Examples of measures for reducing phosphorus and nitrogen losses from agriculture
Converting arable land to extensive grassland
Changing from intensive agriculture to extensive grassland will reduce nitrogen and phosphorus losses. This method suits best in areas which were historically kept as grazing areas and have conservation value.
Converting arable land to extensive grassland is very effective in reducing nitrogen because the low inputs ensure that nitrogen does not accumulate in soil. Conversion to ungrazed grassland can reduce nitrate losses by 95%. However, where the phosphorus content in soil is high, significant reductions in the leaching of soluble phosphorus are not achieved in the short term because the elevated levels of phosphorus will continue to be recycled through the soil. The immediate effect is that a permanent vegetative cover will reduce soil erosion and phosphorus losses in surface runoff. Conversion to ungrazed grassland can result in a 50% reduction in phosphorus.
This is an extreme change in land use that is unlikely to be implemented by farmers without incentives.
Plant cover in winter
Plant cover in winter will reduce nitrogen and phosphorus leaching and soil erosion.
Without the plant cover in winter, nitrate can be lost through leaching by excess winter rainfall and phosphorus through sediment transport in surface runoff. Plant cover in winter protects the topsoil of the fields against the erosive forces of rain, melt and runoff waters during winter. Furthermore, it helps to improve the soil structure by increasing the amount of organic matter in the topsoil of the fields which decreases the topsoil’s susceptibility to silting. Plant cover in winter can reduce erosion 10-40% and nitrate leaching 10-70%.
The method is relatively easy to implement. The costs of this method depend on the chosen plant, area and the possibility to use the farmer’s own machinery or contractor.
Minimal cultivation systems
Using discs or tines to cultivate the soil or direct drill into stubbles (no-till) will maintain organic matter and preserve good soil structure. This will improve infiltration and retention of water and thereby decrease total phosphorus concentrations in surface runoff.
Conversion from ploughing to minimal or no cultivation systems will decrease phosphorus in surface runoff. When using minimal cultivation systems the phosphorus storage concentrates in the shallow topsoil and that can in the long term increase the amount of dissolved phosphorus especially on the steep slopes with high phosphorus content. Buffer zones and more accurate phosphorus fertilisation should be used there. Nitrate leaching is generally decreased to a small extent through reduced mineralisation of organic matter in soil in the autumn.
The costs of this method depend on how it suits to the farm’s crop rotation, how suitable the soils are for this method and whether it is profitable to use a contractor or purchase the machinery for the farm.
Cultivate land for crop establishment in spring rather than autumn
Autumn cultivation of land stimulates the mineralisation of nitrogen from organic matter reserves at a time when there is little nitrogen uptake by the crop, which will increase the potential for over-winter leaching losses. By cultivating in spring, there will be less opportunity for mineralised nitrogen to be leached and the nitrogen will be available for uptake by the established spring crops.
Cultivation of soils results in mineralisation of organic nitrogen and increases the risk of nitrate leaching. The amount of mineralisation is strongly affected by soil temperature, moisture and nitrogen balance under the previous crop. Cultivation in spring is better, because bare soil is not exposed over the winter period and actively growing crop is established soon after cultivation to take up nitrogen and provide surface cover.
Land for spring crops, ploughed late in the autumn, has the winter for frost action and wetting and drying cycles to break down soil clods. Ploughing in the autumn also allows early establishment of the following spring crop. On medium to heavy soils if ploughing is not carried out in late autumn, the delayed cultivation may result in the spring crop being drilled into a drying seedbed. This may impact on establishment and yield.
Catch crops are fast-growing crops that are grown simultaneously with or between successive plantings of a main crop.
Catch crops protect the surface of the soil and catch the extra nutrients. The longer the soil is covered with vegetation, the smaller is the nitrate leaching. Catch crops can also improve the soil structure and increase the amount of organic matter in the soil. According to a Finnish study undersowing of ryegrass with barley reduced nitrate leaching 27-68% depending on soil.
This method is relatively easy to implement. The costs of this method consist of buying the seeds, sowing and finishing the catch crop.
Ploughing of ley on sandy soils in autumn
The time for ploughing a ley is very important to nitrogen leaching. From a leaching point of view, it should be ploughed late in autumn instead of early in autumn. Spring ploughing is also good but nitrogen release from the large amounts of organic-N is often too late for crop demand and might instead be leached in the following autumn. However, ploughing in late autumn or in spring is not possible on many clay soils so this is a method for sandy soils.
Because a lot of organic nitrogen is turned over into nitrate when ploughing a ley, leaching from ley ploughed early in autumn can be considerable, especially if the ley contains clover or if there is a lot of above-ground biomass. In such cases, an effective way to reduce leaching is to delay the ploughing of ley from early to late autumn. On clay soils effectiveness decreases as the clay content in the soil increases up to a limit where the clay content does not make it possible to employ late ploughing or ploughing in spring.
The single largest cost arises if ploughing is done so late in autumn that sowing of winterwheat is no longer possible. Ley is a good crop before winterwheat and often gives a larger yield of winterwheat compared to when cereals are cultivated before winterwheat. If this situation occurs, costs can be of importance but if the timing of ploughing of ley does not influence the choice of the next crop in the crop-rotation the cost is small.
Controlled sub-surface drainage
Controlled sub-surface drainage intensifies the drainage systems so that drainage waters from the arable areas can be efficiently utilised by the plants. The runoff of drainage waters is controlled and they are recirculated back to the arable area for irrigation.
Controlled subsurface drainage will prevent nutrient leaching with ditch waters from the arable areas into watercourses and return the nutrients dissolved in the water back to the plants’ root zone. Controlled subsurface drainage can result in 40% nitrate reduction.
The cost will be covered best in the cultivation of special plants e.g. potato.
Fertiliser and manure management
Preparing nutrient balances provides farmers with a tool for the long-term planning of fertilisation. Nutrient balances provide information on the efficiency of nutrient utilisation and help to identify the cropping phases in which nutrients are lost. The calculation of nutrient balances makes it possible to intensify the water protection measures for each farm and parcel.
Using nutrient balances for fertilisation planning helps to reduce the excess nutrients in the soil to a minimum. It also ensures that the soil is in a sufficiently fertile state to maximise the efficient use of nutrients already in the soil. Improving the accuracy of the use of fertilisers on the basis of the crop, the yield and the characteristics of the parcel to the economic optimum will ensure that the necessary quantities of the essential crop nutrients are only available when required for uptake by the crop.
This method is cost-effective. Nutrient losses are a direct measure of the principal problem, namely excessive nutrients in the environment. Farmers have the freedom to determine the most economical method of nutrient loss reduction. The use of this method will require investment in education and guidance.
Conversion from conventional to organic production
Minimum standards of organic production are regulated by Council Regulation (EEC) No. 834/2007 and starting by 1 January 2009 Council Regulation (EEC) No. 2092/91.
Nutrient input in organic production aims at promoting and maintaining soil fertility rather than crop yield. Organic production aims at closed nutrient cycles. Nutrient use efficiency is regularly higher and nutrient losses to the environment lower than in conventional production.
Organic production systems often use more labour because of new management practices, manual control of weeds, pests, and diseases and applying large volumes of organic fertilisers. They also have potentially increased harvest costs. The combined effect on production costs from increased labour requirements and lower chemical inputs will vary and must be assessed in relation to other factors, particularly yield and price changes.
Reducing the amounts of nitrogen and phosphorus fertilisers by a certain percentage below the economic optimum will reduce the residual nitrate in the soil after harvest and in the short term the amount of soluble phosphorus. In the long term reducing phosphorus fertilisers can reduce the amount lost as particulate phosphorus.
There will be a reduction of residual soil nitrate available for leaching in the autumn but there will be no effect on the nitrate mineralised from soil organic matter. In the long run, when soil phosphorus reserves will be decreased there will be a reduction in soluble phosphorus loss.
This method will have an impact on crop yields and crop quality and therefore there would be a considerable resistance to the method. Reducing phosphorus fertilisers would impact immediately crops that are particularly responsive to phosphorus e.g. potatoes and some vegetable crops. Reduction of nitrate fertilisers would have an immediate impact on all crops other than legumes.
Application techniques of manure
Decreasing of manure surface application and promoting injection techniques and mulching will decrease leaching into the watercourses immediately. These methods will help to prevent the exposure of manure to the surface runoff and drain flow losses.
By injecting the slurry it is possible to apply it directly into the active layer of soil. The slurry can be released into slots cut in the soil and then closing them after application. There are also direct ground injection systems in operation which work by the direct injection of pressurised slurry into the ground. The injection of slurry effectively increases the utilisation of manure nutrients compared with surface application.
The additional cost is the biggest in the small farms. In the big farms the fixed costs will be divided by a bigger amount of manure and additional costs per tonne are smaller.
Integration of fertiliser and manure nutrient supply
Using manure analysis to calculate the amount of nutrients supplied by manure applications will help to determine the amount and ideal timing of additional fertilisers required by the crop. Taking better account of the nutrients in manure can reduce the fertiliser inputs and nitrate and phosphorus losses.
Mineral fertiliser applications are reduced for optimum economic production level and to maintain adequate levels in the soils. The method is effective when mineral fertilisers are used to top-up the nutrients supplied in manure.
This method achieves savings rather than increasing costs. The use of this method will require investment in education and guidance.
Acid soil makes the plant nutrient uptake difficult. Especially the applicability of phosphorus is weakened in the acid soils. Phosphorus is bound tightly to the soil particles and it will easily drift from the fields with runoff waters to the watercourses. Phosphorus intake will increase considerably when the pH is over 6.0.
Liming helps to attain reasonable yields in acid soils with lower phosphorus fertiliser rates. Liming aims to ensure that phosphorus is utilised efficiently and thus to prevent nutrients from leaching into watercourses.
It may take 5 to 10 years after application to recover the cost of lime. The economics of lime use on rented land need special consideration. Profitability of liming on rented land is decreased and depends on the period of the rental agreement.
Avoiding the application of fertilisers and manure to high-risk areas
Not applying mineral fertilisers and manure at any time to high-risk areas helps to prevent the mobilisation and transfer of nitrate and phosphorus to the watercourses. Risk areas can be, for example, areas with flushes draining to a nearby watercourse, cracked soils over field drains or fields with high phosphorus values. Phosphorus risk areas can be estimated by using the phosphorus risk index or certain specified risk elements.
Losses of phosphorus on eroded soil particles and by leaching are greatest on high phosphorus index soils. Applying manure to these areas will increase the excessive phosphorus content of the soil and increase the amounts lost. This method is most effective against losses of phosphorus where the primary mechanism of transport is surface runoff.
The cost of not applying fertilisers to high-risk areas would be in terms of avoiding a drop in production proportional to the lost yield. Not applying manure to high-risk areas will have no costs if land is available elsewhere on the farm. If there is a need for increased manure storage, there would be additional costs.
Avoiding the spreading of fertilisers and manure during high-risk periods
Avoiding spreading mineral fertilisers or manure during high-risk periods reduces the availability of nitrate for loss through leaching and of phosphorus for loss in surface runoff. High-risk periods can be, for example, when there is a high risk of surface flow, rapid movement to field drains from wet soils or when there is little or no crop uptake.
Surface runoff risk is the greatest when rain falls onto sloping ground with saturated, frozen or snow-covered soils. Rapid flow of nutrients through the soil is most likely to occur from drained soils when they are wet and rainfall follows soon after applying fertilisers. Avoiding the addition of nitrogen in the autumn reduces the amount of nitrates available for leaching by over-winter rainfall.
This method will not have any costs in most cases because the fertiliser should not be required during high-risk periods since the crop will not be growing. However there may be indirect opportunity cost if the high-risk periods coincide with crop development in spring.
Increasing the capacity of manure storage
Adequate collection and storage facilities provide the possibility to choose when to apply manure to fields and there will be fewer occasions when lack of capacity forces the farmer to spread manure at unsuitable times. Manure can be spread at times when there is a low risk of runoff and when there is an actively growing crop to utilise the nutrients supplied in the manure.
If there is not enough storage capacity for manure the farmer has to spread it as it is produced. This will inevitably result in applications at times when there is a risk of nitrate leaching and phosphorus being transported to watercourses in surface runoff.
This method is most important on farms that handle their manure as slurry.
Transporting manure to neighbouring farms
Where there is a surplus of nutrients, farm manure can be exported to neighbouring farmland. This reduces the nutrient load on the farm that has an excess of manure thereby reducing the risk of diffuse pollution. It also enables the remaining manure to be managed in a more integrated way.
It is possible to balance the input of nutrients in an effective way so that there will be enough capacity of land to absorb the nutrients.
This method is most easily applied when the receiving farm holding is close e.g. within 5-20 km. The costs increase with distance. The treatment of manure (composting) helps it to be transported over larger distances relatively easily.
In slurry separation, slurry is divided into a liquid and a solid fraction. The liquid part with lower nutrient concentration can be utilised at the production site and the solid with high dry matter content and high nutrient concentration can be transported to the other farms. This can either be done slowly by a weeping-wall system, or more quickly by mechanical separation. There are a number of different types of mechanical separators including rotary screens, roller presses, screw presses, inclined screens and vibrating screens.
Slurry separation does not change the total phosphorus content of the slurry but will help to decrease the cost of transportation to other areas when there is not enough arable land to spread the slurry. Slurry separation allows greater flexibility in spreading times and application and thus can optimise the full nutrient potential of slurry.
In order to get maximum return from the investment, a separator must integrate easily into the existing farm setup with little extra expense and there must be sufficient slurry produced on the farm to justify the outlay.
Composting solid manure
Composting uses aerobic microbial metabolism to increase temperatures to inactivate pathogens and to reduce the readily available nitrate content of manures. Composting results in a more stable product which is easier to spread and more attractive to distribute to greater distances.
The readily available nitrate content of manure is typically reduced from 25% to 10% of the total nitrates, so nitrate losses in land spreading are likely to be lower.
Composting of solid manures can be carried out on individual farms using standard farm equipment.
Biogas production reduces greenhouse gas emissions, provides a source of renewable energy and generates a digest product with reduced odour emissions and pathogen content at land spreading.
Biogas production does not change the total nutrient content of the manure but will help to distribute it to greater distances through improved transport economy. The biogas digest is more easily distributed over a greater distance than the slurry.
High capital costs discourage uptake unless the process is supported by economic incentives or subsidies. The biogas production that is completely based on farm manure production can only be profitable in very large scale. Small farms can get the biogas production to be profitable by handling waste materials that come outside the farm or selling energy or biogas digest or if the energy consumption on a farm is very big.
Pelletisation is most appropriate for manures with a high dry matter content, such as poultry litter or manures that have already been treated and separated to give a high dry matter material.
Pelletisation does not change the total nutrient content of the manure but will help to distribute it to greater distances through improved transport economy.
Pelletisation is generally carried out in centralised plants. The costs are high but the end product can command a good price as a fertiliser.
The incineration process has been identified as one possible method for dealing with poultry litter. The poultry litter is used as a fuel for power plants. The resulting ash can be sold as a phosphate and potash fertiliser.
Incineration does not change the total nutrient content of the manure but will help to distribute it to greater distances through improved transport economy.
The investment costs are high. The running costs of incineration are estimated at around one Euro per tonne of dry solids contained in the waste. Although poultry manure is very dry and readily combustible, it may not be economically feasible to establish an incineration plant solely for solid farm wastes and even more so for slurries owing to the large amount of water present.
Adopting phase feeding of livestock
Livestock at different growth stages or stages of the reproductive cycle have different optimum nutritional requirements. Because of limited labour and housing facilities, livestock with different feed requirements are often grouped together and receive the same ration. As a result some stock will receive higher levels of nitrogen and phosphorus than they can utilise efficiently and will excrete the surplus.
Greater division and grouping of livestock on the basis of their feed requirements allows more precise formulation of individual rations. This will reduce the amount of nitrogen and phosphorus applied in manures and therefore decrease losses in surface runoff and by leaching.
There is a limited scope for improvements in the poultry sector where phase feeding is already widely in use. There is a great potential for phase feeding in the pig sector to reduce nitrogen and phosphorus excretion. However the costs can be considerable without necessarily improving performance.
Reducing dietary nitrogen and phosphorus intakes
Farm animals are often fed diets with higher than recommended contents of nitrogen and phosphorus as a safeguard against a loss of production arising from a deficit of these nutrients. For example, it has been shown that some cows get more protein (nitrogen) in their feed than would be necessary. In practice, however, surplus nitrogen and phosphorus is not utilised by the animal and will be excreted.
Avoiding excess nitrogen and phosphorus in the diet composition of livestock diets can reduce the amount of nitrogen and phosphorus excreted either directly to fields or via manure and thereby minimise additions to the pools of nitrogen and phosphorus that are sources of diffuse pollution.
For example, the protein content in cowfeed can be reduced by one percent unit without decreasing milk yield.
Supplementation of synthetic phytase to pig feed reduces the need for the addition of mineral phosphate. Phytase increases the availability of phosphorus in the feed and allows total phosphorus contents to be reduced without affecting productivity.
With the addition of phytase the phosphorus content of the feed can be reduced by up to 30% for pig feed.
If there is too little phosphorus in the pig feed or the ratio between different minerals is wrong, the condition of pig legs and the ability to move can weaken. This can have an effect on the economic output.
Wet feed and fermentation
Endogenous phytase in grain can be activated by wetting the pig feed some time before feeding thereby reducing or even eliminating the need for mineral phosphorus supplementation. This means that pig production with wet feed systems should be able to utilise feed with lower phosphorus content than normally recommended.
Fermentation of the feed can reduce the need for mineral phosphate supplementation. Fermentation occurs naturally in wet feed after a certain amount of time. The fermentation process is difficult to manage and the method is still to be developed.
Establishment of wetlands
Constructed or established wetlands are used to intercept runoff water from a field or group of fields. Wetlands can be natural or artificial, permanent or temporary, with water that is static or flowing, fresh or brackish. The wetland may be a wet grassland, wet woodland, reed bed, bog, sedimentation pond or lake.
Wetlands act by intercepting pollutant delivery, providing a buffer zone and can potentially clean up polluted water. Wetlands improve water quality by breaking down, removing, using or retaining nutrients, organic waste and sediment carried to the wetland with runoff from the watershed. They can trap sediment and through the retention of runoff reduce nitrates and phosphorus (soluble and particulate). Wetlands reduce the severity of floods downstream by retaining water and releasing it during drier periods and protect stream banks and shorelines from erosion. According to a Finnish study, wetlands have reduced 25-48% phosphorus and 20-90% nitrogen. Swedish studies show that wetlands can reduce phosphorus 90-100% and nitrates 76-90%. The effectiveness depends on the size of the wetland, vegetation, loading and influx.
Wetlands are quite expensive to implement and their construction will often involve the loss of some agricultural land. Constructed wetlands require maintenance due to deposition of sediment and organic matter.
Establishing vegetated and unfertilised buffer zones alongside watercourses decreases erosion and the movement of nutrients into watercourses. Buffer zones can reduce pollution in two ways. They stop agricultural activity on the area and therefore reduce direct pollution from inorganic fertilisers and organic manure additions. They also intercept overland flow from agricultural areas just before it reaches the watercourse.
Buffer zones should be free-draining and have a good surface porosity to intercept surface runoff. According to a Finnish study, buffer zones of 10 meters have proved to be efficient in reducing the leaching of suspended solids, dissolved phosphorus and total nitrogen. During the four years of research, suspended-solid loads were reduced by 50–60%, leaching of nitrogen by 50% and leaching of phosphorus by 30%. The efficiency of buffer zones in removing suspended solids and nutrients is affected by the width of the zone, gradient of the drained field, soil type and particularly by the variety and density of zone vegetation.
Buffer zones require a certain amount of investment to establishment but once established require little maintenance.
Effective purification of runoff waters
For the purification of runoff waters, soil particles in the runoff water are precipitated by Al3+ -ions or aluminium oxide polymers resulting in a low concentration of soluble phosphorus in runoff waters and negligible amounts of exchangeable phosphorus in the precipitated soil aggregates. This method needs further refinement and testing if it is to be used for quantitative determination of redox-sensitive P in runoff.
Systematic on-farm individual advice
Agrotechnical measures are implemented by close co-operation between farmers and advisors. Advisors apply limited stocking density, crop coverage over winter, intercropping, fixed value for nitrogen utilisation of farm manure, limited nutrient budget, fertiliser plans and nutrient balances.
This method can reduce nutrient input by 50% and nutrient losses by 30%.
The method is easy to implement. It requires a dense system of advisors to support farmers.
References to Examples of measures for reducing phosphorus and nitrogen losses from agriculture
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