Biogas residues in substitution for chemical fertilizers: A comparative study on a grassland in the Walloon Region.

To provide sufficient quantities of food and feed, farming systems have to overcome limiting factors such as the nutrient depletion of arable soils. Nitrogen being the main mineral element required for plant growth, has led to the extensive use of chemical fertilizers causing nitrogen pollution of the ecosystems. This field study investigates the use of biogas residues (BRs) as biofertilizers and their contribution to the mitigation of nitrate leaching in agricultural soils, while also demonstrating the polluting nature of chemical fertilizers. Nine different fertilization treatments classified in three schemes and two nitrogen doses were tested for three consecutive years on a grassland in the Walloon Region of Belgium. Residual soil mineral nitrogen, percentage contribution of treatments in residual nitrate and agronomic performance were assessed for each fertilization treatment. The results obtained showed significant differences on treatment and scheme level regarding nitrate accumulation in the soil, with chemical fertilizers posing the highest nitrate leaching risk. BRs did not cause nitrate accumulation in the soil, and were N rate and rainfall independent, while the chemical treatments indicated a cumulative tendency under high N rate and low precipitation. Forage yield did not demonstrate statistical differences on treatment and scheme level but varied with changing precipitation, while the maximum application rate suggested a plateau. Aboveground nitrogen content was significantly higher after the application of chemical fertilizers only in the first year, while all the chemical treatments indicated a dilution effect under elevated annual rainfall. Finally, the partial substitution of chemical fertilizers by raw digestate reduced the concentration of NO3− in the soil without having a negative impact on the yield and N content of the biomass. These results strongly advocate for the environmental benefits of BRs over chemical fertilizers and underline their suitability as biofertilizers and substitutes for chemical fertilizers in similar agricultural systems.