Evaluating the Sustainability of High-Dose Sewage Sludge Application in Fertilizing Szarvasi-1 Energy Grass...

The accumulation of municipal sewage sludge is a worldwide problem, although when properly treated, it can be utilized for various purposes in industry and agriculture. Due to its high nutrient content, one of its possible uses is the application as fertilizer on agricultural or degraded lands with the purpose of non-food plant production. In the present study, the sustainability of dehydrated sewage sludge application was tested in Szarvasi-1 energy grass (Thinopyrum obtusiflorum cv. Szarvasi-1) plantations, with special focus on the turnover of nutrients and trace elements in two experiments conducted outdoors between 2016 and 2019. Experiment 1 was conducted in 1 m³ containers, and the treatment was started on two-year old plants in 0, 15, 22.5, and 30 Mg ha^-1 doses per year applied in two or three portions to reveal the upper limit of sludge application. Experiment 2 was conducted in 100 m² field quadrates with 0, 7.5, 15, and 22.5 Mg ha^-1 doses per year applied once a year, which is in the range of the currently permitted application dose in Hungary. Soil, sludge, and plant samples, as well as physiological data, were collected. Aboveground biomass yield was measured 2-3 times per year. Increasing doses of sewage sludge significantly increased the yield compared to the controls, but the increment between the second and third doses was small. Chlorophyll content (SPAD values) increased tendentiously and partly significantly. The maximal quantum efficiency of PSII and the stomatal conductance was not improved compared to the control, whereas the relative water content of the plants was increased in Experiment 1 but not in Experiment 2 compared to the control. Malondialdehyde concentration was increased by the largest dose in Experiment 1. The concentration of macroelements, Ca, Mg, N, and S, increased in the aboveground biomass with increasing doses of sewage sludge, but even after three years, the cumulative amount removed with the harvested biomass was much smaller than the amount remaining in the soil. The total amount of K in the harvested biomass exceeded that introduced to the soil by the treatments. Micro- and trace-element concentrations did not show increasing tendency in the biomass, suggesting a slower uptake and removal rate than macroelements. The results point to the necessity to assess the real nutrient requirement and trace-element uptake by the plants as compared to the sewage sludge treatment to avoid their uncontrolled accumulation in the soil and ensure a sustainable fertilization of the plantations.