Different N source preferences in the degraded alpine meadow
The N uptake preference changed significantly after alpine meadow
degradation. There is a general, not total, but majoritarian shift from
NO3- to glycine following meadow
degradation (Fig. 1), this suggests that perhaps organic forms are most
available than inorganic forms in degraded meadow. We have not explained
the environmental differences among undegraded and degraded meadow but
all suggest that in degraded meadow the mineralization can be slower, as
for example the amino acid deamination and thus plants must be
specialized in absorbing amino acids or short peptides (Robinson et al.
2022). This is only a possibility, that does not exclude that in
undegraded meadow the nitrificant bacteria can be present in great
amounts, and microbial process from litter to
NO3- are more active in non-degraded
meadow allowing the fast formation of nitrate from organic matter and/or
fixed ammonium than in degraded meadow (Sowerby et al. 2005). This
result strongly suggest two different patterns, a niche segregation with
respect to the forms of N uptake, with a clear specialization in
coexisting species. This is in agreement with the niche segregation
hypothesis and in particular with biogeochemical niche at the level of
different elements, coexisting species should evolve to avoid as much as
possible the direct competition (PeƱuelas et al. 2019; Sardans et al.
2021). At this regard this study provides evidence that this segregation
occurs even at the concrete nutrient level when this is present in the
environment in distinct chemical forms.
There were no significant correlations between the preferences for
different N sources and soil nutrient concentrations, as reported
previously (Wen et al. 2018; Zhou et al. 2021), mainly because the
absorption of nutrients by plants is a complex process related to soil
nutrient substrates as well as microorganisms (Li et al. 2021) and
atmospheric N deposition (Xiao et al. 2021). Therefore, it is not
possible to predict the overall N source preferences of plants based on
soil N concentrations alone. The specific determinants of N source
preferences in plants need to be studied further.
In the past decade, many studies have evaluated N source preferences
using isotope labeling technology. However, inconsistent sampling
methods is an important issue; some studies consider aboveground and
root parts for analyses of N source preferences (Ma et al. 2021), while
others utilize only plant leaves owing to sampling difficulties or
assumptions of a uniform distribution of plant N in aboveground and
belowground parts (Kahmen et al. 2006; Von Felten 2009). Of note, we
observed differences between N sources in the allocation to aboveground
parts and roots after absorption (Table 3, Fig. 1). Therefore, resource
allocation to roots cannot be ignored in N preference studies (von
Felten 2009; Li et al. 2019; Weemstra et al. 2021). The allocation in
the aboveground and belowground and root absorptive capacity of
different N sources varied with meadow succession processes and N
sources, the degraded alpine meadow preferred glycine mainly from the
strongest glycine absorption capacity and greatest glycine storage in
roots (Table 3, Fig. 1). Our results emphasized the importance of
comprehensive aboveground and belowground sampling in studies of plant N
preferences.