Many of the soils in the world’s arid ecosystems are either degraded or experiencing increases in severe anthropogenic degradation¹. Particularly impacted are photosynthetic topsoil communities known as biocrusts² (Fig. 1), which protect drylands soils against erosion and contribute to soil fertilization. Reversing degradation by bringing back the natural biocrust soil cover through interventional re-inoculation has been long considered a viable remedy to dryland restoration and sustainability. Yet, the biocrust restoration technologies already developed are typically labor-intensive and pricy, limiting their use to small spatial footprints (less than hundreds of square meters). This is the biggest obstacle now standing in the way of achieving ecologically meaningful interventions.

Figure 1. A trampled biological soil crust community (biocrust) shows its organo-sedimentary structure, with cyanobacterial filaments holding soil particles and forming a coherent crust layer on the top few mm of the soil that overlays loose soil below.

 Better together

In an approach we named “crustivoltaics” (bio-“crust” and photo-“voltaic”), we took advantage of an existing solar farm in the Sonoran Desert of Arizona to develop a solution to the scaling problem. Here, and during a three-year investigation we showed experimentally that elevated photovoltaic (PV) panels favor growth of biocrust communities in the soils they cover compared to edaphically and climatically similar areas away from PV panel influence (Fig. 2). We also found that compared to the slower natural recovery, which can take anywhere from decades to hundred years^3,4, under PV installations recovery was fast (between one and two years), even under extreme, unexpectedly harsh environmental conditions, further supporting our idea of using PV solar farms as biocrust nurseries.

Figure 2. Influence of solar panels on biocrusts growth. A solar farm in the Sonoran Desert of Arizona showing experimental plots with close-ups of the biocrusts growing inside and outside of PV panel rows, and their corresponding observed chlorophyll a concentration range.

 New opportunities

We showed that solar-farm-grown biocrust communities, exhibited the microbial make-up typical of local natural biocrust communities. This demonstrates that crustivoltaics can be used to produce a biocrust inoculum that matches microbial communities adapted to local environmental conditions in relatively short periods of time, at a low cost, requiring no specialized personnel by taking advantage of existing solar farm infrastructure. Because existing solar farms in many arid lands cover large expanses, the potential of crustivoltaics inoculum production is commensurately large. This is in contrast to greenhouse- and laboratory-based inoculum production where obtaining tailored communities requires multiple optimization and monitoring efforts.

 Dust deposition lowers the power and voltage outputs of solar panels raising operation expenses, and hence solar farms should also benefit from implementing “crustivoltaics” as a continuous dual operating mode by reducing inputs of fugitive dust that is produced in nearby crustless soils. Moreover, by increasing soil carbon due to newly planted biocrusts, solar farms may obtain additional sources of revenue through carbon credits. For example, nursing a hectare of mature biocrust, which can roughly sequester 9 tons of atmospheric CO₂ ⁵ , and at a credit price of US$ 32 per ton of sequestered carbon⁶, translates into an additional profit of about US$ 300 ha^-1. Together, these benefits could contribute to positioning solar farms as net proponents of ecological sustainability. Therefore, encouraging investment into ecological restoration techniques.

 Because everything is not perfect

Continuous harvesting of biocrust will deplete soil and refilling beneath panels will become inevitable during continuous operation, raising operating expenses as a consequence. Moreover, biocrust inoculum tends to perform poorly when used to restore soils with different edaphic or climatic characteristics from where it was sourced, meaning that the use of crustivoltaics may be limited to restoring areas in the vicinity of existing solar infrastructure.

Some take home points.

Working on developing technologies to create biocrust inoculum suitable for long-term, socioeconomically sound ecological restoration of dryland soils has kept us busy over the last decade. We overcame many technical challenges in designing our microbial nursery approach^7,8: establishing best isolation and cultivation techniques for indigenous microbes under controlled conditions in order to produce large quantities of pedigreed (genetically matched to field microbes) inoculum adapted to local environmental settings^7–10. The implementation of crustivoltaics promises to take the microbial nursery out of the laboratory and greenhouse, positioning the field for a tangible leap toward landscape scale restoration. Although implementing crustivoltaics restoration at the landscape scale will involve coordinating a diverse group of personnel, entities and agencies, the results envisioned in the crustivoltaics manuscript encourages us to embrace the task ahead and to push the technological approaches to tackle some of the potential limitations already foreseen in our contribution.

References

1. Nkonya, E., Mirzabaev, La. & Braun, J. von. Economics of Land Degradation and Improvement - A Global Assessment for Sustainable Development. Internationla food policy research institute (Springer Open, 2015). doi:10.1007/978-3-319-19168-3_3.

2. Garcia-Pichel, F. Desert Environements: Biological Soil Crusts. Encyclopedia of Environemental Microbiology (2003).

3. Fick, S. E., Barger, N., Tatarko, J. & Duniway, M. C. Induced biological soil crust controls on wind erodibility and dust (PM10) emissions. Earth Surf. Process. Landforms 45, 224–236 (2020).

4. Li, X. et al. Biocrust Research in China: Recent Progress and Application in Land Degradation Control. Front. Plant Sci.12, (2021).

5. Beraldi-Campesi, H., Hartnett, H. E., Anbar, A., Gordon, G. W. & Garcia-Pichel, F. Effect of biological soil crusts on soil elemental concentrations: Implications for biogeochemistry and as traceable biosignatures of ancient life on land. Geobiology 7, 348–359 (2009).

6. Bernal, B., Murray, L. T. & Pearson, T. R. H. Global carbon dioxide removal rates from forest landscape restoration activities. Carbon Balance Manag. 13, (2018).

7. Giraldo-Silva, A., Nelson, C., Barger, N. & Garcia-Pichel, F. Nursing biocrusts: isolation, cultivation and fitness test of indigenous cyanobacteria. Restor. Ecol. 27, 793–803 (2019).

8. Velasco Ayuso, S. V., Giraldo-Silva, A., Nelson, C., Barger, N. N. & Garcia-Pichel, F. Microbial nursery production of high-quality biological soil crust biomass for restoration of degraded dryland soils. Appl. Environ. Microbiol. 83, (2017).

9. Giraldo-Silva, A., Nelson, C., Penfold, C., Barger, N. N. & Garcia-Pichel, F. Effect of preconditioning to the soil environment on the performance of 20 cyanobacterial strains used as inoculum for biocrust restoration. Restor. Ecol.(2019) doi:10.1111/rec.13048.

10. Nelson, C., Giraldo-Silva, A. & Garcia-Pichel, F. A fog-irrigated soil substrate system unifies and optimizes cyanobacterial biocrust inoculum production. Appl Env. Microbiolgy 86:e00624-, (2020).