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.