Ecosystem Carbon Use Efficiency Is Insensitive to Nitrogen Addition in an Alpine Meadow

Carbon use efficiency (CUE) is an important variable in carbon cycling and a key parameter in biogeochemical models, which is expected to increase with N availability. However, how CUE changes with N input rate remains elusive and lacks field evidence. We implemented a field manipulative experiment to examine effects of N addition with six levels of rate (0, 2, 4, 8, 16, and 32 g·N·m −2 ·year −1 ) on canopy CUE (the ratio of net primary productivity [NPP] to gross primary productivity [GPP]) and ecosystem CUE (the ratio of net ecosystem productivity [NEP] to GPP) in an alpine meadow on the Tibetan Plateau in 2014–2016. Over the 3 years, nitrogen addition treatments did not change canopy CUE or ecosystem CUE in comparing with the control, although it significantly increased GPP, NPP, and NEP; those components of CUE had the same response patterns to N addition rates, which shifted from linear responses in the first year to saturation responses thereafter. Moreover, GPP showed a positive linear relationship with NEP, NPP, ecosystem respiration, and autotrophic respiration under each N addition treatment with no changes in slope. The inherent coupling between GPP and ecosystem respiration, as well as GPP and autotrophic respiration, made CUE less sensitive to N addition than carbon fluxes themselves. The conclusions challenge the widely held hypothesis that ecosystem CUE increases with N addition. This study provides the first empirical evidences on the proposition that CUE is insensitive to N availability, which is of importance in model construction and parameterization.