Landuse and water quality
Adding more nutrients to the soil than is needed by the plants growing in it or by using the land in ways that cause erosion eventually leads to lakes and rivers gaining those nutrients. More nutrients in our water bodies means that more phytoplankton (microalgae) can grow and the water in rivers and lakes becomes murky. Cyanobacteria are a phytoplankton group that can produce toxins, making the water dangerous to drink or swim in.
When trees are removed or grassland is overused or trampled then soil erosion can occur. Soil particles have particulate phosphorus attached to them. As the particles are washed overland into streams, rivers and lakes they carry the particulate phosphorus with them. The soil particles end up settling out of the water on to lake and river beds.
By adding more fertiliser, such as urea, or animal urine than a plant needs, then plants cannot use all of that nutrient. This is especially true when a cow urinates on the grass during winter when plants are growing slowly and not using so much nutrient. Summer drought can also cause grass to grow slowly, so this too can reduce the uptake of nutrients. A cow's urine patch equates to 500 to 1000 kg N ha-1 which is the nitrogen a grass plant needs for two to three years. Urine contains a nitrogenous substance called urea and when an animal urinates then the urea in the urine is transformed to ammonium then nitrate by soil bacteria. Nitrate is soluble so if it does not get used by plants then it leaches into the groundwater. Once nitrate is in the groundwater then it flows underground until it eventually ends up in a stream, river or lake. This can take many years.
In the image below, notice how these animals have disturbed the soil.
Figure 1. Soil disturbance by farm animals (photo, Wendy Paul).
How this affects water quality
Nutrients used by plants on land are the same as the nutrients used by algae (macrophytes and phytoplankton) and plants in lakes. Plants and algae fix carbon through photosynthesis and require nitrogen and phosphorus in forms that can be taken up or are bioavailable. Bioavailable forms of nitrogen include ammonium and nitrate and dissolved reactive phosphorus for phosphorus.
With more nutrients available, then more algae and plants can grow. This leads to more organic matter in the lake which means there is more decaying matter for bacteria to breakdown and use as a source of nutrients. As the bacteria respire they use up all of the oxygen in the bottom of a stratified lake during summer. Under de-oxygenated conditions the bonds between iron cations and particulate phosphorus adsorbed to sediment particles break releasing dissolved reactive phosphorus that algae and plants can use. As bacteria breakdown organic matter, ammonium is produced and it builds up in the bottom waters.
Figure 2. Lake stratification during summer.
During stratification the surface waters are 'cut off' from the bottom waters since the warmer, less dense water at the top cannot mix with the nutrient-rich bottom water. During that time, some cyanobacteria or blue-green algae outcompete other phytoplankton since they have mucilage or gas vesicles that help them float near the surface where they have good access to light, they store more phosphorus than they need and can fix nitrogen from the atmosphere in special cells called heterocytes. Many species of blue-green algae are known to produce toxins that can cause harm to humans and animals. To find out more about cyanobacteria and the toxins they produce click here.
Figure 3. Cyanobacteria bloom on Lake Ngaroto (photo, Warrick Powrie).
Landuse practices
Nutrients, in the form of fertiliser or urine, added to the land in quantities greater than grass plants can take up will increase the nitrate leached into groundwater and eventually into water bodies. Similarly, increased soil disturbance and removal of plants will increase the sediment particles reaching a lake.
Fertiliser
Applying more fertiliser than is required, especially during winter when plants are growing slowly and not using the nutrients is not a good practice and results in unnecessary cost to farmers also. Farmers can use a software package to estimate the amount of fertiliser required for different parts of their farm and produce a nutrient management plan.
Effluent Ponds
After milking, cowsheds are hosed down to remove all the urine and faeces dropped on the concrete pad. Rather than allowing the effluent to run into drains then into waterways, this is stored in effluent ponds. In the effluent ponds, bacteria break the organic matter down and water is evaporated off. The effluent is stored till the warmer months and spread over the paddocks. This practice reduces the need for fertiliser.
Figure 4. Effluent pond on a dairy farm (photo, Wendy Paul).
Break-feeding
Break-feeding is the practice which farmers use to eke out the available feed during winter by confining cattle to small plots of grass using electric wires and moving them around the farm in a controlled manner and using supplementary feed also. This practice intensifies the quantity of urine going on to the pasture whilst reducing the grass down to stubble thus making the ground more prone to soil disturbance.
Figure 5. Break-feeding on a dairy farm (photo, Wendy Paul).
Feed pads
Using feed pads and supplementary food is a better way to manage food supply as they allow for collection of effluent which can then be spread more evenly over paddocks during the warmer, drier season when plants can better utilise the nutrients and the soil is not disturbed by hoof action.
Sediment traps and wetlands
Sediment traps and wetlands are both ways of reducing the amount of sediment, thus nutrient, entering a water-body. They slow the water so that the sediment drops out. Plants in and around wetlands and traps means take up some of the nutrient trapped. Silt from the traps can be removed and spread on the paddocks to recycle the nutrients and reduce the amount of fertiliser required.
Riparian margins
Planting around lakes and along stream edges provides a buffer where sediment particles are 'caught' so that they don't enter the water and the plants can utilise the nutrients before they reach the lake or water course.
Other practices
Some farmers graze their animals outside of lake catchments during winter. Chicory, unlike grass, is a deep-rooted plant that will grow during dry weather, reducing feed problems over summer and providing a plant that can take up nutrients when grass is slow growing.
Useful links
There are case studies on wetland restoration, riparian planting and silt traps on the LERNZ website read more... For further information click here.
NZ Landcare Trust has facilitated projects that involve use Best Management Projects. To find out more click here.
Overseer® is a software package that helps farmers to look at how nutrients are distributed on their farms and produce a nutrient management plan.
In conjunction with AgResearch, Herd homes® have developed a feed pad system with a shelter and effluent collection bunker below it.
Science on the Farm is a web resource that links to the NZ education science curriculum achievement aims.
References
Denyer, K., Peters, M., Berry, N., Dresser, M. 2010. Best Management Practices for Enhancing Water Quality in the Waikato. NZ Landcare Trust, Hamilton, New Zealand.
Gibbs, M.M. 1987. Groundwater contributions to water and nutrient budgets. In Lake managers handbook, Water and Soil Miscellaneous Publication No 103. (ed W.N. Vant), pp. 230 Wellington, Ministry of Works and Development., Wellington, New Zealand.
Hickey, C.W. & Rutherford, J.C. 1986. Agricultural point source discharges and their effects on rivers. New Zealand Journal of Agricultural Science, 20, 104:109.
Ledgard, S.F., Crush, J.R., & Penno, J.W. 1998. Environmental impacts of different nitrogen inputs on dairy farms and implications for the Resource Management Act of New Zealand. Environmental Pollution, 102, 515-519.
Ledgard, S.F., Penno, J.W., & Sprosen, M.S. 1999. Nitrogen inputs and losses from clover/grass pastures grazed by dairy cows, as affected by nitrogen feriliser application. Journal of Agricultural Science, Cambridge, 132, 215-225.
Menneer, J.C., Ledgard, S.F., & Gillingham, A.G. 2004. Land use impacts on nitrogen and phosphorus loss and management options for intervention. AgResearch Ltd, Hamilton, New Zealand.
