6. Environmental aspects of slow- and controlled-release fertilizers and
6.2. Nitrification inhibitors
pivot irrigated potato and maize grown in a large-scale lysimeter, all on different soils.
Nitrous oxide emissions from the controlled-release fertilizer plots were almost one third of those with urea, and N recovery was almost twice that achieved with urea. The authors concluded that the contribution of controlled-release fertilizers and nitrification inhibitors to air and water quality conservation is basically due to greater NUE and reduced N fertilization rates.
Alva and Tucker (1993) and Alva et al. (1993) concluded from experiments in a citrus orchard in Florida that there is no adverse affect on the growth of young citrus trees when the frequency and rate of N application is reduced by using polyolefin-coated controlled-release fertilizers and that nitrate leaching can be minimized also.
Currently, it is estimated that cereal crops recover only about 30 to 50% of applied fertilizer N. Without major improvement in N use efficiency, increasing N inputs may lead to negative environmental impacts as the organic N moves from the soil-plant system to the air or water (Cassman et al., 2002).
Farmers adopting fertilizer best management practices (FBMPs) are already using several methods to adapt the amount of N fertilizers and other plant nutrients applied to meet the needs of plants to avoid N losses, to maintain safe and clean groundwater, and to reduce emissions of ammonia and other environmentally relevant gases to the atmosphere. Such methods begin with testing the soil to avoid any over- or undersupply of N, dividing the total N requirement into smaller applications to match the plant’s N demand pattern, timing the application according to rainfall, side-dressing the fertilizer, injecting or incorporating it into the soil, etc. It will not always be possible – mainly depending on weather and soil conditions, but also on the availability of labour – to choose the optimum amount and the best timing for N fertilizer application.
Nitrification and urease inhibitors can contribute to better NUE and to reducing N losses through denitrification, leaching and volatilization (Prasad and Power, 1995).
Nitrification and urease inhibitors should, therefore, be part of FBMPs.
Minimizing N losses implies an enhanced N-use efficiency. Even in cases where the use of nitrification inhibitors does not lead to an increase in crop yield, there are still environmental benefits (Frye, 2005). The majority of studies, tests and investigations have clearly proved the reduction of nitrate leaching, when N fertilizers have been amended with nitrification inhibitors. There are only very few cases where a reduction in nitrate leaching has not been shown. This has special importance within the EU because of the ‘Nitrate Directive’ (Jonkers and Smeulders, 2005). The latest developments and actual status are given in the following box.
The EU Nitrate Directive
The Nitrate Directive (1991) aims at reducing pollution of surface waters by nitrates from agricultural sources. It sets a maximum threshold of 50 mg NO3–/l. Another EU directive, recently revised, specifically addresses groundwater. Consistency between both directives is ensured by the Water Framework Directive (2000), which defines the principal pollu-tants, standards and monitoring obligations.
A directive is an obligation of result, i.e. the Nitrate Directive mandates achieving maxi-mum 50 mg NO3–/l in surface waters. The directive has to be enforced at the national level, where actions and measures are defined. For the Nitrate Directive, national action plans have to be revised every four years.
Enforcement of a directive may be different from one country to another, and derogations are possible. For instance, The Netherlands, with reference to their grassland farms with N surpluses in the form of animal manure, reached an agreement with the European Commission, laid down in the Third Action Programme (2004). This programme introduces a system of application standards, both for animal manure, total manure and phosphate from 2006 onwards. For animal manure, the maximum application level is of 170 kg N/ha.
The application standards for total N is aimed at reaching 50 mg NO3–/l or less in surface water in 2009. Farms with at least 70% of grassland may request a specific derogation for animal manure application: 250 kg N/ha instead of the 170 kg N/ha.
In the United States, a seven year study at the University of Minnesota showed that the use of nitrapyrin reduced leaching of nitrate-N by about 15% annually (when averaged across seven years), with fall applied anhydrous ammonia compared to fall application of ammonia without nitrapyrin (Huffman, 1997). Yields were increased by 6% for the fall comparison. Fall applied ammonia plus nitrapyrin produced similar yields and similar levels of nitrate-N leaching as the same rate of N applied in the spring. In 2003, Arise Research conducted a trial at Martinsville (Illinois, USA), in which nitrapyrin with spring-applied urea reduced nitrate leaching by 30% (Dow AgroSciences, 2003).
In Germany, nitrate leaching was decreased by 27% with DCD-stabilized fertilizers compared to calcium ammonium nitrate on a podsol-gley soil and by 40% on a loamy soil over a seven-year period (Scheffer, 1991). Furthermore, in various studies it was also emphasized that nitrate leaching can be decreased significantly during humid springs and under crops like maize or sugar beet (Amberger 1991a, 1993a; Amberger and Germann-Bauer, 1990; Gutser, 1991, 1999a).
The use of DMPP with urea reduced nitrate leaching when maize was irrigated with an overhead mobile-line sprinkler system (Diez-López et al., 2008).
Serna et al. (2000), applied ammonium sulphate nitrate with and without DMPP to six-year-old citrus plants grown individually outdoors in containers in Spain with consecutive flood irrigation. The experiment ended after 120 days and the results indicated that DMPP improved N-use efficiency and reduced nitrateleaching by retaining N in the ammoniacal form. An additional effect was the increased ammonium nutrition of the plants, which are able to absorb larger amounts of ammonium compared to nitrate(Serna et al., 1992). Similarly, in 1993 and 1994, Serna et al. studied ammonium sulphate nitrate on citrus with and without DCD. DCD reduced nitrate losses, improved N-use efficiency and minimized environmental risks associated with the irrigated production of citrus.
Figure 22. Nitrate leaching in spring-applied urea (Adapted from Dow AgroSciences, 2003).
0 5 10 15 25
ppm NO3_
20 30 35
Sept 24 Sept 4
Aug 3 July 28
July 22 July 11
June 14
Without N-Serve With N-Serve
Carrasco and Villar (2001) studied the use of manufactured and organic fertilizers with and without DMPP on irrigated maize and wheat in the Northeast of Spain. They concluded that using DMPP could reduce nitrate losses in ‘Nitrate Vulnerable Zones’.
Reduced nitrate leaching was found also by Bañuls et al. (2001). Chaves et al. (2006) incubated cauliflower leaves with DCD and DMPP. Under favourable conditions, DCD inhibited nitrification of the crop residues for 50 days and DMPP for at least 95 days indicating their potential, especially of DMPP, to reduce nitrate leaching after incorporation of crop residues.
The requirement to reduce leaching of nitrate in water catchment areas with restrictions on N fertilizer use may provide an opportunity to increase the use of nitrification inhibitors. In recent years, there has been increasing attention to nitrate leaching from intensively managed grassland grazed by cattle, mainly due to the application of organic manures with the largest proportion being produced from animal urine (McKervey et al., 2005). The urine patch is the primary source of N losses, not fertilizer N (Edmeades, 2004). New Zealand has a lead position in investigating nitrate leaching from grazed grassland (Suter et al., 2006) and there it has been shown that the application of DCD reduces nitrate leaching from grassland by 61% on average (Clough et al., 2007). McKervey et al. (2005) concluded however, that nitrification inhibitors alone will probably not provide a complete solution to the problem of nitrate leaching from pastures, but they could be a useful tool in conjunction with the adoption of good management practices.
According to Gutser (2006) and Ebertseder and Gutser (2006), N losses from slurry, when applied to arable crops, could be largely avoided by choosing the right time of application. Nevertheless, the incorporation of a nitrification inhibitor into the slurry would stabilize the ammonium and reduce the risk of nitrate losses through leaching.