Amitava Rakshit, N C Sarkar, D.Sen and R.K.Maity
Without the soil food web, plants would not obtain the nutrients necessary for growth, and the above ground food web would not continue long .Extensive research has demonstrated the great importance of soil organisms for the fertility of the soil. In one gram of productive soil there is a complex web that can exceed over 100 million microorganisms that may represent over 1000 species. The main components are bacteria, fungi, algae, protozoa, nematodes, earthworms, and insects. Out of these, bacteria and fungi constitute about 80%, the proportions of these two depending on soil type. There is a complex ecological interdependence between all soil organisms. Together they are responsible for the cycle of decomposing and restructuring organic material so that it will be accessible to growing plants. It is also responsible for the nitrogen and water-retaining properties as well as for other factors of great importance for soil fertility. Because of the importance of soil biota in mineralization and immobilization of nutrients, physical and biochemical degradation of organic matter, biological control of plant pests, and as food sources for other organisms, it is crucial to evaluate the potential impacts of transgenic plants on soil ecosystems.
In genetic engineering, a package of novel genes are inserted into the recipient organism. In addition to the desired property gene, a number of other genes have to be added to ensure successful insertion. Among the potentially problematic genes inserted into plants, those that help overcome the barriers against the introduction of foreign genes are of particular interest in the context of soil ecology. They function as vectors for successful insertion and prevent rejection of inserted foreign genes. Transgenic or genetically modified plants possess novel genesthat impart beneficial characteristics such as herbicide resistance.
GE crops and impact on soil microbes
One of the least understood areas in the environmental risk assessment of genetically modified crops is their impact on soil and plant associated microbial communities. The potential for interaction between transgenic plants and plant residues and the soil microbial community is not well understood. The recognition that these interactions could change microbial biodiversity and affect ecosystem functioning has initiated a limited number of studies around the world. The great problem in predicting potential outcomes here is that the suggested horizontal transfer mechanism would have very diverse results, impossible to predict. Only by multiple experiments will it be possible to get some idea whether this can affect soil fertility. We can only speculate on some potential scenarios that might be the result if our hypothesis is correct:
- The scenario that would seem most likely is that there occurs an accumulation of vector DNA in the soil microbiota with repeated GMO cultivation. This would enable horizontal transfer between unrelated species, leading to a cumulative loss of soil biodiversity over repeated harvests. Diversity has been found to be important for soil fertility.
- A second possible complication might come from horizontal transfer of the toxin gene from Bacillus thuringiensis (BT) to soil bacteria from GM crops with this gene.
- A third scenario might be that a new variety or species arises that is able to overgrow or damage some essential species of soil microorganism so that the ecological balance would be disrupted. If it has a good survival and multiplication capacity, including sporulation ability, it might spread widely through wind erosion and through the ground water. New virulent and harmful species might cause reduction of fertility in infected soils.
At this time, studies have shown the possibility that transgenes can be transferred to native soil microorganisms through horizontal gene transfer, although there is no evidence of this occurring in the soil. Furthermore, novel proteins have been shown to be released from transgenic plants into the soil ecosystem, and their presence can influence the biodiversity of the microbial community by selectively stimulating the growth of organisms that can use them. Microbial diversity can be altered when associated with transgenic plants; however, these effects are both variable and transient. Soil and plant-associated microbial communities are influenced not only by plant species and transgene insertion but also by environmental factors such as field site and sampling date. Minor alterations in the diversity of the microbial community could affect soil health and ecosystem functioning, and therefore, the impact that plant variety may have on the dynamics of the rhizosphere microbial populations and in turn plant growth and health and ecosystem sustainability, requires further study
However, it has also been shown that these effects are dependent on field site, seasonal variation, and method of analysis used for assessment. The changes in microbial communities associated with growing transgenic crops are relatively variable and transient in comparison with some other well-accepted agricultural practices such as crop rotation, tillage, herbicide usage, and irrigation. Since minor alterations in the diversity of the microbial community, such as the removal or appearance of specific functional groups of bacteria such as plant-growth-promoting rhizobacteria, phytopathogenic organisms, or key organisms responsible for nutrient cycling processes, could affect soil health and ecosystem functioning, the impact that plant variety may have on the dynamics of rhizosphere microbial populations and in turn plant growth and health, and ecosystem sustainability, requires further study.
It is impossible to judge, without direct experimental investigation, how probable it is for the proposed mechanism to work in practice to a significant extent and to result in altered soil microorganisms and in disturbed soil ecology to such a degree that it will result in a decrease of soil fertility.
The potential problems
‘Problems’ will not be noticed immediately, or even in a year or two. Indeed, it may be many years, or tens of years, before there are noticeable changes in the soil microflora. But changes may well occur, and these will be both genetic in origin and evolutionary in nature. In short, the genetic makeup of the soil bacteria and fungi could well change in the longer term as a result of the cultivation of GM-crops. The nature of these changes, and their consequences, cannot be predicted,and it is this ‘unpredictability’ which should cause concern. There may be many potential ‘problems’ which have been described below:
- The accelerated spread of antibiotic resistance genes in bacterial populations, with the inevitable consequences for human (and animal) medical treatments.
- Changes may occur in the relationships between root nodule bacteria and their host plants. Any genetic variants of these bacteria (caused by horizontal gene transfer from GM-crops) which result in a less symbiotic, and more parasitic, association with the host plant could lead to reduced levels of nitrogen-fixation. For crop plants this could be a real problem, with lower yields and possibly lower soil fertility. For wild species, it could cause changes in local vegetation as the competitive balance between plant species is altered.
- Changes may occur in the relationships between mycorrhizal fungi and their host plants. In recent years the importance of mycorrhizae in the growth of plants has only just been realised. It now seems that the majority of higher plants, including some crop species, form associations with these soil fungi. Any horizontal gene transfer to mycorrhizal fungi from GM-crops which changed the characteristics of their interactions with higher plants could potentially have far-reaching effects on ecosystem structure and dynamics.
Conclusion
Future work needs to address long-term effects of transgenic crops in rotation, while keeping in mind that these effects should not only be compared with a non-transgenic counterpart, but also to other acceptable changes in the agroecosystem, such as growing a novel non-transgenic plant or utilizing a new agronomic practice.
To conclude the key factor here is ‘unpredictability’. It is not possible to predict that horizontal gene transfer will never occur in the soil. If it does occur, no prediction could be made about its possible consequences. The only thing we can predict is that we are unsure of the long-term effects on the biosphere of the cultivation of GM-crops.
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Amitava Rakshit, Department of Soil Science and Agricultural Chemistry, Institute of Agricultural Science, BHU, Varanasi, 221005,India, Email: amitavar@bhu.ac.in
N C Sarkar, Department of Agronomy, Nagaland University, Nagaland, India, Email: ncsiari@rediffmail.com
D.Sen, College of Horticulture and Forestry, CAU, Pashighat, AP, Email: dr.d.sen@gmail.com
R.K.Maity, Professor ,Departamento de Quimica Biologia Universidad de las Americas-Puebla ,
Santa Cacarina Martir, Cholula C. P. 72 820, Puebla, Mexico Email: rkmaiti@yahoo.com
