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Soil Carbon: The Rhizosphere Priming Effect and Second Generation Biofuels


Anthropogenic driven climate change has the potential to dramatically reshape ecosystems worldwide. Rapid increases in atmospheric carbon dioxide and other greenhouse gases have increased global temperatures resulting in catastrophic losses of ecosystem services and biodiversity. Soils are the largest carbon reservoir in the terrestrial ecosystem, yet soil carbon is almost always excluded from global carbon models. Research in this field has been limited due to methodological and logistical difficulties. Understanding the key factors modulating soil carbon fluxes is essential for predicting future climatic conditions.

Soil organic matter (SOM) provides numerous benefits to the soil ecosystem including maintaining soil structure, improving water holding capacity, and increasing nutrient availability. Soils that have relatively low amounts of SOM are less productive and less resilient to environmental change compared to soils with high amounts of SOM. Furthermore, the most recalcitrant carbon has been found to remain in the soil for centuries, therefore is it important to investigate the biological and the physical mechanisms that determine the rate of SOM decomposition. The rhizosphere, the 2mm of soil surrounding roots, is a biological hotspot within the soil ecosystem. The effects of live roots on SOM stores can be quantified by measuring the rhizosphere priming effect (RPE) which is the change in rate of SOM decomposition due to the presence of roots.

The biological mechanisms driving the RPE have been studied in greenhouse and growth chamber experiments since the 1990s. However, much less is known about how the interactions between plant roots and soil structure dynamics effect the magnitude and direction of the RPE. In order to address this question, soybeans were grown at three different densities in a continuous 13C-isotope labeling greenhouse. I measured three indicators of soil structure (1) the amount of clay particles in the leachate (2) the amount of dissolved organic carbon in the leachate and (3) aggregate size distribution. Greater rhizosphere activity was linked with increases in dissolved organic carbon and clay particles in the leachate. There was a greater proportion of large (>2mm) water stable aggregates in the higher density treatments. Together these results indicate that rhizosphere activity increases aggregate turnover, which may physically expose SOM to microbial attack and increase priming effects.

Evidence from greenhouse and laboratory experiments have indicated that plant roots and rhizosphere microbes jointly regulate the rate of SOM decomposition. However, due to methodological challenges, it is unknown how the RPE manifests at the ecological scale. Individual live roots from five woodies species were excavated from the field, inserted into chambers filled with native soil, and were incubated in the field for fifty days. A novel pulse trapping method was used to sample root and soil respiration in the field during a 48 hour period. Root and soil respiration were distinguished based on 13C partitioning. Generally, greater root biomass at the end of the infield incubation was associated with higher priming effects. Species specific effects were also found. These results are consistent with laboratory studies, and suggest that this method can be used in the future in order to gain a better understanding of root-soil interactions in near field conditions.

Preserving soil carbon stores through targeted agricultural practices has been a subject of interest in the U.S. since the 1930s. The Conservation Reserve Program recommends switchgrass for soil remediation due to its tolerance for saline and sodic soils and its extensive root system. Biofuel production in the U.S. has garnered increasing attention since the 1990s due to a national interest in energy independence and potential impacts of fossil-fuel-related climate change. Switchgrass, a grass native to the Midwestern United States, improves soil quality, stores carbon deep in the soil, and provides habitat for birds. Increasing the production of biofuel from switchgrass may possibly decrease carbon emissions, increase carbon sequestration, and improve rural economies. I summarize the current social, technological, and logistical impediments to second generation biofuel production. I then examine the role of uncertainty, both political and economic, and its role in the second generation biofuels industry. I conclude that without crop insurance for farmers growing biofuel crops and increases in subsidies, farmers are unlikely to grow enough switchgrass for biofuel to meet the federal mandate. Instead, it is likely that farmers will meet the cellulosic mandate by selling corn stover.

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