outcomes to readily measurable parameters, we should be able to better optimize management within different We tested the hypothesis that particulate organic matter (POM) regions and cropping systems. and aggregate dry mean weight diameter (DMWD) are related to This work follows up on previous findings suggesting fertilizer-use efficiency (FUE) and leaching susceptibility. Soil cores measures of POM and aggregation are particularly good (15 cm diam. by 50 cm depth) were collected from 12 farm fields representing three cropping systems: conventional (CT) and no-tillage indicators of management practices' influence on soil (NT) management of corn (Zea mays L.)-soybean [Glycine max (L.) quality (Carter, 2002). Both indicators have been found Merr.] rotations, and CT applied to more diversified corn-soybeanto be sensitive to management practices in Illinois (Wanbased rotations (R-CT). Three of the four R-CT farms were organider and Bollero, 1999), and both are likely to be related cally managed. In a 95-d greenhouse trial, cores were seeded with to crop nutrient acquisition, N leaching, and organic corn, amended with 15 N-labeled urea applied at four rates (0, 75, 150, matter dynamics. Measures of POM, which represent 225 kg N ha Ϫ1), and subjected to a stressful moisture regime. Aggrerelatively young (5-25 yr) biologically and physically gate DMWD, which serves as a rough index of pore-size distribution, active organic matter, are thought to be predictive of was greater in NT cores. Although total leached N was similar in all N mineralization potential (Boone, 1994; Yakovchenko cropping systems, increased macropore flow in NT cores led to greater et al., 1998). leaching of fertilizer N and less leaching of soil-derived N, as well as greater moisture stress and decreased plant N uptake. The R-CT cores The influence of management on SOM and SOMhad more POM and organic C in the top 30 cm of soil and higher related outcomes is complex, soil specific, and croppingcrop biomass and biomass-N content. However, FUE in R-CT cores system dependent. In the corn-soybean-based cropping was relatively low since FUE does not account for contributions of systems studied here, increases in surface-soil POM conindigenous N. For the same reason, FUE remained relatively high in tents reported for soils under NT management had come CT systems despite less labile organic matter. Both FUE and SOM at the expense of POM in subsoil, and total POM levels conservation declined with increasing N application rates. Increasing in the profile were similar in NT and CT systems labile sources of N, reflected in POM pools, through crop diversifica-(Needelman et al., 1999). Aggregate DMWD, which tion can substitute for incremental increases in fertilizer N and immay be inversely related to soil physical condition in prove long-term productivity on Illinois Mollisols. the Midwest, was not found to vary consistently between NT and CT soils but was greater in cultivated than uncultivated soils (Wander and Bollero, 1999). Labile

This work builds on a previous study of long-term tillage trials that found use of no-tillage (NT) practices increased soil organic carbon (SOC) sequestration at Monmouth, IL (silt loam soil) by increasing the soil's protective capacity, but did not alter SOC storage in DeKalb, IL (silty clay loam), where higher clay contents provided a protective capacity not affected by tillage. The least limiting water range (LLWR), a multi-factor index of structural quality, predicted observed soil CO2 efflux patterns. Here we consider whether LLWR can predict sequestration trends at a third site, Perry, IL (silt loam soil) where SOC content is lower and bulk density is higher than in previously considered sites, and determine whether pore size characteristics can help explain the influence use of NT practices has had on SOC sequestration at all three locations. At Perry, LLWR was again related with differences in specific soil organic carbon mineralization rates (RESPsp) (2000-2001). Reduced RESPsp rates explain increases in SOC storage under NT management observed only after 17 yr. Trends in RESPsp suggest use of NT practices only enhance physical protection of SOC where soil bulk density is relatively high (approximately 1.4 g cm(-3)). In those soils (Monmouth and Perry), use of NT management reduced the volume of small macropores (15-150 microm) thought to be important for microbial activity. Physical properties appear to determine whether or not use of NT practices will enhance C storage by increasing physical protection of SOC. By refining the functions used to compute the LLWR and our understanding of the interactions between management, pore structure, and SOC mineralization, we should be able to predict the influence of tillage practices on SOC sequestration.

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