Aggregate stability and the spatial distribution of pore networks within macro-aggregates are essential and dynamic mechanisms controlling the retention of water, gases, solutes and Soluble Organic Compounds (SOC) within soil profiles. Highly connected intra-aggregate pore geometries are essential contributors to sustainable plant production and increased carbon (C) sequestration. Process-level mechanisms of aggregate stabilization and soil C sequestration within natural soil profiles of agricultural, forest and grassland ecosystems need to be better understood and contrasted before sustainable land use decisions can be developed at local and national levels. Intensive biomass production by agroecosystems produces a constant source of decomposing cellulose and lignin-based plant residues at the soil surface. These residues combined with existing soil Particulate Organic Matter (POM) and plant root exudates produce substrate-rich solutions that diffuse into highly inter-connected soil aggregate pore networks. Intra-aggregate porosities appear to be of much greater consequence to soil C sequestration than the distributions of aggregate size fractions. Soil solutions rich in SOC compounds diffuse into newly formed micro-fissures formed by repeated Drying and Wetting (DW) cycles. Greater diffusion of SOC compounds into centers of aggregates mobilize a plethora of sequential biophysical processes that stabilize aggregates and sequester more soil C. Our reports of tillage-induced reductions of solution flow and various biological, chemical and physical gradients through macro-aggregates (Park and Smucker, 2005; Horn, et al., 1994) suggest that long term tillage decreases pore network connectivity within aggregates. Greater contents of labile C, observed on exterior layers, combined with the more recalcitrant C discovered within interior regions of aggregates from tilled soils, suggest accumulations of labile C substrates on surfaces of aggregates promote the loss of soil C by aggregates subjected to frequent tillage operations. Greater porosity, higher microbial activity and reduced strength of exterior regions of soil aggregates, compared to their centers, suggest spatial gradients of C control soil microbial respiration processes which promote C losses from aggregate surfaces. Additions of glucose-13C to aggregate surfaces during five DW cycles maintained soil aggregate stability throughout the 35-day incubation and increased C flux into aggregate interiors. Repeated additions of C developed gradients which controlled the spatial distributions of microbial communities within aggregates. Increased rates of glucose C retention by aggregates subjected to frequent incubations indicated greater sequestration of C by either physical protection or chemical stabilization of new C within micro-fissures of treated macro-aggregates. These experiments demonstrate the mineralization of POM and roots are continuous donors of SOC substrates that diffuse into the expanding micro-fissures and form tightly sorbed recalcitrant C that is physically and chemically unavailable to bacterial communities. Intra-aggregate pore structures promote the transport of labile surface C into aggregate interiors. We observed highly stabilized aggregate structures as higher C concentrations accumulated on aggregate surfaces along with concomitant distribution of more C within macro-aggregates from non tilled (NT) and native forest (NF) soils. Less C accumulated on surfaces and less C diffused to interiors of aggregates from conventionally tilled (CT) soils. Therefore, we conclude that greater concentrations of SOC, accumulations of cations, clay minerals, and bacterial populations on aggregate surfaces improve the formation of macro-aggregates which develop highly connected intra-aggregate pore networks that promote the diffusion of substrates into aggregate interiors resulting in more stable macro-aggregates. References: 1. Horn, R. et al., 1994. Denitrification rate and microbial distribution within homogenous soil aggregates. Int. Agrophys. 8:65-74. 2. Park, E.J. and A.J.M. Smucker. 2005. Saturated hydraulic conductivity and porosity within macroaggregates modified by tillage. Soil Sci. Soc. Amer. J. 69:38-45.