Carbon sequestration by macro-aggregates from agricultural ecosystems

Park, E.J. and A.J.M. Smucker

Presented at the All Scientist Meeting (2003-09-12 )

Macro-aggregates composed of micro-aggregates, primary minerals, cations, microbial and plant biomass, and organomineral complexations are stabilized by networks of roots, hyphal and organomineral associations within intra-aggregate pores.  Spatial and temporal fluxes of cations, clay minerals, soluble organic carbon (SOC) and microbes within aggregates develop during multiple wetting-drying cycles as soil solutions diffuse towards aggregate surfaces and into intra-aggregate micropores.  We investigated the C sequestration mechanisms associated with stabilization processes within macro-aggregates by determining the establishment of C and N gradients within multiple concentric layers of macro-aggregates sampled from conventionally tilled (CT), none tilled (NT) agroecosystem soils and nearby native forest (NF) soils. We also compared these C and N contents with mean weight diameter (MWD) indices of aggregate stability responses to wet sieving, their textural compositions, and their saturated hydraulic conductivities (Ks).  We observed 2.2 and 1.6-fold greater C contents in NT macro- aggregates compared to CT aggregates from Wooster silt loam and Hoytville clay loam soils. Carbon contents were positively correlated with aggregate stability and intra-aggregate porosities. There were no significant gradients of C between concentric layers of NT and NF macro-aggregates, whereas C concentrations in exterior layers of CT aggregates from Wooster and Hoytville soil series were 36% and 8% greater than their interior layers. Both greater gradients and increasing total C in macro-aggregates were parallel with increasing porosities within concentric layers, whole aggregate stabilities, and the Ks values within aggregates of each soil type.  These results indicate that C migration into macro-aggregates is influenced by the development and the stabilization of intra-aggregate pores that appear from increased microbial activity and accelerated influxes of SOC and cations into aggregate interiors during multiple wetting-drying cycles.

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