As human population numbers worldwide continue to increase, we are having to be increasingly clever about finding ways to produce enough food for all consumers. One potential technique is converting deserts into agricultural landscapes. Anyone who knows their history will be aware that this practice is by no means new, but modern technology allows desert farming on much larger scales--not only creating larger agricultural areas, but also producing greater quantities of food.
There have been several studies investigating the feasibility of large-scale desert farming, but one variable that has not received much attention is bacteria--specifically, the type and function of bacteria that are present in desert soils. One thing that is known is that deserts have much more bountiful and diverse bacterial communities than once thought. This is impressive given that these creatures have to tolerate extreme temperatures, the threat of desiccation, high soil salinity, low nutrient levels, high summer UV radiation levels, and physical instability caused by wind. But how will they fare against the introduction of agricultural crops? If they form partnerships with plants (by living in or near plant roots), might they have beneficial effects on crops by buffering them from soil pathogens?
(Farmers tilling the soil at SEKEM, an ecological village in Egypt focused on sustainable living.
These are some of the questions recently answered in a study by collaborators from Austria's Graz University of Technology and Egypt's SEKEM and Heliopolis University. The scientists collected soil samples from in and around desert agricultural fields used to grow German chamomile (Matricaria chamomilla L.), pot marigold (Calendula officinalis L.), and Solanum distichum Schumach & Thonn. (an African herb in the nightshade family). By subjecting the samples to different types of DNA analyses and culturing techniques, they were able to compare bacterial communities between farm and desert soil, identify particular species groups and individual species present at the two types of site, and investigate the potential anti-fungal capabilities of different soil species.
(German chamomile, Matricaria chamomilla L.)
Overall, agricultural field soil had a higher diversity of bacteria than desert soil, and only about 40% of resident bacterial strains of the two site types were shared. Interestingly, while desert soil had 3 main bacterial groups (Ochrobactrum sp., Rhodococcus sp., and Bacillus sp.), bacteria in the farm soil tended to be plant-specific, with each plant sharing only about 20% of species with the other two species at the farm.
The dominant phyla at both types of site were Proteobacteria, Firmicutes, and Actinobacteria. However, though bacteria from all these groups were present both in the desert and on the farm, they were found in different numbers at each type of site. There were about three times as many Firmicutes in agricultural soil than in the surrounding desert, but there were about twice as many Proteobacteria, and about 5 times as many Actinobacteria, in the desert as at the farm. Some phyla (Acidobacteria and Planctomycetes) were found only in agricultural soil, while others (Deinococcus-Thermus) were found only in desert sand.
(An example of a Bacillus species--B. subtilis. Bacillus species are in the phylum Firmicutes, which was particularly prevalent in agricultural soil.)
When bacterial isolates from the farm and desert soil were exposed to Verticillium dahliae, Rhizoctonia solani, and Fusarium culmorum fungi in culture, the farm species were about twice as likely to have anti-fungal activity than the desert species. Further, while approximately 17% of isolates from the farm site were effective against all three pathogens, this was true of only 11% or so of desert isolates. Although there were some overlaps in the type of bacteria from each site that had anti-fungal properties, some strains could only be found in one habitat or the other. In total, the researchers were able to pinpoint 45 bacterial strains that show promise as possible soil inoculates that could be used to combat fungal damage to desert crops.
Because there were such obvious differences in the soil bacterial communities at desert and farm sites, the researchers performed an analysis to investigate which environmental features may have driven this divergence. The most important difference was water supply, which, of course, was much greater on the farm than in the desert. Soil pH was also an important factor as, to a lesser extent, were the amount of organic carbon in the soil, and overall soil quality. Thus, the introduction of water, mulch, and organic nutrients from the plants themselves all played a role in transforming the original desert sand into a very different type of habitat suitable for different species of microorganisms--and plants.
Desert agriculture has the potential to have many positive effects, including generating more food in poor regions, producing renewable crops for industrial application, and capturing/restoring carbon dioxide in the soil. One of the known drawbacks of this method is the amount of water required for irrigation; for example, approximately 5 billion cubic meters of water is thought to be necessary for the 40% growth of desert farmland forecast for Egypt over the next 6 years. On top of this, the current study indicates that farming will also alter soil bacterial composition; however, while some of the more extreme-habitat-loving bacteria disappeared from agricultural soils, the overall diversity of the bacterial community was actually higher in farm sites. Thus, without further research, it is hard to say whether the net effect of desert agricultural efforts will be negative or positive. One thing that is clear is that desert soils are home to indigenous bacteria that can help make the habitat more suitable for plant growth. This suggests that, no matter what other materials might be introduced to desert farms, at least farmers won't need to inoculate the substrate with foreign bacteria.
Köberl, M., Müller, H., Ramadan, E.M., and Berg, G. Desert farming benefits from microbial potential in arid soils and promotes diversity and plant health. 2011. PLoS ONE 6(9):e24452.
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