Plant & Soil sample methods
Prior to the sample collection, all the plots were spatially zoned for the sake of long-term monitoring for them. In August 2019 and 2020, plant heights were measured for all the species within each plot, and plant biomass was measured by harvesting all the aboveground live plant material in the two randomly selected 0.5m × 0.5m quadrats within each plot. Biomass of two sub-quadrats were combined, and sorted to species and dried to constant weight at 65℃. Three soil samples (0-10 cm layer) in each plot were collected by using a stain-steel cylinder (5 cm in diameter) and were mixed into a composite sample for the plot and passed through 2 mm sieve before measured. Gravimetric soil water content [%] was measured after the soil subsamples were dried at 105 °C for 48 h. The N concentration in plants was analyzed by an automatic element analyzer (Vario MACRO cube, Elementar Analysensysteme GmbH).