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With increased cropping intensity, one would expect that more crop residue and C would be added to the Soil C sequestration can improve soil quality and reduce agriculsoil than with a crop-fallow system (Campbell et al., ture's contribution to CO 2 emissions. The long-term (12 yr) effects of tillage system and N fertilization on crop residue production and 1995, 2000b; Janzen et al., 1998a; Peterson et al., 1998). soil organic C (SOC) sequestration in two dryland cropping systems As the amount of crop residue returned to the soil is in North Dakota on a loam soil were evaluated. An annual cropping increased, SOC sequestration is expected to increase if (AC) rotation [spring wheat (SW) (Triticum aestivum L.)-winter the residue C is not lost as CO 2 to the atmosphere wheat (WW)-sunflower (SF) (Helianthus annuus L.)] and a spring because of tillage induced decomposition (Larney et al., wheat-fallow (SW-F) rotation were studied. Tillage systems included 1997; Reicosky, 1997a,b). Research in the Great Plains conventional-till (CT), minimum-till (MT), and no-till (NT). Nitrogen has shown that SOC sequestration is enhanced by N rates were 34, 67, and 101 kg N ha Ϫ1 for the AC system and 0, 22, fertilization (Campbell and Zentner, 1993; Campbell et and 45 kg N ha Ϫ1 for the SW-F system. Total crop residue returned

Traditional sampling methods are inadequate for assessing the interrelated physical, chemical, and biological soil properties responsible for variations in agronomic yield and ecological potentials across a landscape. Recent advances in computers, global positioning systems, and large‐scale sensors offer new opportunities for mapping heterogeneous patterns in soil condition. We evaluated field‐scale apparent electrical conductivity (ECa) mapping for delineating soil properties correlated with productivity and ecological properties. A contiguous section of farmland (250 ha), managed as eight fields in a no‐till winter wheat (Triticum aestivum L.)–corn (Zea mays L.)–millet (Panicum miliaceum L.)–fallow rotation, was ECa mapped (≈0‐ to 30‐cm depth). A geo‐referenced soil‐sampling scheme separated each field into four ECa classes that were sampled (0‐ to 7.5‐ and 7.5‐ to 30‐cm depths) in triplicate. Soil physical parameters (bulk density, moisture content, and percentage clay), chemical parameters (total and particulate organic matter [POM], total C and N, extractable P, laboratory‐measured electrical conductivity [EC1:1], and pH), biological parameters (microbial biomass C [MBC] and N [MBN], and potentially mineralizable N), and surface residue mass were significantly different among ECa classes (P ≤ 0.06) at one or both depths (0–7.5 and 0–30 cm). Bulk density, percentage clay, EC1:1, and pH were positively correlated with ECa; all other soil parameters and surface residue mass were negatively correlated. Field‐scale ECa classification delimits distinct zones of soil condition, providing an effective basis for soil sampling. Potential uses include assessing temporal impacts of management on soil condition and managing spatial variation in soil‐condition and yield‐potential through precision agriculture and site‐specific management.

The extreme climate of the northern Great Plains of North America requires cropping systems to possess a resilient soil resource in order to be sustainable. This paper summarizes the interactive effects of tillage, crop sequence, and cropping intensity on soil quality indicators for two long-term cropping system experiments in the northern Great Plains. The experiments, located in central North Dakota, were established in 1984 and 1993 on a Wilton silt loam (FAO: Calcic Siltic Chernozem; USDA 1 : fine-silty, mixed, superactive frigid Pachic Haplustoll). Soil physical, chemical, and biological properties considered as indicators of soil quality were evaluated in spring 2001 in both experiments at depths of 0-7.5, 7.5-15, and 15-30 cm. Management effects on soil properties were largely limited to the surface 7.5 cm in both experiments. For the experiment established in 1984, differences in soil condition between a continuous crop, no-till system and a crop-fallow, conventional tillage system were substantial. Within the surface 7.5 cm, the continuous crop, no-till system possessed significantly more soil organic C (by 7.28 Mg ha −1 ), particulate organic matter C (POM-C) (by 4.98 Mg ha −1 ), potentially mineralizable N (PMN) (by 32.4 kg ha −1 ), and microbial biomass C (by 586 kg ha −1 ), as well as greater aggregate stability (by 33.4%) and faster infiltration rates (by 55.6 cm h −1 ) relative to the crop-fallow, conventional tillage system. Thus, soil from the continuous crop, no-till system was improved with respect to its ability to provide a source for plant nutrients, withstand erosion, and facilitate water transfer. Soil properties were affected less by management practices in the experiment established in 1993, although organic matter related properties tended to be greater under continuous cropping or minimum tillage than crop sequences with fallow or no-till. In particular, PMN and microbial biomass C were greatest in continuous spring wheat (with residue removed) (22.5 kg ha −1 for PMN; 792 kg ha −1 for microbial biomass C) as compared with sequences with fallow (SW-S-F and SW-F) (Average = 15.9 kg ha −1 for PMN; 577 kg ha −1 for microbial biomass C). Results from both experiments confirm that farmers in the northern Great Plains of North America can improve soil quality and agricultural sustainability by adopting production systems that employ intensive cropping practices with reduced tillage management.