Thursday, January 1, 2009

Lawns and Turf - Helpful or Hurtful to Global Climate Change

It is perhaps appropriate that my thoughts turn to climate change and the amount of carbon that I personally emit into the atmosphere, as I drive around the state for Master Gardener and other trainings. From January to March, I am scheduled to deliver 17 talks, that will require that I drive approximately 2612 round trip miles. Together, these trips will emit approximately 2023 pounds of carbon dioxide if I take my own vehicle (a 2006 Ford Escape, that gets 25 miles per gallon) and 1011 pounds of carbon dioxide if I rent a Hybrid sedan from OSU Motor Pool (50 miles per gallon). Because I am often asked to talk about integrated pest management, alternatives to pesticide or some other facet of sustainable gardening, I often feel hypocritical for driving around the state to teach and promote sustainable choices. But, I enjoy meeting the many Master Gardeners from across the state, learning more about Master Gardener Programs in individual counties, and always enjoy the scenary.

Given that my thoughts have turned to how much carbon I emit when I drive to teach, I thought I would blog about how much carbon we can sequester or emit via our lawn care practices.

In 2002, an article in Agronomy Journal (94:930-935) by Qian and Follett attracted much attention from scientists and industry professsionals, alike. Briefly, Qian and Follett measured the soil organic matter (abbreviated, hereafter, as SOM) in 15 golf courses that ranged in age from 1.5 to 45 years old (as of the year 2000). Because carbon is a key component of SOM (approximately 57% of soil organic matter is carbon, by weight), scientists have long used measures of SOM as a proxy for the amount of carbon sequestered by soils. Qian and Follett found that SOM increased with the age of the golf course, and that SOM continued to increase until about 31 years in fairways and 45 years in putting greens. Furthermore, Qian and Follet found that previous land use influenced total SOM. Agricultural fields that were converted into golf courses had lower SOM than golf courses that were converted from native grasslands. This last finding can potentially be explained by another study, conducted by Khan and colleagues (see below).

However, a paper presented at a recent meeting of the American Geophysical Union questions the overall utility of turf to moderate the emission of greenhouse gasses. Amy Townsend Small measured SOM in 4 parks in Irvine, California. The parks were established between 4 and 34 years ago. While Townsend Small found that older parks had more SOM, she also found that older parks emitted more nitrous oxide, which is a potent greenhouse gas. Although the precise cause of this relationship is not known, Townsend Small hypothesizes that a build up of nitrogen in the soils, as a result of repeated fertilizer applications, may contribute to increased nitrous oxide emissions.

In addition, a recent paper published in the Journal of Environmental Quality by Khan and colleagues reports that long term application of nitrogen fertilizers to corn fields decreases SOM. Khan and colleagues suggest that excessive nitrogen applications increases the decomposition of dead organic matter and SOM by soil bacteria. This is perhaps why Qian and Follet found that golf courses converted from agricultural fields had less SOM than golf courses convereted from native grasslands - the native grasslands were not subject to repeated fertilizer applications.

Taken together, what does all of this mean for you, the gardener? Here are the take home points:

1) Plants in general, including turf grasses in lawns, sequester carbon and may help mitigate carbon emissions and global climate change.

2) The benefits we all may gain, from increased SOM and carbon sequestration, can be nullified if we use fertilizers excessively. Excess fertilizer may volatize into nitrous oxide (a greenhouse gas) or may accelerate the decomposition of organic matter in the soil. Neither of these is good for helping to slow global climate change.

3) Khan and colleagues suggest that farmers should get a soil test before applying fertilizers to their fields. Doing so will lessen the likelihood that fertilizers are applied excessively. I suggest the same for you and I, dear gardener. Having your soil tested prior to applying fertilizers will help to ensure that we apply only what is needed, and only when soil nutrients are truly depleted.