Abstract
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.
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Claims 5
Claim · model-written summary
Total biomass rose to N8 and then plateaued.
Not reportedTaxonComparator
Verbatim
e0“Total biomass increased with N addition up to moderate N levels (N8) before reaching a plateau”
Claim · model-written summary
Total plant N uptake peaked at N2 at 3086 mg N m −2 and then declined.
Not reportedComparator
Verbatim
e0“total plant N uptake peaked early (N2) at 3086 mg N m −2 and declined thereafter”
Claim · model-written summary
Plants transiently shifted preference from ammonium to nitrate at N2.
Verbatim
e0“plants exhibited a transient shift in N form preference from NH 4 + to NO 3 − under low N addition (N2)”
Claim · model-written summary
Microbial N uptake declined to 484 mg N m −2 at N2.
Not reportedInteractor role
Verbatim
t0“plant–microbial N competition”
e0“a decline in microbial N uptake (484 mg N m −2 )”
Claim · model-written summary
Plant microbial competition for nitrate intensified at N2 and subsequently weakened at higher N levels.
Verbatim
t0“plant–microbial N competition”
e0“These patterns intensified plant–microbial competition for NO 3 − at N2 but weakened subsequently at higher N levels.”