Any study of the fate of genetically modified microorganisms (GMMs), released into the environment, must examine how far they can disperse from their release site. The likelihood of dispersal will depend on two key factors: 1. . Speed and distance of dispersal. 2. . Survival and/or reproduction, enabling them to reach new environments or hosts in a viable state. This paper reviews factors influencing the speed and distance of dispersal of soil microbes. These will depend on soil management practices, soil physico-chemical properties, and on the activities and movement of members of the soil biota acting as vectors. Models of viral and bacterial dispersal are evaluated. Their limited relevance to natural soils, with their inherent heterogeneity, and applicability to actinomycete and fungal dispersal is discussed. Cultivation practices cause physical disturbance of the soil by ploughing, tyning and hoeing. The movement of soil during these activities can disperse microorganisms within 20-30 cm of the plough depth and several metres horizontally. Ploughing and tyning increase porosity, but heavy traffic compacts the soil and destroys macropores. The former thus aid dispersal and the latter retards it. Plant roots can act as food for microorganisms, and their growth will disperse the microorganisms deeper into the soil. Dispersal may be further increased as root death creates new macropores. As dispersal models have been mainly developed from studies of homogenised soils, many of these factors have not been adequately incorporated in predictions of microbial dispersal. Their are few studies directly pertaining to faunal movement of GMMs. This review highlights our current knowledge in identifying potential key faunal groups which may be influential in enhancing GMM transport in soil. The soil faunal community present a range of species diversity and function. The potential importance of the major soil faunal groups in dispersal of microorganisms is discussed. Larger, mobile animals, such as lumbricids are the main agents of vertical and horizontal dispersal of GMMs. Large epigeic fauna such as diplopods and isopods may be important in horizontal dispersal at the soil surface but information on their abundance in different ecosystems is limited. Geographic distribution of faunal groups inflences their importance at any one location. Generally, the more numerically abundant, but smaller-sized protozoa and nematodes play an insignificant part. The dispersal of microorganisms by soil fauna may be of greater importance than abiotic factors in soils of high clay content or impeded drainage. Models based on artificial microcosms have shown the importance of soil particle size in downward movement of the drainage water which will carry the microbial propagules. This review highlights two major areas where our understanding of interactions of soil structure, biotic components and microbial dispersal is lacking. Firstly, soil heterogeneity is not included as a factor in model systems of abiotic dispersal of microbes. This is a major drawback for their use as predictive tools in natural field soils. Secondly, our lack of knowledge of the interactions between many soil faunal taxa and the microbial community requires further attention. We recommend that further studies should be conducted in intact systems (natural, undisturbed sites or intact soil cores) to improve the predictive power of models of movement of microorganisms in soil.
All Science Journal Classification (ASJC) codes
- Agricultural and Biological Sciences (miscellaneous)
- Soil Science
- Cultivation practices
- Genetically modified microorganisms
- Soil fauna
- Soil structure