Using biomass of microbial guilds and soil exoenzymes to assess sustainability of woody biofuel feedstock production

Background/Question/Methods

Assessing environmental impacts is a critical step for demonstrating sustainability of low carbon alternative transportation fuels.  One way to assess environmental impacts of dedicated woody biofuel cropping systems is to measure changes in soil-quality indicators.  Physiochemical soil quality indicators respond slowly to altered management activities, whereas biological indicators assess potential short-term effects on soil quality.  Our objectives were to monitoring soil biological responses to a shift in management from traditional food and forage crops to poplar biofuel crops.  Measurements tested if changes in management decreased the biomass of soil microbial guilds, or degraded the potential for soil exoenzymes to release nitrogen and phosphorous from organic matter.  The study included three poplar biofuel test sites in Idaho, Washington and Oregon.  The Washington site included both a forage crop and a forest plantation as an additional reference.  We measured biomass of microbial guilds by analyzing phospholipid fatty acids (PLFA) in surface soil, and we measured nutrient release potential by analyzing for soil exoenzymes.  We assume that nitrogen release potential is represented by protease plus chitinase activity, phosphorous release potential is represented by phosphatase activity, and decomposition potential is represented by cellulase activity.

Results/Conclusions

Biomass of microbial guilds decreased in a consistent manner over time, and there was limited within-site variation.  This indicates that the PLFA assessment of microbial-guild biomass is a precise measure of differences between management systems.  Since, there are few consistent differences among management types, we conclude that woody bofuel cropping systems do not degrade microbial biomass .  Similarly, soil exoenzymes showed consistent response patterns, but little differences between management types.  The potential amount of N released was consistently 20% below that of P.  There were no differences among management types for P release potential, yet N release was significantly greater only at the Idaho location.  The forest plantation at the Washington location also had significantly higher nitrogen release potential than both the forage crop and poplar biofuel crop.  The rate of N release per unit cellulose varied significantly between locations, with Idaho and Oregon being greater than the Washington location.  The rate of phosphorous released represented by did not differ among locations or management types. Nitrogen release potential indicated that the poplar biofuel crops may improve nitrogen availability similar to the forest plantation.  Monitoring will continue to confirm observed responses among seasons, locations and management systems.