3D-printed biological habitats for the protection and persistence of Rhizobia species in compacted soils

Microorganisms in soils are responsible for many ecosystem services. However, in degraded soils microbial abundance and function are limited which compromises several biologically facilitated processes. Inoculating soils with desirable microbes can help to re-instate or initiate a viable functioning microbial community. However, establishment success is reliant on the survival of the microorganism in an adverse environment. In this proof-of-concept study, artificial microbial refugia have been developed using resin and light-emitting diode array (LED) 3D printing technology. We assessed whether the artificial refugia, termed a Rhiome, would support better microbial growth in degraded soils. Soil compaction, a form of soil degradation, and Rhizobium, an important microorganism for global agriculture, were both selected as the use case application for this assessment. Different materials together with resin were assessed for their suitability as a 3D printing material and for supporting rhizobial growth. The best result was found in materials constructed with a combination of polylactic acid (PLA) resin, yeast extract and mannitol. In a soil compaction experiment with inoculation of rhizobia, the addition of Rhiome significantly increased bacterium survival in the compacted soil to a level similar to, or higher than, the rhizobial loading in non-compacted soils. Augmentation of the resin with yeast extract and mannitol increased Rhizobium growth significantly compared with the Rhiome constructed only with resin. These results indicate that the Rhiome was highly beneficial for instigating and maintaining significant rhizobia survival and growth in compacted soils. Further work, including near-to-field assessments, are required to assess Rhiome performance in a range of applications and to refine material properties relative to important context-specific performance metrics such as degradation rate. We propose the Rhiome concept as a promising asset in the toolbox for soil ecological restoration as a means of improving soil resiliency.