Early performance and nutrition of two willow species in short-rotation intensive culture fertilized with wastewater sludge and impact on the soil characteristics

Recycling wastewater sludge as fertilizer in short-rotation intensive culture of willows offers a viable opportunity to increase biomass productivity. Salix discolor Muhl. and Salix viminalis L. were planted at two densities (20 000 and 30 000 cuttings/ha), on two sites: a well and a poorly drained site. Three doses of dried and granulated sludge (100, 200, 300, and 0 kg/ha of "available" N) were applied in the spring of the first and the second seasons of growth. The aims of the experiment were to (i) investigate growth and nutritional plant response to fertilization and (ii) estimate the impact of sludge application on the soil. Over two seasons, aboveground biomass was similar for S. discolor and S, viminalis on both sites and for all sludge treatments. Every increment in the sludge dose applied induced an increase in willow productivity. Vector analysis of stem and branch nutrient concentration suggested that nitrogen was the most limiting nutrient. At the end of the second season plots fertilized with the highest doses of sludge indicated soil enrichment in organic matter. However, nitrate concentration in the soil increased with increasing sludge doses. The accumulation of heavy metals from sludge was not hazardous to the environment according to provincial limits. A moderate dose of dried and pelleted sludge (100 kg/ha "available" N) may reduce nitrate leaching and appears to be a good fertilizer during the establishment of willows.