Carbon Gradients Within Soil Aggregates of Agroecosystems

Dell, C.J., Y. Kavdir, D. Santos, and A.J. M. Smucker

Presented at the All Scientist Meeting (1999-07-20 to 1999-07-21 )

Research into the biophysical mechanisms associated with the formation, function, and stability of soil aggregates is a primary focus of the Soil Biophysics Laboratory. The development of mini-Soil Aggregate Erosion (SEA) chambers provides a unique opportunity for separating soil aggregates into concentric layers. Multiple analyses of each layer are used to determine gradients for a variety of chemical, physical, and biological properties within the aggregates. Quantities and forms of C within soil aggregates are important factors controlling aggregate structure. In addition to the measurement of total C, gradients of 13C concentrations are being measured. Plants with C3 and C4 photosynthetic pathways incorporate 13C at different rates, therefore plants with contrasting metabolism deposit C with differing isotopic ratios into soil components. Consequently, delta 13C ratios within concentric layers of soil aggregates indicate the recent plant sources of soil C in contrast to C deposition by previous vegetation. Measurements of C gradients in aggregates of the Kalamazoo silt loam from KBS indicate a two- to three-fold increase in the amounts of C within exterior layers of the aggregates compared to the central regions of aggregates. These concentric layer differences are greater in no-till than in conventionally tilled soils. Delta 13C measurements indicate that up to 67% of the C in the exterior regions of aggregates is derived from recent contemporary plant inputs. Similar measurements are currently being made on aggregates from additional experiments at KBS as well as sites in Michigan, Ohio, Illinois, and Minnesota. Additionally, long-term incubations will be performed to determine the rate of C turn-over from each concentric layer within soil aggregates.

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