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