Are Green Roof Inoculations Worth It?
by Dr. Anna Zakrisson on Wednesday, September 4, 2019
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Are green roof inoculations the magic key to green roof performance?
Are green roof inoculations the future of the living roof industry? Well, yes and no. Many already apply inoculants to green roofs, but with apparently little science to guide them. A thorough look through the scientific literature provides little evidence that the currently available commercial inoculants have clear positive benefits on green roofs despite positive effects in some agricultural systems. Once again, we must remind ourselves that plants are not a homogeneous green population, but very distinct species with different needs and abilities. Microbes are also extremely diverse and their interactions with the plant world exceedingly complex.
I believe that inoculations could be an important part of green roof installations in the future, but we are not there yet. We need much more data-driven research to get to that point.
Soil appears to be an important factor in the development of thriving microbial communities. Thus, simple inoculations of virtually sterile green roof media with no organic components will likely not provide any significant benefits.
Green roof inoculations
Inoculants have, in many cases, been shown to have significant positive effects on agricultural yields. Can this data be directly and uncritically be transposed to green roofs? Well, probably not. At least not for all green roof media, all climates, and all situations. Anyone claiming such generality should politely be asked to provide some data to back up their claims.
The effect and success of inoculations depend on many factors such as inoculant type, substrate type, plant community (some plants from strong associations, others less so), moisture, climate, local natural microbial communities .
There has been a variety of results from green roof inoculations cited in the literature. Young et al.  showed that the application of a commercial arbuscular mycorrhizal fungal inoculum on the plant Prunella vulgaris on a green roof did indeed improve the phosphorus content of the plant, but no effect on growth rate or biomass production was observed.
Molineux et al.  could only provide inconclusive results in their short study on the plant Plantago lanceolata, albeit, this study was short and with low replication.
Also, few studies have been carried out researching how green roof microbial communities develop over time , which is a rather important topic as inoculations are costly, and we need a clear cost-benefit analysis for inoculation-treatments before they should be proposed for a project.
Data on the commonly used sedum plants on vegetated roofs are largely missing with a few notable exceptions .
Thus, the jury is still out with regard to green roof inoculations. Perhaps, we have not found out how to match microbes with plants, and if certain microbe mixes might act antagonistically .
Green roof inoculations on media devoid of organic matter
Microbes need water to thrive. You are more likely to find molds and bacterial slimes in the bathroom and kitchen compared with our living room. At least, I hope so. If you have mold in your living room, you should probably take a close look at the reason for that and call your landlord.
Hence, if you are to inoculate a green roof soil, you should make sure that it has the correct moisture levels when you do. Soil moisture is a critical factor for microbial activity and growth. Both too wet and too dry conditions can have adverse effects. Fungi are vulnerable to drought as they thrive at higher soil moisture contents than bacteria .
Due to these moisture constraints, it is difficult to see how an extremely well-drained media with zero organic matter, designed to drain off at maximum speeds could support a thriving microbial community. Also, due to the lack of media micropores and sticky long-chain molecules in the soil, it is also hard to see how the inoculants should remain in the media. Just take a look at a sandy beach vs. a rich agricultural soil: which looks like it’s supporting the most complex microbial community? For well-drained sand, the expensive inoculants would be very prone to get washed out during even a small-to-moderate rain event. The same would be true for a well-drained green roof media.
So, what is the role of microorganisms in supporting plant health?
The green roof as a hostile environment
A green roof is a hostile environment. Especially extensive green roofs due to their shallow substrates of less than 15cm. The plants are exposed to a range of unforgiving abiotic factors such as solar radiation, wind, and strong temperature fluctuations. Add to this an isolation from the at level habitats with their rich microbial ecosystems and often high nutrient availability. A green roof simply isn’t the best place to be if you are a plant.
Add to this already existing harshness; the FLL green roof media guidelines propose a highly porous, light, and well-drained media with a low organic component. The reason for this is to keep the weight down, to facilitate drainage, and to avoid volumetric soil losses due to high microbial respiration.
Hence, there are short-term engineering gains, provided you want to facilitate fast runoff, for using this type of media. These gains come at a biological cost: poor plant health unless you ensure high biological maintenance such as irrigation and nutrient applications. In the end, this turns into engineering costs as poor plant health means reduced retention capacity and thus reduced roof performance.
So how to improve the soil and simultaneously keep the roof lightweight?
The role of microorganisms in supporting plant life
In natural systems, microorganisms provide plants with essential nutrients such as phosphorus and nitrogen. In return, microorganisms, such as fungi and bacteria, collect sugars from the plants. This symbiotic relationship is especially important in nutrient-poor and hostile environments.
Plant roots can only reach so far, but if they enter into a symbiotic relationship with, for example, an arbuscular mycorrhizal fungus, the reach of the nutrient-collecting network expands immensely. This is, of course, great for both the plant and microorganism. These symbioses increase plant resistance, health, and hence, functionality.
To put microbial importance into context, one single gram of soil may contain 200m fungal hyphae and many tens of thousands of bacteria . These all interact with the plants. Thus, viewing them in separation is, in many cases, not wise.
From an ecological perspective, and perhaps also from an economic perspective, it’s more useful to view the plant/soil system as a “holobiont” – a multi-organism that functions as one. Because, that’s really how these organism work, at least when they are allowed to.
Due to historical reasons, little focus has been on the role of microorganisms on green roofs and their role in nutrient cycling and runoff, as well as how they support living roof plant productivity and hence by extension: roof performance. This, despite the well-studied central role of these organisms in supporting plant life and productivity in natural and agricultural systems .
Green roofs and “sterile” soil media
Newly installed green roofs tend to have very low microbial biomass, and diversity as most of the soil media ingredients are engineered and derived from mineral components. Also, most substrates are sterilized before use as it is vital to kill off viable weed seeds together with any potentially harmful microbes . Natural soil can contain thousands upon thousands of unwanted seeds/m2  that potentially also could destroy the functionality of the roof.
To facilitate more nature-like conditions for the plants, organic matter is often added, e.g. at 10-20% by volume.
However, one study noted that most of the microbes added to a roof originate from the addition of the planters and not the soil . Nonetheless, the soil is critical and facilitates microbial growth and provides a refuge to which the “holobiont” can fully develop. Soil is important!
Green roof soil is important!
There are strong indications that including actual soil in green roof soil media is important for high roof performance. One study compared compost:brick vs. compost:soil media formulations and found that the compost:brick media had both lower microbial activity as well as lower nutrient concentrations than the compost: soil medias tested 
Nutrient solubilization and capture by microorganisms on green roofs
So, what are microorganisms good for?
One important microbial function involves the solubilization of insoluble forms of phosphorus. This perhaps seems like a minor thing, but if we take a look at data from the agricultural sector the issue suddenly becomes one of importance, 75-90% of all added phosphorus fertilizer is precipitated either by iron, aluminum or calcium in the soil . It should be noted that the production of phosphate fertilizers is a costly and energy-consuming process and the global need in 2014 was 42,700,000 metric ton  of which, I repeat, 75-90% was precipitated and much rendered inaccessible for the plants. Microbes can make this precipitated phosphorus accessible to the plants. Microbes are important.
Similarly, on green roofs phosphorus can quickly be lost through these chemical processes or flushed out during rain events. One chemical species that is prone to leeching off green roofs is nitrate. Microbes can help to absorb and temporarily immobilizing such nutrients that are highly soluble and prone to fast runoff.
Keeping nutrients tied into the microbial biomass result in a much more efficient nutrient delivery system, especially when the plants form plant-microbial symbioses. Low nutrient losses through runoff also mean that less nutrients must be applied. The system becomes efficient not only from an ecological but also from a financial perspective due to the lowered maintenance costs.
Green roof nutrient availability and microorganisms
Microbes can provide many types of ecosystem functions. Some bacteria even have the ability to fix molecular dinitrogen (nitrogen gas) from the air, which means that they can directly fertilize the soil by using plain air!
So, let’s add these organisms to our green roof media via inoculations and all will be good, right? No. that’s unfortunately not how these things work.
When it comes to nitrogen fixation, many of the nitrogen fixers are often associated with the root nodules of specific plant families, such as Leguminosae to which peas belong. These plants are not common on extensive green roofs. However, there are also free-living nitrogen-fixing bacteria, but their role on green roofs is largely unknown. These organisms are also often hard to culture for use in inoculations and are generally extremely poorly studied. Also, several human pathogens have been identified as free-living nitrogen fixers high-lightening the complexity of this issue .
You simply cannot just take any old microorganism and hope that it will work wonders for a specific plant.
The future of green roof microbial inoculations
There is a world of undiscovered opportunities out there! But we should also be wary of too fantastic claims and magic products. This is very much a new field for this industry, and every step should be considered with data to back it up.
That is not to say that this will not change in the future once more research has been conducted. It is an amazing field, and I personally cannot wait for more data!
A heartfelt thank you to all researchers who have spent years monitoring and analyzing data. You are true heroes!
Please, don’t hesitate to contact us if you have any questions or comments!
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