| Harmonized methods are necessary to evaluate accurately the impact of pesticides on soil microorganisms, which play an essential role in soil biological processes including nitrogen dynamics, organic matter decomposition and nutrient availability. Although a number of studies on the effects of pesticides, mainly herbicides and insecticides, on soil microorganisms have been reported, very little research has been done to examine the long-term effects of fungicides on soil microbial activities and ecological processes, which are important in maintaining soil fertility. In addition, the impact of fungicides on soil processes, based on interrelationships within microbial populations and community- or system-level interactions, are also poorly understood. In my research, three broad-spectrum fungicides: benomyl, captan, and chlorothalonil, commonly used in current agricultural systems, were chosen to assess their impact on soil ecological processes and to establish harmonized approaches to evaluating their effects. Three experimental stages, laboratory batch incubations, a microcosm technique and a field study were included in evaluating the effects of the three fungicides on soil ecological processes by measuring: (1) soil microbial activity, including substrate-induced soil respiration (SIR) and soil enzymatic activities, (2) nitrogen dynamics, including NH{dollar}sb4{dollar}-N, NO{dollar}sb3{dollar}-N, dissolved organic nitrogen (DON) and microbial biomass nitrogen concentrations (BION), (3) nitrogen transformations including net N mineralization and nitrification, (4) rates of litter decomposition, (5) leaching of nutrients and (6) plant growth and yield. The effects of the fungicides on soil microbial activity, nitrogen dynamics and plant growth depended on the nature and concentrations of the fungicides, the quality of organic amendments, soil types and environmental conditions. Patterns of response of soil microbial activity and nutrient cycling processes to the chemicals were specific to each of the fungicides. Benomyl enhanced soil microbial activity and transformations of NH{dollar}sb4{dollar}-N and DON, that resulted in greater plant biomass and shoot heights. Captan suppressed N mineralization, apparently by reducing microbial activity, as indicated by short-term reductions in SIR and soil dehydrogenase activity. Significant accumulations of soil NH{dollar}sb4{dollar}-N, over 90 days in the captan-treated soils, probably resulted from lower rates of nitrification. Soil microbial activity/biomass and in situ NO{dollar}sb3{dollar}-N concentrations were depressed by the chlorothalonil treatment that resulted in retarded plant growth. Among the three fungicides, captan appeared to have more pronounced overall impact on soil microbial processes than either benomyl or chlorothalonil. The impact of benomyl at typical field application rates on soil microbial processes and the impact on soil environments was much less than that of the other fungicides. Taken together, the techniques and data presented in this study provided not only a much better understanding of the effects of soil applied fungicides and ecological processes, but also an idea how the interaction between the processes influence nutrient availability and plant growth in agricultural systems. |
