PARENT SESSION
Oral Session 96: Forest Ecology V: Soils and Nutrient Cycling.
Presiding: G Newman and S Bigelow
Thursday, August 5, 8:00 AM to 11:30 AM, Meeting Room B 114.

Assessing tree rhizosphere carbon dioxide efflux using a natural ¹³C tracer method.

Cheng, Weixin^*,1, Fu, Shenglei¹, Susfalk, Richard², Mitchell, Robert³, ¹ University of California-Santa Cruz, Sanrta Cruz, California² Desert Research Institute, Reno, Nevada³ Jones Ecological Research Center, Newton, Georgia

ABSTRACT- Carbon dioxide efflux originated from tree roots and their associated rhizosphere microbial processes is an important constituent of the atmospheric carbon pool and is potentially controlled by many climate change factors such as temperature, moisture, and CO₂ concentrations. However, the lack of realistic methods for accessing rhizosphere CO₂ efflux has been a major research. Here we report results obtained from experiments aiming to test a novel natural ¹³C tracer method for measuring rhizosphere CO₂ efflux rates of mature trees in the field. The principle of this ¹³C natural tracer method is based on the difference in ¹³C:¹²C ratio between plants with the C3 photosynthetic pathway and plants with the C4 pathway, and on the subsequent ¹³C differences in soil C derived from the two types of plants. By growing tree roots in a chamber filled with a C4-derived soil, the amount of carbon dioxide produced from tree roots can be partitioned from soil respiration because of their differences in ¹³C abundance. The detection limit of this method is largely determined by the absolute difference between the mean ¹³C abundance values of the two end members and the associated measurement errors. The results indicated that the measurement errors of this method ranged from 2.5% to 3.6 %. A treatment difference of approximately 10% of the mean would be detected as statistically significant. The rates of rhizosphere CO₂ efflux measured in our experiments were well within the range of reported rates of excised tree roots and of whole root systems of seedlings grown in liquid media. However, there seemed a significant effect of soil/medium types on the measured rates. The advantages and the limitations of this method will be discussed.

Key words: rhizosphere, carbon-13, tree roots, carbon dioxide