What effects do mowing, fertilization and irrigation have on carbon sink and sequestration in turfgrass ecosystems?

Public concerns about climate change and a growing number of government and business commitments to emissions reductions have increased focus on carbon sequestration in vegetated landscapes as a nature-based climate solution. More specifically, landscapes dominated by plants could be considered a carbon sink.

A carbon sink is anything, natural or otherwise, that accumulates and stores some carbon-containing chemical compound for an indefinite period and thereby removes carbon dioxide (CO2) from the atmosphere. Turfgrasses are broadly used for sports (golf, football, soccer, baseball, tennis, etc.), residential and commercial areas (home lawns and commercial real estate), and public municipalities (parks, schools, and roadsides), all of which have the potential to mitigate urban carbon dioxide (CO2) emissions.

The overarching objective of this project is to determine the impact of high versus low levels of mowing, nitrogen fertilization and irrigation on carbon sequestration of cool-season turfgrass. Our goal is to evaluate trade-offs between the resources required to maintain healthy turf and its carbon sequestration potential, and to ultimately develop recommendations to minimize the greenhouse emissions associated with turfgrass maintenance. To research this objective, we conducted an extensive review of the current literature and performed a meta-analysis. We also initiated several field trials in Corvallis, Oregon.

Findings produced from the literature review showed that newly established turfgrass is usually a substantial carbon sink, rapidly accumulating carbon in the soil for more than a decade. As turfgrass matures, carbon sequestration rates diminish and reach a carbon neutral state around 50 years after establishment. This is due to the fact that, as soils accumulate more organic matter, soil microbes also acclimate and decompose organic matter more quickly, so that accumulation eventually ceases and the total quantity of organic matter stored in soil levels off.

An estimate of net greenhouse gas emissions determined that recently established turfgrass (less than 10 years old) is a carbon sink on average, even when including emissions from mowing and fertilization practices. This analysis also suggested that the use of synthetic fertilizers is a major source of greenhouse gas productions within turfgrass management. Using locally sourced compost, rather than synthetic nitrogen fertilizer, could potentially help to reduce fertilizer-related emissions.

We wanted to know if turfgrass continued to be a net carbon sink when irrigation, mowing frequency and nitrogen fertilizer amounts were reduced. To quantify this, several field trials were initiated at the OSU Lewis-Brown Farm in Corvallis, Oregon.

For these projects, we are utilizing two methods to quantify CO2 uptake and emissions between turfgrass and the atmosphere. The first method captures high-frequency dynamics on one field and uses an automatic clear chamber system (EoSense Inc., Dartmouth, Nova Scotia, Canada), which measures CO2 exchange every hour under optimum management practices (weekly mowing, fertilization four times per year and daily irrigation during periods of heat stress).

This method of data collection provides a continuous measure of CO2 exchange rate throughout the year and allows us to calculate an annual carbon budget (Figure 1). The second method allows us to measure more plots and treatments, using a portable, manual, clear chamber system (PP Systems, Amesbury, Massachusetts).

We use this device to measure CO2 exchange at midday, when turfgrass is growing most actively, once every two weeks from plots receiving different management practices of mowing (2 inches and 4 inches), fertilization (0 and 4 lbs. nitrogen fertilizer per 1,000 sq. ft. per year) and irrigation (non-irrigated control to 0.6 cm of precipitation applied four times per week, June through September) (Figure 2).

Preliminary data from the field trials suggest that in our local climate (Willamette Valley, Oregon), cool-season turfgrass was able to assimilate atmospheric CO2 at high rates during cool months, which suggested a winter carbon sink. During summer, environmental stresses including heat and drought can largely reduce the ability of cool-season turfgrass to assimilate atmospheric CO2.

Fertilization and mowing height can affect CO2 flux rates during cool months. When differences were observed, turfgrass maintained at the 2-inch mowing height sequestered more carbon on average than the turfgrass mowed at a 4-inch height. This is likely because the lower mowing height is stimulating more lateral growth or turfgrass density. During the cool months, fertilization typically increased turfgrass carbon sequestration. Fertilizer applications did not improve summer carbon sequestration.

These results would suggest that fertilizer applied during periods of peak growth and development will stimulate growth and improve carbon sequestration, and fertilizer applications should be avoided during periods of environmental stress (i.e., the summer months). Summer irrigation provided very little to no improvements in carbon sequestration, likely because irrigation not only increased turfgrass growth but also increased soil microbial activity, prompting decay of soil organic matter and releasing CO2 into the atmosphere.

Conclusions

Our findings suggest that management practices (mowing, fertilization and irrigation) have limited effects on CO2 flux rate. Therefore, low-input turfgrass is likely a good choice for reducing management inputs while maintaining carbon sequestration and other environmental benefits.

We will continue to collect data to calculate an annual carbon balance, which will provide a better understanding of the cool-season turfgrass system in our local environment. This effort will provide information on the net emissions of turfgrass, helping turfgrass professionals explain their environmental impacts to a concerned public, while identifying risks and opportunities associated with emerging carbon programs.

We also hope to identify best management practices for enhancing carbon sequestration and to deliver these results to turfgrass seed producers, turfgrass managers (golf course superintendents, commercial turfgrass managers, school and park employees) and other users (homeowners and master gardener programs).

This work was supported by the USDA Agricultural Research Service through projects to C.L.P. (5030-12000-015 and 2090-11000-008) and by the USDA Agricultural Marketing Service through grant ODA6026GR to A.K.

About the authors

Dr. Ruying “Wrennie” Wang ([email protected]) is a Research Associate within the OSU Turfgrass Science Program, Department of Horticulture in Corvallis, Oregon.

Dr. Claire Phillips ([email protected]) is a Research Soil Scientist at USDA-ARS Northwest Sustainable Agroecosystems Research Unit in Pullman, Washington.

Dr. Clint Mattox ([email protected]) is a Research Weed Scientist at USDA-ARS Forage Seed and Cereal Research Unit in Corvallis, Oregon.

Dr. Alec Kowalewski ([email protected]) is an Associate Professor within the OSU Turfgrass Science Program, Department of Horticulture in Corvallis, Oregon.