Spatial Distributions of E. Coli in Soil Aggregates From Three Contrasting Soil Management Systems.

Differences in micro-scale pore structures are likely to have a substantial effect on the fate of E. coli in soil.  Knowledge of the exact pore locations within soil aggregates opens new opportunities for understanding soil microorganism movement and functioning. The objectives of this study are 1) to characterize and model pore structure of intact soil aggregates using X-ray computed microtomography and to quantify the differences in pore structures of the aggregates from the same soil type but under long-term (~20 years) of different land use and management; 2) to identify areas within soil aggregates from different managements accessible to E. coli; 3) to relate E. coli fate and movement within aggregates to the aggregates’ internal pore structures and pore distributions in the aggregates of the three studied soil treatments. We studied soil aggregates (4-6 mm in size) from sites under conventionally tilled (CT) and no-till (NT) corn/soybean/wheat rotation and native succession vegetation (NS) on Typic Hapludalfs of the Kalamazoo (fine-loamy) series at NSF Long-Term Ecological Research site, southwest Michigan. Intra-aggregate pore images were completed at the Advanced Photon Source, Argonne National Laboratory, IL at resolutions of 14.6 microns. Images were segmented using indicator kriging approach. Pore networks were reconstructed and characterized using 3DMA-Rock software. Aggregates from NS had lower overall porosity than CT aggregates, however, greater amount of large pores and greater pore connectivity led to faster ion and E.coli transport through NS aggregates and lower E.coli intra-aggregate retention. Analysis of accessibility of different regions within the aggregate to E.coli indicated that microbes were uniformly distributed through the pore space of the CT aggregates, while they often failed to reach aggregate centers in NT aggregates and tended to be lower in the central portions of NS aggregates. E.coli distribution within the aggregate was negatively correlated with pore size while not directly related to pore connectivity at the image resolution studied. The results indicate that more E.coli retention within upper soil layer can be expected in the conventionally tilled soil while long-term reduction in soil disturbance may contribute to greater E.coli transport with preferential inter-aggregate flow.