Agrichemicals are synthetic chemical compounds (including fertilizers and pesticides) employed in agricultural practices for improving crop yields. Fertilizers affect the nutrient content, growth and development of plants. Liu et al. (2014), illustrated this with Lactuca sativa L. (lettuce) where an increase in the nitrogen (N) content of the soil and the leaves of this species was related to increased concentrations of nitrogen [N] fertilizer applied. Likewise, Blal et al. (2012), showed how N fertilizer increased the yield of Sesamum indicum L. (sesame) and parameters such as capsule weight, number of capsules produced and seeds per capsule. According to Viik et al. (2012), OptiCrop fertilizer (NPK 21:08:12, + S + Mg + B + Ca) increased the number of flowers, the number of pollen grains and the amount of nectar produced in comparison to the control (i.e no fertilizer) for Brassica napus L. (rapeseed). Since fertilizers can increase the growth of different species in different ways, the same formulations are not necessarily useful across crop types. However in promoting the growth of a particular plant species (usually the crop), changes in plant community structure can occur, which often includes a reduction in plant species richness. Hahn et al. (2015), showed how the application of fertilizer on plants can subsequently affect community structure. An increase in the number of Lepidoptera caterpillars was suggested to be the result of fertilizer inputs promoting the growth of certain plant species that are beneficial for some Lepidoptera individuals. On the other hand, changes in plant community structure can stimulate a reduction in the species richness of specific herbivores and pollinators due to a loss in plant diversity.
N fertilizers are the most extensively used fertilizers globally and often used to excess. The over-use of these agrichemicals can cause leaching of soils and surface run off and these processes result in N losses (Alva et al. 2006 and Duretz et al. 2011). Such losses reduce the efficiency of the fertilizer (Qiao et al. 2012) for crop production and become a source of pollution, for example, in soil acidification (Cai et al. 2014), greenhouse gas emissions (Xu et al. 2012 and de Morais et al. 2013) and eutrophication of nearby waters (Gaufer et al. 2003). The over use of nitrate-based fertilizers increases the osmotic concentration of the soil solution and if the external solution is more concentrated than the internal solution of the plant, this will induce wilting and slow plant growth, as water uptake against the concentration gradient will be heavily strained (Hewitson, 2016). Furthermore, excess nitrate will also induce knock on environmental issues such as soil fertility by increasing salt concentrations of soils, producing
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saline, infertile soils (Hewitson, 2016). In addition to this, the overuse of fertilizer stimulates excess nitrate in vegetable crops, such as Brassica rapa L. (Chinese cabbage) (Hewitson, 2016). High concentrations of nitrate are converted into nitrite when consumed and can accumulate into harmful amounts of nitrosamines, carcinogenic compounds associated with pancreatic, stomach and colon cancers (Dubois, 2015 and Dillan, 2015). The use of fertilizers needs to be monitored carefully not only for environmental reasons but also for human health. Nutrient experiments are conducted to determine optimal concentrations of nutrients for crop species and this helps avoid the excessive use of agrichemicals.
Agricultural policies have been established to prevent the misuse of N fertilizer and the knock-on negative environmental consequences. The Nitrates Directive (91/676/ECE) was implemented in 1991 with the aim of protecting the quality of European waters (Department of Agriculture, Food and the Marine, 2011). Under various implementations, this policy aims to prevent nitrates from agricultural sources (such as fertilizer) polluting ground water sources and through this, establishing good farming practices. By encouraging good agricultural practices, Member States can provide a standard of protection against the pollution of waters in the future (Department of Agriculture, Food and the Marine, 2011). A series of Codes of Good Agricultural Practices aim to reduce the frequency of when N fertilizers can be applied so application can be centred on periods when crops require N thus preventing nutrient run off into waters. This stimulates more efficient use of N fertilizer in agriculture (Department of Agriculture, Food and the Marine, 2011). The Nitrates Directive also identifies eutrophic waters or those at the risk of being polluted i.e those containing >50mg/l of nitrate. This policy designates ‘Nitrate Vulnerable Zones’ (NVZs) which are areas of land that drain into polluted waters or those at risk of pollution (Department of Agriculture, Food and the Marine, 2011). Within these zones, action programmes have been created to help manage them which are directed from the Codes of Good Agricultural Practices. Every four years, all Member States draw up a report outlining nitrate concentrations in ground waters, eutrophication of waters, assessment of any action programmes conducted, NVZs and any predictions regarding future water quality (Department of Agriculture, Food and the Marine, 2011). Such implementations help to control the overuse of agrichemicals and optimizes their application for crop growth