The potential of reusing New Zealand's biowastes combined with native and exotic species for improved environmental and economic outcomes

Biowastes are unwanted materials of biological origin include biosolids (sewage sludge), Treated Municipal Wastewater (TMW), wood-waste, Dairy Shed Effluent (DSE), and composts made from municipal wastes. Potentially, biowastes can improve soil fertility and reduce the requirement for mineral fertilizers for both degraded and productive lands. However, application to soil may result in the accumulation or leaching of the Nutrients and Contaminants Associated with Biowastes (NCAB) in the environment. Nutrients include nitrogen (N) and phosphorous (P) and other macronutrients, while common contaminants include cadmium (Cd), copper (Cu), and zinc (Zn). In New Zealand (NZ), most biowastes are discharged into waterways (e.g. treated municipal effluent) or disposed of in landfills (e.g. biosolids). This is expensive and represents a waste of a potentially valuable resource. While the application of biowastes to pristine agricultural land may be unacceptable, biowastes may be used to enhance the growth on degraded or marginal lands for the production of timber, fibre, energy, essential oils, or even NZ-native honey. Some of the negative environmental effects of adding biowastes to soil may be offset by the overlying vegetation if such plants take up nutrients that would otherwise leach, provided these plants do not accumulate unacceptable concentrations of contaminants. I hypothesised NZ native and exotic plants that were selected for their potential economic or ecological value, may improve environmental outcomes of applying biowastes application to low-fertility soil through increased growth, while accumulating minimal concentrations of contaminants in their aerial parts. I also hypothesised that mixing distinct biowastes would reduce accumulation of contaminants and improve soil quality, thus stimulating growth of the plants. I aimed to determine the plant-soil interactions on biowaste-amended soil using greenhouse experiments and field trials. Specifically, I tested Leptospermum scoparium, Kunzea robusta, Kunzea serotina, Olearia paniculata, Coprosma robusta, Podocarpus cunninghamii, Grisilinea littoralis, Pseudopanax arboreus, Phormium tenax, Phormium cookianum, Cordyline australis, Pittosporum eugenioides, Pinus radiata, Brassica napus, Sorghum bicolor, and Lolium multiflorum. Particular attention was paid to L. scoparium and K. robusta because these NZ-native species produce valuable honey and essential oils. The biowastes included biosolids, TMW, sawdust, DSE, and compost made from municipal green-waste. Mineral fertilisers were used as comparison for some species. I measured the effects of the biowastes on plant growth and elemental uptake as well as the soil quality. Three glasshouse-based experiments and two field trials were conducted to support the objectives of this research. Initially, the response of L. scoparium and K. robusta to individual nutrients was determined using mineral fertilisers on orthic brown soil with a clay-loam texture. Using agronomically-relevant application rates equivalent to 200 kg N ha-1, 100 kg P ha-1, 100 kg K ha-1, 100 kg S ha-1, my experiments showed that only N improved growth. However, the nutrient additions to soil resulted in increased foliar concentrations. Amending the same soil with 2600 kg N ha-1 equivalent of biosolids and 200 kg N ha-1 equivalent of DSE improved the growth of both L. scoparium and K. robusta by 34% and 64%, respectively and increased foliar P, Ca, and S uptake by 33%, 37%, and 32%%. Concentrations of Cd, Cu and Zn increased, but remained within threshold values. A second experiment, using 10 L lysimeters, showed that biosolids applied at 1200 kg N ha-1 equivalent improved the growth of L. scoparium, K. robusta, P. radiata, S. bicolor, B. napus and L. multiflorum by 60%, 27%, 61%, 29%, 61% and 77%, respectively. The beneficial effect of biosolids was slightly offset when it was mixed in equal volumes with sawdust. In general, the biowastes produced a larger growth response than urea applied at 200 kg N ha-1 equivalent, while the N leaching under biosolids was generally lower. There was a significant species effect on N-leaching, with L. scoparium and K. robusta leaching significantly less N than the other species. None of the species accumulated unacceptable concentrations of contaminants. In a field trial on a Pawson Silt Loam, the irrigation of TMW at 500 mm yr-1 improved the growth of some, but not all species tested. A trial comprising 11 native species, namely L. scoparium, K. robusta, O. paniculata, P. arboreus, C. robusta, P. cunninghamii, G. littoralis, P. eugenioides, C. australis, P. tenax, and P. cookianum was established on ca. 1000 m2 of land near the town of Duvauchelle. Trees irrigated with TMW grew better than or the same as unirrigated trees. There were no signs of toxicity. The plants with the greatest positive response to TMW were L. scoparium, O. paniculata, C. robusta, Podocarpus cunninghamii, Cordyline australis, and Phormium tenax. A second field trial at the former Eyrewell forest showed that only K. serotina responded positively to the application of municipal compost (1200 kg N ha-1 equiv.) and a DSE-sawdust mixture (2400 kg N ha 1 equiv.). This thesis shows that a diverse range of NZ biowastes can be used to promote the growth of NZ-native and exotic species, without resulting in unacceptable concentrations of contaminants in the plants or soils. Whereas TMW and DSE could be continually applied to plants, the continual application of biosolids may result in the accumulation of contaminants in soil. Therefore, the biosolids application would be more suited to a single application to restore a low-fertility or degraded soil. Mixing the biosolids with sawdust may further reduce plant contaminant uptake or NO3- leaching. This beneficial reuse of biowastes will reduce disposal costs, while providing valuable economic or ecological benefits. There was some evidence in this thesis that some NZ-native plants, namely L. scoparium and K. robusta, may alter nutrient cycling in soil and therefore further reduce NO3- leaching. These rhizosphere studies should be the subject of future research

Biowaste mixtures affecting the growth and elemental composition of Italian ryegrass (Lolium multiflorum)

Biosolids (sewage sludge) can be beneficially applied to degraded lands to improve soil quality. Plants grown on biosolids-amended soils have distinct concentrations of macronutrients and trace elements, which can be beneficial or present a risk to humans and ecosystems. Potentially, biosolids could be blended with other biowastes, such as sawdust, to reduce the risks posed by rebuilding soils using biosolids alone. We sought to determine the effect of mixing biosolids and sawdust on the macronutrient and trace element concentration of ryegrass over a 5-mo period. Lolium multiflorum was grown in a low fertility soil, typical for marginal farm areas, that was amended with biosolids (1250 kg N ha⁻¹), biosolids + sawdust (0.5:1) and urea (200 kg N ha⁻¹), as well as a control. Biosolids increased the growth of L. multiflorum from 2.93 to 4.14 t ha⁻¹. This increase was offset by blending the biosolids with sawdust (3.00 t ha⁻¹). Urea application increased growth to 4.93 t ha⁻¹. The biowaste treatments increased N, P, Cu, Mn, and Zn relative to the control, which may be beneficial for grazing animals. Although biowaste application caused elevated Cd concentrations (0.15-0.24 mg kg⁻¹) five- to eightfold higher than control and urea treatments, these were below levels that are likely to result in unacceptable concentrations in animal tissues. Mixing biosolids with sawdust reduced Cd uptake while still resulting in increased micronutrient concentrations (P, S, Mn, Zn, Cu) in plants. There were significant changes in the elemental uptake during the experiment, which was attributed to the decomposition of the sawdust

Effect of pine waste and pine biochar on nitrogen mobility in biosolids

Humanity produces ~27 kg of dry matter in biosolids per person per year. Land application of biosolids can improve crop production and remediate soils but may result in excessive nitrate N (NO₃⁻N) leaching. Carbonaceous materials can reduce the environmental impact of biosolids application. We aimed to ascertain and compare the potentials for Monterey pine (Pinus radiata D. Don)-sawdust-derived biochars and raw sawdust to reduce NO₃⁻-N leaching from biosolids. We used batch sorption experiments 1:10 ratio of material to solution (100 mg kg⁻¹ of NH₄⁺ or NO₃⁻) and column leaching experiments with columns containing biosolids (2.7% total N, 130 mg kg⁻¹ NH₄⁺ and 1350 mg kg⁻¹ NO₃⁻) mixed with soil, biochar, or sawdust. One type of low-temperature (350°C) biochar sorbed 335 mg kg⁻¹ NH₄⁺, while the other biochars and sawdust sorbed <200 mg kg⁻¹ NH₄⁺. None of the materials sorbed NO₃⁻. Biochar added at rates of 20 to 50% reduced NH₄⁺-N (<1% of total N) leaching from columns by 40 to 80%. Nitrate leaching (<7% of total N) varied little with biochar form or rate but was reduced by sawdust. Incorporating dried sawdust with biosolids showed promise for mitigating NO₃⁻-N leaching. This effect likely is due to sorption into the pores of the biochar combined with denitrification and immobilization of N rather than chemical sorption onto surfaces