Sunflower growth and yield response to sewage sludge application under contrasting water availability conditions

Good knowledge of crop response to biosolids is necessary for optimizing their use as soil amendments. The present 2-yr study evaluated the impact of soil application of municipal sewage sludge on growth, dry matter translocation, achene and oil yield as well as on achene Fe, Cu, Zn and Mn content of field-grown sunflower (Helianthus annuus L.) under contrasting water availability in terms of seasonal rainfall. Treatments included three sewage sludge rates (9, 18, and 36 Mg dry weight ha(-1) yr(-1)) compared with inorganic fertilizer (138 kg N plus 24 kg P ha(-1) yr(-1)) and a non-amended control. Adding sewage sludge increased stem diameter, height, and early growth of sunflower plants compared with control. Dry matter at flowering was positively correlated with translocation of dry matter to achenes (r = 0.855**). However, high early dry matter was translated into enhanced achene yield only with adequate water availability along growth cycle, while water shortage resulted in fewer achenes and poor seed-filling. Sewage sludge application increased achene yield even under water shortage, suggesting a beneficial effect of sewage sludge on soil water status. Achene yield with added sewage sludge was similar to or greater than with the inorganic fertilizer. Sewage sludge decreased achene oil concentration and increased achene N concentration, compared with the non-amended control, without affecting Zn, Fe, Mn, and Cu content in achenes even at the high application rate of 18 Mg ha(-1). Findings support that sewage sludge addition in soil at the rate of 9 Mg ha(-1) was agronomically adequate and could replace inorganic fertilizer in sunflower production, irrespective of seasonal rainfall amount and distribution patterns.