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

We conducted a four-level N addition experiment over 2011-2016 in a Tibetan alpine grassland, and collected data of aboveground net primary productivity (ANPP) from other five N addition experiments at the same site but different time periods over 1976-2012. Nitrogen response efficiency (ANPP response per unit N addition) generated from these six experiments was quantified to test the N sensitivity of plant productivity. Syntheses of these six experiments revealed that ANPP displayed a linear-plateau response to N addition and saturated at 9.3 g N m^-2yr^-1. N response efficiency was significantly affected by mean annual temperature and monthly precipitation at the key growing stage (i.e., leaf-out phase in April and fast-growing phase in June), and decreased over the past four decades by a rate of -0.68 g g^-1 N yr^-1. Such a decrease in N response efficiency was likely due to the consequences of improving soil nutrient availability, aggravating water limitation and shifting plant species composition along with global change. 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.<br>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.<br>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. <br>