Investigating the Role of the Soil Microbiome in Ecological Restoration

Abstract

Human impact has extensively transformed and degraded more than two-thirds of terrestrial land, highlighting the urgent need for restoration. To improve restoration outcomes in harsh environments, there is a pressing need to develop effective and economically viable techniques. Soil microorganisms, known for their vital roles in soil nutrient cycling, soil structure stabilization, and plant-soil interactions, have gained considerable attention in academia. Nonetheless, their application in restoration projects remains limited. Many of previous attempts to incorporate soil microbial organisms into restoration were driven by the progress in understanding soil microbial functions rather than the practical demands of restoration practitioners. Additionally, the spatial heterogeneity of soil microbial communities often leads to inconsistent effects of soil microorganisms in restoration projects. To overcome two barriers above and achieve practitioner-oriented microbial methods, it is essential to anchor microbial studies locally, address unresolved issues in current restoration strategies, and validate the effect of potential functional microorganisms through field experiments.Two fundamental questions must be addressed to determine the direction for applying soil microorganisms to restoration. First, should soil microbial communities be indirectly managed through existing restoration methods, or should they be directly manipulated? Second, should soil microbial communities be monitored and used as restoration indicators or manipulated to provide specific ecosystem functions? In response to the first question, soil microbial communities from eight ongoing revegetation field experiments across the American Southwest were analyzed. Revegetation with one species and four species resulted in similar microbial community richness, composition, and relative abundance of functional groups, suggesting that direct manipulation of soil microbial communities seems more appropriate. For the second question, the increased relative abundance of nitrogen fixers and ureolytic bacteria associated with cheatgrass invasion suggests that soil microorganisms are likely driving the increased nitrogen, rather than passively responding to it. The findings from both studies support the potential of direct manipulation of soil microbial communities to restore nutrient cycling disrupted by plant invasion. The previously proposed strategy was implemented in a case study focusing on invasive beachgrass control in Point Reyes National Seashore, California, USA. Despite the successful eradication of beachgrass through herbicide treatment, the undecomposed litter of invaded beachgrass continued to impede the reestablishment of native plant communities even after five years. We found significant differences in soil organic matter level, nitrogen level, relative abundance of bacteria functional groups related to nitrogen cycling, fungal community composition, and relative abundance of saprotrophic fungi between treated and uninvaded sites, with minimal signs of recovery observed after herbicide treatment. In response, a field experiment was conducted to restore soil microbial communities and potentially accelerate decomposition. Soil inoculation was used to manipulate soil microbial communities through transferring varying quantities of soil from uninvaded sites with abundant saprotrophic fungi to herbicide-treated sites. Although the decomposition rate of the native shrub with high litter quality increased as expected with high rates of inoculation, the decomposition rate of beachgrass remained unaffected. Thus, while soil inoculation serves as a valuable field research method for manipulating soil microbial communities, its cost-effectiveness limits its utility as a restoration tool.