Concentric Layers of Soil Carbon and Microbial Populations in Soil Aggregates from LTER Interactions Tillage and Poplar Sites at KBS

Smucker, A.J.M., D. Santos, J. Wagester, and E.A. Paul

Presented at the All Scientist Meeting (1998-07-21 to 1998-07-22 )

Surface accumulations of recently deposited and labile soil C on soil aggregates establish heterogenous C gradients along planes of weakness within soils. Carbon deposited by plant roots were evaluated by the natural abundance of 13C in whole aggregates and peeled fractions from concentric layers of aggregates ranging from 7 – 11 mm across. Surface layers can be peeled from aggregates ranging from 0.5 to 15 mm across by the new soil aggregation erosion (SAE) chambers developed at Michigan State University. Twice as much C is located on surface layers than is within interior regions of soil aggregates from agricultural soils. Following 20 months of alfalfa (C3-C), nearly 80% of newly deposited soil C originated from the concurrent alfalfa crop. Less than 25% of C sequestered within aggregates originated from the alfalfa. New C inputs by corn and rye roots were 0.41 and 1.45 g C kg-1 soil for the 0-5 cm depths. Increased C coupled with N gradients, reported earlier, result in 200% increases in the gradients of C:N from the exterior to interior regions of soil aggregates, Table 1. These and other factors control microbial populations of fungi within soil aggregates of the Kalamazoo loam soil. Fungal biomass, measured by the direct microscopy method, was 40 % greater in the exterior layers of aggregates from the LTER poplar site.Table 1. Soil C:N in aggregates sampled at 0-5 cm from the LTER Interactions tillage site as evaluated by the CHN analyzer. n = 4.Concentric soil aggregate layerExternalTransitionalInternalReturn to Contents

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