Submitted to: Acres USA
Publication Type: Popular Publication
Publication Acceptance Date: 9/20/2012
Publication Date: 10/1/2012
Citation: Kremer, R.J. 2012. Soil microbiology under drought stress. Acres USA. 42(10):18-21. Interpretive Summary:
Technical Abstract: he severity of the 2012 drought affecting much of the Midwestern U.S. is readily observed in the extremely stressed conditions of crops and natural vegetation. However, we may not realize that the extent of drought effects is just as severe on the biology below the soil surface. Detrimental effects on soil organisms due to drought conditions have consequential impacts on important biological cycles in soil. The soil microorganisms that drive decomposition to form organic matter and release essential nutrients including nitrogen, phosphorus, potassium and sulfur for plant uptake are affected mainly by the lack of adequate soil moisture under drought. Despite potential survival mechanisms available to soil microorganisms, many microbes succumb to heat stress under severe drought conditions. Some studies report decreases of more than two-thirds of the original microbial biomass following drying of soils. Heat stress damage may be caused by desiccation - the removal of water from living cells during the drying process. Different microbes are affected differently under heat stress and the overall microbial diversity in soil changes. High microbial diversity means that all biological processes in soil are functioning as a balanced system. However, if a component of the microbial community is highly sensitive to drought stresses, then the system becomes imbalanced due to loss of the function carried out by the sensitive microbes. Soil moisture deficit not only adversely affects microbial diversity, it also affects the availability of organic substances for microbial metabolism. Organic substances must be in a dissolved or in a soluble form for movement to microbial cells and then transferred across cellular membranes for ultimate breakdown into nutrients and energy. Thus we can readily see how extreme soil temperatures and moisture deficits profoundly affect soil biological processes by reducing, if not inhibiting, availability of organic food sources (substrates) to microbes. Soil microorganisms are resilient and eventually return to pre-stress population levels when conditions adequate for growth return. However, we do not know whether all processes associated with a healthy ecosystem will be immediately and fully operational. Isolated areas of green vegetation within the drought-affected landscape indicate that soil management practices were previously established that allowed the plant-soil biological system to withstand drought stress better than those sites under less intensive management. When we amend soils with organic materials and/or maintain living roots by growing continuous plant cover, soil aggregates form that not only promote soil structure but also protect microbes. Crop rotation is also important in maintaining the water status of the production or cash crop. Recent studies show that wheat growing after legume cover crops extracts 67% more soil water compared with continuously grown wheat because the root systems were more extensively developed under rotation. Such management practices as these for moderating effects of drought on soil microbial activity may well be applicable in most agroecosystems for improving crop and forage growth and development and providing preventive measures against effects of drought. Amendment with organic materials increases soil organic matter, which influences physical, chemical, and biological properties, all of which work together to maintain high soil quality. High soil quality under sustainable management may greatly aid vegetation and soil microbes in withstanding effects of drought.