Despite considerable attention over the last 25 yr, the importance of early protein break-down products to plant nitrogen (N) nutrition remains uncertain.We used rhizosphere injection of15N-,13C- and14C-labelled inorganic N and aminoacid (L-alanine), with chase periods from 1 min to 24 h, to investigate the duration ofcompetition for amino acid between roots (Triticum aestivum) and soil microorganisms.We further investigated how microbial modification ofL-alanine influenced plant carbon(C) and N recovery.From recovery of C isotopes, intact alanine uptake was 0.2–1.3% of added. Soil microbesappeared to remove alanine from soil solution within 1 min and release enough NH4+toaccount for all plant15N recovery (over 24 h) within 5 min. Microbially generated inorganic orketo acid C accounted for < 25% of the lowest estimate of intact alanine uptake.Co-location of C and N labels appears a reasonable measure of intact uptake. Potentialinterference from microbially modified C is probably modest, but may increase with chaseperiod. Similarly, competition forL-alanine is complete within a few minutes in soil, whereasNO3added at the same rate is available for > 24 h, indicating that long chase periods biasoutcomes and fail to accurately simulate soil processes.