Light Competition and Biodiversity Loss Cause Saturation Response of Aboveground Net Primary Productivity to Nitrogen Enrichment
Fangfang Ma, Bing Song, Quan Quan, Fangyue Zhang, Jinsong Wang, Qingping Zhou, Shuli Niu
Abstract
Abstract 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·m −2 ·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 NH 4 + ‐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.