Use of a Nitrogen Balance to Assess Nitrogen Dynamics in Dryland Organic and Conventional Cropping Systems.

Achieving adequate soil nitrogen levels for competitive grain production is one of the primary challenges encountered by organic growers. Animal manure compost and green manures are common sources of nitrogen in organic cropping systems; however, nitrogen from these sources is released slowly over time and each fertilizer’s contribution to the available nitrogen pool is unclear. To monitor plant-available nitrogen levels within cropping systems, net nitrogen change was calculated for each year of crop production in two organic systems and one conventional system under dryland production in eastern Washington. System 1 is a five-year organic alfalfa (three years)/winter wheat/spring barley rotation; System 2 is an organic winter wheat/spring wheat/winter peas (hay) rotation receiving poultry manure, and System 3 is a conventional winter wheat/spring wheat/winter peas (hay) rotation. Net nitrogen change was negative in organic System 1 and positive in Systems 2 and 3. All three years of alfalfa production produced negative net nitrogen change, indicating nitrogen withdrawal from the system; however, total soil nitrogen was greatest in System 1 as compared to other systems. Net nitrogen values were greatest for organic System 2, largely as a result of the animal manure amendments. The winter pea crop in both Systems 2 and 3 also resulted in negative net nitrogen. In no cases did the green manure crop contribute to positive net nitrogen at the end of the season, whereas animal manure resulted in positive net nitrogen values ranging from 30 to 200 kg ha^-1. The ability of green manure crops to build up available nitrogen in dryland organic systems is not promising; however, green manure and perennial crops were shown to contribute to the soil organic nitrogen pool, possibly providing a source of nitrogen in the long-term.