Enhancing Turfgrass Carbon Sequestration to Improve Sustainability and Market Access

Enhancing turfgrass carbon sequestration to improve sustainability and market access
Ruying “Wrennie” Wang1, Clint Mattox1, Claire Phillips2 and Alec Kowalewski1
1Department of Horticulture, Oregon State University
2United States Department of Agriculture

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Summary 

Oregon State University will evaluate the impacts of turfgrass maintenance practices (nitrogen fertilization, irrigation, mowing height, and mowing frequency) on turfgrass carbon balance and soil carbon accumulation. This project will investigate how to enhance accumulation of soil organic carbon in order to reduce the climate footprint of turfgrass, which can provide ways of addressing regulatory burden imposed by greenhouse gas reduction programs and improve market acceptance of natural turfgrass. Results of this research will be disseminated to turfgrass seed producers, turfgrass managers (golf course superintendents, commercial turf managers, school and park employees), and other users (homeowners and master gardener programs) through extension activities including field days, presentations, and written materials.  Preliminary fundings from this work determined that cool-season turfgrass uptakes CO2 at high rate during cold and cool weather compared to summer time, and fertilizing at 4 lbs N and mowing at 2 inches can increase CO2 assimilation rate in the winter/spring time.  Findings from this research have been presented at the state, regional and international level. 

Introduction 

Criticisms of the environmental impacts of lawns, including a high climate footprint, pose challenges to the market acceptance of turfgrass domestically and internationally. However, turfgrass is a perennial crop, and turf systems have been shown to accumulate soil organic carbon (SOC) at high rates, comparable to regenerating forests and fallowed cropland. The emissions associated with turfgrass are primarily from its maintenance, and include fuel emissions from mowing, fertilizer production, and water pumping, and N2O emissions following nitrogen fertilizer application. Nevertheless, home lawns in the U.S. have been estimated to provide a carbon sink for 66-199 years before the emissions from maintenance overcome the benefits of SOC sequestration. There are few assessments, however, of how to enhance turfgrass SOC accumulation in order to more efficiently offset maintenance emissions. The goal of this study is to evaluate how turfgrass maintenance can be modified to enhance SOC accumulation, and to characterize trade-offs between carbon sequestration, aesthetic characteristics, and maintenance intensity. 

OBJECTIVE 1: Determine the impact of high versus low levels of mowing, nitrogen fertilization, and irrigation on carbon sequestration of cool-season turfgrass.

OBJECTIVE 2: Evaluate trade-offs between carbon sequestration, weed populations, and maintenance intensity, including water use.

OBJECTIVE 3: Communicate research findings through extension activities.

Objective 1 Methodology:

We will study C-sequestration in a subset of treatments from the Oregon School IPM field trials. From the mowing trial, we will examine two mowing heights (2 and 4 inch) at two frequencies (weekly and monthly). From the fertilization trial we will compare 0 and 9.76 kg m-2 of nitrogen applied annually, and from the irrigation trial we will compare a non-irrigated control to 0.6 cm applied four times per week. Four replicates of these eight treatments will provide a total of 32 plots (each 5 × 10 ft). 

Carbon-sequestration will be determined during years 4 and 5 post-establishment using a biometric approach to estimate net annual ecosystem CO2 exchange (NEE). NEE is a measure of whether a plant + soil system is a net sink or source of CO2 at an annual time step. Over short timescales (<10 years), NEE provides a more sensitive approach for quantifying C-sequestration than measuring changes in SOC. However, based on the high rates of SOC accumulation that have been reported for turfgrass [0.14% yr-1 (Qian and Follett, 2002)], we expect to have sufficient analytical power to detect changes, and will therefore also measure changes in SOC stocks to 50 cm depth from an initial sample in Feb 2020 and in Feb 2023, providing a comparison from years 3-6 post-establishment.

The biometric approach to estimating NEE is based on the equivalence NEE = NPP – Rh, where NPP is the net primary productivity, or annual shoot and root growth, and Rh is heterotrophic soil respiration. We will measure these components following the methods described by Belilli-Marchesini et al. (2011). We will measure shoot NPP by collecting and weighing grass clippings each time the plots are mowed, and will also quantify the growth of shoot biomass below the mowing height by sampling 10 × 10 cm quadrats to ground level on a quarterly basis (Feb, May, July, Oct). All shoot material will be returned to the plots as mulch after the mass is determined. We will measure root NPP from quarterly measurements of root growth increment at 0-15 and 15-30 cm depths. Rh, which describes the CO2 emissions from soil microbes only, and excludes root respiration, will be estimated by creating a small root enclosure in each plot, by driving a 6-inch diameter PVC pipe into the ground to 30 cm depth. Respiration from these enclosures, as well total soil respiration adjacent to the exclosures, will be measured in twice-monthly campaigns using a soil flux chamber coupled to a portable CO2/CH4 gas sensor. A regression model relating Rh to soil temperature and moisture will be developed from sensors buried in each plot, and these regressions will be used to estimate Rh between measurement campaigns, and to calculate cumulative Rh on an annual basis. 

In summary, these data will quantify over two years the annual uptake of CO2 through plant growth, and the annual loss of CO2 through respiration. Additionally, twice-monthly measurements of CH4 fluxes, and quantification of gasoline and fertilizer consumption needed to maintain the plots, will be used to complete carbon budgets for each treatment.

Objective 2 Methodology: 

We will evaluate turf aesthetic quality, labor requirements and costs, and weed incidence in each treatment, to identify synergies and trade-offs with carbon sequestration.  

Visual turfgrass quality and color ratings will be assessed monthly using the National Turfgrass Evaluation Program scale of 1-9, with 6 or greater considered acceptable. Weed populations will be measured in the spring, summer, fall and winter using the point intercept method and a 91 × 122 cm quadrant with 36 intersections. At these intersections the presence or absence of turfgrass, broadleaf weed, or grassy weed will be documented.  

Objective 3 Methodology: 

Findings from this research will be presented at several outreach events that are regularly hosted or attended by the OSU Turfgrass program. These include the Oregon Golf Course Superintendents Annual Meeting, the Oregon Sports Turf Mangers Association Annual Meeting, the Northwest Turf Association Meetings, the Annual Oregon Seed Association Meeting, Master Gardener Training, School IPM Coordinator Annual Training, and “OSU Turf Field Day” held annually in September in Corvallis, OR. 

Findings (2021) – Objective 1 and 2 

As expected, the CO2 flux rate of turfgrass ecosystem varied by season. In general, higher CO2 assimilation rates were observed in the winter/spring for mowing, irrigation, and fertilization trials compared to rates measured in summer (Figure 1, 2 and 3). 

Mowing frequency had no significant effect on CO2, whereas mowing at 2 inches (5.1 cm) had higher assimilation, quantified using net ecosystem CO2 exchange, than mowing at 4 inches (10.2 cm), such effect only occurred in Jan and once in May (Figure 1).

Fertilization at an annual rate of 4 lbs per 1000 ft resulted in higher uptake of CO2 by turfgrass compared to 0 lb rate early in the year (Figure 2). Fertilization had no benefit in the summertime for fixing carbon. 

Surprisingly, irrigation generally did not affect the turfgrass ecosystem CO2 exchange (Figure 3).  Because irrigation promotes turfgrass photosynthesis but at same time increases system respiration, particularly the respiration by soil microbes, which cancels out the benefit of uptaking CO2 by photosynthesis. 

Extension (2021 and 2022) – Objective 3

Findings from this research have been presented at the virtual American Geophysical Union meeting from December 13-17, 2021 reaching 50 attendees, and in person at the Summer OSU Turf Field Day in Corvallis, OR on Aug 26, 2021 to 85 attendees, Northwest Turf Association Meeting on October 29, 2021 in Bandon, OR to 60 attendees, ASA-CSA-SSSA International Meeting on Nov 10 and 11, 2021 in Salt Lake City, UT reaching 210 attendees, and the OSU Turf Winter Field Day on March 3, 2022 in Corvallis, OR to 85 attendees. 

On Feb 23, 2021, the OSU Turf Program produce an extension video on “Enhancing Turfgrass Carbon Sequestration” https://www.youtube.com/watch?v=gYt6HFZmlCo  Currently this video has been viewed 227 times. 

Figure 1: Mowing frequency (A) and mowing height (B) effects on net ecosystem CO2 exchange (NEE), positive number = net CO2 sequestration, while negative number = net CO2  emission. Error bars indicate standard deviation; *, *** denote significant differences at p < 0.05 and 0.001, respectively. 

Figure 2: Fertilizer effects net ecosystem CO2 exchange (NEE), positive number = net CO2 sequestration, while negative number = net CO2  emission. Error bars indicate standard deviation; *,** denote significant differences at p < 0.05 and 0.01, respectively. 

Figure 3: (A) Irrigation effects on net ecosystem CO2 exchange (NEE), positive number = net CO2 sequestration, while negative number = net CO2  emission. Error bars indicate standard deviation; ** denote significant differences at p < 0.01. (B) Precipitation in Corvallis, OR.