Biowastes promote essential oil production on degraded soils

Abstract Biowastes (wastes of biological origin) can improve soil fertility but may render the land unsuitable for food production because they introduce contaminants, including heavy metals, pathogens and xenobiotics. We investigated whether sewage waste (pond sludge from Kaikoura and anaerobically-digested biosolids from Christchurch) and Dairy Shed Effluent (DSE) could improve degraded soils for the production of essential oils (EOs). We grew lavender (Lavandula angustifolia Mill.), rosemary (Rosmarinus officinalis L.) and thyme (Thymus vulgaris L.) in two greenhouse experiments in Lismore stony silt loam soil (LSL) amended with pond sludge or biosolids (500–4500 kg N ha−1 equiv.) or DSE (200 kg N ha−1 equiv.). Pond sludge application (2800 kg N ha−1 equiv.) increased the biomass of L. angustifolia and T. vulgaris by 60 % and 62 %, respectively. Christchurch biosolids application up to 1500 kg N ha−1 equiv. to L. angustifolia and R. officinalis increased the biomass of both plant species by up to 86 % and 80 %, respectively. The effect of treatments on EO concentration was insignificant in most cases except for DSE (200 kg N ha−1 equiv.) and Christchurch biosolids at rates >1500 kg N ha−1 equiv., which decreased the EO concentrations in R. officinalis and L. angustifolia. This decrease in EO concentration offset some of the increase in EO production resulting from the increased biomass of the biowaste-amended plants. The ideal EO production increase occurred when Christchurch biosolids were applied at 1500 kg N ha−1 equiv. The benefits of biowaste additions to degraded soils are greater than would occur if they were added to high-fertility soils. Heavy metal concentrations in all treatments were below food safety standards. Biowastes could rebuild degraded soils and produce valuable EOs, thereby reducing the economic and environmental costs of biowaste disposal, while improving soil fertility and generating revenue from otherwise underproductive land.