USGA-style root zones in greenhouse cells were used to investigate elevated soil carbon dioxide effects on creeping bentgrass.

As carbon dioxide enters the plant cell, resulting low pH can injure root systems and stunt growth. Carbon dioxide levels of 10 percent decrease bentgrass turf quality significantly. In poorly aerated soils, carbon dioxide remains in the soil, potentially building to toxic levels.

The Transition Zone is the area of the eastern and central United States where both cool- (C₃) and warm-season (C₄) grasses can grow. Turf management in this zone is difficult, however, because severe winters challenge warm-season species, and extreme summers make cool-season species suffer (5).

Cool-season species such as creeping bentgrass are C₃ plants, which are physiologically adapted to summertime temperatures of 16 to 24 C (1). In the Transition Zone, temperatures often exceed 30 C, placing creeping bentgrass outside its region of optimal growth.

Summer bentgrass decline has become an all-inclusive term used to describe deterioration of bentgrass during summer months. Several environmental and plant physiological factors explain this process; however, it still is not completely understood.

Obvious factors include high temperatures and humidity causing heat stress, heavy foot traffic, compaction, increased disease incidence and excessive soil moisture levels. These can combine to create a problem often overlooked: poor soil oxygenation and increased levels of soil carbon dioxide (CO₂).

Gases make up approximately 20-30 percent of a typical soil volume (2,6). The soil atmosphere is defined as "the gaseous phase of the soil, being that volume not occupied by solid or liquid (3)." Generally, the composition of soil air is similar to that of the atmosphere, although oxygen and carbon dioxide fluctuate in the soil inversely, with an increase in carbon dioxide resulting in a decrease in oxygen (3,6,8).

Atmospheric and soil gases interact in the overall quality of intensively managed turfgrass areas, such as golf greens. The relationship between soil gas composition and turfgrass physiology is not fully understood. Roots require oxygen for root respiration; however, several other interactions remain undefined, such as the varying levels of CO₂, especially during warm summer months.

Soil carbon dioxide
Carbon dioxide is found at very low levels in atmospheric air (approximately 0.03 percent). Much higher levels are found in soils (3). Carbon dioxide is produced in the soil by microbial and root respiration with the uptake of carbon and oxygen to produce carbon dioxide. Increased soil microbial activity occurs with increased temperature, often elevating carbon dioxide concentrations during warmer weather. In poorly aerated soils, carbon dioxide remains in the soil, potentially building to toxic levels (8). Well-aerated soils generally have reduced carbon dioxide concentrations because it is replaced by oxygen and it readily diffuses out of the soil.

Soil moisture influences carbon dioxide levels. When a soil becomes highly saturated, oxygen diffusion slows, allowing the buildup of carbon dioxide. In saturated soils, excess soil moisture fills pore spaces, reducing soil oxygen and limiting its diffusion.

Gases move through soil in two ways: the air phase or water phase. Diffusion rates via the air phase are more rapid than through the water phase (6). Therefore, soil aeration depends largely on soil moisture content and water-filled pore space. Additionally, when oxygen diffusion slows, the lack of atmospheric oxygen replenishment can decrease soil oxygen and increase toxic gases such as carbon dioxide.

Generally, native soil golf greens have higher soil moisture content and compaction potential than sand-based greens, therefore increasing carbon dioxide and decreasing oxygen (7). However, when highly saturated or compacted, all green soil construction types can have high carbon dioxide levels.

Several theories exist regarding how carbon dioxide damages roots. The most widely accepted hypothesis is that high carbon dioxide levels decrease the cytoplasmic pH of root cells, thereby interfering with water and nutrient uptake (4). Distilled water saturated with carbon dioxide has a pH around 4 as a result of the production of carbonic acid (H₂CO₃) (4). As carbon dioxide enters the plant cell, the low pH can injure root systems and stunt growth (9). In addition, increased carbon dioxide levels reduce water and nutrient uptake by roots (4).

Inflated soil CO₂
A study at Clemson University during the summer of 1999 investigated Crenshaw creeping bentgrass's (Agrostis palustris) response to inflated carbon dioxide concentrations. The study examined inflated soil carbon dioxide's effects on bentgrass root growth and turf health.

Soil carbon dioxide levels rise as soil temperatures and moisture levels increase. However, at what level does soil carbon dioxide degrade root growth and turf quality? To answer this question, creeping bentgrass was exposed to differing levels of inflated soil carbon dioxide and depleted soil oxygen. The study was performed in a greenhouse where cells 25 cm in diameter by 40 cm deep were constructed to simulate a USGA - specified golf green. Crenshaw creeping bentgrass was seeded at 59 kilograms per hectare (1.5 pounds per 1,000 square feet) and mowed at 5 mm (³/16 inch) to simulate golf green condtions.

Treatments included:

	Untreated control
	17.5 percent oxygen, 2.5 percent carbon dioxide
	15 percent oxygen, 5 percent carbon dioxide
	10 percent oxygen, 10 percent carbon dioxide

Gas mixtures were added to the root zone through the bottom of the soil columns for 1 minute every 2 hours to maintain desired levels. Root mass (grams) and length (centimeters) were measured at the end of the three-week study by removing a 2-inch by 12-inch root core from each cell. Root length (cm) was measured by averaging the two longest roots.

Root mass was measured by washing sand and organic matter from the roots with an automated root washer separating sand and peat from root tissue via water and air pressure. Samples were dried at 80 C for three days, then weighed. Turf quality ratings (9 highest quality, 1 lowest) -- which include color, density and health -- were taken every four days. The study was replicated twice during the summer.

Results
Reduction in turf quality was noted one week after the study started. Carbon dioxide levels of 10 percent decreased turf quality significantly from the untreated turf throughout the study. Lesser levels of carbon dioxide did not reduce turf quality.

Root mass and length were reduced by all inflated carbon dioxide levels. Inflated carbon dioxide (greater than or equal to 2.5 percent) decreased root length by ⁴/5 inch and root mass by 40 percent. No rooting differences were observed among carbon dioxide concentrations of 2.5, 5 and 10 percent.

Conclusions
Inflated levels of soil carbon dioxide are detrimental to creeping bentgrass root growth and turf quality. Decreased root mass and length followed three weeks of soil carbon dioxide equaling or exceeding 2.5 percent. Unacceptable turf quality resulted from 10 percent soil carbon dioxide. Creeping bentgrass carbon dioxide toxicity was found at these levels in a greenhouse situation.

Similar levels in a golf course green may potentially cause greater bentgrass damage from increased stress incurred in the day-to-day rigors of golf course management. Further research is necessary to investigate inflated soil carbon dioxide on a golf course green. Additional research may be useful to observe soil moisture and temperature effects on bentgrass response to inflated carbon dioxide.

Management of soil CO₂
In addition to proper golf green construction and adequate drainage, soil carbon dioxide is best controlled by cultural practices. Frequent aerification and core cultivation alleviate compaction and open the soil for increased oxygen exchange. Another innovative method of maintaining a healthy soil atmosphere is the use of subsurface air movement. Pulling or pushing air through the soil column of a golf green may potentially increase drainage and soil oxygenation.

Currently, research at Clemson involves investigation of year-round aerification practices using different tine diameters and lengths for increased aeration and root growth and decreased compaction, layering and soil carbon dioxide levels. Additional research observes various timings and intervals of subsurface air movement and its effect on plant growth and the soil atmosphere.

Acknowledgments

Thanks to the Clemson turf team, especially Jim Camberato, Ph.D.; Hoke Hill, Ph.D.; and my fellow students.

Literature cited

1. Beard, J.B. 1997. Dealing with heat stress on golf course turf. Golf Course Management 65(7):54-59.
2. Brady, N.C. 1984. The nature and properties of soils. Macmillan, New York.
3. Bremner, J.M., and A.M. Blackmer. 1982. Composition of soil atmospheres. p. 873-901. In: A. Klute and A.L. Page (eds.), Agronomy Monograph No. 9. Methods of soil analysis, Part 2. Chemical and microbiological properties. 2nd ed. American Society of Agronomy, Madison, Wis.
4. Chang, H.T., and W.E. Loomis. 1945. Effect of carbon dioxide on absorption of water and nutrients by roots. Plant Physiology 20:221-232.
5. Christians, N.E. 1998. Fundamentals of turfgrass management. Sleeping Bear Press, Chelsea, Mich.
6. Hillel, D. 1980. Fundamentals of soil physics. Academic Press, San Diego.
7. Kavanagh, T., and R.M. Jelley. 1980. Soil atmosphere studied in relation to compaction. p. 181-188 In: R. W. Sheard (ed.), Proceedings of the Fourth Inter-national Turfgrass Research Conference, Guelph, Canada.
8. Williamson, R.E. 1964. The effect of root aeration on plant growth. Soil Science Society of America Proceedings 28:86-90.
9. Williamson, R.E. 1968. Influence of gas mixtures on cell division and root elongation of broad bean. Agronomy Journal 60:317-321.

Thomas Nikolai is a research assistant in the department of crop and soil sciences at Michigan State University. John Rogers III, Ph.D., is associate professor in turfgrass management. Douglas Karcher and John Hardy are research technicians at MSU. Paul Rieke, Ph.D., is a professor in the department.