Crop and Soil Nitrogen Dynamics of Nine Rotation Systems During the Transition and Certified Organic Phases of Dryland Grain Production.

Crop and soil nitrogen dynamics of nine rotation systems during the transition and certified organic phases of dryland grain production Kristy Ott-Borrelli1, Richard Koenig1, Ian Burke1, Robert Gallagher2 and E. Patrick Fuerst1 1Washington State University, Department of Crop and Soil Sciences, Pullman, WA 99164 2Penn State University, Department of Crop and Soil Sciences, University Park, PA 16801 Producing certified organic crops in the dryland region of Eastern Washington is difficult. One main challenge is the unavailability of manure or other inexpensive sources of nutrients. As a result, growers must rely on legume-based, rotations to meet the high nitrogen (N) needs of organic cereal crops. A long-term research study was initiated in 2003 in Pullman, WA, to evaluate nine rotation systems designed to transition dryland cereals from conventional to organic production. The systems ranged from intensive grain production to intensive legumes for forage or green manure, as well as systems with alternating cereal grains and legumes. Additional inputs from commercial organic fertilizers were also included in some systems. The entire study was sown to certified organic spring wheat in 2006 and winter wheat in 2006-07. Samples of soil, cover crop and cash crop biomass were collected annually from each system and tested for inorganic and total N. Nitrogen inputs, net gains or losses, and soil inorganic N during the transition and certified organic phases were compared for each rotation system. Increasing the frequency and intensity of perennial or winter annual legumes managed as green manure or forage resulted in improved N fertility, certified organic wheat yields, and weed control than rotations with a higher frequency of spring cereals or spring peas. A better understanding of the N dynamics in each rotation system can assist development of a comprehensive organic fertility and soil quality management plan for organic dryland crop production in the Pacific Northwest.