4.1 Effects of N addition on plant community ANPP
Consistent with previous reports from both field N addition experiments (Bai et al. 2010; LeBauer and Treseder 2008; Seabloom et al. 2021; Yue et al. 2020) and investigations of atmospheric N deposition gradients (Stevens et al. 2015), we found that plant community ANPP was enhanced under N addition. By providing long-term empirical evidence (continuous addition of N for 10 years), our findings confirm that N is a limiting factor of ecosystem productivity in temperate grasslands (Bai et al. 2010; Lü et al. 2018; Zhang et al. 2015).
Moreover, we found that grasses, which contributed 82.4% to the community, mediated the response of the plant community ANPP to N addition, in line with previous studies (La Pierre et al. 2016; Lü et al. 2018; Tian et al. 2020; Van Sundert et al. 2021) and in support of the mass ratio hypothesis (Grime 1998). In particular, the ANPP of grasses, not forbs, was promoted under N addition. It has been shown that N enrichment always promotes the ANPP of grasses (Bai et al. 2015; Hao et al. 2018; La Pierre et al. 2016; Tang et al. 2017; Van Sundert et al. 2021). For the ANPP of forbs, positive (La Pierre et al. 2016), neutral (Ren et al. 2021; Tang et al. 2017; Van Sundert et al. 2021), and negative (Bai et al. 2015; Lu et al. 2021) effects under N enrichment have all been reported in previous studies. On a global scale, a meta-analysis has shown that N enrichment has few impacts on alterations in the ANPP of forbs (You et al. 2017). The distinct responses between grasses and forbs may be attributed to their root morphology and proliferation. Higher specific root length and specific root area (Ravenek et al. 2016; Zheng et al. 2019; Zhou et al. 2018) can help grasses occupy N-rich patches (Šmilauerová and Šmilauer 2010) to promote the growth of aboveground parts. Meanwhile, faster growth in the ANPP of grasses may restrict forbs through light competition (Hautier et al. 2009; Van Sundert et al. 2021), resulting in non-significant increases in the ANPP of forbs under N-enriched conditions.
We further found that the ANPP of the plant community and grasses, not forbs, was positively associated with soil inorganic N availability, especially soil NO3−–N, indicating that plant communities, in particular grasses, prefer NO3−–N. This finding supports the theory that with long-term evolutionary adaptation, preferential N form absorption by local plant species matches the characteristics of N cycles (Wang and Macko 2011; Zhang et al. 2018a; Zhang et al. 2016). In arid/semi-arid ecosystems, native herbaceous plants prefer NO3−–N, which has been reported in deserts (Zhuang et al. 2020), grasslands (Ashton et al. 2010; Kahmen et al. 2006; Wang et al. 2021; Xu et al. 2011), and forests (Ma et al. 2021). We also found that both the plant community and grasses were driven by the soil NO3−–N concentration, even though the soil NH4+–N concentration was higher (Fig. 2d); grasses mainly absorb NO3−–N in adjacent grasslands where soil NO3−–N is the most abundant form after N addition (Cao et al. 2021; Xi et al. 2017). From a physiological perspective, NO3−–N is the main N source for photosynthesis in chloroplasts (Heldt and Piechulla 2021; Tischner 2000), resulting in plant-specific NO3−–N preference. Furthermore, annual and biennial forbs, which are opportunistic species, do not show an apparent preference for N forms (Ren et al. 2021; Zhang et al. 2015). Together, these findings indicate that evolution (Zhang et al. 2018a), rather than the N-induced NO3−–N increment, determines N uptake preferences.
In addition, by employing stepwise regression and path analyses, we determined that the SWC and soil NO3−–N concentration jointly affected the increase in grasses ANPP, in which SWC had a stronger effect (a larger standard coefficient). These results confirm that in semi-arid grasslands, plant community biomass production is co-limited by water and nitrogen (Lü et al. 2018; Ren et al. 2017; Xu et al. 2018; Zhang et al. 2021), with water as the primary limiting factor (Bai et al. 2004; Li et al. 2020; Ren et al. 2018). More importantly, by conducting a decadal N addition field experiment, we showed that in combination with SWC, soil NO3−–N, not total inorganic N, codetermines the plant community ANPP. Hence, our study extends the water and N co-limitation hypothesis in arid/semi-arid natural ecosystems.
4.2 Effects of the frequency of N addition on plant community ANPP
Interestingly, we found that the plant community ANPP increased with a decrease in the frequency of N addition. The alterations in the plant community ANPP based on the frequency of N addition were attributed to changes in the ANPP of grasses, which were associated with the N addition frequency-induced soil NO3−–N concentration (Fig. 3). The frequency of N addition with seasonal events affects the soil NH4+–N, NO3−–N, and inorganic N concentrations. First, little N is absorbed by herbaceous plants (Joseph and Henry 2009; Ma et al. 2021) and immobilized by microbes in the non-growing season, that is, winter (Joseph and Henry 2009; Ma et al. 2018). Second, N addition in the winter tends to be lost from the rhizosphere through denitrification and leaching when soil moisture becomes saturated during freeze–thaw dynamics (Joseph and Henry 2009; Li et al. 2021; Müller et al. 2002). Third, Zhang et al. (2014a) reported that increased soil ammonia volatilization under a high frequency of N addition results in lower soil NH4+–N concentrations in the surface soils during the growing season. These ecological processes may have caused the reduced soil N accumulation in the surface soils during the growing seasons under the higher frequencies of N addition in our study. Therefore, stronger eutrophication at a lower frequency of N addition promotes increased plant community biomass production. This implies that experiments with a lower frequency of N addition may overestimate the positive effect of atmospheric N deposition on ecosystem productivity in the long term. Future research should clarify how the frequency of N addition affects ecosystem functioning to provide reliable parameters to accurately evaluate the ecological impacts of N deposition based on N addition field experiments.