CO-Horts

Your Lawn...and its Global Warming Potential

Posted by Tony Koski, Extension turf specialist

There’s nothing like a -15 F night to get a person thinking about global warming, right? Well, that, and the fact that it’s my turn to write for our blog … and there’s not much else exciting to write about in the turf world in early January. I recently read on one of our favorite blogs, the Garden Professors, about a cool experiment that one of the Garden Professors (Jeff Gillman) conducted to demonstrate the potential effect of increasing CO2 levels on plant growth – in which he used perennial ryegrass, a commonly used turf species. When grown in a high CO₂ atmosphere, the ryegrass grew measurably faster than under ambient conditions. Fun, interesting, and not unexpected. Also not unexpected were some reader comments suggesting that growing and mowing the faster-growing turf might lead to increased levels of atmospheric CO₂ - and possibly contribute to global warming. 

You can find hundreds of anti-turf rants on the internet (no proof...but who needs that?) which state that, of course, bluegrass lawns and their maintenance MUST contribute to global warming and will cause the end of civilization as we know it - along with the hundreds of other sources of greenhouse gases, including automobiles, bottled water, burping and farting cows, rice farming...and even owning a dog or cat. But I digress...

Here at Colorado State University, my colleague, Dr. Yaling Qian, and her graduate students have conducted research for years on the carbon sequestration (fancy way of saying “removing carbon dioxide from the atmosphere via photosynthesis and storing it in plant parts that are slow to decompose…like roots, thatch and soil organic matter”) potential of lawns and golf courses. 

In a turf system, carbon is sequestered in
its roots, thatch, and soil organic matter

For years we have known that turfgrass systems, whether they be golf courses or home lawns, can store relatively large (compared to agricultural systems, anyway) amounts of carbon. However, the “carbon cost” of maintaining  those turf systems (referred to as “hidden carbon costs” in carbon world lingo) has been less well-understood – thus begging the question: Does the maintenance of a lawn or golf course emit more carbon dioxide (and other greenhouse gases, like nitrous oxide) into the atmosphere than is sequestered in the soil by those turf systems? In a carbon neutral turf system, the carbon “costs” of maintenance are equally offset by the amount of carbon sequestered by the turf. In a carbon negative turf system, it would store more carbon than is released as a result of its maintenance…and the opposite for a carbon positive system. It’s a bit confusing but, when it comes to carbon sequestration, negative is good and positive is bad. So…..are lawns (or golf courses…or parks…or…any turf system) carbon negative, neutral, or positive?

As a lawn matures, it needs less nitrogen to remain
healthy because N is stored in (and released from)
its soil organic matter.

In a recent journal article published by Dr. Qian, she clearly proves that a properly-maintained Kentucky bluegrass home lawn (moderate levels of nitrogen, water and mowing) will be largely carbon negative (remember, that’s good!) for the first 10-20 years of its existence, carbon neutral or slightly carbon negative during (approximately) years 20-30, and carbon neutral to slightly carbon positive after 30 years. As any turf system matures, its rate of carbon storage begins to slow – and then levels off when the turf is around 30-40 years of age. Their research shows that these mature turf systems require approximately half (or less) the nitrogen needed during establishment and early years. By reducing N fertilization rates on older lawns (from 4 lbs. N per 1000 square feet per year down to 1 or 2 pounds annually), nitrous oxide (a powerful greenhouse gas) emissions can be reduced by as much as 50% - bringing those very old turf systems closer the goal of being carbon neutral or negative. 

So, YES, urban lawns can act as carbon “sinks”, thus being beneficial in terms of reducing greenhouse gas emissions – even when the hidden carbon costs of mowing, fertilization and irrigation are factored in – if the lawn systems are maintained using best management practices. Yes….lawns are good for the environment…and we have research to prove it!

The "hidden carbon costs" of tree production should

be considered in discussions of the potential for trees
to sequester carbon in our urban landscapes.

Another comment on the Garden Professors blog suggested to “… get rid of the lawn and plant trees.” as perhaps an alternative (to lawns) approach to carbon sequestration. But, as my co-blogger Alison O’Connor recently wrote, merely planting a tree and assuming it is instantly providing an environmental benefit in terms of carbon sequestration is a faulty assumption – because you must consider the very real hidden carbon costs of growing and maintaining that tree – as well as, eventually, removing it.

I guess the bigger point to consider here is this: all of our landscape plants, by virtue of photosynthesis, remove some carbon from the atmosphere. Whether or not they are individually carbon negative or positive is determined not by just the plant themselves, but what it “costs” in terms of carbon to produce, plant, and maintain that plant. And when it dies, it will eventually decompose and return some of that stored carbon back to the atmosphere. For those of us in the green industry, whether teachers or practitioners, the use of best management practices will make for healthier plants, probably save (someone) money, and perhaps even keep some greenhouse gases in a form which many people find more acceptable – non-gaseous.