By Carlos Ocampo, Project Hydrologist - Centre of Excellence for Ecohydrology, The University of Western Australia
When fertilizer chemicals (phosphorus and nitrogen) escape farm boundaries and enter waterways, they can end up in protected biodiversity hotspots such as wetlands and estuarine environments, where they cause problems by disrupting natural nutrient cycles. The end result can be catastrophic toxic algal blooms (due to nutrient enrichment) which may affect the functioning of the whole ecosystem.
The annual costs associated with nutrient pollutants are estimated to be from $100 million in the UK and Wales and $240 million in Australia, to a staggering $2.2 billion in the U.S. These figures include not only spending on recovery of threatened species and drinking water, but also components such as the value losses in recreation, real estate and tourism. However, worldwide total spending on responding to the problem is minute in comparison to these annual costs. If simple changes can offer an economic benefit to the landholder and avoid these significant downstream costs then everyone wins.
Until now, the common approach to the problem has focused on protecting and restoring riparian buffer zones (natural strips close to waterways). These buffer zones provide vegetation that trap nitrogen and phosphorus, and have proven effective in landscapes where water runoff moves mainly via surface pathways or within the root zone.
Researchers still struggle with how to deal with phosphorus losses from farming systems that operate in landscapes where water runoff moves via subsurface pathways (i.e a shallow water table) and within permeable coarse soils dominated by sand. Such landscapes are found in coastal plain areas from Southern Florida to around Australia, and commonly linked to nutrient-poor environments and biodiversity hotspots. Often, these landscapes were converted from natural wetlands to agricultural uses. Riparian buffer zones are risky to rely on here, as heavy water runoff from farms may overwhelm or bypass these buffer zones and nutrients can enter agricultural drainage ditches and other waterways.
An exciting study, led by Associate Professor Megan Ryan of the School of Plant Biology at the University of Western Australia (UWA), sponsored by Greening Australia and Harvey River Restoration Taskforce and funded by the Alcoa Foundation Advancing Sustainability Research Program, is investigating how to use native plants to reduce fertilizer runoff into waterways in these landscapes. While the ultimate goal is to protect wetlands of international significance from pollution by nutrients, the project will look for suitable plants with an income stream for farmers and address the problem of toxic algae blooms worldwide.
Participants in the study - farmers, landholders, community groups and academic researchers – are looking at subsurface nutrient pathways from upland to midslope to riparian zones to find places, times and methods to intercept nutrients before they reach waterways.
The first research component of the study has tackled how water and phosphorus move in the subsurface, and how this water is discharged into artificial drains in field sites of Western Australia. Since riparian zones are physically connected to other landscape units, it seems logical to first discover how water runoff and nutrients move before implementing any nutrient trapping strategy.
Following a buildup of phosphorus in the water table in late summer-early autumn 2012 (Jan-Apr) at the riparian zones, our study asked, “How would this phosphorus make its way to the drainage ditch? Would the drain waters intrude into the water table (via the drain’s bed and banks) and reach it or would it be pushed out to the drain by a rising water table in riparian areas?” Miss Hani Mohamad, a Bachelor of Environmental Engineering student from the School of Environmental Systems Engineering (UWA) undertook the first attempt to explore the issue.
Using water level and water temperature data, Mohamad computed how much water and heat were exchanged between the drain and the water table in riparian zones, helped by the large difference in temperature between the cold water in the drain (average 12 oC/53.6 oF) and warm water in the water table (average 19 oC/66.2 oF). Mohamad combined several technologies to gather the information: a camera taking pictures of a level staff in the drain, automatic water level sensors monitoring the water table in shallow bores and temperature sensors buried in the drain’s bed and banks.
So what is the net result? Riparian buffer zones showed a tendency to accumulate water and created favorable conditions for plant uptake of nutrients during the spring season (Sep-Nov 2012) with the water table close to ground surface and phosphorus-enriched water. Management of vegetation at this time may, therefore, be a way in which phosphorus could be intercepted before reaching the drain.
In a similar way, we can now move to the big landscape picture and find the answer to the question: Which plants will work the best to reduce nutrients leaving the farm? If so, where and when do they have the best chance of success?
The team is currently assessing the exact mechanisms by which phosphorus changes across the landscape. So if conditions are favorable away from riparian zones for trapping phosphorus, the project will adapt opportunistic management strategies that take full advantage of the conditions provided by the environment.The School of Plant Biology team is currently assessing which plant species will be the best value for farmers, and the environment, in terms of reducing phosphorus losses to waterways. The research program will develop recommendations based on this holistic view of the problem. So stay tuned for further developments on our website.