Nitrogen addition enhanced water uptake by affecting fine root morphology and coarse root anatomy of Chinese pine seedlings

Global nitrogen (N) deposition can affect root morphology, anatomy and thus the water uptake by trees. The variation in uptake with the induced changes in root morphology and anatomy, however, is not clear. We grew seedlings of Pinus tabuliformis for two years across an N gradient (0–12 g N m−2 y−1) and analysed the associated changes between the root hydraulic conductance (RHC) and the morphology and anatomy of different root orders. (1) The RHC increased with N addition and reached the peak at 9 g N m−2 y−1. (2) The number, length, biomass and mycorrhizal colonization rates of the fine roots (first two order roots) decreased with N addition. (3) The stele diameter and tracheid diameter of the coarse roots (third and higher root orders) increased with N addition. (4) Decreasing root quantity and increasing physiological activity (i.e. RHC and respiration rates) induced by N addition may be a trade-off between root morphology and physiology plastic. The morphological, anatomical and physiological responses of the different root orders to N addition increased the water uptake, transport, and whole-seedling growth. These heterogeneous responses should be incorporated into models of root uptake and growth in forest ecosystems.