Nitrogen dynamics differed among the first six root branch orders of Fraxinus mandshurica and Larix gmelinii during short-term decomposition

Fine root (<2 mm) decomposition provides a substantial amount of available nitrogen (N) that sustains plant growth. The N release pattern during litter decomposition is generally controlled by initial N concentrations or C/N. Because root branch order and mycorrhizal colonization (related with branch order) are both highly related with different initial chemistry, a hypothesis was proposed that N dynamics during root decomposition varied among different branch orders. Using the litterbag method, decomposition of the first six order roots for Fraxinus mandshurica (an arbuscular mycorrhizal species) and Larix gmelinii (an ectomycorrhizal species) was studied in Northeast China during a 513-day period. Results showed a similar pattern for the two species with contrasting mycorrhizal type: lower-order roots (the lateral root tips), which had an initial C/N of 17–21, continuously released N without any immobilization and maintained a consistently low C/N (<20), whereas higher-order roots, which had an initial C/N of 28–48, periodically immobilized N, leading to a declining C/N over time. In addition, the magnitude of N dynamics is different between species for lower-order roots, but no different for higher-order roots. These results suggest that fine root N dynamics are heterogeneous among branch orders and that species-specific differences depend on the behavior of lower-order roots.