Effects of long-term nitrogen addition on water use by Cunninghamia lanceolate in a subtropical plantation

The deposition of reactive nitrogen (N) has substantially increased in subtropical regions due to human activities. However, the effects of long-term N addition on the water-use efficiency of subtropical forests are poorly understood. Here, we conducted an 11-year experiment in a subtropical Cunninghamia lanceolate plantation with four N-addition levels: N0, N1, N2, and N3 (equivalent to 0, 6, 12, and 24 g of N m-2 yr-1, respectively). A thermal dissipation probe system was used to calculate sap flow and plant biomass carbon was assessed by field investigation. The whole-plant water use and water-use efficiency were estimated. In addition, the δ13C of tree rings was used to indicate the plant intrinsic water-use efficiency. The results showed that N3 significantly increased the annual sap flow velocity, especially in summer and winter. Annual water use, plant growth, and water-use efficiency did not significantly differ among the N treatments, but water use tended to be higher in N3 tha...