Spatial distributions of soil properties at the field and watershed scale may affect yield potential, hydrologic responses, and transport of herbicides and NO−3 to surface or groundwater. Our research describes field‐scale distributions and spatial trends for 28 different soil parameters at two sites within a watershed in central Iowa. Two of 27 parameters measured at one site and 10 of 14 parameters measured at the second site were normally distributed. Spatial variability was investigated using semivariograms and the ratio of nugget to total semivariance, expressed as a percentage, was used to classify spatial dependence. A ratio of <25% indicated strong spatial dependence, between 25 and 75% indicated moderate spatial dependence, and >75% indicated weak spatial dependence. Twelve parameters at Site one, including organic C, total N, pH, and macroaggregation, and four parameters at Site two, including organic C and total N, were strongly spatially dependent. Six parameters at Site one, including biomass C and N, bulk density, and denitrification, and 9 parameters at Site two, including biomass C and N and bulk density, were moderately spatially dependent. Three parameters at Site one, including NO−3 N and ergosterol, and one parameter at Site two, mineral‐associated N, were weakly spatially dependent. Distributions of exchangeable Ca and Mg at Site one were not spatially dependent. Spatial distributions for some soil properties were similar for both field sites. We will be able to exploit these similarities to improve our ability to extrapolate information taken from one field to other fields within similar landscapes.

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Soil organic matter (SOM) is of primary importance for maintaining inherently low SOC contents (typically below 1%, Hunt soil productivity, and agricultural management practices may signifiet al., 1982). Consequently, small changes in the SOM cantly influence SOM chemical properties. However, how SOM chemical characteristics change with agricultural practices is poorly under-content are significant to the agricultural production of stood. Therefore, in this study, we evaluated the impacts of tillage the region. An evaluation of tillage and crop residue (conventional vs. conservation) management on the structural and management practices to rebuild SOC levels has been compositional characteristics of SOM using cross-polarization magicconducted by Hunt et al. (1996). These researchers monangle-spinning (CPMAS) and total sideband suppression (TOSS) itored changes in SOC levels in numerous small tillage solid-state 13 C nuclear magnetic resonance (NMR) and diffuse reflecplots and found that after 9 yr of CnT, the SOC content tance Fourier transform infrared (DRIFT) spectroscopy. We characin the top few centimeters was significantly higher than terized both physically and chemically isolated SOM fractions from a the soil under CT management. Campbell et al. (1999) Norfolk soil (fine-loamy, siliceous, thermic Typic Kandiudults) under reported that over an 11-to 12-yr period, increases in long-term tillage management (20 yr). The solid-state 13 C NMR results C storage in the 0-to 15-cm soil depth, because of indicated that humic acid (HA) from conventional tillage (CT, 0-5 cm) was less aliphatic and more aromatic than HA from conservation adoption of no-tillage, were small (0-3 Mg ha Ϫ1). Most tillage (CnT). The aliphatic C content decreased with increasing depth of the differences were observed in the 0-to 7.5-cm soil (0-15 cm) for both CT and CnT treatments. The reverse trend was depth, with little change in the 7.5 to 15 cm. However, true for aromatic C content. Based on reactive/recalcitrant (O/R) the short and long-term influences of disturbance on C peak ratio comparisons, HA was more reactive in the top soil (0-5 mineralization are complex and may vary depending cm) under CnT than CT. Both soil organic C (SOC) and light fraction on types of soil and plant residues (Hu et al., 1995; (LF) material were higher in the 0-to 5-cm soil of CnT than CT Franzleubbers and Arshad, 1996; Alvarez et al., 1998). treatment. Our results show that long-term tillage management can The strong influence of soil management on the amount significantly change the characteristics of both physical and chemical and quality of SOM was also reported by others (Janzen fractions of SOM.