Monday, February 26, 2018

Will Carbon Sequestration Redeem the Lawn?

Landscaping-wise, few things offend my aesthetic and ecological sensibilities quite so much as grandiose sward of tightly mown, heavily chemically-treated grass punctuated by one or two trees and a couple of bushes. (Leaving aside, of course, the “mulch garden” topped off with a few lonely perennials.) Recently, though, I’ve learned to see some good in the approximately 40 million acres of lawn that engulf the residential landscape in the US. Caveats remain, serious ones, which I’ll get to in a bit; but the truth is, your lawn, my lawn, that of the business down the street or the corporate campus in a nearby suburb, serves as a carbon sink of modestly robust proportions. 

This is important, because these days any plan for keeping the average global temperature from going up more than 3.6 degrees (2 degrees C) now involves active carbon sequestration. We’ve dilly-dallied so long that while heading off extreme global warming and all the misery it will entail could once have involved fairly modest efforts, now much more is required, and faster. So much CO2 and other greenhouse gases permeate the atmosphere that cutting emissions to zero will no longer suffice. We must go carbon negative; but there still exists no feasible technological way to do this at any kind of scale. Yet carbon sequestration is what natural ecosystems do all the time; in general, the healthier the ecosystem, and the more full of life its soil, the more carbon it sequesters.  Hence, if we expect to be successful, it will be by working with natural systems, through reforestation, grassland restoration, natural area conservation and expansion, and changing agriculture practices to enhance soil health and thus carbon sequestration.  

Here is where lawns come in, and where anyone with a lawn can help, within certain parameters. In the US, lawns, including golf courses and playing fields, are considered part of croplands, 5% of the total, to be exact. On average, lawns sequester about 0.5 to 1.5 Mg carbon, per hectare per year (a hectare equals 2.2 acres) as opposed to an average cropland rate of 0.3 MG C per hectare per year. A standard comparison by defenders of turfgrass is that soil carbon sequestration rates compare favorably with natural grasslands, especially if the lawn is looked after through mowing, watering, and adding inputs of various kinds. Inputs vary widely and could be anything from simply leaving clippings on the lawn after mowing and other organic measures to the full panoply of fossil fuel derived fertilizers and pesticides on offer, and what they are is rarely specified in the literature. 

According to one study, even after subtracting the hidden carbon costs involved in using power lawn mowers, other power tools and fossil-fuel-based chemical inputs, lawns still sequester carbon. Another study comparing the carbon sequestration capabilities of lawns in Colorado with natural shortgrass prairies found that lawns actually beat the prairies in their carbon storage amounts. But as the authors point out, this is with careful tending and involves using quite a bit of water. Lawns would not survive in that arid region without human intervention, whereas the prairie has been self-sufficient and sequestering carbon for thousands of years. Furthermore, turfgrass has an average root-depth of only four inches or so, leading me to wonder about lawns’ capabilities for deep carbon storage of the kind at which deep-rooted prairies excel, with their diverse and prolific soil communities. And in healthy woodlands, though much of the carbon is locked up in the trees, vast underground networks of fungi and other denizens of the living soil enable carbon to translocate deep in the ground. 

When I first heard of lawns’ carbon sequestration potential, it seemed hard to believe, since a classic high-input lawn is a sort of biological null, at least as far as general biodiversity is concerned, but this actually makes sense. Grass is a perennial, one that “armors” the soil. The natural death and regeneration of its roots add biomass, as does leaving the clippings on after mowing, which enables the soil biome to thrive and carbon to be stored, at least within the top fifteen inches. Planted in a geographically favorable location, a lawn can be maintained with little effort and, once planted and established, there’s little to no soil disturbance. So a lawn, once planted and reasonably cared for will immediately start sequestering carbon, and keep doing so for at least twenty-five to thirty years, until a saturation point is reached. 

Unlike some folks I know, I’ve never been absolutely against an appropriately sized and placed lawn, though intensely against the use of synthetic fertilizer and pesticides. Surrounded as it is by large beds of native plants, my own backyard lawn is a small part of the garden, not the extent of the landscaping—a crucial difference. Though it grows well and stays green without irrigation or synthetic inputs, my lawn would fail in the eyes of the pristine lawn brigade, since within its boundaries grow a medley of white clover, early spring bulbs, violets, a few dandelions and a little plantain and creeping Charlie. Thus it earns its ecological keep. Those other species of plants both increase the lawn’s carbon storage potential and help increase ecosystem interactions. The clover’s deep roots fix nitrogen and help the soil retain moisture during dry periods. When in bloom, the bulbs (snowdrops and scilla), violets, clover and even dandelions and creeping Charlie benefit pollinators. The whole expanse thrives with only organic inputs: clippings left on after mowing, overseeding with grass and clover seed every few springs, and top-dressing with home made compost spring and fall (if I get to it). The black, carbon-rich soil soil is eminently healthy, and earthworms abound. There’s no nitrogen or phosphorus run off, little to no methane or nitrous oxide emissions, and no hidden carbon costs either: we use a push lawn mower and hand rakes.

All of this is easy because the Chicago area where I live is one of the few areas of the US that qualify as a turfgrass sweet spot. It’s one of the places temperate enough, with enough natural rainfall and good enough soil conditions that grass can grow happily, all by itself, with little to no help from humans. In much of the country, however, irrigation and intensive inputs, synthetic or organic, are required to keep lawns lush, at a steep environmental cost exacerbated by mono-culture-only aesthetic requirements. These costs are so high that in many cases the soil carbon sequestration benefits lawns offer do not, overall, balance the scales in their favor. If a lawn can’t survive without a serious care regime, especially extra watering, perhaps its best not to have a lawn, per se, regardless of how much carbon it may sequester. 
Credit: Melisi et al

Because of these high costs, a movement way from lawns is slowly taking place. In arid regions across the western half of the continent where lawn irrigation can easily take 50-75% of a household’s water usage, over the past few years communities have been restricting outdoor water use.  Graywater systems are on the rise. Residents are encouraged to put in ecosystem-appropriate xeriscaping as a substitute for grass, which can thrive without stressing town water supplies. Fertilizers and pesticides are being looked at, too, since they continue to contribute to a variety of ecological ills such as failing soil health, release of nitrous oxide, coastal dead zones and toxic algae blooms. Golf courses, schools and park districts are starting to reassess the use of synthetic inputs and some are switching to organic methods. Yet homeowners are proving slow to follow suit, and the ideal of a green front lawn, even where it geographically doesn’t belong, is proving surprisingly resistant to revision.

It’s interesting to imagine a US where manicured turfgrass lawns ceased being a universal requirement in landscaping, supposedly applicable to all regions, everywhere (though other species of grass, such as St. Augustine, might be used in the south). What other kinds of carbon-sequestering plants could be used in the area in front of the typical suburban house? There are native groundcovers and sedges that thrive in Texas, and shortgrass prairie flowers and grasses such as buffalo grass flourish across the plains. What kinds of regionally appropriate landscaping would arise? Could thirsty Colorado lawns be replaced with native prairie species? 

Even in a grass-friendly region, grass is not appropriate in every situation. My small front yard became too shady for a lawn, thanks to a pair of maples in the parkway. It now sports a small, spring blooming serviceberry tree surrounded by woodland sedges interspersed with low growing spring wildflowers. It is very much a work in progress and will take a few years to fill in. Yet I feel confident that carbon sequestration is going on in and will continue to do, no inputs required. Let golf course owners and playing field managers wrestle with the correct balance of water and inputs that would achieve carbon sequestration while keeping their fields green.  At least there the grass is being used for something besides viewing purposes only. In general, in most parts of the country, a landscaped area of low maintenance, eco-system friendly plants (preferably species native to the area) remains a better choice than a lawn. 


Jason said...

I'm a little surprised to read this about carbon sequestration by lawns. I wonder how they compare to tall grass prairie grasses and forbs that grow in our garden. I still have some lawn, full of "weeds" like yours. I would only add that there is no such thing as a "little" creeping charlie.

Adrian Ayres Fisher said...

Hi Jason,
Thanks for your comment. I agree about the surprise factor. There is a lot of work that needs to be done on this topic, especially deep carbon sequestration. Re creeping Charlie, yes it can get rampant, particularly in years with mild winters. In my own yard, I don't have much of a problem with it in the sun; damp shade is another issue entirely--one reason I decided to stop trying to grow grass in the shady front. For some reason it seems easier to control in the areas thickly planted with native plants.

Larry Vickerman said...

Adrian: This article and "What does synthetic fertilizer have to do with it?" hit the proverbial nail on the head. After years of research and experimentation, I can tell you there is little hope for lawn carbon sequestration until we put the synthetic fertilizer away, diversify the typical lawn in terms of plants, and inoculate the soil typically with good compost to restart the incredible soil microbe and plant (root) interaction required. After hearing Dr. Christine Jones speak and studying the Morrow Plot data of University of Illinois, it is obvious we started down the wrong road 50 years ago and it is hard for us to turn back. Thanks for the great information!

Adrian Ayres Fisher said...

Hi Larry, Thanks. Yes, and if we could only add managed grazing to suburbia! :)

It would be interesting to find out from suburban chicken owners how their birds impact their lawns. I've got a friend who does a three-year rotation in his (extensive) vegetable garden that incorporates his laying hens foraging in the fallow section. I'll have to talk to him more about this.