ends of the residues (especially the fresh residue) may have produced adsorption capacities that would not be observed under field conditions. Fresh or thatch switchgrass residue in vegetative filter strips can help abate atrazine and metolachlor by intercepting and sorbing some of the herbicides.

REMOVAL AND DEGRADATION OF ATRAZINE AND METOLACHLOR BY VEGETATIVE FILTER STRIPS ON CLAY LOAM SOIL

The effectiveness of filter strips, with and without vegetation, in removing dissolved atrazine and metolachlor in runoff was investigated using aluminum tilted beds set at 1% slope on Cullen clay loam soil. Runon containing atrazine and metolachlor was applied on the up-slope end of the simulated filter strips. Water samples from surface runoff, lateral subsurface movement, and leachates as well as filter strip soil samples were collected and herbicide concentrations determined. The filter strips reduced the amount of dissolved atrazine and metolachlor in runoff by about 6% of the amount applied. The absence or presence of switchgrass did not affect the amount of herbicide filtered. About 56 to 82% of the runon volume leached through the 30-cm soil depth of the filter strips. In the leachate, about 72 to 88% of the amount of applied herbicide was filtered or adsorbed to the soil. The presence of switchgrass reduced the amount of runoff volume and increased the amount of leachate volume. In total, about 53 to 73% of the amount of herbicide applied was removed by the filter strips. The primary mode of dissolved herbicide removal in applied runon was by infiltration and soil adsorption mechanisms. Soil herbicide concentrations were greatest at the 0 to 10 cm depth, decreased to less than 50 μg kg−1 over a 7-week period. In the filter strip soil, the presence of switchgrass significantly increased the degradation rate of metolachlor, but not atrazine. Infiltration of runoff into the filter strips is key to reducing dissolved herbicides from moving offsite. The presence of surface connected macropores is important in facilitating this process on heavier textured soils.

Degradation of metolachlor in bare and vegetated soils and in simulated water-sediment systems

A study was conducted to determine the half-life (t1/2), degradation rate, and metabolites of metolachlor in a water-sediment system and in soil with and without switchgrass. Metolachlor degradation in a laboratory was determined in sediment from Bojac sandy loam soil incubated at 24 degrees C. The study also was conducted in a greenhouse on tilted beds filled with Bojac soil and planted with switchgrass. In both experiments, samples were collected at days 0, 7, 14, 28, 42, 56, and 112 and analyzed for metolachlor and its major metabolites. The water-sediment oxidation-reduction potential took 28 d to reach -371 mV and the pH increased from 5.6 to 6.5 by the last sampling day (day 112). The average soil temperature of the tilted beds with or without switchgrass during the study was 21degrees C and the soil moisture content was 23% by volume. The t1/2 of metolachlor was 34 d in sediment and 8 d in the water phase. The t1/2 of metolachlor in soil from the switchgrass filter strip (6 d) was not different from that in soil without grass (9.6 d). The metolachlor metabolites ethanesulfonic acid (ESA) and oxanilic acid (OA) were detected in the water-sediment system and in soil from tilted beds. In both sediment and soil from tilted beds, the two metabolites peaked by day 56 of incubation and declined after that, indicating transformation to other products. In the water-sediment system, greater quantities of OA and ESA were detected in sediment than in the aqueous phase. The production of OA and ESA in the watersediment system occurred in the first 28 d, when the system was at an aerobic redox state. Metolachlor can degrade in sediment and the relatively high soil temperature and moisture level accelerated its breakdown in beds with and without switchgrass. Under warm and moist soil conditions, the presence of switchgrass has no effect on the degradation of metolachlor.

Effect of temperature, disturbance and incubation time on release and degradation of atrazine in water columns over two types of sediments

Abstract The availability of sediment-bound atrazine to overlaying water is key in assessing its risk to aquatic organisms. A laboratory study was conducted to determine the potential release and degradation of sediment-bound atrazine in overlaying water columns. Sediments were generated from Cullen (1.3 % organic carbon, 38% clay) and Emporia (0.63% organic carbon, 12% clay) soils and were incubated at two temperatures (5 and 24 °C) for 336 days. The concentration of atrazine in the water columns was determined before and after disturbance of the sediments to simulate dredging of water bodies. At 5 °C, the release of atrazine from sediment to water columns was biphasic, whereby a short period of rapid diffusion was followed by a longer period of slow release. With no sediment disturbance, about 40% of the atrazine applied to sediment diffused into the water columns at 5 °C. At 24 °C, less than 15% of theapplied amount entered the water columns; this was due to the greater degradation rate of atrazine at this temperature. Initially, the disturbance of sediments caused greater amounts of atrazine to be released into the water columns at 5 °C, but disturbance caused no effect after 112 days. At 24 °C, the amount of atrazine released from sediments after disturbance was less than at 5 °C, and was minimal after 28 days. There was some diffusion of metabolites out of the Cullen sediment at 24 °C. In Emporia sediments however, metabolites were released only after disturbance of the sediment. Release of sediment-bound atrazine decreased with time and the decline was rapid at 24 °C, because of degradation of the herbicide. Disturbance of sediments, such as dredging, is unlikely to remobilize aged atrazine in warmer environments.

Response of beans and hairy fleabane leaves to ozone and paraquat with and without the antiozonant, ethylenediurea

Abstract Growth chamber experiments were conducted to determine if there is a pattern of cross-tolerance to paraquat and ozone (O 3 ) using an O 3 -insensitive bean ( Phaseolus vulgaris L.) genotype, and paraquat-resistant hairy fleabane [ Conyza bonariensis (L.) Cronq.] biotype. The additional objective was to determine if the antiozonant, EDU, protects seedlings against paraquat injury. Bean and fleabane seedlings were treated with EDU and exposed to O 3 at 0, 0.1, 0.3 and 0.5 ppm (v/v) for 6 hr or paraquat at 0, 0.1, 0.3 and 0.5 mM. Responses were evaluated by measuring fresh weight and chlorophyll content. EDU protected the O 3 -sensitive bean genotype and both hairy fleabane biotypes from O 3 injury. However, EDU did not improve the tolerance of the paraquat-susceptible hairy fleabane or the two bean genotypes to paraquat. The paraquat-resistant hairy fleabane was not tolerant to O 3 , and the O 3 -insensitive bean genotype did not show resistance to paraquat. No cross-tolerance between O 3 and paraquat in beans and hairy fleabane was detected.

Metolachlor fate and mobility in a tidal wetland soil

A study was conducted to determine the fate of the herbicide metolachlor in a tidal wetland soil located along the James River in Virginia, USA. Soil adsorption/desorption and mineralization characteristics and mobility of metolachlor were determined on the Levy tidal wetland soil. The metolachlor Kd value was 65.8 L kg−1 and Koc value was 810 L kg−1 C−1, which are much greater than would be found on typical agricultural soils. After four 24-h desorption periods, the total amount of metolachlor that desorbed ranged from 16 to 22% of the amount initially adsorbed. Metolachlor mineralization was about 0.46% of the amount applied after 84 days, which indicates that it does not mineralize very readily in the Levy soil. The amount of metolachlor that leached from the soil columns averaged 1.64% of the amount applied after 84 days, showing a low degree of mobility. These results suggest that the Levy tidal wetland soil can function as an effective filter of metolachlor and other similar herbicides that enter via agricultural runoff, protecting surface and ground waters.

Generation mean analysis of inheritance of insensitivity to ozone injury in two crosses of beans (Phaseolus vulgaris L.)

The inheritance of bean (Phaseolus vulgaris L.) foliar insensitivity to ozone (03) was studied using generation mean analyses (GMA) as the genetic design. The following GMAs were evaluated: ‘Bush Blue Lake (BBL) 290’ x PI 304833, and ‘BBL 254’ x PI 300657. The six populations (P,, P2, F,, F2, F, x P,, and F, x P2) comprising a GMA from each cross were planted in 15 cm plastic pots as a randomized complete block design with three replications. When unifoliate leaves were about two-third expanded and maximally sensitive to 03 they were fumigated with non-filtered ambient (NF) + 0.06 ppm 03 concentration in open top field-fumigation chambers. Data on percent leaf 03 injury were recorded. The mean rated values of the incidence of O, injury in the F, generation were lower than the mid-parent values in both crosses, suggesting that insensitivity may be dominant over sensitivity. A genetic model with additive-dominance effects was fitted to generation means in ‘BBL 290’ x PI 304833, and ‘BBL 254’ x PI 300657 and...