Effects of microbial inocula on soil nutrient cycling and the productivity of bioproduct crops in marginal soils

Background/Question/Methods

Due to the land area needed to meet the food needs of a growing population, there has been increased interest in the use of marginal lands to grow crops for other bio-based products like bioenergy. Utilizing marginal lands to grow bioproduct crops such as giant miscanthus (Miscanthus x giganteus) and switchgrass (Panicum virgatum) could improve soil quality, minimize negative environmental impacts on the surrounding ecosystems, and offer economic benefits to growers in rural areas. Because marginal soils tend to produce low crop yields, directly manipulating the soil microbiome with microbial inocula (i.e., biofertilizers) could provide plants with better access to nutrients and lead to increased yields. We hypothesized that biofertilizer would have greater persistence in the low-quality soil due to reduced competition with the resident microbiome leading to larger impacts on plant performance and soil biogeochemistry. Additionally, we hypothesized that biofertilizers would have a similar effect on Panicum and Miscanthus. We performed a greenhouse experiment with marginal and sub-marginal soil containing either Miscanthus, Panicum, or no plant. The soils received either a commercial biofertilizer, a ‘microbiome transplant’, or served as uninoculated controls. The ‘microbiome transplant’ was a microbial inoculum derived from an established and highly productive Miscanthus stand on marginal land.

Results/Conclusions

Neither Panicum plants nor their microbiome were affected by either microbial inocula. However, in Miscanthus plants belowground biomass was increased by both the commercial biofertilizer and the microbiome transplant in the marginal soil. This increase in belowground biomass could allow Miscanthus to better resist disturbances such as drought. The microbial transplant treatment also led to increased soil respiration rates in Miscanthus rhizosphere soils in the marginal soil but not the submarginal soil. Miscanthus and Panicum increased the rates of nitrogen fixation in rhizosphere soil, but this was unaffected by biofertilizer. This increase in nitrogen fixation suggests a prevalence of free-living nitrogen fixing bacteria in soil planted with Miscanthus and Panicum which could help to improve soil quality of the marginal soils. In summary, our results suggest microbial inocula may enhance the performance of bioproduct crops on marginal soils and that bioproduct crops can improve the fertility of low-quality soil by enhancing nitrogen fixation.