Tuesday, 11 July 2006 - 10:45 AM

Managing Precipitation Use in Dryland Systems to Enhance Productivity and Sustainability.

Gary A. Peterson1, Dwayne Westfall1, and Lajpat Ahuja2. (1) Colorado State University, 200 West Lake St., Fort Collins, CO 80523, (2) USDA-ARS, Great Plains Systems Research Unit, 2150 D, Centre Ave., Fort Collins, CO 80526

In the Great Plains of North America potential evaporation exceeds precipitation during most months of the year. About 75% of the annual precipitation is received from April through September, and is accompanied by high temperatures and low relative humidity. Dryland agriculture in the Great Plains has depended on wheat production in a wheat-fallow agroecosystem (one crop year followed by a fallow year). Historically this system has used mechanical weed control practices during the fallow period, which leaves essentially no crop residue cover for protection against soil erosion and greatly accelerates soil organic carbon oxidation. This paper reviews the progress made in precipitation management in the North American Great Plains, synthesizes data from an existing long-term experiment, and demonstrates how the management principles involved can be applied in other climatic environments. The long-term experiment used to elucidate the principles was established in 1985 with the objective to identify dryland crop and soil management systems that maximize precipitation use efficiency (maximization of biomass production per unit of precipitation received), improve soil productivity, and increase economic return to farmers. Embedded within the primary objective were sub-objectives that focus on reducing the amount of summer fallow time and reversing the soil degradation that has occurred in the wheat-fallow cropping system. The experiment consists of four variables: 1) Climate regime; 2) Soils; 3) Management systems, and 4) Time. Cropping system intensification increased annualized grain and crop residue yields by 75 to 100% compared to wheat-fallow. Net return to farmers increased by 25 to 45% compared to wheat-fallow. A key physical property, soil bulk density, was reduced by 0.01g cm-3 for each 1000 kg ha-1 of residue addition over the 12-year period, and each 1000 kg ha-1 of residue addition increased effective porosity by 0.3%. A key chemical property, soil organic C content, was increased by more than 1000 kg ha-1 by increasing cropping intensification compared to the wheat-fallow system. All cropping system effects were independent of climate and soil gradients, meaning that the potential for C sequestration exists in all combinations of climates and soils. Soil C gains were directly correlated to the amount of crop residue C returned to the soil. Improved macroaggregation was also associated with increases in the C content of the aggregates. No-till practices have made it possible to increase cropping intensification beyond the traditional wheat-fallow system, and in turn precipitation-use efficiency has increased by 30% in West Central Great Plains agroecosystems. The concept of “fallow stages” was derived from our data synthesis. In this paper we apply the “fallow stage” concept to other geographic areas. Cases from a summer precipitation climate (Great Plains), winter precipitation climate (Pacific Northwest) and a Mediterranean climate (Morocco) are compared. Scientists can use this approach to analyze precipitation and temperature distributions in relation to various crop species and thus identify the best intervention points for improved management.

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