Root production and tissue quality in a shortgrass steppe exposed to elevated CO2:: Using a new ingrowth method

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. Root soluble concentrations increased an average of 11% and lignin concentrations decreased an average of 6% with elevated CO2, while nitrogen concentrations decreased an average of 21%. However, most tissue quality responses to CO2 varied greatly among years, andC:N ratios were higher in only one year (22 ambient vs. 33 elevated). Among years, root nitrogen concentrations declined with increasing aboveground plant nitrogen yield, and increased over the study period. Estimates of root production by the ingrowth donut method were much lower than previous estimates in the shortgrass steppe based on C-14 decay. We discuss reasons why all ingrowth methods will always result in relative rather than absolute estimates of root production.