Plant roots have been recognized to be modular, and a third order root cluster has been proposed to be the basic root module unit based upon the life cycle. This experiment examines root modularity of the nutrient-uptake function using stable isotope ^(15)N. Solidago canadensis root clusters of second or third orders—from the same third or fourth order roots, respectively—were treated with^(15)NH_4^(15)NO_3, NH_4NO_3, and de-ionized water for 15 and180 min. The δ^(15)N values of the root clusters were then analyzed and compared. The δ^(15)N values of ^(15)N-treated root clusters of both second and third orders were hundreds of times higher than that of the ^(15)N untreated root clusters.However, the differences of the δ^(15)N values among ^(15)N untreated root clusters(though expressed some significant variations), did not indicate the ^(15)N shared by the sister root clusters came from a common higher-order root. These results demonstrated functional modularity of root nutrient uptake, revealed a second order root, perhaps even a first order root to be a base module unit in terms of root nutrient uptake. The results also suggested that the concept of rootmodularity is function-specific. This experiment further revealed the importance of treatment timing in stabilizing the internal ^(15)N/^(14)N ratio in roots and avoiding top-down transportation of ^(15)N back into roots to secure unbiased measurements.
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