Assessing Soil Carbon Sequestration in Turfgrass Systems Using Long Term Soil Testing Data

R.F. Follett  - Co-Investigator: Y. L. Qian ¹

PROBLEM: Throughout the United States (US) urban landscapes are continuing to expand onto former farms, pastures, and native areas and is being converted into turfgrass, such as home lawns, parks, commercial landscapes, recreational facilities, golf courses, and other greenbelts.  Recent global concerns over increased atmospheric carbon dioxide (CO₂) that can potentially alter the earth•s climate systems have resulted in rising interest in studying SOM dynamics and C sequestration capacity in various ecosystems.  Because of high productivity and lack of soil disturbance, turfgrass may be making substantial contributions to sequester atmospheric carbon.  Determination of C pools in urban turfgrass soils will shed light on the role of turfgrass systems in contributing to terrestrial C sequestration.  Turfgrasses are reported to occupy about 20 million hectares in the US and the area is expanding.  This land area exceeds the 14.7 million ha enrolled under the USDA conservation reserve program and is equivalent to about 14% of the 141 million ha of privately-owned, cultivated cropland in the United States.  Understanding soil organic matter (SOM) dynamics in turfgrass systems is of great importance because of its profound influence on ecosystem sustainability, soil fertility, and soil structure.  Soil organic matter content is also of particular interest to turf managers because: 1) changes in SOM influence the air-filled porosity, water retention, and percolation in the sand-based root zone, which is used intensively in putting greens, tees and sports fields; and 2) SOM serves as a major repository and reserve of plant nutrients, especially nitrogen (N), phosphorus (P), sulfur (S), and potassium (K). 

APPROACH:  To study SOM dynamics, measurements over years and decades are necessary since changes in SOM occur slowly and annual changes are generally small.  Historic data may be most effective in assessing C sequestration in urban turfgrass lands.  Many golf courses analyze soils (including SOM measurement) on a yearly basis and possess long-term soil testing results and may serve as an effective surrogate for evaluating the broader issue of the importance of urban landscapes for sequestering soil organic carbon (SOC).  Long-term climate records, documented management activities, and soil data are also available for many well-managed golf courses.  Such information is invaluable in revealing the dynamics of SOM, assessing C-sequestration, and interpreting soil C changes in golf courses.

To determine the rate and capacity of soil carbon (C) sequestration, we compiled historic soil testing data from parts of 15 golf courses that were near metropolitan Denver and Fort Collins, CO and one golf course near Saratoga, WY.  In addition, we compiled a total of about 690 data sets on previous land use, soil texture, grass species and type, fertilization rate, irrigation, and other management practices.  The oldest golf course was 45 years old in 2000, and 12 years of soil testing results were available; the newest golf course was 1.5 years old.  In the fairway sites, existing soils were subjected to shaping and topsoil replacement prior to establishment. Soils from most green and tee sites were not indigenous; about 30 cm of sand was placed over a gravel layer or native soil.  The purposes of the study reported here were to: 1) conduct a survey to compile data on soil testing results from different golf courses in chronosequences, spanning a wide range of duration of turf management to examine the SOM dynamics in golf courses, and 2) generate regression models to predict the rate of SOM changes, and to help identify factors important to C sequestration.

RESULTS: Nonlinear regression analysis of compiled historic data indicated a strong pattern of soil organic matter (SOM) response to decades of turfgrass culture.  Total C sequestration continued for up to 45  years in putting greens at average rates approaching 0.9 and 1.0 ton ha-1 year-1 for fairways and putting greens, respectively.  Our study also found that past land use imparted a strong control of SOM baseline; fairways converted from agricultural lands exhibited 24% lower SOM than fairways converted from native grasslands.  Golf course SOM was 20-45% higher in fairways and greens than on tees.  We concluded that C sequestration in turf soils occurs at a significant rate that is comparable to those reported for US land that has been placed in the conservation reserve program.

¹ Dept. of Horticulture and Landscape Architecture, Colorado State Univ., Fort Collins, CO 80523-1173