Biowastes to establish plants for essential oil production on low fertility soils : A dissertation submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Biowastes are unwanted materials of biological origin. They include biosolids (from sewage works), Dairy Shed Effluent (DSE), sawdust and Treated Municipal Wastewater (TMW). When applied to soil, biowastes can provide plant nutrients, but also introduce heavy metals, pathogens, or xenobiotics. Biowastes could improve degraded or low-fertility soils and generate revenue through the production of non-food products such as essential oils (EOs). I grew NZ native plants, mānuka (Leptospermum scoparium J.R. Forst & G. Forst) and kānuka (Kunzea robusta de Lange & Toelken) as well as Lavender (Lavandula angustifolia Mill.), Rosemary (Rosmarinus officinalis L.) and Thyme (Thymus vulgaris L.) in series of greenhouse experiments in medium-to-low fertility soils. Soils used in the experiments were Bideford clay loam (BCL), Lismore stony silt loam (LSL), Pawson silt loam (PSL) and Craigieburn silt loam (CSL), ordered from highest to lowest fertility, that were amended with either biosolids (up to 13500 kg N ha-1 equiv.), biosolids + sawdust (1:0.5, 1250 kg N ha-1 equiv.) and DSE (200 kg N ha-1 equiv.). Two types of biosolids from Kaikoura (KB) and Christchurch City Council (CB) were used in the experiments. Moreover, biosolids were applied in different methods including incorporation to the soil, surface application and using patches of the biosolids in soil. I had a field experiment to evaluate the effect of TMW (30 kg N ha-1 equiv.) on the EO yield and compositions of L. scoparium and K. robusta. Field surveys were used to evaluate the EO concentration and composition in natural populations of L. scoparium and K. robusta.

In the greenhouse experiments the maximum biomass increase was related to CB application (3000 kg N ha-1 equiv.) that enhanced the biomass of L. scoparium by up to 29-fold in the lowest fertile soil (CSL). Generally, the optimum biosolids application rate was 1500 kg N ha-1 equiv. in the greenhouse experiments that increased the biomass of L. scoparium, K. robusta, L. angustifolia and R. officinalis by up to 120%, 170%, 86% and 70 % in PSL and LSL (low fertile soils), while DSE only increased the biomass of L. scoparium and R. officinalis and not the other plant species. Adding sawdust to KB increased the biomass of L. scoparium and K. robusta although it offset the L. scoparium growth increase by the KB-only treatment. Applying TMW increased the canopy volume of L. scoparium and K. robusta in the field experiment. Biowastes increased foliar concentrations of some macronutrients (e.g. N, P and S). Generally, the concentration of TEs including Zn was increased by biosolids and no other biowastes application. Maximum CB application rate increased the Zn concentration of L. scoparium and L. angustifolia leaves by 4 and 3 times (from 24 to 102 and 38 to 115 mg kg-1 dry matter), respectively. Cadmium concentration increased by up to 11-fold and 31-fold (from 0.03 to 0.22 and 0.01 to 0.31 mg kg-1 dry matter) in the L. scoparium and K. robusta leaves when biosolids were applied in high rates. Concentration of the TEs in all treatments and all experiments stayed in the safe range of the food safety standards. Generally, the treatments had a negligible effect on oil concentration except for DSE (200 kg N ha-1 equiv.) and biosolids application at rates higher than 1500 kg N ha-1 equiv. that decreased the R. officinalis and L. angustifolia EO concentrations, respectively. This would offset the effect of biomass increase in terms of oil production. Contrasting methods of biosolids application had similar effects on the EO concentration of L. scoparium. Most of the essential oils’ evaluated components were unaffected or slightly affected by biowaste addition. The incorporation of biosolids with soil had a greater effect on the EO composition compared to surface application. The field survey showed that the EO concentration of L. scoparium and K. robusta was significantly higher than the plants grown in the greenhouse. This could be the result of environmental conditions that would induce the EO production of the plants.

This study indicates that biowastes that are disposed into landfills or waterways may be beneficially used to restore native ecosystems on low-fertility or degraded soils to produce essential oils. Applying biosolids (up to 1500 kg N ha-1 equiv.) could establish native ecosystems dominated by L. scoparium and K. robusta that annually would produce up to 103 kg ha-1 and 98 kg ha-1 of essential oils worth NZ$38400 and NZ$34300, respectively. Further field trials are warranted to elucidate critical ecological variables and production economics in biowaste management. In particular, the effect of adding biowastes to established stands of L. scoparium and K. robusta should be determined.