Shifts in Microbial Community Composition and Physiological Profiles across a Gradient of Induced Soil Degradation

Abstract

A disturbance gradient was created to evaluate the relationship between changes in soil physical and chemical properties with microbial community composition. Plots of a 12‐yr fallow Ultisol in Northeastern Brazil were arranged in a Latin Square design and disturbed by tillage events applied zero, one, two, three, or four times over a 3‐mo period. Sixty days after the last event, plots were sampled (0–20 cm) and soil analyses performed. Tillage resulted in a linear disturbance gradient characterized by significant declines in soil available water, saturated hydraulic conductivity, soil aggregation, organic C, and microbial biomass. The disturbance gradient also affected soil microbial communities, but the changes observed were dependent on the profiling technique used. The application of one tillage event significantly altered terminal restriction fragment length polymorphism (T‐RFLP) profiles, which remained unchanged on further disturbances. In contrast, community level physiological profiles (CLPPs) continued to change as disturbance levels increased and were significantly correlated with most soil properties associated with soil degradation. Phospholipid fatty acid (PLFA)‐based assessment of microbial community structure was the least sensitive to disturbance, showing significant changes only between the control plots and those subjected to four tillage events. Nevertheless, the linear shift observed in a PLFA biomarker of bacterial stress (ratio of 19:0cy to 18:1w7c) in response to the disturbance gradient was consistent with physiological changes in the CLPPs. The results clearly illustrate that microbial community responses to increasing levels of soil physical disturbance can be detected at physiological (CLPPs), biochemical (PLFA), and genotypic (T‐RFLP) levels; however, they are not synchronously coupled. Further studies are needed to determine if there is functional significance to the physiological responses to disturbance that are, or are not, accompanied by community composition changes.