Park, E.J. (Michigan State University, Crop and Soil Sciences, Michigan State Univ., East Lansing, MI, 48824; Phone: 517-355-0271, ext. 247; Fax: 517-355-0270; Email: parkeun2@msu.edu)

Spatial distribution of lignin biopolymers and aggregate stability of forest and tilled soil types

E. J. Park *, K. Dria, D. Gamblin, T.R. Filley, A.J.M. Smucker

Soils are large reservoirs that mitigate global warming by removing greenhouse gases from the atmosphere and sequestering them as soil organic matter (SOM). One proposed mechanism is that carbon (C) is sequestered within soil aggregate interiors during the aggregation process. Stable soil aggregates preserve intra-aggregate porosities that do not collapse during wetting and drying cycles. Stable micropore networks increase the retention of carbon, which feeds back into the formation of more stable aggregates. Repeated wetting-drying cycles change internal pore geometries and associated microhabitats and creating more stable macro-aggregates. Research by Smucker and coworkers (EGU Abstracts, 2004) suggest that the exterior portions of aggregates contain greater concentrations of C and N than their interiors, establishing gradients of δ¹³C values across these aggregates.

We investigated aggregate stability by wet sieving and by polar tensile strength crushing resistances of aggregates from agricultural ecosystems in Hoytville clay loam and Wooster silt loam soils. Carbon contents, lignin biopolymers, textural distributions and intra-aggregate porosities were compared. Samples of Hoytville and Wooster soils from forest, conventional tillage (CT) and no tillage (NT) agriculture ecosystems were gently sieved into various size fractions. Soil macro-aggregates (6.3-9.5mm) were peeled, by mechanical erosion chambers, into concentric layers and separated into exterior, transitional and interior regions. Alkaline CuO oxidation was used to determine the composition of lignin, suberin, and cutin biopolymers to determine changes in source and degradative states of the SOM.

Soil carbon contents in aggregates from no tillage (NT) systems were 1.6 and 2.2 fold greater than conventional tillage (CT) systems from Hoytville and Wooster, respectively. Greater soil C contents increased water stability of soil aggregates. Aggregate stabilities increased as C contents exceeded 4%. C content and water stability of aggregates showed positive correlations with intra-aggregate porosities that decreased by 6-7% in CT aggregates. Polar tensile strengths of air-dry aggregates increased with increasing bulk density, C, and clay content. Preliminary results indicate that both soils show similar relative yields of lignin and hydroxyl fatty acids with a greater abundance of lignin than cutin and suberin acids. Greater abundances (per 100mg organic carbon) of CuO products were observed in the native forest than in either agricultural system and in general the lignin in the forest soils was less oxidized. For both soils, slight trends in biopolymer concentrations were observed between the exterior, transitional and interior regions of the aggregates from the forest and CT or NT ecosystems. These results suggest that non-disrupted and higher intra-aggregate porosities retained more internal carbon. Compound specific isotopic analysis is currently being utilized to better understand the source of lignin within the aggregates.