Sewage sludge, digestate, and mineral fertilizer application affects the yield and energy balance of Amur silvergrass

Abstract

Sewage sludge and digestate can be applied as fertilizers to reduce the use of mineral fertilizers and reliance on non-renewable resources in their production. A field experiment was conducted in north-eastern Poland (2016–2018) to evaluate the biomass yield and the energy efficiency ratio (EE) of Amur silvergrass [Miscanthus sacchariflorus (Maxim.) Hack] in production technologies that differed in the form and rate of nitrogen fertilization: (i) unfertilized control; (ii) 100 kg N ha−1 applied as mineral fertilizer; (iii) 100 kg N ha−1 applied as sewage sludge; (iv) 100 kg N ha−1 applied as digestate; (v) 160 kg N ha−1 applied as mineral fertilizer; (vi) 160 kg N ha−1 applied as sewage sludge; (vii) 160 kg N ha−1 applied as digestate. The application of 100 kg N ha−1 increased dry matter yield (DMY) by 15% relative to the control treatment (8.5 vs. 9.7–9.8 Mg ha−1). An increase in the nitrogen rate to 160 kg N ha−1 did not increase biomass yields. The yield-forming effects of digestate and sewage sludge were comparable with those exerted by mineral fertilizers. The energy inputs (EI) in the production technology involving mineral fertilizers ranged from 16.3 (100 kg N ha−1) to 20.9 GJ ha−1 (160 kg N ha−1). Organic wastes decreased EI in the production of Amur silvergrass by 34–40% (sewage sludge) and 41–48% (digestate). Energy output (EO) peaked when M. sacchariflorus was supplied with 100 kg N ha−1, regardless of fertilizer type. Energy gain (EG) was highest (131 GJ ha−1 y−1) when Amur silvergrass was fertilized with digestate at a rate equivalent to 160 kg N ha−1. Nitrogen fertilization decreased the EE of biomass production by 25–32% (sewage sludge and digestate) to 54–65% (mineral fertilizer). The EE was highest in the production technology involving digestate (13.2–14.4), followed by sewage sludge (11.4–13.0), and it was lowest under the influence of mineral fertilizers (6.7–8.8).