Rhizosphere Microbial Community and Activity of Zero-Tillage Grasses Cultivated for Biofuel Feedstock in Hawaii.

The community composition and activity of soil microbes often serve as early indicators of soil quality due to a shift in management because of the important links between microorganisms, organic matter decomposition, and plant nutrient availability. The objective of this work was to study the effect of management practice, and irrigation on microbial communities of two potential biofuel feedstocks: sugarcane and energycane. Soil was sampled from pits at four depths, two within and two below the rhizosphere. Root biomass was measured volumetrically, microbial community composition and activity were analyzed by phospholipid-derived fatty acids (PLFA) and enzyme assays respectively. Two management practices were compared: 1) commercial - burning the crop on two-year rotations, and 2) four years of ratooning which is a form of zero-tillage harvest. Based on the PLFA analysis, which can serve as a measure of functional diversity, the ratooned plots hosted a more diverse microbial community compared to the commercial plots. Root biomass, microbial biomass and enzyme activity decreased with depth. The upper zone within the rhizosphere had a greater presence of fungi, mycorrhizal fungi, and gram-negative bacteria – root symbionts and digesters of labile plant material. With depth, the communities shifted to gram-positive bacteria and actinomycetes, which metabolize the more structurally complex and degraded organic matter. Conservation agricultural practices influencing the functional diversity of microbial communities can be an important contributor to the resilience of agricultural systems.