Leaching potential of metals and nitrates from digestate-amended soils

Abstract

Digestate is a pre-determined mix of nutrients produced from the anaerobic digestion of municipal waste. Although it is a good source of nutrients and organic matter, its use as a soil fertiliser may cause increased leaching due to its high content of heavy metals, salinity and nitrates and thus could be a threat to groundwater quality. Three soil samples of contrasting textural characteristics were taken from both Malta and Gozo and the effect of digestate application on these soils was studied in soil columns under laboratory conditions. The soil was placed in 20cm columns having a 10cm diameter and digestate was added to the top 5cm soil and mixed. No digestate was added to control soil. Following an incubation period of 6 weeks with constant water application to stimulate irrigation, a soil sample was taken for all control and digestate amended soil columns from 7 cm below the soil surface and analysed for: AB-DTP A-extractable metal, electrical conductivity, pH and water soluble potassium. A free water sample was also taken and analysed for nitrates. When compared to the control soil, all soils that were amended with digestate showed an increase in nitrate (1280 ppm max), water-extractable potassium (950 ppm max) and salinity levels (478 µS/cm max), in the sampled soil and free-water. No significant increase in AB-DTPA -extractable heavy metals was registered, indicating that under these experimental conditions and during this short incubation period, the metals in the digestate, although in some instances present in relatively high quantities, were not mobile in soils.