of greenhouse gases in the atmosphere (IPCC, 2001), and ability of our agricultural systems to sustain produc-Society is facing three related issues: overreliance on imported fuel, tion at rates needed to feed a growing world population increasing levels of greenhouse gases in the atmosphere, and producing sufficient food for a growing world population. The U.S. De-

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Recent studies of SOC storage and turnover have employed 13 C natural abundance (␦ 13 C) as an in situ Soil organic carbon (SOC) is sensitive to management of tillage, marker of relic and recent SOC pools. Mass concentraresidue (stover) harvest, and N fertilization in corn (Zea mays L.), tions of SOC and the ␦ 13 C signature are sufficient to but little is known about associated root biomass including rhizodeposition. Natural C isotope abundance (␦ 13 C) and total C content, mea-calculate the amount of SOC originating from a C 4 crop sured in paired plots of stover harvest and return were used to estimate (e.g., corn) or from a C 3 crop [e.g., soybean, Glycine corn-derived SOC (cdSOC) and the contribution of nonharvestable max (L.) Merr.] when the initial soil organic carbon biomass (crown, roots, and rhizodeposits) to the SOC pool. Rhizo-(SOC i) has a different 13 C signature than the current deposition was estimated for each treatment in a factorial of three crop (Balesdent et al., 1987). The ␦ 13 C technique has tillage treatments (moldboard, MB; chisel, CH; and no-till, NT), two shown that tillage influences the depth distribution of N fertilizer rates (200 and 0 kg N ha Ϫ1), and two corn residue manage

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Tillage‐induced soil roughness is an important consequence of tillage because water and air transport phenomena are affected. Characterizations of roughness have been statistical in nature and have lacked a physical connection to the configuration features affected by roughness. A new method of analyzing microrelief data is presented that improves the description of the soil surface and should lead to better information about the physical processes affected by tillage. The new procedure results in two surface configuration parameters, limiting slope (LS) and limiting elevation difference (LD), that are based on the slope or inclination of the surface of soil clods and to the average relief (elevation difference), respectively. These two indexes are directly related to the configuration of the surface and also sensitive to differences in roughness. They should help allow roughness to be used simultaneously with other soil properties to predict transport phenomenon.

The effects of tillage‐surface residue interactions on soil upper boundary temperatures are described. Regression curves are developed between the maximum and minimum soil surface temperatures vs. maximum and minimum air temperatures at the 2‐m height. Soil surface temperature variations between various residue and tillage treatments were mainly due to the change in surface residue cover. During fall and spring, the maximum temperature differences were 12 and 19°C, respectively, between no residue and surface residue treatments for the same tillage condition. Soil surface temperature variation due to the disturbance of soil by tillage for the similar residue cover condition were relatively small. During the growing season, maximum and minimum soil surface temperatures under corn canopies were approximately the same for all tillage‐residue interactions. A procedure to normalize diurnal upper boundary temperatures with respect to daily maximum and minimum soil surface temperatures is suggested. This procedure simplifies the upper boundary temperature data for easy comparison between treatments, seasons, and locations. Comparison of the average normalized soil surface temperature curves suggests that the shape of the daily soil surface temperature curve is slightly different for various tillage and residue interactions, with and without a corn crop. These differences in the average normalized soil surface temperature curves suggest that residue cover delays cooling more than heating of the soil surface. Based on the regression and the average normalized soil surface temperature curves, a procedure is described to estimate the soil upper boundary temperatures for various tillage and surface residue conditions from daily maximum and minimum air temperatures. Application of these soil upper boundary temperatures is in the physically based soil temperature models to estimate the root zone soil temperatures for various tillage and residue management systems with and without a corn crop.

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