Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/2004
Publication Date: 1/10/2005
Citation: Milchunas, D., Mosier, A., Morgan, J.A., Lecain, D.R., King, J. 2005. Root production and tissue quality in a shortgrass steppe exposed to elevated CO2: using a new ingrowth method. Plant and Soil Journal. 268:111-122. Interpretive Summary: Rising atmospheric levels of carbon dioxide (CO2) are known to contribute to climate change, but they also have direct effects on plant physiology, by increasing photosynthesis and improving the plant water relations. Much less investigated and understood are the effects of elevated atmospheric CO2 on root biology. In this field experiment, root dynamics in a native shortgrass prairie exposed to twice today’s levels of CO2 were evaluated using a new method for quantifying root growth to determine how continuing rising atmospheric CO2 will affect the ecology of this and other semi-arid grasslands. We found that plant root growth was stimulated at elevated CO2 by 60% during years of intermediate precipitation compared to long-term site averages, with small to no effects of CO2 on root growth in either very wet or dry years. Estimates of root production by this new method, which is a modification of previous standard in-growth root methods, were lower than previous estimates based on using radioisotope labeling of roots. The results suggest that rising atmospheric CO2 will have important affects on the growth and turnover of native plant roots in grasslands around the world.
Technical Abstract: A modified root ingrowth method was developed to minimize destructive sampling in experiments with limited space, and used to estimate belowground net primary production and root tissue quality in a native semiarid grassland exposed to elevated CO2 for five years. Increases in root production of over 60 % were observed with elevated CO2 during years of intermediate levels of precipitation, with smaller effects in a very wet year and no effects in a very dry year. Aboveground to belowground production ratios, and the depth distribution of root production, did not differ between ambient and elevated CO2 treatments. Increased tissue quality with elevated CO2 was observed in terms of increases in root soluble concentrations and decreases in lignin concentrations, whereas nitrogen concentrations decreased indicating a decrease in quality. Root tissue quality responses to CO2 often occurred in only a couple years, and C:N ratios were higher in only one of four years sampled. Estimates of root production by the in-growth donut method were much lower than previous estimates in the shortgrass steppe based on 14C decay. We discuss reasons why all ingrowth methods will always only result in relative rather than absolute estimates of root production.