The shift in key functional traits caused by precipitation under nitrogen and phosphorus deposition drives biomass change in Leymus chinensis

Abstract Background Under changes in biological and abiotic factors, the trade-offs between key functional traits in plants have a decisive impact on biomass production. However, how precipitation and nutrient deposition affects the trade-offs in traits and ultimately productivity is still unclear. Here a mesocosm experiment was conducted to explore the relationships between biomass production and aboveground and belowground key functional traits and their trade-offs under changes in precipitation (average precipitation, increased precipitation) and nutrient depositions (nitrogen: 0, 10 g N m-2; phosphorus: 0, 10 g P m-2) in Leymus chinensis, a monodominant perennial rhizome grass that is widespread in the eastern Eurasian steppe. Results 1) Aboveground biomass, plant height, and carbon assimilation rate were all significantly increased with the increase in precipitation, whereas the water use efficiency was decreased with precipitation changes; 2) Simulated nitrogen (N) deposition and simultaneous N and phosphorus (P) deposition significantly increased aboveground biomass, plant height, specific leaf area and water use efficiency under both average and increased precipitation conditions. However, P deposition alone had no significant effect on the aboveground biomass under average rainfall conditions; 3) Moisture is the key factor regulating the effect of N and P deposition on the increased biomass production. Under conditions of average precipitation, water use efficiency was the key trait determining the biomass of L. chinensis, however, under increasing precipitation, plant height determined L. chinensis biomass. The reason for this result is that under average precipitation conditions, there were obvious trade-offs between water use efficiency and leaf area, specific leaf area, leaf thickness and leaf dry matter. Conversely, under increasing precipitation, the effect of restricted soil water on leaf traits was relieved, and the key limiting trait changed from water use efficiency to plant height. Conclusions The shift of fundamental traits of photosynthetic carbon gain induced by precipitation under N and P deposition is the key ecological driving mechanism for biomass production of typical dominant species in semi-arid grassland.