Functional tipping points in plant–microbial N competition revealed by form‐specific uptake in alpine grasslands

Abstract Nitrogen (N) deposition drives nonlinear ecosystem responses, yet the underlying below‐ground mechanisms that govern functional tipping points remain poorly understood. We conducted an in situ 15 N labelling experiment using NO 3 − and NH 4 + across six N addition levels (0–32 g N m −2 year −1 , designated N0–N32) in an alpine grassland on the Qinghai–Tibetan Plateau. Total biomass increased with N addition up to moderate N levels (N8) before reaching a plateau, while total plant N uptake peaked early (N2) at 3086 mg N m −2 and declined thereafter, revealing a decoupling between structural growth and functional N acquisition. Notably, plants exhibited a transient shift in N form preference from NH 4 + to NO 3 − under low N addition (N2), concurrent with a decline in microbial N uptake (484 mg N m −2 ) and a similar shift in their preference. These patterns intensified plant–microbial competition for NO 3 − at N2 but weakened subsequently at higher N levels. Synthesis . Our results demonstrate a threshold shift in N acquisition strategies that precedes biomass saturation. We propose that N form preference patterns and associated shifts in plant–microbial competition can serve as early‐warning indicators of ecosystem N saturation, providing mechanistic insights to improve threshold‐based nutrient management in alpine grasslands.