Comparative assessment of five cellulosic biofuel management strategies: Implications to soil carbon and nitrogen dynamics.

摘要

Potential changes in cropping practices to accommodate the direct utilization of plant biomass for the production of second-generation cellulosic biofuels have raised concerns over unintended environmental effects. We focused on changes in: i) soil C pools, ii) N cycling capacity, and iii) microbial community structure. We hypothesized that cropping practices (e.g. plant selection, residue removal) would influence soil microbial community structure and functional capacity, altering soil C and N turnover compared to conventional agricultural practices. Replicated plots at the Purdue Water Quality Field Station (WQFS) were used in a comparative assessment of soil quality for cellulosic biofuel management and conventional systems, with surface soils sampled four times over a three-year establishment period. The biofuel systems were no-till continuous corn (Zea mays; CR), Miscanthus x giganteus (MS), tall grass prairie dominated by big bluestem (Andropogon gerardii; PR), dual-purpose sorghum (Sorghum bicolor; hybrid PU8168X; SG), and upland switchgrass (Panicum virgatum; c.v.Shawnee; SW). Soil C pools were evaluated by characterizing the physiochemical properties, including total organic C (TOC), permanganate oxidizable C (POXC), total nitrogen (TN), and soil pH, and the biological measures of microbial biomass using total phospholipid fatty acid - phosphate (PLFA-PO4), microbial C utilization via basal respiration (BR), and response to glucose addition (SIR). Shifts in N cycling pathways were estimated by soil assays of dehydrogenase (DH) and urease (UR) enzyme activity, net N mineralization (NM), acetylene reduction (AR), nitrification potential (NP) and denitrification potential (DP), and by profiling the diversity of functional genes essential for nitrogen fixation (nitrogenase, nifH), nitrification (ammonia monooxygenase, amoA), and denitrification (nitrite reductase, nirK) by PCR-denaturing gradient electrophoresis (PCR-DGGE). Changes in the size and structure of the microbial community were judged by quantifying differences in PLFA signatures and by evaluating fungal diversity via PCR-DGGE targeting the internal transcribed spacer (ITS) rRNA region. We observed no significant differences between systems in the TOC or POXC. However, cellulosic harvesting in CR, MS and in PR caused relative declines in BR and SIR. Based on multivariate statistical analysis of physiochemical (TOC, POXC, TN, pH) and microbial (PLFA-PO 4, SIR, BR) parameters, we highlight the strength of BR as an early predictor of changes in soil C dynamics. The N cycle was impacted by plant selection and management, showing a distinct polarization in both multivariate analysis of N transformations and composite analysis of N functional gene diversity between the minimal management in the PR system and the high intensity practices of CR. With time, SG, SW and MS developed unique N functional community structures and trended towards similarity with the PR system. We noted that residue removal from continuous corn did not alter soil enzymatic activity, but did shift functional community diversity. Evaluation of PLFA and ITS-DGGE profiles supported the overall distinction of PR, SW and to a lesser extent SG from all other cropping systems. It is clear that cropping system selection is highly influential on the soil microbial community and critical belowground C and N processes, which may influence long-term ecosystem function. The intermediate nature of dual-purpose sorghum with respect to soil C and N dynamics supports this crop as an alternative annual biomass cropping system. Furthermore, pursuit of dedicated perennial biofuels may offer an avenue to enhance agroecosystem services while supporting a bioenergy economy.