Biomass yield and quality of bioenergy crops grown with synthetic and organic fertilizers

• Sludge increased biomass accumulation and gross energy yield of bioenergy crops.
• Sludge improved Nitrogen and Phosphorus uptake.
• Sludge improved feedstock quality: reduced alkali metals, Cl, S & Si mass fraction.
• Sludge resulted in the optimal biomass C:N ratio for methane production.
• Sludge increases sustainability of energy cropping through reduced energy inputs.

Utilization of sludge is an attractive option for fertilizing bioenergy crops, since in addition to its valuable macro- and micro-nutrients, it may also contains some pollutants. Therefore, a three-year field experiment was conducted to investigate the suitability of sewage and digested sludge in comparison to synthetic fertilizer on biomass production and its quality in three crop species, with specific attention to the relationship between N availability and biomass accumulation. Fertilizer treatments for each crop species were standardized on the basis of total Nitrogen as follows: maize 120 kg ha−1, fibre hemp 60 kg ha−1 and oilseed rape 90 kg ha−1 Nitrogen. The highest biomass yield of maize (30 Mg ha−1 Dry Mass) and hemp (15 Mg ha−1 Dry Mass) was obtained from the sewage sludge treatment. The sludge treatment also gave the highest leaf Nitrogen mass in biomass of both maize (14 g kg−1) and hemp (18 g kg−1) at 60 DAS (flowering stage). Sewage sludge increased slightly gross energy yield of maize (438 GJ ha−1) and hemp (272 GJ ha−1) in comparison to other treatments. The mass fraction of ash and the combustion-relevant elements such as Chlorine, Potassium, Sodium and Calcium were lower in crops fertilized with sewage sludge than in those fertilized with synthetic fertilizer. In conclusion, sewage sludge produced as high biomass yield as synthetic fertilizers. In addition, it improved growth and quality of biomass.