Excess nutrients in the soil can potentially lead to pollution of groundwater, eutrophication in streams, rivers and estuaries and excessive greenhouse gas emissions (such as nitrous oxide – N2O).
The scientists are investigating a number of ways farmers can manage their farms to reduce excess nutrient loss to help prevent water and air pollution.
Ross and Selai have been investigating nitrogen inhibitors. Farmers can apply nitrogen inhibitors to prevent nitrification from taking place (the process of ammonium converting to nitrate). This limits the amount of nitrogen compounds (nitrate) that can be leached from the soil, lessening the risk of nitrogen loss to the environment. Farmers can also apply inhibitors to prevent denitrification taking place (the process of nitrate converting back to gaseous nitrogen – N2). Quite often, the process of converting nitrate to N2 is not completed, and nitrous oxide (N2O), a greenhouse gas, is released instead. The inhibitors reduce the amount of this greenhouse gas that is produced.
Collecting and spreading effluent
Effluent from animals is rich in the nutrients nitrogen and phosphorus. Dung is loaded with phosphorus, while urine from animals (cows in particular) contains concentrated amounts of nitrogen. Although the grass may initially die or lie dormant after contact with urine, once the nutrients are processed in the soil, the plant life above flourishes, as shown by dark green patches of grass in paddocks. These ‘urine patches’ indicate an excess of nutrients is present. The scientists suggest farmers collect effluent (urine and cow dung) and then redistribute it at a low rate, evenly across paddocks where it is needed. This prevents a build-up of nutrients in one place. This takes place already to some degree, as excreta is collected from the milking shed and reapplied across paddocks.
Herd shelters or stand-off pads are being trialled on the AgResearch farm near Balclutha as a way to collect effluent. Stock are taken off paddocks and put on stand-offs – particularly in the winter – for extended periods of time. Winter is a high-risk time when higher rainfall causes increased run-off and loss of nutrients. Plant uptake of nutrients is low at this time of the year so fewer nutrients are required.
Excreta is deposited by stock onto the stand-off pad, which is designed to drain the excreta to a collection point. The effluent is then watered down, mixed and piped back to effluent spreaders set up in paddocks where it is needed.
Fertilisers are applied at a rate that allows plants to utilise the nutrients so fertiliser is not wasted.
Protecting riparian zones
Riparian zones are strips of vegetation along the edges of waterways. Fencing off riparian zones prevents stock from urinating and spreading dung close to surface water.This also prevents stock from treading and compacting the soil and stream banks, thus protecting the stream habitat. Riparian zones may also help to filter surface run-off in some situations.
Developing wetlands/riparian zones
Scientists from NIWA (the National Institute for Water and Atmosphere) have developed the wetlands and riparian zones on their experimental farms by planting out around them. The extra plants act as filters as run-off comes through. The plants catch sediment erosion, take up excess nutrients and provide a food source for soil bacteria to remove nitrate nitrogen via denitrification.
Regulation and helpful tools
The Invermay scientists suggest that regulation for farmers shouldn’t be too stringent. Farmers should be encouraged to make changes for themselves and be provided with plans and tools to assist them to make decisions about nutrient management.
The Overseer® Nutrient Budgeting model is an example of an online tool that has been developed by AgResearch scientists to help guide this management. It examines nutrient use and movements within a farm. The computer model calculates and estimates the nutrient flows in the farming system and identifies risk for environmental impacts through nutrient loss, including run-off and leaching, and greenhouse gas emissions.
Nature of science
Science is embedded and practised in the context of larger social surroundings. Although scientists may understand the science accompanying environmental nutrient overload problems and possible solutions, there are other considerations involved such as cost and loss of production to the farmer. Scientists make suggestions based on science research, while other considerations help to determine the final outcome.