Mycorrhizal symbiosis and nutrient transfer into plants
2.7.2 The consequences of the use of each agricultural system alone
While conventional agriculture relies on synthetic inputs which may not be sustainable, there is much evidence that a shift from conventional to biological agricultural systems can result in a significant reduction in crop yields (Oberson et al., 1993, Reganold et al., 2001). Several studies pointed out that after a long period of using conventional systems there is a risk to the sustainability of the productivity of agricultural soils and environmental consequences, including: an imbalance in the microbial biomass of the soil, increased soil erosion, declining soil fertility, reduced biodiversity, groundwater contamination and the impact on the constituents of the atmosphere such as nitrogen oxides and carbon dioxide (Matson et al., 1997, Barrow, 2012).
Welbaum et al. (2004) note that following the development of chemical fertilisers they proceeded to replace organic farming systems; however, the downside of this was that these fertilisers affected micro-organisms whose importance was still unrecognised at that time. Following the initial introduction of chemical fertilisers to agriculture after World War 2 crops flourished, with increased yields and improved quality. However, this success was short-lived as continuous and intensive use of agricultural fields eventually depleted the soil’s nutrients which were either absorbed by the crops or washed away by rain or irrigation. The result was a decline in soil quality. This prompted even heavier applications of synthetic fertilisers and so on, in a vicious cycle that, while producing the desired results of increased crop quality and quantity, fixed large amounts of mineral nutrients to the soil, which causes damage to the soil, environment and human health in the long term.
Conventional agricultural practices often cause an imbalance in the population of organisms in the soil (Franke-Snyder et al., 2001, Araújo et al., 2009, Bainard et al., 2011). The effect may be competition among different species for available resources; conventional practices also involve high usage rates of elements such as N or P which may impede the growth of
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some species, for example, fungal hyphae of mycorrhizal fungi (Egerton-Warburton and Allen, 2000, Egerton-Warburton et al., 2001, Adesemoye et al., 2008). In addition, pesticides may also interfere with the growth of fungal hyphae on the roots of host plants or impact on the establishment of bacterial colonies by inhibiting the growth of the roots of the host plant (Bethlenfalvay et al., 1996, Rejon et al., 1997). The use of fungicides in conventional practices can also kill beneficial fungi (Beyer‐Ericson et al., 1991).
Biological systems also have some disadvantages. For example, mechanical weeding, or tillage, that is carried out when herbicides are not used can wipe out mycorrhizal colonies (Douds et al., 1995, Gosling et al., 2006, Rasmann et al., 2009). However, in waterlogged soil weed control through tillage enhances soil quality by improving aeration, thus increasing microbial activity. However, ploughing during the growing season and/or during the period of mycorrhizal fungi activity may produce undesirable results, such as severing of the fungi’s hyphae. It can be concluded from this that colonisation of beneficial fungi is best maintained by careful management of the timing of agricultural processes, such as tillage. Fontaine et al.
(2003) reported that the nutrient content of organic matter used as fertiliser is dependent on the materials used. For instance, the result of the decomposition of straw residues often produces limited N (Fontaine et al., 2003).
A considerable body of research has indicated that non-traditional systems, such as organic regimes, cannot promote plant growth by providing the required nutrients with the same speed as conventional systems. This is because organic fertilisers may provide sufficient N for the soil, and increase soil pH as a result of C-mineralisation, the production of OH- ions through ligand exchange and the introduction of base cations such as K+, Ca2+ and Mg2+ (Hargreaves et al., 2008). An increase in soil pH may enable some of the nutrients to be absorbed or exchanged by the plants. However, organic fertilisers will not be able to provide enough of all nutrients, or facilitate increased availability of nutrients that are not present in sufficient quantities in the soil by improving soil pH.Therefore, dispensing with conventional systems is difficult when intensive production with high yields is needed in order to provide food in large quantities. On the other hand, when there is limited agriculture that does not strain the resources of agricultural soils, and when appropriate agricultural cycles include land fallow, it may be possible to dispense with such systems. The search is continuing to confirm the feasibility of replacing conventional practices, or merging them with organic and bio-systems, in order to minimise the environmental risks posed by the use of artificial or manufactured fertilisers, and to protect the soil from erosion and degradation. However, the
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shift from traditional agriculture to biological agriculture leads to a significant reduction yield in agricultural crops (Oberson et al., 1993), because plant nutrition management differs between the two types of agricultural systems. In traditional agriculture, the goal of fertilisation is the direct feeding of the plant through the supply of artificial sources of nutrients, often applied in large quantities. On farms managed using biological farming principles, the emphasis is on addition of organic matter and bio-inoculants to aid in mineralisation and feed the soil. Finding alternative ways of meeting the nutrient needs of plants is likely to improve both crop quality and soil fertility. Despite the fact that alternative methods may mean an increase in production costs in the short term (e.g. labourers are required to remove weeds instead of using herbicides), the long-term benefits are likely to be reduced labour costs and improved sustainability (Reganold et al., 1990).