Moderate water stress does not inhibit nitrogen remobilization, allowing fast growth in high nitrogen content Quercus...

Remobilization of stored nitrogen (N) plays an important role in the early growth of deciduous trees in spring. Several environmental factors can modulate N remobilization, but whether water stress is one such factors is unknown. This study analyzes how the size of N storage in Quercus variabilis Blume seedlings interacts with water stress to affect N remobilization, uptake and new growth. This information is important for improving success of forest tree plantations under dry spring conditions. During the first growing season, we produced seedlings with distinct N content by applying two fall N fertilization rates (12 or 24 mg N per seedling) using 15N-enriched fertilizer. At the beginning of the second growing season, a new experiment was started where seedlings were transplanted into larger pots and subjected to two watering levels (85 or 40% of field capacity). The plants were sampled at 4 weeks (T1), 8 weeks (T2) and 12 weeks (T3) after transplanting. Low watering reduced the growth of high and low N seedlings, but high N seedlings showed greater growth than low N seedlings. During bud burst and initial shoot elongation (T1), restricted watering, which induced a moderate water stress, did not affect the amount of N remobilized from roots, the major source of stored N source at this growth stage. This suggests that high N storage can partially counteract the negative effect of moderate water stress on early growth. At T1, water stress did not affect N uptake, and high N content seedlings absorbed significantly less soil N than did low N content seedlings. At T3, in contrast, water stress was the main determinant for N uptake, with drought-stressed plants showing lower uptake than well-watered plants. We conclude that moderate drought does not inhibit N remobilization from the major storage organ at early growth stages in spring, and that increasing N storage of planted seedlings through fall fertilization can mitigate the negative effect of moderate spring drought on growth.