Responses of sunflower to traditional and conservation tillage under rainfed conditions in southern Spain

Nitrogen (N) availability is one of the most crucial aspects of conservatio n tillage. Therefore, the advent of this system makes it advisable to study nutritional crop response for given site-specific environments. We have st udied for three years (1993, 1995 and 1997) the effects of a traditional ti llage (TT) and a conservation, reduced tillage (CT) system on the growth an d nutrition of a sunflower crop, in a wheat (Triticum aestivum L.)-sunflowe r (Helianthus annuus L.) rotation, established under rainfed agriculture in a sandy clay loam soil (Xerofluvent) of southern Spain. The TT method cons isted mainly of the use of mouldboard ploughing, and CT was characterized b y leaving the crop residues on the soil surface as mulch, chiselling after wheat and disc harrowing after sunflower, before sowing wheat. Sunflower, t he crop studied, was not fertilized. In both tillage treatments, seedlings (separated into shoots and roots) and plants at flowering were collected fo r analysis. Plant growth and yield were also determined. After harvesting i n 1995 and 1997 soil samples were collected (0-5 and 5-30 cm depth) for oxi dizable organic matter (OM) and available nutrients analysis. For the third crop, soil nitrate nitrogen (NO3-N) was determined (0-10 and 10-30 cm dept h) before sowing and four times after sowing. The CT method usually increas ed OM and N, and other nutrient contents in the top soil (0-5 cm), in relat ion to TT. However, for the third crop, soil NO3-N at the seedling stage wa s similar in both treatments (within the range 25-42 kg ha(-1), 0-30 cm dep th). Nevertheless, sunflower plants showed better early growth and higher N concentration in TT than in CT. Concentration and total content of NO3-N o f both shoots and roots in the seedlings of the third sunflower crop were a lso greater in TT (more than 50% and 100%, respectively). This seems to sho w that factors other than N-availability could affect the early plant nutri tion and growth. These nutritional differences disappeared later (early flo wering), except in the second sunflower crop (1995), which could not comple te normal growth in the TT treatment due to the severe drought of that year The lower early growth and N-uptake in CT did not affect either yield or s eed quality for the three years studied. It is concluded that CT was effect ive in increasing OM and N in the top soil, that CT greatly reduced early g rowth and N uptake of the crop but favoured yield formation during drought. Those are important aspects to be considered in the long-term. (C) 1998 El sevier Science B.V. All rights reserved.