Are Recalcitrant Biomacromolecules Potential Sinks in the Global Carbon Cycle?.
Klaus Lorenz and Rattan Lal. Carbon Management and Sequestration Centre, School of Environment and Natural Resources, FAES/OARDC, The Ohio State Univ, 2021 Coffey Rd., Columbus, OH 43210
The major reservoirs of carbon on Earth are the oceans (38,400 Gt), the lithosphere (75,000,000 Gt) and the terrestrial biosphere (2,500 Gt). The atmospheric CO2 exchanges rapidly with the C pool in the oceans and the terrestrial biosphere. Fossil fuel burning, however, unlocks C from the lithospheric pool (i.e., kerogen, coal, petroleum) while land-use changes may release C from the soil organic C (SOC) pool. The resulting increase in atmospheric CO2 by human activities contributes to the climate change. Until CO2-neutral technologies for energy production are available, managing the SOC pool is an opportunity to slow the rate of increase of atmospheric CO2. The proportion of the stable SOC increases with increase in soil depth due to physical stabilization and changes in microbial processes but also due to increase in inputs of recalcitrant plant-derived biomacromolecules and their selective preservation. By comparing the occurrence of biomacromolecules preserved in fossil fuels and fossil plant remains for thousands to millions of years (i.e., terrestrial biomarkers) with those stored for thousands of years in sediments and in soil profiles in the stable SOC pool, plant biopolymers with a high residence time can be identified. The n-alkyl compounds are found in recent and ancient sediments, vascular plant leaf extracts, fossil plant tissues and petroleum. However, only the very high range of the n-alkane odd-over-even predominance in the number of C atoms is a potential chemotaxonomic tool for inputs from higher plants. A variety of pentacyclic triterpenoids are used as general tracers of higher plant input (e.g., in crude oils, marine sediments). Steroids are common in higher plants and attributed to higher plant inputs in petroleums and ancient sediments, but not in marine sediments. Cutin-derived compounds are used as chemotaxonomic tracers for higher plant inputs to younger marine sediments. Lignin commonly occurs in woody tissues and cell walls of vascular plants, but lignin-derived records for marine and lacrustine sediments can be biased by selective preservation. Furthermore, lignin is not as stable in soil as previously thought. Resin-derived compounds (i.e., sesquiterpenoids and diterpenoids) are commonly found in petroleum, and inputs by gymnosperms and conifers can be distinguished. Suberins/suberans and tannins may also contribute to the stable SOC pool. By selecting plant/cultivars with higher concentrations of biomacromolecules showing a high preservation potential, the residence time of C in the soil can thus be directly managed. The scientific knowledge, however, about biomacromolecules contributing to the stable C pool in biosphere and lithosphere is scanty. With the development and application of new techniques for the elucidation of structural features of organic compounds in the earth's crust, the potential of biomacromolecules as C sinks in the global C cycle and their role in mitigating climate change can be evaluated.