A field method of determining NH4+ and NO3- uptake kinetics in intact roots: Effects of CO2 enrichment on trees and crop species

Models describing plant and ecosystem N cycles require an accurate assessme nt of root physiological uptake capacity for NH4+ and NO3- under field cond itions. Traditionally, rates of ion uptake in field-grown plants are determ ined by using excised root segments incubated for a short period in an assa y solution containing N either as a radioactive or stable isotope tracer (e .g., (ClO3)-Cl-36 as a NH4+ analogue, (CH3NH3)-C-14 as an NO3- analogue or (NH4+)-N-15 and (NO3-)-N-15). Although reliable, this method has several dr awbacks. For example, in addition to radioactive safety issues, purchase an d analysis of radioactive and stable isotopes is relatively expensive and c an be a major limitation. More importantly, because excision effectively in terrupts exchange of compounds between root and shoot (e.g., carbohydrate s upply to root and N transport to shoot), the assay must be conducted quickl y to avoid such complications. Here we present a novel field method for sim ultaneous measurements of NH4+ and NO3- uptake kinetics in intact root syst ems. The application of this method is demonstrated using two tree species; red maple (Acer rubrum) and sugar maple (Acer saccharum) and two crop spec ies soybean (Glycine max) and sorghum (Sorghum bicolor). Plants were grown in open-top chambers at either ambient or elevated levels of atmospheric CO 2 at two separate US national sites involved in CO2 research. Absolute valu es of net uptake rates and the kinetic parameters determined by our method were found to be in agreement with the literature reports. Roots of the cro p species exhibited a greater uptake capacity for both N forms relative to tree species. Elevated CO2 did not significantly affect kinetics of N uptak e in species tested except in red maple where it increased root uptake capa city, V-max, for NH4+. The application, reliability, advantages and disadva ntages of the method are discussed in detail.