As the human population continues to grow, agricultural systems face more and more pressure to increase crop production. This demand has resulted in the domination of several staple crops, such as wheat, corn, and soy, in human diet and thus in the fields of many industrial farms. This transition has led many farmers to plant and replant large areas of land with a single crop, a practice known as monoculture. However, while monoculture may initial yield impressive results, over time, the health of the crops decline and production is reduced (Biodiversity and Agriculture, 2016). Much effort has been put into understanding this process to optimize the health of the crops to ensure the future of producing enough food to meet the demands of the human population.
A new study suggests that the practice of monoculture shifts the structure of the soil microbiome, which in turn negatively impacts the health of the crop (McNear, 2013). The microbial life inhabiting the soil immediately surrounding the roots of a plant, an area known as the rhizosphere, is known to play important roles in the health of the plant from nutrient acquisition to defense against pathogens (Zhao, 2016). Likewise, roots often secrete compounds that attract or repel microbes, thus shaping the population of microbes that they associate with. In this study, the authors hypothesize that the successive replanting of the same crop alters the compounds that roots secrete, which consequently disrupts the soil microbiome and the function that it serves to the crop. They used the replanting of Pseudostellaria heterophylla, a crop commonly used in Chinese medicine, to investigate the effects of monoculture on soil microbes. P. heterophylla grows best in a specific part of southeast China, but intensive cultivation in the same area has started to result in declining health and production of the crop. Attempts to grow the crop in less desirable areas have had little success, so there is considerable need to understand the relationship between monoculture and crop health. The authors looked both at the overall impact on the abundance and diversity of soil microbes, as well as the specific impact on a few species with known beneficial or pathogenic relationships to the crop.
The authors analyzed the soil in fields that had not been planted, fields that had been planted with P. heterophylla for the first time, and fields that had been replanted with P. heterophylla. They found that the replanted soil had fewer species of microbes than the unplanted or newly planted soil, and that the replanted soil had fewer species in common with the unplanted soil than the newly planted soil had. In other words, replanting P. heterophylla reduced the diversity of microbes and altered which species were present. The authors looked more closely at the second finding and found that replanted soil contained less beneficial bacteria, such as those that aid in element cycling and nutrient absorption, and more harmful bacteria, such as denitrifying bacteria and pathogens, than unplanted or newly planted soil. The authors also identified specific species of microbes with known functions and analyzed their response to replanting. Pathogenic fungal and bacterial species increased in the replanted soil from the unplanted and newly planted soil, and beneficial bacterial species decreased. The authors provide ample evidence to suggest that the detrimental effects of monoculture result from the disruption of the soil microbe population after successive replanting of the same crop. This study introduces many more questions around how to sustainably maintain healthy crops. Is there a way to restore the population of beneficial microbes to maintain the health of the plants? We are only beginning to learn about the abundance of life that is too small for us to see, and much more research is required before we will be able to employ them to our benefit. Even so, the soil microbiome offers an exciting new avenue for research into how to optimize the health of crops so that they can be grown sustainably in the numbers required to meet the needs of our growing population.
“Biodiversity and Agriculture.” The Center for Health and the Global Environment. N.p., n.d. Web. 16 June 2016.
McNear Jr, D. H. “The rhizosphere-roots, soil and everything in between.”Nature Education Knowledge 4.3 (2013): 1.
Zhao, Yong-Po, et al. “Insight into structure dynamics of soil microbiota mediated by the richness of replanted Pseudostellaria heterophylla.”Scientific reports 6 (2016).