How microbes can, and cannot, be used to assess soil health

Abstract

Healthy soils are critical to the health of ecosystems, economies, and human populations. Thus, it is widely acknowledged that soil health is important to quantify, both for assessment and as a tool to help guide management strategies. What is less clear is how soil health should actually be measured, especially considering that soil health is not exclusively a product of soil physical and chemical characteristics. Given their well-established importance to many aspects of soil health, microbes and microbial processes are often used as metrics of soil health with a range of different microbe-based metrics routinely used across the globe. Unfortunately, it is our opinion that many of these pre-existing microbial measurements are not easy to interpret and may not necessarily provide credible inferences about soil health status. Here we review the microbial indices used to assess or monitor soil health and discuss some of the broader issues associated with their use. We provide recommendations to more effectively guide and improve how microbial information could be used to yield relevant and actionable assessments of soil health.

An evaluation of biological soil health indicators in four long‐term continuous agroecosystems in Canada

“…The fungi/bacteria (AMF/TB, SF/TB, TF/TB) ratios differed between the systems, but were significantly lower under monocultures, except for SF/TB where the corn was similar to the grasses. Although many factors can cause variations in fungal/bacterial (F/B) ratios, thus making their interpretation difficult (Fierer et al., 2021), the evaluation of these ratios along with multiple indicators can provide valuable information on how soil health is affected by management practices. Our results showed a highly significant treatment effect, and are consistent with previous studies showing the association of F/B ratios with differences in C sequestration (Malik et al, 2016) and with more sustainable agricultural practices such as grasslands with reduced N fertilization (de Vries et al., 2006) and mixed annual–perennial cropping (Mann et al., 2019).…”

“…Two of the most important, as well as sensitive, indicators of soil health are the soil microbial community (SMC) and soil organic matter (SOM), which are intrinsically related. Microbial abundance, diversity and activity regulate many essential soil functions, including biogeochemical transformations that are key to nutrient cycling and SOM dynamics, although there are challenges in interpreting and linking these parameters to overall soil health (Fierer et al., 2021). Inherent SOM dynamics, in turn, define the microbial component of the soil ecosystem (Cotrufo et al., 2013; Lehman et al., 2015; Stott, 2019) and include more known measures of soil health such as labile C. Thus, any comprehensive evaluation of soil health as affected by management and climate should involve the evaluation of both the SMC and SOM.…”

Role of soil in the regulation of human and plant pathogens: soils' contributions to people

“…Understanding the complex interactions and ecological phenomena that govern suppressiveness in a particular place and time will provide a foundation for new tools and indicators to predict potential outbreaks and develop preventative solutions [41,184]. Better knowledge of which tools to use, and when to stay out of the way of soil's native ability to regulate detrimental organisms, will come from ecological insights gained from studies of soil biodiversity [185].…”

Introducing the Microbes and Social Equity Working Group: Considering the Microbial Components of Social, Environmental, and Health Justice

“…For example, plant biodiversity, a primary driver of soil microbiome diversity and community structure, is lower in communities disenfranchised through “redlining” (inequitable development districting), demonstrating how systemic racism affects ecosystem biodiversity ( 143 ). Soil microbiomes offer promising metrics for assessing soil health across environmental change and through time ( 144 ). It is critical to understand how anthropogenic climate change impacts environmental microbes, as well as how microbes drive global change processes ( 8 ), and to consider this during environmental impact studies for infrastructure development or industry permitting to avoid ecosystem disruption.…”