The coupling effects of water deficit and nitrogen supply on photosynthesis, WUE, and stable isotope composition in Picea asperata

Abstract

Water stress and nitrogen (N) availability are the two main factors limiting plant growth, and the two constrains can interact in intricate ways. Moreover, atmospheric N depositions are altering the availability of these limiting factors in many terrestrial ecosystems. Here, we studied the combined effects of different soil water availability and N supply on photosynthesis and water-use efficiency (WUE) in Picea asperata seedlings cultured in pots, using gas exchange, and stable carbon and nitrogen isotope composition (δ 13C and δ 15N). Photosynthesis under light saturation (A sat) and stomatal conductance (g s) of P. asperata decreased as the soil moisture gradually diminished. Under severe water-stress condition, N addition decreased the A sat and g s, whereas the positive effects were observed in moderate water-stress and well-watered conditions. The effect of N addition on the intrinsic WUE (WUEi) deduced from gas exchange was associated with soil water availability, whereas long-term WUE evaluated by leaf δ 13C only affected by soil water availability, and it would be elevated with soil moisture gradually diminished. Water deficit would restrict the uptake and further transport of N to the aboveground parts of P. asperata, and then increasing δ 15N. Therefore, δ 15N in plant tissues may reflect changes in N allocation within plants. These results indicate that the effect of N enrichment on photosynthesis in P. asperata is largely, if not entirely, dependent on the severity of water stress, and P. asperata would be more sensitive to increasing N enrichment under low soil water availability than under high soil moisture.