[Effects of warming and nitrogen addition on community production and biomass allocation in an alpine meadow.]

A long-term experiment, involving exogenous N addition and simulated warming, was conducted in an alpine meadow in Damxung, northern Tibet, to study how warming and N addition influence community production and biomass allocation. The results showed that warming resulted in a warm but dry microsite, that was, air temperature increased by 1.6 ℃ and soil surface temperature increased by 1.4 ℃, and soil water content decreased by 4.7%. Under no N addition treatments, warming significantly decreased plant aboveground biomass by 61.5%, 108.8% and 77.1% in 2012, 2013 and 2014, respectively. Under high N treatments (40 and 80 kg N·hm^-2·a^-1), warming had no significant effect on aboveground biomass. These findings indicated that the effect of warming might be dependent on N addition level, and N addition could compensate for soil N loss caused by warming. Warming led to an increase in root/shoot by 98.6%, 60.7% and 97.8% in 2012, 2013 and 2014 under no N addition treatments, respectively. Under the ambient condition, the change percentages of aboveground and belowground biomass of plant communities first increased and then decreased along an N gradient, with the saturation thresholds of above- and below-ground biomass for N addition 56.0 and 55.5 kg N·hm^-2·a^-1, respectively. Under the warming condition, above- and belowground biomass increased linearly with increasing N addition. These findings suggested that warming modulated the response patterns of alpine meadows to exogenous N input, which was mainly caused by decreased soil inorganic N under warming. The estimation of N thresholds highlights that alpine meadows are more sensitive to future N deposition than other types of grasslands.