Tradeoffs between nitrogen- and water-use efficiency in dominant species of the semiarid steppe of Inner Mongolia

In water-limited environments, photosynthetic carbon gain and loss of water by transpiration are in a permanent tradeoff as both are contrarily regulated by stomata conductance. In semiarid steppe grasslands water limitation may covary with nitrogen limitation. Steppe grassland species are capable of optimizing their use of limiting resources, water and nitrogen, but regulation is still poorly understood. In a two-year experiment with addition of water (irrigation simulating a wet year) and nitrogen (0, 25, and 50 kg urea-Nha(-1)) we assessed intrinsic water use efficiency (WUEi), nitrogen use efficiency (NUE), and related plant functional traits (PFTs) of four dominant C-3 species (Leymus chinensis, Agropyron cristatum, Stipa grandis, and Artemisia frigida). Water and N fertilizer supplementation significantly increased plant primary production, and N effect was more pronounced under irrigated conditions. Parallel with the responses of plant production, a strong tradeoff between WUEi and NUE was detected: water supply increased NUE but decreased WUEi, whereas N addition slightly increased WUEi at the expense of NUE. This tradeoff occurred at the leaf level, and involved the responses of leaf N concentration and specific leaf area. WUEi of species changed among treatments in a predictable manner by the parameter of leaf N content per area. Dominant plant species commonly achieved a higher utilization efficiency of the more limiting resource via altering PFTs, which was an important mechanism of adaptation to variable resource limitation in semiarid grasslands.