Microbial Community Changes of a Flooded Nature Trail along the Rio Grande

In July 2010, Laredo, Texas was flooded throughout the riparian zone adjacent to the Rio Grande, potentially affecting the soil microbial community.  The purpose of this research was to assess changes that occurred in the microbial community post flooding within a primarily-grassland nature trail that was subjected to contamination from U.S. and Mexican municipal sources.  Our hypothesis was that the soil heterotrophic bacteria would be impacted by the flood due to contaminants deposited.  At 0, 6, 14, and 24 weeks post-flood, composite soil samples were collected within 1m² quadrats at three depths (0-2, 2-20, and 20-30 cm) in flooded and unflooded sections along 25- and 50- meter transects.  Soils were serially diluted and cultured on R2A nutrient agar followed by incubation for five days at 23^ºC.  In samples collected the week of the flood (week zero), it was determined that flooded areas closest to the river were the most affected as the mean bacterial count was 3.38 X 10^-6 CFU/g dry soil, which was significantly different (p < 0.0001, ANOVA) from the mean of 9.45 X 10^-6 CFU/g dry soil in the upper unflooded sections.  Six weeks post-flood, flooded sections showed recovery with a 2.78-fold increase in bacterial counts, with minimal changes in the unflooded sections.   Flooded soils sampled closest to the river contained high levels of ciprofloxacin and triclosan (antimicrobials), and atrazine (an herbicide). The data suggests that the lower bacterial counts in the flooded samples were due to disturbance by the flood as well as contaminant deposition.