Soil represents a favorable habitat for microfauna and is inhabited by a wide range of microorganisms, including bacteria, fungi, algae, viruses, and protozoa. According to one study, there is about one and ten million microorganisms present per gram of soil with bacteria and fungi being the most dominant. A renowned scientist once remarked, “Soil deprived of microorganisms is a dead soil”. However, these microbial communities may not be physiologically active all the time in soil due to limited nutrients availability. The importance of these microorganisms is depicted by their active involvement in almost every chemical transformation taking place within the soil. They also play a vital role in recycling of nutrients by decomposing organic matter entering in soil e.g. animal and plant litter hence restores depleting soil fertility. Certain soil microorganisms such as mycorrhizal fungi enhance the nutrients available to plants, while others uplift the nutritional status of the soil. Those microorganisms, which improve the fertility status of the soil have been termed ‘biofertilizers’ and are receiving increased attention for their utilization in agriculture as microbial inoculants. Some microbes also trigger the production of various plant hormones and improve plant growth, hence termed as ‘phytostimulation’. Some pathogenic microorganisms are also present in soil which damages crop plants through their roots. However, these harmful microbes are effectively controlled by some other native microorganisms which hinder their growth by producing certain inhibitory compounds and also stimulate the natural defensive mechanism of plants. This specific class of microbes are collectively called ‘biopesticides’ which serves as an evolving alternative to synthetic pesticides for the protection of crops against certain pathogens and pests.

The widespread use of transgenic plants within modern agriculture has initiated major debates on the possible negative effects of these plants on the ecosystem, particularly soil microbial fauna. Most of the modern era transgenic plants produce toxins or chemical compounds that have insect and herbicide resistant properties. These compounds enter the soil either by the plant remains ploughed after crop harvest or by direct exudation of plant roots. However, most of the studies indicated that there is no significant variation in the diversity and population size of microbial species in soils harvesting transgenic and non-transgenic Bt cotton plants. Similarly, another study also showed that there was no difference in the microbial fauna of soil bearing herbicide resistant rice and non-transgenic rice. Furthermore, another study confirmed that richness of the microbial communities in rhizosphere soil did not differ between Bt and the non-Bt cotton.

In contrast to the above findings, a scientist concluded that an insecticidal compound produced by a transgenic cotton plant can remain in the soil for as long as 140 days after its harvest. Once in the soil, the toxin could be adsorbed or adhere to clay particles, humic components or organic-mineral complexes and then be prevented against degradation by soil microorganisms. Gradually, it could accumulate to a certain concentration that might affect the activity of soil microbial communities. Moreover, another study revealed that fewer microbes were isolated from the roots of glyphosate-resistant oilseed crop as compared to non-transgenic line. Moreover, low soil pH and higher clay contents increased the persistence of Bt toxin in soil. In another study examining this question, no degradation of the toxic protein in transgenic Bt corn leaves was observed during the first month, after which the toxin concentrations decreased to 20% of their initial values during the second month. There was no further degradation during winter and the toxin continued to degrade slowly when temperatures again increased in spring. These results suggested that the transgene protein can remain in soil for a long time. Soil is a complex system where trophic and non- trophic interactions among organisms are difficult to monitor by short-term researches. At this time, it is still not known whether continuous planting of transgenic plants over many years will lead to increased accumulation of transgenic proteins, or what impact this might have on soil microorganisms. Keeping in view the increasing deployment of different pyramided transgenes in various crops, a considerable amount of research is needed to investigate the long lasting effect of transgenic plants on beneficial soil microbes.

This article collectively authored by  Asim Abbasi, Dr. Muhammad Sufyan, Iqra, Muhammad Ibrahim Shahid, Muhammad Ashfaq.