Direct assessment of symbiotically fixed nitrogen in the rhizosphere of alfalfa

TL;DR: In this article, the authors used 15N-labeled N2 gas to tag recently fixed N in three alfalfa lines [cv Saranac, Ineffective Srinac, and an ineffective Sranac line] growing in 1-m long polyvinylchloride drainage lysimeters in loamy sand soil in a greenhouse.

Abstract: Rhizodeposition has been proposed as one mechanism for the accumulation of significant amounts of N in soil during legume growth The objective of this experiment was to directly quantify losses of symbiotically fixed N from living alfalfa (Medicago sativa L) roots to the rhizosphere We used 15N-labeled N2 gas to tag recently fixed N in three alfalfa lines [cv Saranac, Ineffective Saranac (an ineffectively nodulated line), and an unnamed line in early stages of selection for apparent N excretion] growing in 1-m long polyvinylchloride drainage lysimeters in loamy sand soil in a greenhouse Plants were in the late vegetative to flowering growth stage during the 2-day labelling period We determined the fate of this fixed N in various plant organs and soil after a short equilibration period (2 to 4 days) and after one regrowth period (35 to 37 days) Extrapolated N2 fixation rates (46 to 77μg plant−1 h−1) were similar to rates others have measured in the field Although there was significant accretion of total N in rhizosphere compared to bulk soil, less than 1% was derived from newly fixed N and there were no differences between the ‘excreting’ line and Saranac Loss of N in percolate water was small These results provide the first direct evidence that little net loss of symbiotically-fixed N occurs from living alfalfa roots into surrounding soil In addition, these results confirm our earlier findings, which depended on indirect 15N labelling techniques Net N accumulation in soil during alfalfa growth is likely due to other processes, such as decomposition of roots, nodules, and above ground litter, rather than to N excretion from living roots and nodules