The results of a three year study into the effects of organic and conventional farming practices on soil microbial diversity have been published by NEFG scientists. Former PhD student Caroline Orr (pictured) used soils from the Nafferton Factorial Systems Comparison trial to test the effects of organic versus conventional crop management including fertility management practice (compost versus NPK fertiliser) and crop protection method (organic versus conventional) on the general bacterial population and the subset of this population that fixes nitrogen (free-living N fixers). She also looked at temporal effects since soils were collected over three years, on three dates in each year (spring, summer and autumn).
Dr. Orr extracted nucleic acids (both RNA and DNA) from soil samples and used a variety of techniques to measure the size, diversity and activity of soil bacterial communities. Crop protection had a consistent effect across the three years with slightly smaller N fixing bacteria populations measured when pesticides were used, although the expression of the gene for N fixation was not affected by pesticides. This is consistent with other studies which have shown that free-living N fixing bacteria are particularly sensitive to pesticides. In contrast to other studies, fertility management practices did not affect the general bacterial population in any consistent way, although in some years free-living N fixing populations were larger and more active when compost was used as a nutrient source.
In spite of these interesting management effects, the overriding factor affecting both the general and free-living N fixing bacterial populations was sample time. This means that the year that the samples were taken and the date within a given year, tended to have a larger effect than crop management on soil bacteria.
Currently NEFG researchers are exploring the impacts of crop management on microbial populations in the immediate root zone (the rhizosphere). Rhizosphere populations are thought to be of particular importance since they can directly impact on the uptake of nutrients by the crop. It is hoped that some consistent effects of crop management on these populations can be identified that will lead to recommendations about best practices to optimize soil bacterial population function.
The full results of this study are available on the PLoS ONE website via this link: http://dx.plos.org/10.1371/journal.pone.0052891
Orr CH, Leifert C, Cummings SP, Cooper JM (2012) Impacts of Organic and Conventional Crop Management on Diversity and Activity of Free-Living Nitrogen Fixing Bacteria and Total Bacteria Are Subsidiary to Temporal Effects. PLoS ONE 7(12): e52891. doi:10.1371/journal.pone.0052891