Long-term monitoring indicates no adverse effects to intertidal habitats from biosolids applications onto an adjacent coastal forestry plantation

Abstract Stabilised organic solids derived from sewage sludge (‘biosolids’) are applied to land in many countries as an alternative to disposal as landfill. This study evaluates the long-term effects of biosolids applied to forestry plantations on the adjacent intertidal habitats of Moturoa / Rabbit Island (Aotearoa New Zealand). On this island, biosolids are applied to enhance soil fertility and growth of trees ( Pinus radiata) . However, applications may affect the quality of groundwater and surface water (e.g., increasing concentrations of nutrients and trace metals) and, via seepage into adjacent coastal areas, potentially adversely affect intertidal habitats and biota. Shoreline topography, macroalgal cover, sediment grain size, concentrations of nutrients, trace metals and faecal indicator bacteria, and benthic infaunal community characteristics were studied on three occasions (2008, 2014 and 2019) at twelve intertidal transect sites (four ‘reference’ and eight ‘application’) adjacent to forestry blocks where biosolids have been applied intermittently over a period of 24 years. No long-term adverse changes to the environment and community characteristics attributed to biosolids application were detected between reference and application sites. The study provides evidence that biosolids application can co-occur without detectable adverse effects on the nearby intertidal environment.