Can Home Lawns Store Carbon?

To determine the net carbon capture benefit and/or cost associated with lawns, a basic model was developed to identify the potential for carbon storage, or sequestration, in home lawns within the United States. The model compared carbon accumulated by turfgrass to the energy associated with homeowner lawn maintenance practices or carbon costs. The formula to calculate total carbon sequestration was as follows: Carbon Accumulation – Carbon costs = Total Carbon Stored.

• Carbon Accumulation
• Lawn Management Inputs
• Carbon Costs
• Carbon Stored
• Conclusions

Carbon Accumulation

All green plants, including grasses, use carbon dioxide during photosynthesis. In addition to capturing carbon from the atmosphere, grasses are very efficient at storing carbon dioxide in the soil. Since a lawn is a permanent ground cover with an extensive root system that is continually breaking down and regenerating, lawns are able to accumulate carbon which makes up soil organic matter and this is essential in the development of a healthy soil structure. It improves numerous soil processes and properties including plant available water and nutrient holding capacities, runoff and erosion reduction, and filtering of pollutants.

A wide range of carbon accumulation rates from peer reviewed scientific literature were employed as model inputs. These rates covered grasses found across all regions of the U.S and included both warm and cool season grass species.

Management Inputs

A review of homeowner lawn maintenance practices and consumer product use patterns supplied by The Scotts Miracle-Gro Company were the basis for the development of management scenarios in this model. The following statistics are national, with minor variations across individual states:

Of the estimated 80 million home lawns in the US:

• Lawns are mown an average of once a week during the active growing season.
• Approximately 50%, or 40 million homeowners fertilize the lawn.
• 30 million homeowners fertilize 1 – 2 times per year (1 – 2 lbs N/1000 ft2)
• 10 million follow university best management practices or hire a lawn service

Lawns were divided into 3 categories to mimic a low to high management scenario summarized in Table 1. The categories included Minimal input, Do-it-Yourself (DIY), and University BMP (Best Management Practices for high activity turf). Minimal Input is limited to only mowing, with no fertilization, pesticide, or irrigation practices performed on the lawn. The DIY category was based on homeowners average lawn maintenance practices (1-2 feedings per year, minimal supplemental irrigation) previously described. The majority of home lawns are maintained under the low input scenario. The University BMP recommendations were used as a high management scenario. Mowing, irrigation, and pesticide use data was compiled from available literature.

Table 1. Summary of parameters, data, and assumptions used in the model development

Carbon Costs

Every lawn management practice uses energy that can be converted to a carbon cost. The carbon costs for turfgrass operations are not well documented, so farm operation energy conversions were used (Lal, 2004)¹. In terms of gas to operate mowers, electricity to run irrigation, and fertilizer and pesticide production and transportation, these conversions should be similar to maintenance practices carried out in a home lawn situation (Table 2). The highest carbon cost associated with lawn management practices was irrigation due to the energy required to pump water. However, only 10 – 20% of home lawns use supplemental irrigation. This makes sense when you look at a precipitation map of the U.S. Over half of the U.S. receives enough rainfall in a given year to support grass growth. The other half may require supplemental irrigation depending on regional climate conditions.

Table 2. Carbon Equivalents for Management Practices

Carbon Stored

Any management activity that increases plant growth can increase carbon storage; therefore, maintaining a healthy lawn can significantly influence carbon storage. Mowing high, returning clippings, feeding, and conservative watering can actually increase the ability of a lawn to store carbon.

Overall, a healthy lawn can sequester as much as 300 lbs C/yr or 1500 lbs C/acre/yr. This is more than carbon stored by conventional agricultural and is comparable to the carbon stored in prairie land from the conservation reserve program and some of the natural forested areas across the U.S. If you compare that to car fuel emissions, one average size lawn can capture enough carbon to offset driving a standard-sized car 3,000 miles per year. It may not sound like much, but when you look at the estimated 40 million acres of turfgrass in the U.S., every little bit of grass can make a difference.

Golf courses have been documented to store 892 lbs C/acre/yr in Colorado and farm land converted to golf courses 2,230 – 3,211 lbs C/acre/yr in Ohio. The high rate in Ohio is most likely due to the grass supplying a permanent ground cover to the previous tilled farm soil, as well as fertilizer and irrigation management of the course.

Research has been done to compare fertilized fine fescue (irrigated and non-irrigated), Kentucky bluegrass (irrigated), and creeping bentgrass (irrigated) for differences in carbon accumulation rates. Irrigated fine fescue sequestered the most carbon at 2,989 lbs C/acre/yr while the sequestered carbon from non irrigated fine fescue, Kentucky bluegrass, and creeping bentgrass were 1,240, 1,829, and 1,543 lbs C/acre/yr. All turfgrass species were found to exhibit significant amounts of carbon accumulation over the 4 year researching period.

With all of this said, carbon storage does not accumulate indefinitely. Soils have a saturation point, but soils can take hundreds of years to reach this point in a turfgrass system. Saturation first occurs in the topsoil, and then gradually accumulates into the layers below.

Conclusion

So what does all of this mean for the turf industry and the reduction of greenhouse gases? Grass is a valuable resource and lawns can play a role in removing carbon from the atmosphere and storing it for extended periods of time. Efficient fertility and management practices optimize the net carbon benefit.

Turf is a good and valued resource for sequestering carbon. There was a net positive in carbon storing under all lawn management scenarios investigated, from low maintenance lawns with minimal activity to highly managed lawns used in high activity areas.

Carbon storage can be maximized with maintenance of healthy turf with a dense roots system. Annual feeding (approx 2 lbs N/M), mulching grass clippings and minimizing supplemental irrigation help to maximize turf as a carbon sink.

This means that turf is not a carbon intensive landscape nor is it a major source of carbon. In fact, managing grass with the appropriate amount of fertilizer and irrigation can actually increase the amount of carbon stored. Since the concepts of carbon credits and carbon trading are not being regulated in the U.S., carbon sequestration is an additional benefit of turfgrass that we can continue to promote.

¹Lal, R. 2004. Carbon emissions from farm operations. Environmental International 30:981 - 990

This article was also published in the Ohio Turfgrass Foundation Magazine Turf News in the July/August 2011 edition.