Effect of fertilizer treatment on the biomass production and nutrient uptake of short-rotation willow on cut-away peatlands.

The effects of fertilizer treatment on the soil nutrient concentrations, biomass production and nutrient consumption of Salix x dasyclados and Salix ‘Aquatica’ were studied in five experiments on three cut-away peatland sites in western and eastern Finland during three years. Factorial experiments with all combinations of N (100 kg ha–1 a–1), P (30 kg ha–1 a–1) and K (80 kg ha–1 a–1) were conducted. The application of P and K fertilizers increased the concentrations of corresponding extractable nutrients in the soil as well as in willow foliage. N-fertilization increased foliar nitrogen concentration. An increase in age usually led to decreases in bark and wood N, P and K concentrations and increases in bark Ca concentrations. N-fertilization increased the three-year biomass yield 1.5–2.7 times when compared to control plots. P-fertilization increased the yield only in those experimental fields whose substrates had the lowest phosphorus concentration. K-fertilization did not increase the yield in any of the experimental fields. The highest total biomass yield of NPK-fertilized willow after three growing seasons, 23 t ha–1, was distributed in the following way: wood 42%, bark 19%, foliage 17%, stumps 6% and roots 16%. As the yield and stand age increased, more of the biomass was allocated in above-ground wood. Three-year-old stands (above-ground biomass 18 t ha–1) contained as much as 196 kg N ha–1, 26 kg P ha–1, 101 kg K ha–1, 74 kg Ca ha–1 and 37 kg Mg ha–1. By far the highest proportion of nutrients accumulated in the foliage. The bark and wood contained relatively high proportions of calcium and phosphorus respectively. With an increase in age and size, the amount of nitrogen and potassium bound in one dry-mass ton of willow biomass decreased while that of phosphorus remained unchanged.