Soil Microbes as the HEI
When EPA did the Part 503 risk assessment for its "Standards for the Use or Disposal of Sewage Sludge," they modeled risks of biosolids to plants, animals, water and people. The risk assessment was done several decades ago and has passed muster with two subsequent re-valuations by the National Academies of Science, with some calling the regulations too conservative. The highly-exposed individual (HEI) that scientists used in risk assessment was unlikely to be found in the real world. The assessors were protecting someone who didn’t even exist.
In the decades since, our ability to understand the world beneath our feet has grown exponentially. We can now extract DNA from soils and then analyze the DNA to figure out exactly what types of microorganisms are living in the soil. We can measure populations and population shifts. These types of analysis are new enough that the answers we get from a single study are not sufficient to provide overarching conclusions. We are not sure how fast the world beneath us changes and morphs and what factors are responsible for observed changes. Only now can we understand the level of diversity that this world contains.
Mix these new tools and biosolids and you get a new HEI. Soil microbes are the true HEIs, as the biosolids we add to the soil goes directly into their living rooms. Research papers now address the impacts of biosolids applications on microbial communities and soil ecology. So, let’s have a look at these unanticipated HEIs and how they fare with biosolids.
The first paper in the library comes from Colorado State University. Long-term plots with either a single application of biosolids or multiple applications, at or above agronomic rates, were applied to grasslands. The authors report on microbes, but also report on changing soil conditions following the biosolids applications. Think of it like a major home remodel, which is, in fact, what the microbes got. They went from a stark Danish Modern look to more of an Art Deco elegance. What that means in terms of soils is that, at the higher biosolids application rates, total soil carbon, nitrogen and generally phosphorus were increased. Soil pH did decrease, perhaps due to those microbes spilling wine in their opulent settings. As a result of the increased soil quality, the plant community shifted with increased biomass and decreased species richness (fewer types of plants and generally larger plants). Some changes occurred, too, in the soil microbes. There was an increase in Gram+ bacteria; that was only significant on a short-term basis. No shifts in Gram- bacteria or fungi or to the % fraction of arbuscular mycorrhizal fungi (AMF) for one year. However, it is not clear if the shifts were the result of the new décor, changed plant community, well above normal rainfall for one year of the study, or what. The authors make a big point on the decrease of AMF and suggest that this may be the result of the biosolids. They do note that a previous study on THE SAME PLOTS had shown the opposite effect. The same opposite effect is seen in the second paper in the library.
The second study was done in Scotland where biosolids were applied at agronomic rates to pasture and tilled systems. AMF colonization of roots for multiple crops over multiple times was measured. The authors saw no impact of biosolids with variation due to time and season. So, perhaps those microbial abodes in the first study had leaky roofs with the heavy rainfall and that factor explained the observed difference.
For the third study, we go to a paper that used complex techniques to measure changes in diversity in ammonia oxidizing archaea (a primitive form of microbes that also play a critical role in anaerobic digestion) and bacteria. This study was primarily funded by the US DOE with some additional funding from the Northwest Biosolids Management Association and King County (Seattle) biosolids program. You read this study and wonder why biosolids are even mentioned in the article. Here we measured changes in communities in wildland or native soils as well as agricultural fields on the same soil series. We looked at switchgrass, fertilized with synthetic fertilizers or biosolids as well as other agronomic crops in irrigated fields in Eastern Washington. Biosolids didn’t make it into the final paper because only minimal changes in community were seen between the biosolids soils and the fertilizer soils. The very significant shift was seen between the native soils and the farmed soils. It turns out that farming is very detrimental for microbial diversity in soils. Much like, along with the plow, we bring in whole new populations to the soil. Change may or may not be good, but it seems to be one of the consequences of growing food. Here change may be related to added fertility, changes in soil pH, or the increase in available water associated with irrigation.
If you want to hone in on changes in community for a crop grown under the same management practices on the same soil at the same time, look at the fourth paper. Here either biosolids N or synthetic N was added to corn. Both ammonia-oxidizing archaea and ammonia-oxidizing bacteria were measured. The biosolids-amended soils had higher numbers and higher activity of ammonia-oxidizing archaea than the fertilizer amended soils, so the analyses showed more rapid ammonia transformations with an increased population of a particular group of microbes. I would call this an enhancement, rather than a concern.
The takeaway so far on microbes as the HEI is that, while we now have the tools to monitor them, we aren’t yet clear on what these analyses mean. It would appear that introducing crops and irrigation to the soil ecosystem has a huge impact on the organisms inhabiting our soils. In comparison to that, changes associated with using biosolids are minor. These changes can vary from site to site and over time. In no instances have biosolids hurt microbial communities; the biosolids has just altered population dynamics. These alterations are more likely a result of changes in the soil following biosolids addition than any specific reaction to the biosolids themselves.
For the final paper, I picked an older publication where we tested plant diversity on restoration plots where biosolids had been added with different rates of a high carbon material. We did this to see if increasing the C:N ratio might make for a less ‘rich’ soil environment, one that was more conducive to native plant communities. It did. A ratio of 20:1 seemed to do the trick. Perhaps the minor differences in the first paper in this study were related to the richer soil promoting a richer and less diverse plant community. If that is a concern, just add some carbon with your biosolids. This is a perfectly acceptable option in HEI home décor, when your HEI Is a microbe.
Sally Brown, University of Washington