E.R.J. WUBSDonor-soil microbes drive—and can speed up—the restoration of degraded farmland, scientists at the Netherlands Institute of Ecology in Wageningen have shown. The results of a six-year field test, published today (July 11) in Nature Plants, show the greatest ecosystem repair in formerly arable fields in which the team removed a thick layer of existing topsoil before applying a thin layer of microbe-rich donor soil.
“Of course, seeds of plants were also present in the donor soil,” study coauthor Jasper Wubs of the Netherlands Institute of Ecology told reporters during a press briefing. “But our study shows that it is in fact the soil organisms—such as the bacteria, fungi, and roundworms—which determine the direction of ecosystem restoration.”
Wubs and colleagues tested various soil inoculation approaches in plots carved from a 160-hectare field in Reijerscamp, the Netherlands, that had been farmed for nearly 60 years. In control plots, the researchers left the land as it was. In experimental plots, they left behind existing topsoil or removed up to 50 cm before spreading a 1 cm-thick layer of donor-grassland or -heathland soil. While all the plots that received donor soil fared better than the control plots, the donor-soil–recipient plots in which existing topsoil had been excavated prior to inoculation showed more improvement, and faster too.
The researchers also reported that the type of donor soil drove the resulting microbial community’s composition toward that of the donor site—either grass or heathland. “This is similar . . . to the use of fecal transplants to restore disrupted gut microbiomes in humans,” said Wubs.
“This is pretty cutting-edge research,” said Neiunna Reed-Jones, a fellow at the Oak Ridge Institute for Science and Education who studied enteric human pathogens in agricultural environments while a postdoc at the University of Maryland. While the approach has been discussed in the literature, “I hadn’t heard of anyone doing soil transplantation before.”
“Biocontrol, plant growth promotion, and nutrition by microbial inoculants have a long tradition in agriculture,” noted Gabriele Berg of the Graz University of Technology in Austria who was not involved in the work. “Therefore it is not surprising that . . . soil transfer works well.”
While the study provides an important proof of concept, Berg would like more details on the mechanisms of action. “For me, the most important fact is to understand the quality of soil, especially the microbiota,” she wrote in an email to The Scientist. “To work with a ‘black box’ of soil, which contain thousands of different microbial species, is difficult and not up-to-date.”
Both Berg and Reed-Jones questioned whether the soil-inoculation approach might transfer potentially harmful microbes along with the beneficial ones. “We have to consider that each soil contains a lot of plant and potentially human pathogens,” wrote Berg.
“Will this soil inoculation cause any destruction to already-established soil communities?” asked Reed-Jones. “If this was taken on a larger scale, transplanting soil to a different area, would that introduce new soil microbiota and would they start competing? Would that competition allow some other soil disease to occur?”
Still, the approach shows promise for agriculture. Through microbial transplantation, “enriching soils that may be not be very productive . . . may help with plant proliferation, nutrient management, and carbon fixation,” Reed-Jones said.
E.R.J. Wubs et al., “Soil inoculation steers restoration of terrestrial ecosystems,” Nature Plants, doi:10.1038/nplants.2016.107, 2016.