## 1 Introduction  
The factors affecting the physiological processes controlling the amount of terrestrial carbon (C) sequestered in terrestrial ecosystems mainly include atmospheric CO\({}_{2}\) concentration, climatic variability, land use change and nitrogen (N) fixation [1; 2; 3]. In terrestrial ecosystems, the important processes of determining whether an ecosystem is a C sink or source are C fixation and release through photosynthesis and respiration, respectively. For N-limited terrestrial ecosystems, additional N supply can affect these processes, and in turn affect the strength of C sinks or sources [4; 5; 6; 7; 8; 9]. Research has shown that the response of C exchange to N fertilization in terrestrial ecosystems is positive or negative depending on the ecosystem N status [10; 11; 12; 13]. Many researchers have studied the effects of N enrichment on net ecosystem productivity (NEP) in the forest ecosystem, but it remains uncertain as to how much N addition contributes to the magnitude of terrestrial NEP [1; 14; 15]. Many studies have found that N addition increases NEP [16; 17; 18; 19; 20; 21]. [PERSON] _et al._[22] found NEP was highly correlated with N deposition in forest ecosystems and the response of NEP to N in wet deposition was approximately 725 kg C kg\({}^{-1}\) N. However, [PERSON] _et al._[23] found that the response of NEP to N was about 50-75 kg C kg\({}^{-1}\) N based on a model re-analysis across 22 European forest sites and considering the impacts of climatological differences among stands. [PERSON] _et al._[24] also found that the response of NEP to N would be 30-70 kg C kg\({}^{-1}\) N based on a multi-factor analysis of European forest measurements at nearly 400 intensively monitored forest plots in Europe. The results of both [PERSON] _et al._[23] and [PERSON] _et al._[24] are much smaller than the estimates of [PERSON] _et al._[22]. Therefore, there is still some controversy over the magnitude and sustainability of NEP resulting from N enrichment and its potential mechanisms [25; 26; 27]. However, other studies have reported that N addition may cause a relatively small increase in NEP [12; 28], have no effect [29], or have a negative effect by altering plant and microbial communities, including threatened and endangered species [30]. [PERSON] _et al._[31] pointed out that the influence of N enrichment on ecosystem C storage should be minor in forest ecosystems in the northern temperate zone. [PERSON] _et al._[32] stated that small increases in C storage occurring primarily in living and dead wood may result from elevated N deposition over the next few decades. Using meta-analysis from global N addition experiments, [PERSON] and [PERSON] [33] concluded that the increases in short-term C sequestration below ground caused by N enrichment are due to increased C storage in the surface organic layer, but it appeared to be difficult to predict the response of C storage to N enrichment through a long-term experiment. [PERSON] _et al._[34] reported that the ability of grassland ecosystems to sequester C late in the growing season was decreased by N enrichment because of increased growing season water use and earlier leaf senescence. Furthermore, water and C cycles in terrestrial ecosystems are closely coupled. A global sensitivity analysis for an integrated forest ecosystem model [35] also showed the intrinsic coupling between water and the C cycle through stomatal conductance, which affect both water and C fluxes between the biosphere and atmosphere. Nutrients may not only affect productivity and foliar biomass but are also associated with evapotranspiration (ET) in forests and other ecosystems [36]. Increased productivity and foliar biomass induced by increased nutrient availability are probably responsible for the increase in ET resulting from increased rainfall interception and transpiration. As a consequence, more attention should be paid to water use efficiency (WUE), defined as the ratio of C gain (usually gross primary productivity, GPP) to water loss, _i.e._, ET [37,38].