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Saturday, July 9, 2016

Is Lawn a Carbon Sink?

Many people and organizations these days are looking for ways to reduce excess carbon dioxide in the atmosphere, in the hope that this might slow the warming of the planet. Even the turf industry claims that “well-managed” lawn absorbs carbon and thus should be considered a good thing.

“Lawn is a valued resource for sequestering carbon, with a net positive carbon storage for all lawn management practices.” That’s what they say at Scotts Miracle Gro, which calls itself the world’s largest marketer of branded consumer lawn and garden products.

Both the Lawn Institute and the Outdoor Power Equipment Institute (who knew there was such a thing?) echo this opinion. Ditto for countless lawn product wholesalers and retailers across this country who are eager to proclaim this newly-discovered virtue of the American lawn.
Yet… is it true? This claim appears to be based on a single research report: “Technical Assessment of the Carbon Sequestration Potential of Managed Turfgrass in the United States.” This report was prepared by Dr. Ranajit Sahu in 2008, at the request of the Outdoor Power Equipment Institute. (hmm...)

Before going further, we need to be clear about some basic concepts:
-  As the EPA defines it: “Terrestrial carbon sequestration is the process through which carbon dioxide from the atmosphere is absorbed by trees, plants and crops through photosynthesis, and stored as carbon compounds in biomass (tree trunks, branches, foliage and roots) and as organic matter in soils.”
-  A carbon sink is any system that stores more carbon molecules than it releases.
-  A carbon source is any system that releases more carbon molecules than it stores.

Let’s assume that Dr. Sahu’s data are accurate and that in fact “well-managed lawns capture four times more carbon from the air than is produced by today’s typical lawnmower.” According to Sahu and the turf industry, a well-managed lawn is one that gets mowed regularly, treated with chemicals and watered as needed to keep it growing vigorously. So, let’s take a closer look at this whole system.

How much carbon is actually stored in lawn?

Sahu’s report combines data from several studies to produce this averaged result (stated on page 12, if you’re looking for it): one acre of managed turfgrass will hold about 1.03 Mg/ha/yr. In regular language, this is 0.46 tons, or 920 lbs. of carbon in a year.

How much carbon do mowers emit?

This is the only kind of emission that Sahu includes in his calculations. Citing a “Major Equipment Manufacturer” and the World Resources Institute, Sahu concludes that the average amount of carbon emitted from a mower is 0.08 tons, or 160 lbs. per acre per year. So… 920 lbs. sequestered, minus 160 lbs. emitted by mowing, equals 760 lbs. of carbon storage. And voila…a carbon sink! Could this be the end of the story? No, it is not.

What about lawn chemicals?

We all know that some amount of CO2 is emitted in the manufacture and application of lawn chemicals. How much?  Sahu admits that his report does not “evaluate indirect carbon impacts such as due to creation and transportation of the fertilizer necessary for the management of turf.”

Apparently, in his mind, this is a minor point. He does footnote, though, a  study by NASA’s Christine Milesi, which says carbon emissions due to fertilizer inputs range from 11-28 percent of the total sequestration potential. If we use the mid-point of that reported range, say 20 percent, we can estimate that another 184 lbs. of carbon should be subtracted from the total. This is more than the carbon emitted from mowing! And it brings our theoretical carbon sink down to 576 lbs.

And watering?

Well-managed lawns must also be watered. Why? Because fast-growing lawns need to be mowed often, and the resulting grass clippings, when left on the surface to decompose, will increase soil carbon. (Another Milesi study shows that without these grass clippings, carbon sequestration is dramatically lower. Yet consider: how widespread is the belief that removing the clippings is actually best for the lawn.)

Anyway, irony aside… how does irrigation factor into the equation? The carbon emissions from obtaining clean water  are tricky to figure out. Luckily for us, planners in Portland, Oregon have conducted a detailed audit of that city’s water budget, and they conclude that their production of potable water emits 0.01 lb. of CO2 per gallon of water. Portland’s water supply is partly gravity-fed, so communities that rely entirely on pumped water would likely emit more CO2 than Portland. To be on the safe side, though, let’s use this conservative figure of 0.01 in our calculations.

Hang in here with me now, while we work our way through a few more figures. We are commonly advised to give lawns one inch of water a week. On an acre of lawn, this equals 27,154 gallons of water. Let’s say we irrigate our theoretical acre only six times a year (an arbitrary number, on the low side between “never” for some lawns and 15-20 times a year for others). This rather infrequent watering consumes almost 163,000 gallons per year.

Multiplying this amount of water by 0.01 (from the Portland audit), the CO2 emissions amount to 1,630 lbs. per acre. Because 1 lb. of CO2 equals 0.28 lbs. of carbon, this is equivalent to 456 lbs. of carbon molecules emitted due to occasional irrigation.

We are now down to a theoretical carbon sink of 120 lbs, in a whole acre of lawn.

To put this in perspective…

An acre of established temperate forest can hold from 2,000 lbs. up to  6,000 or more lbs. of carbon per year, depending on the age of the trees and other conditions. Mature grasslands sequester 2,400-3,600 lbs. per acre each year. And, except through natural disturbances (decay, fire, etc), these ecosystems emit only small amounts of CO2, when they respire at night.

Surely we can do better than a meager hundred-pound carbon sink in our managed landscapes. And this is not even the whole story. Sahu’s calculations, and the turf industry’s proclamations, also fail to account for:
- CO2 emitted by the manufacture and distribution of relentlessly marketed mowers, blowers, trimmers, edgers, shredders, clippers, sprinklers and sprayers.
- The carbon emissions of lawn care companies lugging their equipment all over town, day after day throughout the growing season.
- The emission of nitrous oxide – a greenhouse gas that is 300 times more heat-trapping than carbon dioxide – which results from using nitrogen fertilizers, an essential in “properly-managed” lawn.

So: is lawn a carbon sink?

Clearly, with all these factors in mind, there is no way that conventional lawn – lawn treated with pesticides, herbicides and fertilizers, lawn that gets frequently watered and mowed – can be considered a carbon sink. We are a society that excels at failing to grasp the big picture. The only way to believe Scotts’ claim that lawn is “a good foot soldier in the fight against climate change,” is if we ignore the entire carbon-spewing lawn-care process that the turf industry so avidly promotes.

On the bright side, we do have options for creating open spaces that actually are great carbon sinks: wildflower meadows; pocket prairies; clover lawns; “low-mow” grass mixes; lawns made up of native grasses, mosses and sedges; lawns full of violets, thyme, yarrow and dandelions; grass maintained organically and chemical-free; grass mowed with sheep; grass allowed to go dormant during dry periods. We can still have lawns…just not the kind that the turf industry has worked so long and hard to convince us to want. Now they’re trying to tell us that their style of lawn is good for the environment. Don’t believe it.

The alternatives to lawn are beautifully infinite… … and infinitely beautiful.

 © Sue Reed