PARENT SESSION
Oral Session #52: Carbon and Nitrogen Cycling: Soil processes and patterns. Presiding: F. Gilliam.
Wednesday, August 8, 2001. 1:00 PM to 5:00 PM. Hall of Ideas E.

Carbon saturation in soil fractions: How can it occur?

Conant, Richard¹, Six, Johan¹, Paustian, Keith¹, Paul, Eldor¹, ¹

ABSTRACT- With uniform decomposition rates, soil C content is widely postulated to vary directly with C inputs. This expectation is usually met; C sequestration in soil is common in all types of ecosystems with increases in inputs due to increased productivity. However, in some cases no change in soil C occurs even when inputs are increased two to three fold, suggesting that mechanisms that stabilize C in soil have reached their capacity. What conditions lead to instances where soil C doesn't respond to changes in C inputs? What systems/locations are at or near soil C saturation level? We reviewed the literature to address these questions for a number of soil C fractions. Variability in total soil C content for a range of long-term agricultural experiments with varying degrees of residue return to the soil is explained substantially better with a hyperbolic relationship (r²=0.56) than with a linear relationship (r²=0.29), indicating that total C stabilization capacity of soil is limited. Similarly, silt- plus clay-associated C is strongly related to silt plus clay content of the soil. Therefore, C stabilization by association with silt and clay particles is constrained by soil texture. Light-fraction material responds in a similar manner to varying inputs. A major implication of this research is that with continued increases in inputs soils can reach a saturation point beyond which additional C will not be sequestered.

KEY WORDS: soil carbon, soil organic matter, soil texture