ARS | Publication request: The response of soil carbon stocks to changing atmospheric CO2 concentrations are soil-type-dependent

Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: November 12, 2010
Publication Date: December 13, 2010
Citation: Hockaday, W.A., Gallagher, M.E., Masiello, C.A., Pyle, L.A., Polley, H.W., Baldock, J.A. 2010. The response of soil carbon stocks to changing atmospheric CO2 concentrations are soil-type-dependent. In: Proceedings of the American Geophysical Union, San Francisco, California. B41F-0382.

Technical Abstract: Global soil C stocks (2 × 1018 g C) are large enough that a minor climate-induced change in the cycling of the soil C pool would constitute a major climate feedback. The responses of soil carbon stocks to experimental manipulations of atmospheric carbon dioxide concentration ([CO2]) and temperature vary widely in direction and magnitude across different ecosystems. The mechanisms for differences in soil C cycle response to climate change are not well understood. In particular, little is known about the potential role of soil genetic factors such as mineralogy and structure in the climate response. To address this, we examined [CO2]-induced changes in soil organic matter (SOM) quantity and quality at the USDA Lysimeter CO2 Gradient facility (in Temple, TX), which comprises 3 major soil orders (Mollisol, Alfisol, and Vertisol). Temperature, precipitation, and vegetation type are controlled variables across soil orders. We used 13C nuclear magnetic resonance to study the chemical structure and composition of SOM under a native tallgrass prairie community exposed to CO2 concentrations ranging from 270 to 480 ppm. A mixing model (Baldock et al., 2004) was used to estimate soil biochemical stocks. The relative magnitude of biochemical inputs (from grassland roots and shoots) follows the order: carbohydrates >> lignins > proteins = lipids. However, the relative chemical abundances in the soil C pool are: carbohydrates = protein > lipid > lignin > charcoal. These discrepancies in the relative magnitude of the biochemical fluxes and stocks highlight the selectivity of SOM preservation and show that increased primary production (mainly carbohydrate synthesis) in response to elevated [CO2] may not lead to long-term soil C storage unless a carbohydrate preservation mechanism exists in the soil. Indeed, carbohydrate stocks in the Alfisol and Vertisol decreased despite greater inputs at high [CO2]. Only the Mollisol exhibited a capacity to store additional carbohydrate carbon at high atmospheric CO2 levels. Soil protein stocks in the Mollisol, and lignin stocks in the Alfisol, doubled in response to the doubling of atmospheric [CO2]. Soil lipids decreased with increasing [CO2] in all 3 soil orders. These [CO2]-induced changes in the soil biochemical stocks suggest that soil genetic factors could play an important role in the soil C storage potential under different climate regimes. The molecular basis for carbon preservation in soils of distinct genetic origin should inform efforts to model C cycle-climate feedbacks.