Green Roofs - Living Architecture Monitor Winter 2020 - 17

RESEARCH

KEEPING THE " N " IN GREEN ROOFS:
PRACTICAL RECOMMENDATIONS
FOR IMPROVING NITROGEN RETENTION
AND REDUCING FERTILIZER APPLICATIONS
BY MARK E. MITCHELL, PHD, GREATER OHIO LIVING ARCHITECTURE CENTER OF EXCELLENCE

Nitrogen is a critical nutrient for plant health in green roof systems, but if levels
of nitrogen are too high in a green roof, nitrogen, when it rains, may leak from the
roof and pollute downstream waterways.

A

dditionally, fertilizers containing nitrogen added to a roof
to maintain a healthy plant community can be environmentally and economically costly. To make green roofs
even more sustainable, we need to improve their ability to
capture and retain nitrogen through improvements in design
and management.
Nitrogen can enter a green roof via fertilizer inputs, dissolved
nitrogen in precipitation, uptake of nitrogen oxides in the surrounding air, and nitrogen-fixation. Nitrogen-fixation, a microbial process, is of particular interest because it converts nitrogen
in the air that is inaccessible to plants into a form that is accessible for plant growth and maintenance. Some nitrogen-fixing
bacteria form symbiotic relationships with plants and fungi. The
symbiotic relationship provides usable nitrogen directly to the
plant or fungi. In the case of lichens, fungi and bacterial partners
work together to build soil on bare rock surfaces. Bacteria-plant
partnerships have been widely exploited in agriculture (soybeans
for example) to enrich soils without nitrogen fertilizer additions.
There are a large number of plants that partner with
nitrogen-fixing microbes and are capable of inhabiting green
roofs, including mosses, clovers, and vetch. Lichens can also fix
nitrogen and survive on green roofs. Nostoc, a type of bacteria
that forms greenish-brown, jelly-like colonies on green roofs,

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is also capable of fixing large
amounts of nitrogen. By planting, inoculating, or simply
allowing these organisms
to colonize a green roof, we
may be able to enrich the soil
and plant community while
reducing fertilizer additions.
However, any nitrogen entering a green roof, either by
fertilizers or natural processes,
must be retained tightly in the
green roof system or risk being
lost to leaching over time.
Nitrogen can be retained on
a green roof in living plant material, in the soil as dead plant
material (litterfall), bound
to substrate particles (physicochemical immobilization),
or held within the cells of
microorganisms like bacteria,
archaea, and fungi (Figure 1).

Therefore, we may be able to
design and manage green roofs
to be less leaky with regards
to nitrogen by: 1) choosing
plants that are highly effective
at taking up large amounts
of nitrogen, or incorporating
mycorrhizal fungi that act
to expand a plant's fine root
network and nutrient uptake
capacity; 2) incorporating
materials in the substrate like
biochar that may immobilize
nitrogen; and 3) incorporating
chemicals called nitrification
inhibitors that reduce the conversion of a relatively sticky
form of nitrogen, ammonium,
to a readily leachable form,
nitrate.
Hopefully these practical
suggestions for designing and
managing green roofs are help-

LIVING ARCHITECTURE MONITOR / WINTER 2020 / 17


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Green Roofs - Living Architecture Monitor Winter 2020

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Green Roofs - Living Architecture Monitor Winter 2020 - Table of Contents
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