INFLUENCE OF TILLAGE, CROP-ROTATION AND NITROGEN-FERTILIZATION ON SOIL ORGANIC-MATTER AND NITROGEN UNDER RAIN-FED MEDITERRANEAN CONDITIONS

The long-term effects of cropping systems and management practices on soil properties provide essential information for assessing sustainabi lity and environmental impact. Field experiments were undertaken in so uthern Spain to evaluate the long-term effects of tillage, crop rotati on and nitrogen (N) fertilization on the organic matter (OM) and miner al nitrogen (N,,) contents of soil in a rain-fed Mediterranean agricul tural system over a 6-year period. Tillage treatments included no till age (NT) and conventional tillage (CT), crop rotations were of 2 yr wi th wheat (Triticum aestivum L.)-sunflower (Helianthus annuus L.) (WS), wheat-chickpea (Cicer arietinum L.) (WP), wheat-faba bean (Vicia faba L.) (WE), wheat-fallow (WF), and in addition, continuous wheat (CW). Nitrogen fertilizer rates were 50, 100, and 150 kg N ha(-1). A split-s plit plot design with four replications was used. Soil samples were co llected from a depth of 90 cm at the beginning of the experiment and 6 yr later. Soil samples were also collected from a depth of 30 cm afte r 4 yr. These samples, Like those obtained at the beginning of the exp eriment, were subjected to comprehensive physico-chemical analyses. Th e soil samples that were collected 6 yr later were analyzed for OM, NH 4+-N and NO3--N at the 0-30, 30-60 and 60-90 cm soil depths. The tilla ge method did not influence the OM or N-min contents of the soil, nor did legume rotations increase the OM content of soil relative to CW. A longer period may have been required for differences between treatmen ts to be observed owing to the small amount of crop residue that is re turned to soil under rain-fed conditions of semi-arid climates. The WF rotation did not raise the N-min content of the soil relative to the other rotations. The consistent significant interaction between tillag e and crop rotation testifies to the differential effect of the manage ment system on the Oh? content and N status of the soil. The ammonium levels clearly exceeded those of NO3--N throughout the soil profile. T he high N-min content of the soils reveals the presence of abundant N resources that should be borne in mind in establishing N fertilization schemes for crops under highly variable climatic conditions including scant rainfall such as those of the Mediterranean region. (C) 1997 El sevier Science B.V.