Root Biomass and Microbial Response to Irrigation Deficit Treatments in the Rhizosphere of Biofuel Feedstock Cultivation in Hawaii.

Belowground root biomass and microbial activity in the rhizosphere is highly correlated to water resources, soil fertility, and management choice. In turn, soil microbial activity regulates the dynamics of organic matter decomposition and plant nutrient availability thereby providing feedback to productivity. Tropical, perennial C4 grasses cultivated for biofuel produce large amounts of both above- and belowground biomass and have the potential to accumulate soil organic carbon (SOC) rapidly under conservation tillage. These grasses can be grown by ratooning, a form of zero-tillage harvest, which leaves the roots and soil intact, undisturbed, and accumulating SOC. The objective of this work was to study the impact of water deficit treatments on root distribution and microbial activity of two biofuel crops: sugarcane (Saccharum officinarum cv. 65-7052), and energycane (Saccharum officinarum x Saccharum rubustom cv. MOL-6081). Each crop was subject to two levels of irrigation: 100% (standard level of irrigation used in the surrounding commercial fields) and 50% (half of standard level). Root biomass was determined volumetrically from excavated soil pits by depth (0-40, 40-80 and 80-120 cm) which were opened and sampled for both ratooned and commercial plant crops. The dead and live roots from these pits were sorted and quantified. For the microbial study, the soil was sampled from each pit along the base of the stalks at four depths based on the root distribution and density: surface soil, main rooting zone, underneath the main rooting zone, and below the deepest root. Each soil sample was measured for phospholipid-derived fatty acids (PLFA), microbial carbon and microbial nitrogen. The results showed that the total root mass of energycane was larger than sugarcane, and the 100% irrigation had a larger root-biomass than 50%. The microbial activity results yielded differences between crops, irrigation levels and management techniques, and the most substantial differences were based on sampling depth. Quantifying belowground biomass and determining microbial activity based on water deficit treatments and management are important to understanding any sustainable cropping system. However, the results from the rhizosphere environment raise more questions as to what other drivers could be impacting the distribution and abundance of the soil microbes.