Phytomanagement
Phytomanagement uses plants and soil conditioners to control the movement of nutrients and contaminants. This process stabilises, degrades, or removes contaminants, improves soil health, and can produce commercial products. This generates value from the land and creates a circular bio-economy.
We can establish native ecosystems to manage agricultural and municipal waste. These systems absorb excess nutrients and produce native products such as honey or essential oils. Their root zones accelerate the breakdown of organic contaminants and promote pathogen die-off. The restored vegetation also provides ecosystem services, including pollinator support, shelter, fibre, supplementary stock fodder, and improved aesthetics.
Phytomanagement works on soils with a wide range of contaminants, including trace elements from fertilisers and persistent organic pollutants. We can amend soils deficient in essential trace elements with biological wastes and specific crops to correct micronutrient deficiencies in humans and livestock.
Soil Contamination
Healthy soils underpin the production and quality of food and water. High concentrations of contaminants can damage soil health. Soil contamination can arise from natural processes or, more commonly, from human activities. Some unwanted elements, such as nickel, boron, and fluorine, occur naturally at high concentrations during soil formation or from events like volcanic eruptions. Human activity, including agriculture, industry, and transport, continually adds contaminants to soil.
For most contaminated soils, the cost of cleanup far exceeds the land's value. Therefore, instead of returning soil to a pristine state, contaminants are usually managed to minimise risk to humans and ecosystems.
Trace Elements
Trace elements are immutable. Most bind strongly to soil particles, limiting their uptake by plants and their movement downwards. These properties cause trace elements to accumulate in soil. At high concentrations, all trace elements are toxic. Therefore, land-use practices that add trace elements to soil are inherently unsustainable. Many contemporary practices, both conventional and organic, use trace elements to control pests and diseases or to supply nutrients. For example, certified organic systems permit copper-based fungicides, which increases copper concentrations in topsoil over time.
A critical question is what level of trace element accumulation we should tolerate. One could argue that accumulation exceeding threshold values after a century might be acceptable, as new management practices or low-cost remedies may become available within that time.
Cadmium Accumulation
Maintaining soil fertility for agriculture requires regular phosphate fertiliser applications. The rocks used for these fertilisers can contain high concentrations of cadmium, a toxic heavy metal. Repeated applications lead to cadmium accumulation in the topsoil, where it is readily taken up by plants, particularly leafy vegetables.
Our research focuses on using low-cost biowastes as soil conditioners to reduce cadmium uptake by plants. Some biowastes not only reduce plant cadmium but also improve soil fertility, thereby reducing the need for high-cadmium phosphate fertilisers.
Metalliferous Soils & Hyperaccumulators
Metalliferous soils can occur naturally, as in ultramafic (serpentine) soils, or result from human action. In both cases, trace elements may be undesirable due to their toxicity. Alternatively, the soil may be a potential source for commercial extraction of these elements. Plants on metalliferous soils have elevated trace element concentrations in their shoots, creating an exposure pathway into the food chain.
Hyperaccumulator plants gather large amounts of one or more trace elements in their above-ground biomass, often 100 times more than non-hyperaccumulators in the same soil. For most trace elements, the threshold for a hyperaccumulator is 0.1% of dry biomass. For zinc and manganese, it is 1%, and for cadmium, 0.01%. Over 400 hyperaccumulator species are known, many of which grow only on metalliferous soils.
Using Biowastes in Phytomanagement
Biowastes are unwanted materials of biological origin, such as biosolids (sewage sludge), animal effluent, wood waste, and green waste. They contain high concentrations of organic matter and plant nutrients, making them valuable soil conditioners. However, they can also contain pathogens and contaminants. Incorrect disposal is expensive and harms the environment. When applied correctly to degraded lands, biowastes create economic and environmental value.
Biofortification
Approximately 40% of agricultural soils are deficient in zinc, an essential micronutrient. Zinc deficiency reduces agricultural productivity and affects the health of one-fifth of humanity. Applying biowaste can alleviate zinc deficiency and improve soil fertility. In poor countries, applying correctly treated human waste to land both corrects zinc deficiencies and protects waterways.
Biofortification has advantages over adding micronutrients to the final product or using dietary supplements. Physiologically accumulated elements in plants provide a constant dietary source with less risk of toxicity from overdose or deficiency from supply gaps.
Case Studies
Municipal Wastewater (Duvauchelle)
In New Zealand, applying Treated Municipal Wastewater to land is preferred over discharging it into waterways. Plant root zones remove nutrients, mitigate pathogens, and immobilise contaminants. A 2014 trial determined the suitability of soils near Duvauchelle to receive this water. The wastewater significantly enhanced pasture growth, and a field trial of 11 native species showed that irrigated trees grew as well as or better than unirrigated trees.
Public perception often leads to disposing sewage products in landfills or waterways rather than applying them to agricultural land. Using biowastes to establish native vegetation breaks the direct link to the food chain.
Phytomanagement of biowastes using native ecosystems
Native plants and animals are often displaced from agricultural and silvicultural lands. Biowastes can enhance the reintroduction of native ecosystems into such environments, particularly if the soil has become degraded. In addition to providing shelter and ecological benefits, biowaste-assisted native ecosystems can generate revenue through the production of endemic products such as manuka honey and essential oils. Often, the products of sewage treatment are disposed into landfills or waterways because of negative public perception of their application onto agricultural land. When biowastes are used to enhance the establishment of native vegetation, the direct link to food is broken. We are investigating the use of native vegetation in farming systems, where the trees receive biowastes in the form of animal effluents. We are also researching establishment of native vegetation on degraded land using sewage sludge. In both cases, the role of the vegetation is to create value, either through saleable products or via ecosystem services.
Wood-Waste (Kopu)
The Kopu timber-waste pile is a 3.6-hectare site where sawdust was dumped for 30 years, contaminating a local stream. In July 2000, a trial began using poplar and willow clones to phytoremediate the site. Two poplar hybrid clones were chosen for their survival and biomass production. The trees reduce water drainage during summer, mitigating contamination. This phytoremediation cost an estimated NZ$200,000, compared to over NZ$1.2M for traditional capping.
The Tui Mine Tailings
The Tui mine tailings was considered New Zealand’s worst environmental disaster from mining. The site held 100,000 cubic metres of toxic waste with high concentrations of lead, cadmium, and mercury, and a surface pH below 3. To remediate it, a 100m² plot was established in 2001. Biowastes and lime were added to raise the pH and reduce metal availability, allowing native plants to establish quickly. In 2012, the site was fully re-engineered: the tailings were stabilised with cement and capped with topsoil.
Phytomining (Agromining)
Phytomining uses plants to exploit sub-economic ore bodies. A crop of a metal-hyperaccumulating plant is grown, harvested, and burned to produce a bio-ore. The first experiments were done by the US Bureau of Mines using the nickel hyperaccumulator Streptanthus polygaloides, which yielded 100 kg/ha of sulphur-free nickel.
The nickel-hyperaccumulators Alyssum bertolonii from Italy and Berkheya coddii from South Africa show even greater potential because of their high biomass and nickel content. On many ultramafic soils, Berkheya coddii can yield over 20 tonnes/ha with a 1% nickel concentration in its dry matter.
Phytomining is not yet applied at a large scale. It requires clearing large areas of native vegetation for a hyperaccumulator crop. After a few crops, the topsoil becomes depleted in the target element and must be removed. The underlying soil would then need substantial modification for plant growth. However, continual innovation may yet result in profitable operations.
