Mineralization of C sequesteredwithin concentric layers of soil aggregates

Dell, C.J., A.J.M. Smucker, and E.A. Paul

Presented at the Campus Symposium (2001-04-19 )

Organic matter is a key contributor to the formation, function, and stability of soil aggregates.  Recent research in our laboratory has shown that gradients in total C content frequently occur from the surfaces to centers of soil aggregates.  However, measurements of total C provide no specific information about the changes in the nature of soil C compounds.  Therefore, additional information is needed to better define the composition of organic materials within soil aggregates.Determination of the size of soil organic C pools, based on their turnover rates, provides valuable information describing the nature of soil organic matter.  the use of first-order kinetics and a three pool model has been shown to ban an effective method for describing soil organic matter dynamics (Paul et al., 2000).  These pools are frequently referred to as active, slow, and resistant.  the active pool is comprised of the most labile C compounds with residence times in soil of a few days to a few years.  the slow pool contains moderately available C compounds with residence times ranging from a few to 100 years.  The resistant pool contains highly recalcitrant C compounds with residence times of hundreds to thousands of years.  A combination of acid hydrolysis and carbon mineralization measurements provide the data necessary to quantify the size of the three pools and turnover rates for the active and slow pools.Measurement of 13C content in respired CO2 provides information about the age and source of C in SOM.  Plants using C3 and C4 photosynthesis discriminate against the fixation of 13C-CO2 to different degrees.  Therefore, SOM derived from C4 plant material have distinctly different 13C contents than those derived from C3 plants.  At sites where the current vegetation uses a photosynthetic pathway which contrasts that used by the previous vegetation, 13C content in respired CO2 reflects the proportions of C derived from each plant source.

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