THE ROLE OF ABIOTIC FACTORS, CULTIVATION PRACTICES AND SOIL FAUNA IN THE DISPERSAL OF GENETICALLY-MODIFIED MICROORGANISMS IN SOILS

Any study of the fate of genetically modified microorganisms (GMMs), r eleased into the environment, must examine how far they can disperse f rom 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 v iable state. This paper reviews factors influencing the speed and dist ance of dispersal of soil microbes, These will depend on soil manageme nt 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 practi ces cause physical disturbance of the soil by ploughing, tyning and ho eing. The movement of soil during these activities can disperse microo rganisms within 20-30 cm of the plough depth and several metres horizo ntally. Ploughing and tyning increase porosity, but heavy traffic comp acts the soil and destroys macropores. The former thus aid dispersal a nd the latter retards it. Plant roots can act as food for microorganis ms, and their growth will disperse the microorganisms deeper into the soil. Dispersal may be further increased as root death creates new mac ropores, As dispersal models have been mainly developed from studies o f homogenised soils, many of these factors have not been adequately in corporated in predictions of microbial dispersal. Their are few studie s directly pertaining to faunal movement of GMMs. This review highligh ts our current knowledge in identifying potential key faunal groups wh ich may be influential in enhancing GMM transport in soil, The soil fa unal community present a range of species diversity and function. The potential importance of the major soil faunal groups in dispersal of m icroorganisms is discussed. Larger, mobile animals, such as lumbricids are the main agents of vertical and horizontal dispersal of GMMs. Lar ge epigeic fauna such as diplcpods and isopods may be important in hor izontal dispersal at the soil surface but information on their abundan ce in different ecosystems is limited. Geographic distribution of faun al groups influences their importance at any one location. Generally, the more numerically abundant, but smaller-sized protozoa and nematode s play an insignificant part. The dispersal of microorganisms by soil fauna may be of greater importance than abiotic factors in soils of hi gh clay content or impeded drainage. Models based on artificial microc osms have shown the importance of soil particle size in downward movem ent of the drainage water which will carry the microbial propagules. T his review highlights two major areas where our understanding of inter actions of soil structure, biotic components and microbial dispersal i s lacking. Firstly, soil heterogeneity is not included as a factor in model systems of abiotic dispersal of microbes. This is a major drawba ck for their use as predictive toots in natural field soils. Secondly, our lack of knowledge of the interactions between many soil faunal ta xa and the microbial community requires further attention. We recommen d that further studies should be conducted in intact systems (natural, undisturbed sites or intact soil cores) to improve the predictive pow er of models of movement of microorganisms in soil. (C) 1997 Elsevier Science B.V.