Santos, D. 1998. Contributions of roots and organic mater to soil aggregate stabilization. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

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This study examines three specific objectives which address the inter-relationships between soil aggregates and the retention and spatial distributions of carbon (C) contents within individual aggregates. First, a field study was sampled to assess crop rotation and conventional- (CT) and no-tillage (NT) modifications of total C contents and the concomitant incorporation of 13 C into soil macroaggregates. Accumulations of C were greatest in aggregates sampled at the 0-5 cm depth. Analysis of 13 C-natural abundance indicated that 65% of the C in the outer layers of CT aggregates (-26.5[per thousand]) originated from alfalfa roots (δ 13 C of -27.9[per thousand]), while 25% of the NT aggregate C, of the outer layer, originated from the alfalfa. The second field study was designed to investigate the distribution of original or native C 3 – and more recent or contemporary C 3 – and C 4 -derived C in aggregates having different sized-fractions. The proportion of more recent crop-derived C was 53 to 76% higher in NT soil aggregates at the 0-5 cm depth. Depositions of crop-derived C in CT soils ranged from 22 to 45% and appeared to not be affected by soil depth. Aggregate stabilities were lower in CT and NT soils than in never tilled grassland soils. Never-tilled soils contained proportionally greater quantities of water-stable aggregates among fractions >0.5 am, whereas CT and NT soils contained more stable aggregates having fractions <0.5 am. Thirdly, a greenhouse study was designed to investigate amounts and distributions of recently released root C in soil aggregates by ryegrass plants under water stressed and non-stressed conditions. Water availability during a six week growth period significantly changed ryegrass root parameters to depths of 10 cm. Higher C contents were detected in aggregates associated with non-stressed plants. New inputs of C 3 -C into whole aggregates, at 0-5 cm depth, increased by as much as 62% when ryegrass was subjected to drought stress and 25% in the non-stressed treatment. Aggregates associated with non-stressed ryegrass roots, at the 0-5 cm depth, developed higher MWDs than aggregates from unplanted treatments. Soil aggregates associated with non-stressed ryegrass roots also had greater MWDs than aggregates from ryegrass roots under water stress.

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