No abstract

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.

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

Research techniques that have greatly advanced agronomic science in the 20th century, and have enhanced our knowledge of cause and effect, have been accompanied by reductionism. As a result, much valuable research has been accomplished, but less synthesis of results has occurred that provides answers to landscape-level research questions. We propose that a systems approach to the study of soil and crop management problems is a useful technique that tests our present research knowledge in a way that answers practical agricultural problems and simultaneously identifies gaps in basic research knowledge. We have used a problem common to the Great Plains to demonstrate the systems approach: minimizing the soil-and environment-degrading practice of summer fallowing and simultaneously increasing farm profitability. Although much is known about summer fallow at specific locations, less is known about extrapolating this knowledge across soil and climatic gradients. We show that intensified crop sequences under no-till techniques can replace summer fallow in many environments, and increase the productivity per unit of water received. Simultaneous reductions in the damaging factors of summer fallowing, in particular soil erosion and organic matter losses, also have occurred. We are researching the problem at a landscape level without losing the ability to detect causae and effect, and are able to simultaneously conduct basic research on soil nutrient cycling and water budgets. The approach also is being used effectively as an adult educational tool, thereby facilitating the transfer of technology from the researcher to the producer.

Variable-rate technology (VRT) has been developed to variablyThe potential for improved profitability due to variapply crop inputs to manage in-field variability. Although growers have able-rate N application depends on identifying areas in begun to adopt VRT, its profitability is uncertain in N management. the field where additional N inputs will increase revenue The objective of this study was to assess the economics of uniform vs. variable-rate N fertilizer application under two N application scenarios on a scale that is greater than the added costs and/or (farmer vs. custom applications). On-farm studies were conducted on identifying areas where reducing N inputs will decrease two continuous corn (Zea mays L.) fields in northeastern Colorado costs on a scale that is greater than potential revenue under furrow and center-pivot irrigation during the 2000 and 2001 reduction correlated with lower grain yield (Snyder et al., growing seasons. The N management strategies were uniform, grid-based, 1999). Therefore, the economic feasibility of variable-rate site-specific management zone-constant yield goal (SSMZ-CYG) and N application is focused on whether increases in gross site-specific management zone-variable yield goal (SSMZ-VYG). "Profit revenue or decreases in N input costs outweigh the added and loss" software was used to analyze the economics of each N cost of technologies or services needed for variable-rate N management strategy and determine which N strategy was most profitapplication ( Ferguson et al., 1999;Thrikawala et al., 1999). Thrikawala et al. (1999) reported that the profitability N fertilizer (6-46%) was used with the SSMZ-VYG N management strategy when compared with uniform N management. Net returns from of variable-rate N application increased above that of the SSMZ-VYG N management strategy were $18.21 to $29.57 ha Ϫ1 uniform N application as area and in-field soil variability more than uniform N management. Results of this study suggest variincreased. However, additional information and appliable-rate N application utilizing site-specific management zones are cation expenses are involved when managing spatial more economically feasible than conventional uniform N application. variability occurring throughout a field. Review of literature suggested that most studies incorporated the information costs (i.e., soil sampling, developing variable-rate able. Results from three site-years consistently indicated that less total