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<OL> <LI> To characterize and model pore structure of intact soil aggregates using X-ray computer microtomography and to quantify the differences in pore structures of the aggregates from the same soil type but under long-term (>18 years) differential land use and management settings, namely: a)conventionally plowed row crop agriculture; b)conservationally managed (no-till) row crop agriculture; and c)native vegetation on land with previous agricultural use. <LI>To study E. coli distribution within the aggregates, namely: a)identify areas within the aggregates that are accessible to E. coli; b)assess the potential for E. coli of moving out of the aggregate after the aggregate is rewetted and subjected to saturated water flux; and c)conduct comparisons among the three soil treatments listed in Objective 1. <LI>Based on the results from objectives 1-3 a)relate E. coli fate and movement within aggregates to the aggregate internal pore structures and pore distributions in the aggregates of the three studied soil treatments; b)assess E. coli potential for reentering the water current during a water flow event through the aggregate as related to the aggregate pore network structure; and to compare these potentials in the aggregates from the three studied soil treatments.
NON-TECHNICAL SUMMARY: Presently, the fate of Escherichia coli in soils in rural agricultural settings is still not completely understood. It is clear that filtration, adsorption and die-off are the processes which impact the transport of these bacteria, however, differential survival, desorbtion and regrowth are occurring as well and likely are dependent on the micro environment within soil aggregates. Without improved assessment of the mechanisms associated with E. coli transport and survival in the soil, the ability to develop best management practices for manure application to minimize E. coli transport to surface and ground waters will be limited. Understanding the influence of the within aggregate micro environments on E. coli transport and survival can now be greatly facilitated by recent advances in X-ray computer tomography allowing obtaining high resolution images of interiors structure of undisturbed soil aggregates. The main objective of the proposed project is to relate E. coli transport and survival in soil aggregates to the aggregate pore networks and structures as delineated through X-ray computer topography. Three-dimensional pore structures of soil aggregates representing soils under widely used in Midwest agricultural practices and native forests obtained with 3-15 micron resolution will be related to E. coli adsorption and desorbtion, transport and survival within aggregate interiors. The proposed project addresses the first priority of the 26.Water and Watersheds program by contributing to understanding "the sources, fate, and transport of pathogens, such as bacteria, protozoa, and viruses in soil, surface and ground water" with special emphasis on E. coli.
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APPROACH: Soil aggregates will be collected from three treatments at the Long Term Ecological Research Site (LTER-KBS) Michigan: 1) a conventional tillage (chisel-plowed) corn-soybean-wheat rotation with conventional chemical inputs (T1), 2) no-till corn-soybean-wheat rotation with conventional chemical inputs (T2), and 3) native successional treatment, abandoned after spring plowing in 1989 (T7). Preselected soil aggregates will be X-rayed and then subjected to several experiments with Escherichia coli. Aggregate image data will be collected on the bending magnet beam line at the Advanced Photon Source (APS), Argonne National Laboratory (ANL), IL. The first step of the experiments with aggregates will consist of the Escherichia coli application to all the aggregates. Then, the aggregates will be randomly subdivided into three groups (15 aggregates per group). Aggregates of the first group will be subjected to peeling procedure that will separate exterior, interior and intermediate (transitional) aggregate layers. E. coli measurements will be conducted separately for soil of each layer. Aggregates of the second and third groups will be subjected to saturation and subsequent 5 minute saturated water flow, one flow event in group one and two flow events in group two. Then these aggregates will be peeled and E. coli determined in the three layers of each aggregate. In addition, soil pore networks within aggregates will be visualized, described and quantified. The pore network information will be used to explain the results of the E.coli experiments and to model E.coli transport within soil aggregates. By looking with computer microtomography tools at a soil aggregate as a key building block of soil matrix, we can answer a number of questions clarifying fate and transport of pathogens in soil. For example, how does the soil pore structure affect the ability of E. coli to enter soil aggregates? Once within, which parts of the aggregate the E. coli can reach in the aggregates with different pore structures, and what are the environmental conditions within the aggregate for it survival and regrowth? Later, when the aggregate is subjected to saturated water flow during rainfall and runoff events, what are the E. coli numbers from within the aggregate that can leave the aggregate and become a potential surface water and groundwater contaminant? How are these numbers related to the aggregate pore structure?