Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/6/2008
Publication Date: 6/21/2008
Citation: Regitano, J., Koskinen, W.C. 2008. Characterization of Nicosulfuron Availability in Aged Soils. Journal of Agricultural and Food Chemistry. 56:5801-5805.
Interpretive Summary: The amount of pesticide in soil that is available for movement to the target pest or for movement into surface and ground waters is controlled by the degree of binding of the chemical to the soil and the rate at which it degrades. Binding and degradation of pesticides are usually characterized for pesticides freshly added to soil, however we have shown in previous studies that the length of time the pesticide is in contact with soil, aging, can affect these processes. While there is limited information on binding and degradation of some aged pesticides, there is no information on binding of a newer class of sulfonylurea herbicides, such as nicosulfuron, which are applied at low rates to soil. The present study was conducted to determine the changes in distribution between soil bound and solution phases of nicosulfuron, with incubation time. We found that while nicosulfuron degraded in soil, the remaining chemical became more tightly bound to the soil. Therefore, the aged residues would be less available for movement to surface and ground waters. These results are further evidence that increases in binding of the pesticide to soil during pesticide aging should be taken into account during characterization of the environmental fate of pesticides and in the development of mathematical models of pesticide degradation and transport.
Technical Abstract: Sorption-desorption interactions of pesticides with soil determine the availability of pesticides in soil for transport, plant uptake, and microbial degradation. These interactions are affected by the physical and chemical properties of the pesticide and soil, and for some pesticides, their residence time in the soil. Changes in sorption/availability of nicosulfuron (2-[[[[(4,6-dimethoxy-2-pyrimidinyl]amino]carbonyl]amino]sulfonyl]-N,N-dimethyl-3-pyridinecarboxamide) with aging in different soils were determined. Soil moisture was adjusted to -33 kPa and 14C- and analytical-grade nicosulfuron were added to the soil at a rate of 1.0 mg/kg. Spiked soils were incubated at 25 oC for up to 41 d. Replicate soil samples were periodically extracted successively with 0.01 N CaCl2, and then twice with methanolic (NH4)2CO3. Sufficient nicsulfuron remained (>10 % of applied) during incubation to allow calculation of sorption coefficients. Aging significantly increased sorption as indicated by increased sorption coefficients. For instance, for nicosulfuron remaining after the 41-d incubation, Kd increased by a factor of 2 to 3 in Mollisols from the MidWestern United States and by a factor of 5 to 9 in Oxisols from Hawaii and Brazil. It appears the increase in sorption was because the rate of degradation in solution and on labile sites was faster than the rate of desorption of the neutral species from the soil particles. It may have also been due to nicosulfruon anion diffusion to less accessible sites with time leaving the more strongly bound neutral species for the sorption characterization. Regardless of the mechanism, these results are further evidence that increases in sorption during pesticide aging should be taken into account during characterization of the sorption process for mathematical models of pesticide degradation and transport. These data show the importance of characterization of sorption-desorption in aged herbicide residues in soil, particularly in the case of prediction of herbicide transport in soil. In this case, potential transport of nicosulfuron herbicide would be over predicted if freshly treated soil Kd values were used to predict transport.