BACTERIAL UPTAKE AND SOIL SORPTION OF IONIZABLE AGROCHEMICALS

Authors: HULTGREN, RYAN - UNIV OF ILLINOIS; ELVERSON, ELIZABETH - UNIV OF ILLINOI; CUPPLES, ALISON - UNIV OF ILLINOIS; HUDSON, ROBERT - UNIV OF ILLINOIS; SIMS, GERALD

Submitted to: Proceedings of American Chemical Society National Meeting
Publication Type: Proceedings
Publication Acceptance Date: 11/21/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: A number of pesticides exhibit pH-dependent soil sorption. In some cases sorption is thought to relate persistence by reducing bio-availability to microorganisms, thus pH may indirectly affect persistence through effects on sorption. For such compounds, uptake by soil bacteria may also be controlled by pH, though this topic has received much less attention. An experiment was conducted to assess the soil sorption and bacterial uptake of three organic compounds by three species of soil bacteria in pure culture over a range of pH. Two of the compounds studied, prosulfuron and cloransulam-methyl, are herbicides that exhibited pH-dependence in both soil sorption and bacterial uptake. The third compound, naphthalene, is a simple hydrocarbon and exhibited no pH-dependence in either sorption or uptake. Conditions favoring uptake of both herbicides also favored sorption, thus suggesting that persistence could occur over a wide range of fconditions due to either soil sorption or insufficient uptake limiting microbial access to the herbicides. These results provide new insight into reasons for pesticide persistence, and help us to predict persistence over a wide range of environmental conditions. Improved ability to predict fate is essential to designing improved management systems. Thus, the primary impact of this research will be in the development of management hydrolysis, thus dissipation is much faster under acidic conditions. Soil persistence of a related herbicide, flumetsulam has been shown to exhibit striking dependence on bioavailability, which is in turn, dependent upon pH. The results reported suggest a potential for modeling pH-dependent uptake and sorption of ionizable agrochemicals.

Technical Abstract: A number of pesticides contain ionizable functional groups, and thus exhibit pH dependent soil sorption. Ionizable pesticides are thought to sorb to soil or be taken up by cells as a neutral species, thus both processes should exhibit similar pH trends. An experiment was conducted to assess the uptake of three organic compounds by three species of soil bacteria (Pseudomonas testosteroni, Acinetobacter sp., and Streptomyces griseolus) in liquid media over a range of pH. Two of the compounds studied, prosulfuron and cloransulam-methyl, are ionizable herbicides. The third compound, naphthalene, is a non-ionizable hydrocarbon. Isotherm studies were performed using a collection of soils representing a variety of organic carbon (OC) contents and pH values. Bacterial uptake of both pesticides was pH dependent, with permeation decreasing as solution pH values increased. As expected, permeability to naphthalene was independent of solution pH. Both prosulfuron and cloransulam-methyl exhibited strong trends of decreasing Kd values with increasing solution pH. Based on results herein, rates of uptake of prosulfuron from solution and subsequent degradation would be expected to decrease as pH increases. Similarly, sorption-induced bioavailability limitation would appear likely in acid soils, however, prosulfuron is weakly sorbed, and undergoes acid hydrolysis, thus dissipation is much faster under acidic conditions. Soil persistence of a related herbicide, flumetsulam has been shown to exhibit striking dependence on bioavailability, which is in turn, dependent upon pH. The results reported suggest a potential for modeling pH-dependent uptake and sorption of ionizable agrochemicals.