Growth And Carbon Balance Of Kentucky Bluegrass In Response To Mowing And Elevated CO₂

Kentucky bluegrass(Poa pratensis cv. Baron) is a cool-season turfgrass which widely distributes in cool-wet areas such as northern, northwest, northeast China and the middle and lower reaches of Changjiang River in China. Kentucky bluegrass is a bunch type grass with strong reproductive capacity, well-reproducibility, long growth period, powerful resistant to cold, drought and heat. It is extensively used in home lawns, parks, athletic fields, golf courses and pasture. Growth, physiological and carbon balance of Kentucky bluegrass are very sensitive to increasing temperature, and influenced by mowing height, drought stress and CO2levels. Therefore, taking Kentucky bluegrass as target material by field and laboratory simulation, we studied (1) effect of mowing height (7.6cm, high mowing height, HM; and3.8cm, low mowing height, LM) on turf growth and carbon balance responses to seasonal changing temperatures in mature (8year old) turfgrass fields;(2) effects of increasing temperature (15,20,25,30and35℃) on turf growth and carbon balance responses under ambient and elevated CO2levels, whether elevated CO2cause shift of optimal temperature, whether elevated CO2-mitigation of heat damages was associated with the maintenance of positive carbon balance and the accumulation of photosynthetic and respiratory metabolites;(3) effect of elevated CO2on turf growth in shoot and root, photosynthesis and respiration responses to drought and heat stress alone or combined stress; and effect of carbon balance responses to drought and heat stress alone or combined stress under ambient and elevated CO2. The main results include as follows:(1) Both LM and HM plots displayed significant decline in turf quality, shoot biomass, and canopy photosynthetic rate (Pn) with increasing air temperature above23-24℃in both years, and the decline was more pronounced at LM. Turf plots exhibited carbon loss due to greater total respiration rate of shoots, roots, and soil (Rtotal) than canopy Pn under high temperatures (≥31℃in2012and28℃in2013for HM turf;≥28℃in2012and25℃in2013for LM turf) during July-September, but maintained net carbon gain during cooler seasons (May and June) due to greater Pn/Rtotai ratio. Lowering mowing height accelerated carbon loss due to reduced canopy Pn, particularly under high temperatures. Our results suggested that whether mature turfgrass stands sequestrate or loss carbon was dependent on seasonal temperatures and increasing mowing height could help offset the deleterious effects of high temperature and maintain positive carbon balance within the whole turfgrass system.(2) Increasing temperatures to25℃and above inhibited leaf photosynthetic rate (Pn) and shoot and root growth, but increased leaf respiration rate (R), leading to a negative carbon balance and a decline in soluble sugar content under ambient CO2. Elevated CO2did not cause shift of optimal temperatures in Kentucky bluegrass, but promoted Pn, shoot and root growth under all levels of temperature (15,20,25,30, and35℃) and mitigated the adverse effects of severe high temperatures (30and35℃). Elevated CO2-mitigation of adverse effects of high temperatures on Kentucky bluegrass growth could be associated with the maintenance of a positive carbon balance and the accumulation of soluble sugars and total nonstructural carbohydrates through stimulation of Pn and suppression of R and respiratory organic acid metabolism.(3) Elevated CO2increased root and shoot growth, as well as root/shoot ratio under all treatments. Elevated CO2enhanced leaf net photosynthetic rate (Pn) to a greater extent under drought or heat alone, whereas it reduced respiration rate (R), to a larger degree under heat and the combined stress, leading to greater ratio of Pn/R. Elevated CO2mitigated adverse physiological effects of drought or heat stress alone whereas less effects were observed under the combined drought and heat stress. The positive effects of elevated CO2were associated with the development of roots biomass and the maintenance of a positive carbon balance under either stress alone or the combined drought and heat stress. This result confirmed that elevated CO2played a protective role response to drought and heat stress in Kentucky bluegrass.