Monday, November 13, 2006

Within-Season Nitrogen Dynamics in a Long-Term Mediterranean Cereal/Legume Trial.

John Ryan, Samir Masri, Murari Singh, and Mustafa Pala. ICARDA, Tel Hadya, Aleppo, Syria

In dryland Mediterranean-type agriculture, which is dominated by cereal cropping, fallow in alternate years was a common strategy to cope with limited soil moisture resulting from the low rainfall that characterizes the region. With increasing land-use pressure, the practice of fallow has given way to continuous cereal cropping. Sustainability of such a system is clearly questionable due to disease buildup and decreasing soil organic matter. The rationale behind many long-term crop rotation trials establishes in the 1980's at the International Center for Agricultural Research in the Dry Areas in northern Syria was to find biologically and economically acceptable cropping alternatives to cereal monoculture. In the "Cropping Systems Productivity" trial described here, the crops assessed in rotation with durum wheat were chickpea, lentil, vetch, medic, a summer crop (watermelon), in addition to comparisons with continuous cereals and fallow. Subsidiary treatments in this 14-year trial involved nitrogen at 0, 30, 60, and 90 kg/ha, as well as cereal stubble grazing at three levels of intensity: heavy, moderate, and no grazing. During 3 seasons at the end of the trial, N dynamics were assessed throughout the entire growing season by sampling from both un-cropped and cropped micro-plots. Soil temperature and moisture were monitored continuously. Mineral N was measured to a depth of 1-m and N uptake measured in the crop, while biomass and labile forms of N were measured in the top 20 cm. While the N-release patterns were consistently highest for medic and vetch, followed by chickpea and lentil, and least for the fallow plots, the patterns varied considerably during the season, especially for biomass and labile N. The N data paralleled increases in organic carbon forms as well. Thus, legume-based rotations can improve soil quality, adding to the argument for using legumes in rotation with cereals, in addition to increased yields and water-use efficiency.