A weakening nitrogen sensitivity of plant productivity over four decades (1976-2016) in a Tibetan alpine grassland

Our study highlighted that temporal shifting in ecosystem processes and driving factors on plant productivity should be considered in modelling to project carbon and N cycling under global change scenarios.Fig. 1 Responses of aboveground net primary productivity (ANPP, a) and N response efficiency (NRE, b) to N addition in our own manipulative experiment over 2011-2016.Fig. 2 Responses of aboveground net primary productivity (ANPP) to N addition (a) and changes in N response efficiency (NRE, b) across the six N addition experiments spanning from 1976 to 2016 at the Haibei station.

Fig. 3 A closing ANPP gap (the grey area) between the ambient (ANPP ambient ) and upper boundary of varied N additions (ANPP upper ) over 1976-2016 in a Tibetan alpine grassland. The solid line indicates a significant exponential increase in ANPP ambient and the dash line represents a stable level of ANPP upper . ANPP upper , the top 10 percentile of ANPP N observed under varied N additions (Fig. S2); and ANPP ambient , ANPP from the control of the six N addition experiments and the long-term monitoring study.

Fig. 4 Changes in climate, soil nutrients and plant functional group composition over 1976-2016 in a Tibetan alpine grassland. (a-c) MAT (mean annual temperature) and MAP (mean annual precipitation), and humidity index; (d-f) soil total N, Olsen-P and pH in the 0-10 cm; (g-i) relative abundance of grass, sedge and forb in plant community biomass.