From Dirt to Gold! Green Roof Soil Biodiversity

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Soil biodiversity - a forgotten player for green roof performance

Currently, little is known of green roof soil biodiversity. Up until now, the soil compartment has generally been viewed through the eyes of engineers and not through the eyes of biologists. We think both sets of eyes are necessary for the development of a sustainable roof with a clear green roof ROI.

If green roof biodiversity is ever discussed, only plants and insects are often mentioned. This despite the multitude of associations plants have with the inhabitants of the soil medium.

Let’s expand our view of green roof biodiversity!
Enter soil biodiversity!

Green roof biodiversity vs. natural soils

Natural soils are teeming with life! A multitude of organisms such as bacteria, protozoa, nematodes, worms, fungi, can be found.

High diversity is often a sign that the system is stable and healthy.

A stable system is resistant to stresses. This resilience means that it is less likely for one organism to take over and uncontrollably multiply. Such uncontrolled growth can lead to disease, such as when pathogenic fungi cause root rot.

A stable, diverse system is less likely to crash. This translates to money saved for green roof customers.

Nutrient release and green roof biodiversity

The release of nitrogen through the biochemical breakdown of the organic patches in the soil is mainly driven by the microbial community (bacteria and fungi), but the transport of nitrogen away from these nitrogen hotspots is attributed to the movement of microbial-feeding fauna such as nematodes and protozoa.

Do you begin to see how diversity is important? Each and every organism plays a role. If one disappears, the chain breaks and the system becomes wobbly.

Let’s take a look at some of the main groups of organisms that we find on a green roof, what they do and why they are important!

Mycorrhizal fungi

A fascinating microbial group is the mycorrhizal fungi. These fungi form symbiotic relationships with plants. They swap minerals and nutrients for plant-derived sugars.

Mycorrhizal fungi have been found on green roofs and seem to play an essential role in establishing resilient roofs. These fungi, mainly so-called arbuscular mycorrhizal fungi (AM), have been shown to make the associated plants more drought tolerant. The extensive network of fungal hyphae crisscrosses through the soil and tap into water sources the plants may not reach. The same is true for nutrients.

We know that these associations exist, but we know very little about how they are formed and what happens to these associations after the application of broad-spectrum pesticides. Could uncareful broad-spectrum pesticide applications disrupt these symbioses and so reducing roof resilience making it more sensitive to further infections?

We don’t know yet, but it might be good to investigate if we want to provide the best possible maintenance to our customers.

Nitrogen-fixing bacteria

In natural soils, there are small free-living bacterial that can pull nitrogen from the air and turn it into nitrogen-species that can be used by other organisms. This is no small feat as molecular nitrogen, found in air, is remarkably inert.

Smashing that N2-triple bond into smithereens requires an enormous amount of energy or, alternatively, some specialized proteins (enzymes) that these small organisms harbor. This makes these small free-living bacteria very special and able to significantly affect the microbial communities around them. They basically are able to supply themselves with fertilizer pulled from the air!

Sometimes, these bacteria infect plants and form symbioses with them. At least a few of these plant species are used on intensive green roofs, and the symbiotic relationships with the nitrogen-fixing bacteria may result in better plant growth, nutrient, and water uptake.

Of course, it would be neat if we could pull this off on extensive roofs as well. However, as of today, no nitrogen-fixing bacteria are known colonializing the plant types used on extensive green roofs.

Nonetheless, there have been some suggestions to inoculate green roof media with free-living nitrogen-fixing bacteria to reap the benefits of these organisms. Albeit, the data so far has been very inconclusive.

Other interesting fungi and bacteria

There are many different types of fungi and bacteria present in natural soils. The diversity is plainly mind-boggling. This is why we biologists separate these organisms into major groups that describe their action, or behavior, rather their family relationships.

One critical group is decomposers/saprotrophs. These small players can be fungi or bacteria, and they decompose soil organic materials.

When they decompose and respire, they give off a lot of CO₂ (carbon dioxide), so they are intricately linked to the carbon budget of the roof. But, we don’t want the decomposition rates to be too high on the roof. High respiration rates will result in volumetric soil losses.

High temperatures and moist conditions coupled with high nutrient concentrations and organic material mean optimal conditions for the decomposers. Therefore, it is essential to consider these little buggers when designing soils for different climates or your roof might turn into a shrinking down hot compost heap instead of a well-functioning green roof.

Nematodes and amoebas

High nematode abundance may result in significant micro-pore formation of the soil which can aid root development, as well as potentially enhancing soil water retention.

Nematodes and amoeba require a water film around them to survive, which could be an issue on a highly drained green roof.

Both protozoa and nematodes are generally able to enter “resting stages” in which they become extremely resistant to extreme environmental conditions, which would enable them to survive in the extreme environment that is a green roof.

It has been shown that amoebas often increase 5 to 10-fold in numbers after rain events after bacterial growth has been stimulated by the precipitation. Hence, these organisms could have a profound effect in evading nutrient losses from the roof systems.

Currently, we know little about the abundance of nematodes or protozoa on green roofs, but it sure is an interesting topic to dig into!

Other Invertebrates

Yes, insects, spiders, collembolans and many more should be found on green roofs too. Why? Invertebrates are very good at stabilizing the soil as they ensure that plant litter is broken down into smaller pieces that can then be decomposed.

They ensure that the nutrient cycle keeps turning.

Often, invertebrate diversity is used as an indicator of soil quality in urban areas, which underlines the importance of these organisms in stabilizing the ecological system.

Final comment

Green roof biodiversity is important not only from a green perspective but also from a hydrological and financial perspective. If we want to produce roofs that are up to par with future environmental and stormwater regulations, we’d better take a good look at the soil biodiversity!

Soil biology minimizes nutrient runoff naturally, retains more water, and protects the plants from disease and nutrient shortages. Also, we should not forget the other positive aspects of improving the soils such as increased stormwater retention. Similarly to agricultural fields, increasing the humus fraction by a tiny 1% leads to significant increases in water holding capacity, which also lowers the need for irrigation.

We are missing a huge opportunity to retain a load of water for free!

Furthermore, we should ask us the question if the German-style FLL soils, that are used across the US green roof industry, really are a sufficient one-solution-fits-all due. Geographic regions have different climates and rain patterns affecting decomposition rates as well as experienced organism heat stress.

We believe that we have a lot to gain from developing soils that are optimized for climatic regions!

Let’s make millions upon millions of small organisms create a financially sustainable roof for us.
These organisms work 24/7, through all holidays, for free!

Don’t hesitate to contact our experts if you have any questions!

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Reading tip: green roof biodiversity

Rumble, Heather, and Alan C. Gange. "Soil microarthropod community dynamics in extensive green roofs." Ecological engineering 57 (2013): 197-204.