Biosolids are treated sewage sludge, the solid fraction resulting from sewage treatment. In countries with Wastewater Treatment Plants (WWTPs), ca. 27 kg of dry biosolids person⁻¹ yr⁻¹ are produced. In New Zealand, 40% of the biosolids are disposed of into landfills, where they generate methane (CH₄) and nitrous oxide (N₂O). Beneficial reuse of these biosolids would mitigate the environmental and economic costs of landfilling. When added to soil, biosolids improve soil fertility but may introduce unacceptable loads of contaminants, especially trace elements (TEs), xenobiotics, microplastics, and pathogens, that may endanger food safety. Potentially, biosolids may be used to establish NZ-native vegetation, particularly in low-fertility or degraded environments. In this role, a single application of biosolids would be used, reducing the risk of accumulation of TEs in the soil. Given that most NZ-native species are not used for food, there is little risk of contaminants entering the human food chain. It is unclear, however, how much biosolids would increase the growth of NZ-native species, since most of the NZ flora is adapted to low fertility soils, and previous research has shown variable results depending on plant species. Their TE uptake profiles are largely unknown. NZ-native species growing in biosolids-amended soil may change plant chemistry, potentially changing ecosystem nutrition and resulting in facilitating the entry of contaminants into food webs. This thesis sought to determine how the establishment of NZ-native plants could be enhanced by using biosolids in low-fertility substrates. Moreover, it aimed to determine the effect of biosolids on the fluxes of nutrients and TE-contaminants from biosolids-amended substrates to plants.

A seedling experiment aimed to determine the potential use of biosolids in plant nurseries. Three distinct biosolids and one pond sludge from separate WWTPs in NZ were combined with shredded Pinus radiata bark at five rates from 0% and 50%. Six native species Veronica salicifolia (Banks & Sol. ex Benth.) L.B.Moore, Poa cita Edgar, Corokia cheesemanii Turril, Phormium tenax J.R.Forst & G.Forst, Griselinia littoralis Raoul and Cordyline australis (G.Forst.) Endl was grown for four months. All species, except Griselinia littoralis, Corokia cheesemanii and Poa cita showed accelerated growth at rates up to 10% ‘fresh’ biosolids and 30% ‘aged’ addition; these rates were considered as optimal for the specific plant species. Higher rates gave inconsistent growth results; therefore, plants grown at optimal rates were analysed for element uptake. The type and rate of biosolids were important factors that affected overall plant health, growth, and plant chemistry. Biosolids addition increased the plant concentrations of N, P, K, S, Mg, Ca, and Zn at optimal rates. None of the TEs exceeded risk levels for elements in animal fodder, indicating they are unlikely to pose a risk to local ecosystems. Lastly, the pond sludge was unsuitable as a growing substrate, due to low pH, lack of nutrients and proportionally high TEs.

A field trial was set up at Christchurch Wastewater Treatment Plant (CWTP), where biosolids from the CWTP were added to a recent sandy soil at two rates equivalent to 500 kg N ha⁻¹ and 1,500 kg N ha⁻¹. Biosolids were added either as a surface application (Surface), mixed into the top 200 mm (Mixed), in trenches (Band) or as a mass placed underneath (at Depth) the seedling. With four replicates per treatment, the experiment comprised 32 plots with 15 NZ plant species in each. Following one year of growth, the plant growth was assessed by plant volume. Soil samples were collected in each plot and chemical analysis was performed to determine fertility and TEs concentration. Pittosporum eugenioides A.Cunn, Sophora microphylla Aiton and Phormium tenax J.R.Forst & G.Forst had significantly different canopy volume on some of the biosolids treatments. The rest of the species had no significant growth response to biosolids addition. The prevalence of exotic weeds was dependent on the configuration of the biosolids addition: there was a higher coverage when biosolids were surface applied and Mixed at a High rate, requiring more weed control. There were no other significant variations for other soil parameters responsible for soil fertility such as TC or Total N, and there were no changes in soil TEs concentrations, except when these were compared to its nearest control. Given that soil C was positively correlated with P, Cu and Zn along the experimental plot, the high spatial variability of the soil characteristics was likely the result of previous applications of pond sludge from the nearby maturation ponds.

Field sampling was conducted at an active coal mine (Stockton mine) where biosolids have been used for rehabilitation and revegetation since 2012. The chemical properties of biosolids-amended substrates with those from other restoration practices were compared and, determined the uptake of nutrients and contaminants by native and exotic species both in biosolids-amended substrate and other restoration methods to determine the risk of TEs transfer to food chains. The natural soil collected from an undisturbed forest contained high TC and total N (23% C, and 0.5% N) and low Olsen-P 10 mg kg⁻¹ compared with the substrates of the rehabilitated areas. Trace element concentrations in forest soil were similar to suggested soil ecological guideline values for NZ’s soils (ECO-SGVs). Concentrations of Olsen-P were tenfold higher in the biosolids-amended substrates than in forest or other rehabilitation substrates. Biosolids-amended substrates contained Cu and Zn at concentrations 7 to 9-fold higher than the unamended substrate. In plant species planted for rehabilitation across several sites, the concentrations of Mn and Zn in the foliar tissue broadly reflected the soil concentrations. There were significant interspecific variations in the uptake of both essential and non-essential elements. Members of the Asteraceae family accumulated significantly higher concentrations of P, K, Mn, and Zn. These species could be proscribed in areas where high rates of biosolids addition have increased the concentrations of these elements in the substrates.

At the rates of biosolids used in this study, the plant TEs concentrations did not exceed animal tolerance levels for fodder, indicating they are unlikely to pose an undue risk to local ecosystems. Nevertheless, biosolids significantly change the nutrient concentrations in the NZ-native species and this may result in changes in trophic systems e.g. herbivore nutrition.

This study showed that biosolids can be added to substrates at rates of ca. 1,500 kg N ha⁻¹ (or 10% – 30% w/w in potting substrate) to augment the growth of most, but not all, NZ-native species. Given that the field trial (at CWTP) and field sampling (at Stockton Mine) indicated the accelerated development of weeds while in contact with biosolids, it is likely that biosolids accelerate the growth of exotic weeds more than NZ-native species. This is likely due to the high rates of available N and P contained in the biosolids. While these could be reduced by applying lower rates, this would offset the benefits of replenishing organic matter in the low- fertility substrate. Future work should investigate the blending of biosolids with high-C, low-nutrient materials, such as wood-waste, to produce a mixture that favours the growth of NZ-native plants over exotic weeds. Such mixtures would also reduce NO₃⁻-N leaching losses that may otherwise occur with high rates of biosolids addition. Research should also determine the long-term effects of biosolids addition: this study tested seedlings and other native species <12 years old.