The response of manuka (Leptospermum scoparium) to homogeneous and heterogeneous distribution of biosolids in soil

Abstract

Potentially, biosolids (sewage sludge) could be added to soil to enhance the growth of manuka (Leptospermum scoparium) for the production of honey, essential oils, and ecosystem restoration. Given that manuka is a pioneering species that is adapted to low fertility soils, it was unclear whether there would be a positive growth response to biosolids addition. I aimed to determine the effect of biosolids addition on the biomass, root morphology and elemental composition of manuka. Pots (2.5 L) and Rhizoboxes (15 x 30 x 2.5 cm) were filled with low-fertility soils from Eyrewell Forest (Lismore brown soil) and Kaikoura (sand). Biosolids from Kaikoura (10% of the total weight by mass containing 22g N/kg) were applied either homogeneously or heterogeneously to the surface of the pots and in a 5 cm vertical strip on one side of the rhizoboxes. There was also a control (no biosolids). Each treatment was replicated thrice. Manuka seedlings were grown for 12 weeks and then the biomass, root distribution and chemical composition was determined. The addition of biosolids increased the biomass in both soils. The increases in biomass were not significantly affected by the distribution of the biosolids. However, the distribution of the biomass affected root distribution, with roots proliferating in the biosolids patches in the heterogeneous treatments. In the Kaikoura sand, the addition of biosolids increased the plant concentrations of N, C, P, S, Zn, and Cd, whereas in the Eyrewell soil the biosolids increased N, Zn, Cd and Ni. In Kaikoura there were differences between homogeneous and heterogeneous treatments in plant Zn, Cu and Ni and in Eyrewell differences occurred in Zn and Cd. None of the trace element concentrations in manuka were likely to pose a risk to herbivores or ecosystems. My experiment demonstrated that manuka responds positively to the addition of biosolids and that the positive growth response was not affected by the distribution of biosolids on two soil types. Furthermore, the addition of biosolids did not cause manuka to take up unacceptable concentrations of trace elements. Future research should investigate the performance of manuka over a longer timescale and include treatments where biosolids are applied to the soil surface of existing manuka stands. Root morphology should also be investigated for deeper understanding of foraging behaviour.