Mānuka (Leptospermum scoparium) roots forage biosolids in low fertility soil

Potentially, biosolids could be applied to low fertility or degraded soils to establish mānuka (Leptospermum scoparium), an economically important plant species used for honey and essential oil production. Given that this pioneering species is adapted to low-fertility soils, it is unclear whether it would respond positively to biosolids. We aimed to determine the growth, root morphology and elemental uptake of L. scoparium in contrasting soils amended with biosolids, distributed either homogeneously or heterogeneously. Pot and rhizobox experiments revealed that the roots of L. scoparium morphologically foraged patches of biosolids in soil. This finding is in contrast to previous reports that foraging is uncommon in plants adapted to low fertility soils. In a low-fertility sand, biosolids increased the growth 40-fold, irrespective of the distribution of biosolids. This increase was lower (60%) in an orthic brown soil. In the biosolids-amended soils, the foliar concentrations of N, P, K, S, Mg and Ca were above 2%, 1.5 g kg−1, 0.8%, 2.0 g kg−1, 1.7 mg kg−1 and 0.8% respectively, which is within the range of concentrations found in native species in their natural habitat. In the control soils, foliar concentrations of N, P & S were significantly lower, indicating that these elements may be limiting. The maximum concentration of Mn (660 mg kg−1), Zn (211 mg kg−1), and Cd (1.5 mg kg−1) in leaves of plants growing in biosolids-amended soil should not cause concern to plant health, but it should be taken into account for their potential effect on trophic networks. Further experiments should focus on the design of field-scale applications of biosolids for improving L. scoparium growth and determine the effect of biosolids distribution on nutrient losses.