by Bob Kremer, Ph.D.

The spirit and philosophy of the famous Missouri soil scientist, Dr. William A. Al-brecht, is alive and well in the soils depart-ment he made famous at the University of Missouri in Columbia.

The university, realizing the impact of Albrecht’s work with soil, started selecting professors who used the principles devel-oped by Albrecht and applied them to management practices that maintain and improve soil quality.

In 1957 when Albrecht stepped down after 43 years of service, Dr. George Wag-ner was chosen to carry on Albrecht’s work. Wagner served in that capacity until 1992, when Dr. Diann Jordan, who was hired in 1990, assumed the role for 10 years. In 2001 Jordan passed the baton to Dr. Bob Kremer, who became the third individual whose job it was to espouse the Albrecht principles at the university.

Kremer, like Albrecht, Wagner and Jor-dan, is a soil microbiologist. He received his bachelor’s and master’s degrees under the guidance of Dr. Wagner at the University of Missouri and his doctorate at Missis-sippi State. Unlike the others, Kremer is not an employee of the university, but rather an employee of the USDA Agricultural Research Service, and is a faculty member under a “courtesy appointment” by the university as an adjunct professor.

— Ralph Voss

The severity of the 2012 drought af-fecting much of the Midwestern United States is readily observed in the ex-tremely stressed conditions of crops and natural vegetation. However, many of us do not realize the drastic effects of drought on the biology below the soil surface. Detrimental effects on soil or-ganisms due to drought conditions have consequential impacts on important

biological cycles in soil. As moisture becomes deficient, organisms involved in the first stages of decomposition of plant and animal remains on or just be-neath the soil surface shift their activities from cycling nutrients to surviving the harsh and stressful environment. For ex-ample, earthworms that normally break up crude organic materials for further decomposition by soil microbes tend to abandon the organic-rich surface layer and move deeper within the soil profile toward moister habitats when faced with drought conditions. Protozoa, nema-todes and miniscule insects (micro- and mesofauna) that take advantage of water films or relatively high moisture content for attacking organic residues cease ac-tivity and produce dormant, inactive but viable structures such as cysts and egg masses from which new generations of organisms arise when moisture returns to optimum levels.

Soil Microbiology Under Drought Stress

Reprinted from October 2012 • Vol. 42, No. 10

The soil microorganisms that drive decomposition to form organic matter and release essential nutrients includ-ing nitrogen, phosphorus, potassium and sulfur for plant uptake are affected mainly by the lack of adequate soil moisture under drought. The vast majority of soil microorganisms (bacteria and fungi) that function in the temperate climate re-gion (as in the Midwestern United States) are “mesophiles” — they grow within a temperature range of 15 to 30°C (58 to 88°F). Other microorganisms that are very similar to bacteria, known as “ar-chaebacteria,” thrive at temperatures as high as 95°C (200°F), but these are lim-ited to extreme environments such as hot springs and deep sea thermal vents. Surprisingly, activity of mesophilic mi-croorganisms can increase in soils as the temperature increases to a maximum of about 40°C (103°F). For example, activity of enzymes (the biological catalysts that drive decomposition and nutrient release from organic materials) of soil microbes doubled as soil temperature increased from 22 to 34°C (72 to 90°F) in the up-per two inches of a turfgrass soil. This approximates the classic “Q10 rule” that states for every 10°C (about 18°F) in-crease in temperature, activity is doubled. However, and this is a key principle, we must remember that increases in micro-bial activity at higher temperatures occur under optimum soil moisture.

UNDERSTANDING DROUGHT EFFECTS ON MICROBES

The extreme moisture deficiency that we observe with the current prolonged drought can have dire consequences on soil microorganisms and their activities. Before discussing some of these poten-tially deadly effects, we should realize that, through evolution, microorganisms have developed many indirect and direct mechanisms that protect them against environmental stresses. In the soil envi-ronment, microbial cells may be coated

with clay minerals or organic substances that provide indirect protection of cellular structures and contents. Some bacteria (Bacillus spp. and actinobacteria) form heat-resistant, dormant spores that with-stand dry conditions and high tempera-tures of 60°C (140°F) or more allowing them to subsequently germinate and pro-duce actively growing bacterial cells upon exposure to future optimum conditions. Many fungi also produce various types of spores and other “resting structures” that survive high temperatures. Some mi-croorganisms form gummy substances or slime layers that encase individual cells or clusters of cells that congregate to form massive slime layers or biofilms that adhere to inorganic (rocks, soil pore walls, water conduits) or organic (roots) surfaces to insulate that entire microbial community against effects of high tem-perature. As an illustration, we are prob-ably more familiar with a biofilm known as dental plaque on the surfaces of teeth, which shelter numerous types of oral bac-teria. Regardless, our perception of how microbes live in soil and other environ-ments has changed as new technologies for their study were developed from as-suming random and individually distrib-uted cells to what we now view as groups or communities of cells in defined niches. Viewing microbes from this perspective helps us understand how they withstand and how they perish under stress condi-tions including drought.

Despite the potential survival mecha-nisms 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 dam-age may be caused by desiccation, which basically is the removal of water from liv-ing cells during the drying process. When a microbe is desiccated rapidly, cellular contents including proteins and nucleic acids (DNA and RNA) are irreversibly denatured or damaged. Damages to these materials will inhibit microbial growth. In worst cases, membranes and cell walls are ruptured and the cellular contents spill out, effectively killing the microbe. Dur-ing the period June 28 through July 27, 2012, bare soil temperatures at Sanborn Field on the University of Missouri cam-pus measured at 2- to 4-inch depths aver-aged between 28 to 34°C (80 and 94°F); under current dry conditions, desiccation of soil microorganisms is very likely hap-pening. Different microbes are affected differently under heat stress and the over-all microbial diversity in soil changes. High microbial diversity means that all biological processes in soil are function-ing as a balanced system. However, if a component of the microbial commu-nity is highly sensitive to drought stresses, then the system becomes imbalanced due to loss of the function carried out by the sensitive microbes. For example, we know that about 180 days of drought suppresses carbon (C) and nitrogen (N) cycling (performed by bacteria and fungi during decomposition). In this case C and N contents of soil may actually increase due to this decreased activity. Changes in microbial diversity caused by drought also strongly affect the physiology or the functions of the microbes, leading to slower nutrient turnover. For example, activities of enzymes that cycle nitro-gen from proteins and urea; phosphorus

Despite the 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.

Reprinted from October 2012 • Vol. 42, No. 10

from phospholipids and nucleic acids; and carbon from cellulose are reduced by 80 percent or more. These processes are important for plant growth but we do not know how quickly these can re-cover to benefit plants when soil moisture is replenished. Soil moisture deficit not only adversely affects microbial diversity, it also affects the availability of organic substances for microbial metabolism. Or-ganic substances must be in a dissolved or 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. The degradation of organic compounds (both natural sources and synthetic compounds such as pesticides) by microorganisms is the most important process for complete transformation into mineral components. The detrimental effects of drought can effectively slow or even stop the biodeg-radation process by eliminating available soil water.

Soil microorganisms are resilient and eventually return to pre-stress popula-tion levels when optimum conditions re-turn. However, we do not know whether all processes associated with a healthy ecosystem will be immediately and fully operational. Some processes are associ-ated with specialist microbial groups that do not make up a large segment of the microbial community, and may be highly susceptible to stress relative to other mi-crobe groups. For example, nitrogen-fix-ing bacteria (those retrieving atmospheric N and converting to ammonia for use by plants) are sensitive to high temperatures. This suggests that nitrogen transforma-tion in soils could be disrupted because fixed nitrogen that feeds into the nitrogen cycle is reduced. Losses of many microbes due to heat stress could also lead to losses in genetic traits involved in alternate (and perhaps unknown) biological processes that might promote efficiency of nutrient cycling, biological control of pathogens or plant growth enhancement. Because we can only culture 0.1 to 1.0 percent of the microorganisms in soil to identify their roles in biological and ecological soil processes, the unknown losses to drought stress is magnified. Further, the genetic potential lost among sensitive microbes destroyed by drought is immeasurable.

Farmers observing apparent dead or ‘burned up’ vegetation across the land-scape have led to concerns about the relative amounts of soil organic matter remaining after drought. As plants shut down metabolism, including photosyn-thesis, C that naturally leaks from plant roots into the soil is reduced or eliminat-ed. Plants may release 20 to 50 percent of the C fixed via photosynthesis, so suspen-sion of this process, known as rhizode-position, reduces a significant amount of C normally funneled to the soil or-ganic matter pool. Conversely, because microbial activity drastically decreases due to cellular desiccation and forma-tion of insoluble carbon, decomposition becomes stagnant and causes little change in amounts of soil organic matter already in place. Thus changes in organic matter levels may not be as drastic as might be expected. The dead microbial biomass accumulating from drought stress may contribute to a minor extent to both soil organic matter and available N. If we as-sume that about 600 pounds of microbial biomass is present per acre and 90 percent of the biomass is killed during drought, then 540 pounds of biomass may be decomposed by living microorganisms when optimum conditions return by a process known as microbial turnover. Microbial biomass has a C:N ratio of 10:1, implying that over 50 pounds of N per acre could be available due to microbial turnover. This N, after providing a por-tion for microbial growth, could be used by plants as they recover from drought. Many factors will determine the extent of vegetative re-establishment in the spring following drought; these include the type of forage species and their relative sensi-tivity to drought stress; amount of rainfall received needed to revitalize microbial activities and for solubilizing C needed for metabolism and impacts on “micro-bial specialists” such as nitrogen-fixing bacteria and the mycorrhizal fungi.

IMPLICATIONS FOR ‘DROUGHT MANAGEMENT’

Amidst the devastation of the drought some farmers have noted areas within the landscapes on their farms where vegeta-tion is seemingly surviving. The apparent vigorous growth of vegetation in these sites often results from previous land management that promoted establish-ment and development of healthy and extensive root systems. In one case the application of high rates of composted

manure and hay prior to seeding the area with a mixture of fescue, grazing alfalfa and clovers was linked to these green areas within the drought-affected land-scape. The likely scenario, based on ob-servations of similar systems, would be as follows: added organic matter improved soil structure, increased water-holding content and provided dissolved or soluble C necessary for microbial activity; as grass grew, abundant forage protected the soil surface to attenuate detrimental effects of high temperature; trees in the area also contributed to shading the soil surface; deep-rooted alfalfa and clovers added N through fixation and available C via root exudation; these roots also ex-tract moisture from deep within the soil profile and translocate water to shallow rooted grasses and maintain soluble C necessary for microbial activity concen-trated in the plant rhizospheres. When we amend soils with organic materials and/or maintain living roots by growing continuous plant cover, soil aggregates are formed that not only aid soil structure but also protect microbes. The center of soil aggregates harbor microbes and resist drying since this part of the aggregate typically remains moist. Crop rotation is also important in maintaining the water status of the production or cash crop. Recent studies showed that wheat grow-ing after legume cover crops extracted 67 percent more soil water compared with continuously grown wheat because the root systems were more extensively devel-oped under rotation. Such management practices as these for moderating effects of drought on soil microbial activity may well be applicable in most agroecosystems for not only improving crop and forage growth and development but also as po-tential 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 man-agement would seem to withstand the effects of drought.

Dr. Bob Kremer is a soil microbiologist cur-rently employed as a research microbiologist with the USDA Agricultural Research Service at the Cropping Systems & Water Quality Research Unit in Columbia, Missouri and is an adjunct professor of soil microbiology in the Department of Soil, Environmental & Atmospheric Sciences at the University of Missouri.

Reprinted from October 2012 • Vol. 42, No. 10