Introduction
The Qinghai-Tibet Plateau is a fragile ecosystem, and grassland accounts for 60.73% of the area (Fu et al. 2021). The alpine meadow is a typical vegetation type on the Qinghai-Tibet Plateau, accounting for more than 80% of the total grassland area, and is seriously degraded (Fu et al. 2021). The degradation of alpine meadow leads to a series of changes, such as the disappearance of topsoil mattic epipedon, the change of dominant species of plant community from Cyperaceae and Gramineae to forbs, the decrease of plant productivity and the decrease of soil organic carbon, nitrogen and phosphorus nutrients (Song et al. 2007; Bai et al. 2020; Bardgett et al. 2021). This degradation has been a major issue in the context of climate change and overgrazing (Bardgett et al. 2021). The restoration of severely degraded alpine meadows, typically involving the construction of artificial grasslands by artificial-seeding Gramineaes, has been carried out for more than 40 years on the Qinghai-Tibet Plateau (Dong et al., 2022), however, it has not been reported to recover the severely degraded alpine meadows by itself or by the natural restoration measures of grazing reduction or enclosure. The decreasing in soil nutrients, especially soil N, in the degraded alpine grasslands is generally believed to be the main factor limiting restoration (Song et al. 2007; Gao et al. 2019). However, some studies have found that plant communities in degraded alpine meadows are highly stable, as are the soil carbon and N contents, even under fertilization or enclosed conditions (Gao et al. 2019; Bai et al. 2020). Thus, despite in recent years a variety of measures, such as grazing reduction or fencing, to recover its previous non-degraded structure have been implemented in Sanjiangyuan National Park in China, these degraded wetlands cannot be restored to the pre-degradation state. This is the basis to assume that the degraded alpine meadow community has reached a unique successional stable state, by not known mechanisms, fact that warrants to be investigated. To determine how community stability is maintained under low N conditions and to prioritize conservation efforts, it is important to characterize plant N utilization in degraded alpine meadows.
In a low N environment, plant adaptation will undergo directional selection of increased N absorption capacity, increased N concentrations in leaves and roots (Wang et al. 2019; Wen et al. 2022) and increased plant root resource allocation (von Felten 2009). Soil N chemical forms that can be absorbed by plants includes NH4+, NO3-, and organic N (McKane et al. 2002). In general, plants in environments with high soil N contents preferently absorb inorganic N, especially NO3- (Kahmen et al. 2006; Harrison et al. 2007). However, the direct absorption of organic N by plants is increased in N-deficient environments (McKane et al. 2002), this is an effective strategy for the stable coexistence of nondominant plants (von Felten 2009). Therefore, such niche complementary of interspecific N absorption preferences is an adaptive strategy for plants to coexist under low soil N (McKane et al. 2002; Kahmen et al. 2006), and taxa with a preference for a rich N source in soil may be dominant in plant communities (Andersen et al. 2017; Gao et al. 2019). For alpine meadow plants, a number of studies have reported N utilization preferences (Song et al. 2007).
Grassland degradation is usually accompanied by spatial heterogeneity of the landscape (Adler et al. 2001). Patchiness is a common feature of degraded alpine meadows after disappearance of the original mattic epipedons, in addition to decreases in soil nutrients and changes in plant community structure (Li et al. 2021). While the distribution of plants in undegraded alpine meadow is relatively dense and uniform, and the uniform distribution of plant community in degraded alpine meadow is loose and uneven. This is the most intuitive landscape change of degraded alpine meadows. Plant community landscape characteristics, especially small-scale landscape characteristics, are often ignored in studies of species coexistence (Dutilleul 1993), despite the potential for spatial heterogeneity to have a profound impact on vegetation (Dobbert et al. 2021). Despite a number of studies of the effects of local microzone conditions on plants, such as local microzone temperature (Klinges et al. 2021), snow cover, and microtopography (Dobbert et al. 2021), the effects of microscale landscape features, such as patchiness, on species coexistence are often ignored.
Phenotypic plasticity, the ability of a species to exhibit phenotypic changes depending on environmental conditions, is closely related to the size of a species distribution (Callaway et al. 2003; Dobbert et al. 2021). Plasticity in N source preferences in alpine meadow plants is a key mechanism underlying complementary (Song et al. 2007), and plants with significantly plasticity tend to be widely distributed in different environments (Andersen et al. 2017). Two types of plants make up degraded alpine meadows: species remaining from undegraded alpine meadows (classified as generalist species in degraded and undegraded alpine meadows) and species unique to degraded alpine meadows, which enter degraded alpine meadows and become dominant species (Li et al. 2021). It is generally believed that generalist species show greater plasticity than that of unique species (Liang et al. 2020).
Anyway, N uptake preference effects on niche complementary, and plasticity in uneven communities of degraded alpine meadows have not been clearly characterized. Therefore, we evaluated the N absorption capacity and differentiation in N source preferences in plant communities between an undegraded and degraded alpine meadow in the Sanjiangyuan National Park on the Tibetan Plateau by15N marked soil injection with different N sources (glycine, NO3-, and NH4+). Our specific aims were as follows. 1) To determine whether plants in degraded alpine meadows showed higher N absorption capacity under low N conditions compared with undegraded alpine meadows. 2) To evaluate whether the N preference differs between degraded alpine meadow plants and undegraded alpine meadows plants. 3) To evaluate whether generalist species have greater N-preference plasticity and N source differentiation than those of unique species in degraded alpine meadows. 4) To determine whether microscale plant distribution heterogeneity contributes to plant N niche differentiation.
Materials and methods