Incinerated byproducts of industrial, agricultural, or municipal processes have been used for centuries to improve soil pH and physical properties, as well as to recycle nutrients to soil and food systems. These incinerated products include coal fly ash (CFA), biomass ash, manure ash, and sewage sludge incineration ash (SSA). In particular, SSA is generally land applied to recycle phosphorus (P) or other nutrients. While the chemical and physical impacts of recycled ash are well documented, the biological impacts to the soil microbial community are less well understood. Microbes play an essential role in the soil environment by mediating nutrient availability and transforming organic matter. However, given the relatively recent development of advanced soil microbial community analyses and the sheer number, functional capacity, and interconnectedness of soil microbes, the complexities of the soil microbial communities and microbial interactions are actively being explored.

Older microbial studies often relied on traditional methods of analysis, including culturing and Sanger sequencing, the measurement of respiration rates, enzymatic activity, and total microbial biomass. However, over the last 20 years, the development of rapid and inexpensive next-generation sequencing (NGS) techniques for genomic analysis has revolutionized the analytical scope for soil science, which can now examine microbial community dynamics in response to changes in environmental parameters. NGS is high throughput, allowing for hundreds of millions or more of nucleic acid fragments to be sequenced at once, and has been vital in understanding complex microbial communities, such as those in soil, which may contain thousands of microbial species per gram [1].

All bacteria and archaea have a taxonomically significant region of DNA encoding for 16S rRNA, while all fungi have 18S rRNA and an internal transcribed spacer region between this and 23S rRNA. For analysis of DNA extracted from the environment, the variations in these ubiquitous genomic regions are a common way to identify unique taxonomic units, generate phylogenetic trees, quantify relative abundances of microbes in the environmental sample, and compare samples to understand microbial community dynamics in response to different environmental conditions [2]. In this review, the totality of DNA in environmental samples is referred to as the metagenome (or microbiome), while the total microbes themselves are referred to as soil microbiota.

The significant role played by soil microbial communities in mediating soil health and functionality and promoting ecological resiliency is well documented, as are the potential effects of soil amendments on microbial community composition [3]. While a robust microbial community is not the sole regulator of the health of a soil system, it is one of many biotic factors that are required for a stable and sustainable system. Studies have found that a vigorous microbial community with functional redundancy can improve the resiliency of a soil system during various types of environmental pressures and perturbations, such as drought or fumigation [4]. In a soil environment, many factors can affect microbial populations, including soil type or parent material 5, 6, 7, pH [8], climate, organic matter, texture, as well as the interaction of these factors 9, 10, 11. The application of soil amendments such as recycled ash products can also impact microbial community composition (Figure 1).

With the exploration of SSA as a recycled P source, the impacts of this P amendment on the soil metagenome have been mostly unexamined. We performed a ‘proof-of-concept’ soil incubation with a wide range of application rates of SSA to identify what rate would result in a detectible microbial metagenomic response. In this review, we will summarize recent results regarding the impacts of incinerated products on soil microbial communities and the deployment of NGS techniques as context for possible impacts from SSA soil amendment. We will also provide data from this SSA–soil incubation and discuss its implications for shifting P flows through P recycling, as well as further opportunities for research in this field.