To develop simplified methods of hydraulic characterization of field soils and effects of management, frequency distribution of macroporosity (or effective porosity) in a soil is investigated as a measure of its saturated hydraulic conductivity distribution. The effective porosity (φe) of a soil is related to its saturated hydraulic conductivity (Ks) by a generalized Kozeny‐Carman equation. The exponent of this relationship is assumed to vary within a narrow range (value of 4 or 5). The equation is then combined with scaling theory to derive the frequency distribution of Ks scaling factors from the φe distribution. These concepts are tested on experimental data for two widely different soils, a mollisol and an oxisol. The φe is defined as total porosity minus soil water content at −33 kPa pressure head. The exponent of the Ks‐φe relationship is found to be nearly 4 for the soil‐core data of both soils, while for a smaller set of in‐situ field data for oxisol, which was within a narrow range of φe, the value of the exponent was smaller. There was a considerable scatter in the relationships. However, with the exponent set equal to 4 or 5 the distribution of Ks scaling factors derived from φe distribution closely matched the experimental Ks‐derived distribution. The approach has a promise for large‐scale applications.

Soil organic C (SOC) has decreased under cultivated wheat (Triticum aestivum)‐fallow (WF) in the central Great Plains. We evaluated the effect of no‐till systems of WF, wheat–corn (Zea Mays)‐fallow (WCF), wheat–corn–millet (Panicum miliaceum)‐fallow, continuous cropping (CC) without monoculture, and perennial grass (G) on SOC and total N (TN) levels after 12 yr at three eastern Colorado locations. Locations have long‐term precipitation averages of 420 mm but increase in potential evapotranspiration (PET) going from north to south. Within each PET location, cropping systems were imposed across a topographic sequence of summit, sideslope, and toeslope. Cropping intensity, slope position, and PET gradient (location) independently impacted SOC and TN to a 5‐cm soil depth. Continuous cropping had 35 and 17% more SOC and TN, respectively, than the WF system. Cropping intensity still impacted SOC and TN when summed to 10 cm with CC > than WF. Soil organic C and TN increased 20% in the CC system compared with WF in the 0‐ to 10‐cm depth. The greatest impact was found in the 0‐ to 2.5‐cm layer, and decreased with depth. Soil organic C and TN levels at the high PET site were 50% less than at the low and medium PET sites, and toeslope soils were 30% greater than summit and sideslopes. Annualized stover biomass explained 80% of the variation in SOC and TN in the 0‐ to 10‐cm soil profile. Cropping systems that eliminate summer fallowing are maximizing the amount of SOC and TN sequestered.

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crops every 3 yr) and even continuous (annual) cropping in some instances. For example, annualized grain yields Water is the principle limiting factor in dryland cropping systems. for WCF are 75 to 100% greater than WF (Peterson et Surface soil physical properties influence infiltration and cropping al., 2000). Cropping intensification has been possible systems under no-till management may affect these properties through because no-till practices improve soil water storage effiresidue addition. The objectives of this study were: (i) to determine how cropping intensity and topographic position affect soil bulk den-ciencies in the early phases of fallow (Farahani et al., sity, porosity, sorptivity, and aggregate stability in the surface 2.5 cm 1998). Since nearly 75% of the annual precipitation in of soils at three eastern Colorado sites; and (ii) to relate these properthis region occurs during April to September, relatively ties to crop residue returned to the soil surface. No-till cropping small net increases in soil water storage can provide the systems had been in place on three slope positions, at three sites, for necessary water to sustain crop growth between rainfall 12 yr prior to this study. Wheat (Triticum aestivum L.)-corn (Zea events. Thus water capture via increased water infiltramays L.)-fallow (WCF) and continuous cropping (CC) systems were tion rates becomes a significant factor in maximizing wacompared with wheat-fallow (WF) on summit and toeslope positions ter storage at all points in the system. An added benefit at two sites (Sterling and Stratton), and at the third site (Walsh) of cropping intensification is that increased amounts of wheat-sorghum [Sorghum bicolor (L.) Moench]-fallow (WSF) recrop residue are returned to the soil capared with WF. placed WCF. Cropping systems (CC and WCF or WSF) that returned We believe this residue may greatly improve soil physimore crop residue decreased bulk density and increased total and cal properties resulting in increased water infiltration effective porosities compared with WF. Site and slope positions that and capture efficiency. produced more crop residue also improved these properties. However, Soil physical properties such as bulk density, porosity, sorptivity developed no significant differences as a result of cropping system. Macroaggregates made up a higher percentage of total aggre-sorptivity, and aggregation dictate the infiltration chargates in CC and WCF or WSF compared with WF in proportion to acteristics and potentials of the soil. Most important are residue added and were also a function of clay content of the soil at the physical properties of the surface soil (top 2.5 cm), different sites and slope positions. These factors enhance the potential as this is the initial soil-water interface. However, longfor greater infiltration and hence greater water availability for crops. term infiltration can be affected by the hydraulic conductivity characteristics of deeper soil layers. Site latitude (evaporation potential), landscape slope, and cropping

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