Light Competition and Biodiversity Loss Cause Saturation Response of Aboveground Net Primary Productivity to Nitrogen Enrichment

The continually increasing nitrogen (N) deposition is expected to cause a saturation response of ecosystem aboveground net primary productivity (ANPP). However, its underlying mechanisms, especially for the decrease of ANPP under high N addition rate, remain poorly understood. A field manipulative experiment was conducted to investigate the response of ANPP to six levels of N addition rate (0, 2, 4, 8, 16, and 32 g N center dot m(-2)center dot year(-1)) in an alpine meadow during 2015-2017. We specifically explored four possible mechanisms, light limitation, biodiversity loss, soil acidification, and ammonium toxicity, underlying the saturation response of ANPP to increasing N addition. The results showed that ANPP increased linearly with N addition rates in 2015, while converted to a saturation response with N addition rates in 2016-2017. With increasing N addition rate, species richness and soil pH significantly reduced while standing litter, light limitation, and NH4+-N content significantly increased. Under low N addition rate (N0 to N4), increases in N availability significantly improved ANPP although it was partly offset by the indirect N effect via increasing litter accumulation and thus light competition. Under high N addition rate (N8-N32), the decreases in species richness mostly explained the N-induced reduction in ANPP, leading to a saturation response. This study provides empirical evidences on interpreting N saturation response of ANPP in the alpine meadow. The findings will advance our current understanding of N enrichment effects on ANPP and benefit biogeochemical models in parameterization and benchmark analysis.