Influences of N and P additions on soil bacterial diversity
The present study supplies some new insight into the impacts of N addition and P addition on the communities of bacteria and fungi in a temperate meadow. N addition highly enhanced soil N concentration and aboveground biomass, but reduced plant species diversity. Several results found that N enrichment reduced soil bacteria richness in the tropical forest, temperate steppe, and arctic tundra ecosystem (Campbell et al., 2010; Zeng et al., 2016; Wang et al., 2018; Yan et al., 2018), however, one earlier result found that N addition had few effects on the soil bacterial diversity at Cedar Creek (Ramirez et al., 2010). The addition of N also had no influence on soil bacterial OTU richness and α-diversity in the current study, (Fig. 2), suggesting that the response of soil bacterial species richness to N addition was very strong in Songnen meadow, which is inconsistent with our first hypothesis. It might be explained partly by the background nutrient availability or soil pH because many previous results have demonstrated that the diversity of soil bacteria is frequently affected by soil pH, the decline of soil pH decreased the soil bacteria richness (Fierer and Jackson, 2006; Wang et al., 2018). The soil pH in our experiment site was much higher than the above-studied ecosystem, and N addition had no influence on soil pH, suggesting that N addition had few effects on soil bacterial α-diversity because no significant changes in soil pH resulted from N addition in the highly alkalized N-limited temperate meadow (Fig. S3a). Moreover, soil microbial diversity is likely to increase with the enhancement of plant diversity (van der Heijden et al., 2008). N addition highly decreased plant species diversity, but no impact on soil bacterial richness, suggesting that the sensitivity of plant species to N addition might be faster than soil bacteria. The present results also suggest that the controls on species diversities of plant and soil bacteria may not be the same (Fierer et al., 2012).
P addition significantly reduced soil bacterial OTU richness and α-diversity which is not in agreement with the earlier results that the P addition did not affect soil bacterial richness (Eo and Park, 2016), and the soil bacterial diversity is enhanced with P addition in an agricultural ecosystem (Tan et al., 2013). However, the reduction in the soil bacterial diversity might have no negative influence on soil functionality because of the functional redundancies in soil bacteria community (Pan et al., 2014). A significant negative correlation between soil bacterial richness and soil P concentration was detected (Fig. S3b). The present result also indicates that P addition might increase the competition for nutrients between soil bacteria and plants, and then reduce soil bacterial richness (Zhang et al., 2014). Moreover, P addition increased soil available P concentration which might reduce phosphate solubilizing bacteria, and then decrease soil bacterial richness, indirectly. However, P addition had no influence on plant species diversity and aboveground biomass, suggesting that P is not the most important limited nutrients.
Influences of N and P additions on the soil fungal diversity
Previous studies showed that the influences of N addition or P addition on soil fungal diversity are controversial. One previous result demonstrated that N enrichment decreased soil fungal diversity which depended on the types of ecosystem and the dose of nutrient addition (Zhou et al., 2016), but other results found that N fertilization highly enhanced soil fungal richness in N-limited ecosystem (Weber et al., 2013; Mueller et al., 2014). In the present study, the addition of N increased soil fungal richness which is in agreement with the above later results which might suggest that some of the fungal species are more adaptable and tolerant to N addition, and soil N and P concentrations had significant positive influences on soil fungal richness (Fig. S3c, S3d). Therefore, a small amount of nitrogen addition might contribute to the growth of soil fungi because that can improve plant growth and reduce the N competition of soil fungi indirectly.
The previous study reported that P fertilization reduced the soil fungi species richness (He et al., 2016). P addition significantly enhanced the soil fungal richness in our present result, which is inconsistent with the above results. The possible reason is that the addition of P could mitigate the P deficiency which improves the biomass of soil fungi (Liu et al., 2012). Moreover, in the studied meadow soil available P was usually severely limited (Zhao et al., 2019), P addition increased fungal diversity which suggests that the P deficiency might be used for predicting the positive impacts of P addition on soil fungal richness directly.
Influences of N and P additions on the soil bacterial community composition
Several earlier results suggested that the addition of N shaped the community composition of soil bacteria, for instance, increased Proteobacteria abundance (Ramirez et al., 2012) but decreased the abundance of Chloroflexi and Acidobacteria (Zeng et al., 2016). Nevertheless, the N addition influences on soil bacterial community composition are inconsistent in grassland across the globe (Leff et al., 2015). Our results showed that N addition had no impact on the community composition of soil bacteria, which confirmed the consistency of N enrichment impacts on the bacterial community in soil. The possible reason is that N addition did not decline soil pH in the present study because many previous results reported that the decline of pH is a crucial factor that changes the bacterial community (Wang et al., 2018; Guo et al., 2019). Moreover, the db-RDA result indicates that the changes in bacterial community were mediated mainly by soil available P concentration, suggesting that the small changes of soil N addition caused by N input could not directly affect soil bacterial community composition by declining pH and affect soil C/N ratio. Although the addition of N significantly affected the structure of plant community and productivity in this studied ecosystem (Zhao et al., 2019), the changes of plant community structure and ANPP had no significant impact on the soil bacteria community, suggesting that the responses of soil bacteria and plant community might be dissimilar.
A previous study showed that P addition played a crucial role in soil bacterial community structure in tropical forests (Wang et al., 2018), however, another result suggested that P addition did not manipulate soil bacterial community composition in a temperate steppe (Ling et al., 2017). In the current research, the addition of P significantly altered soil bacterial community structure, increased Actinobacteria and Acidobacteria abundances, while declined the abundances of Proteobacteria and Gemmatimonadetes. Several studies have shown that changes in pH caused by P addition play a vital role in influencing the community composition of soil bacteria (Ling et al., 2017; Wang et al., 2018). In the current study, P addition had few effects on soil pH, however, the soil bacterial community was strongly determined by soil P concentration in the additions of P and NP, suggesting that the increase in soil P availability and nutrient balance might play a critical role in shaping the soil bacterial community composition. Besides, the increase of Acidobacteria and Actinobacteria might improve plant growth because Acidobacteria has highly metabolically activities in rhizosphere soil (Lee et al., 2008) and Actinobacteria could improve plant growth by alleviating soil disease suppression (Palaniyandi et al., 2013), which would increase C input from plant to soil with the enhancement of soil bacterial activities (Li et al., 2015). However, the influence of C/N/P ratios caused by the additions of N and P on the community of soil bacteria needs further study.
Influences of N and P additions on the community composition soil fungi
Previous results have demonstrated that the addition of N could alter the community structure of soil fungi (Leff et al., 2015; Yan et al., 2018; Wu et al., 2021). In the current present study, we found N addition highly increased Ascomycota abundance, but decreased Basidiomycota abundances. Ascomycota is saprotrophic fungi that can accelerate soil C decomposition (Xiong et al., 2014), the increase of Ascomycota induced by N addition would increase soil C decomposition and speed up the C cycle. The present result might be used to explain the reason that the increased litter decomposition of Leymus chinensis caused by N addition in the studied ecosystem (Gong et al., 2015). N addition declined Basidiomycota abundance, which might reduce the pressures of competition on Ascomycota for resources (Weber et al., 2013). Furthermore, Glomeromycota abundance remarkably declined under the N addition suggesting that N addition would reduce plant species diversity because most of the species are mycorrhizal plants.
In the present result, P addition significantly affected the community composition of fungi in the soil, which is in agreement with the earlier results (Liu et al., 2012; Nielsen et al., 2015; He et al., 2016). The abundances of Ascomycota and Mortierellomycota significantly increased, but Basidiomycota and Chytridiomycota abundances declined significantly under P addition. Moreover, the db-RDA results showed that N and P addition effects on soil fungal community composition were possibly determined by soil P concentration (Liu et al., 2018). The current results support the above results, and the changes in soil fungal community structure would alter the ecological functions of soil. For instance, Mortierellomycota could defend against soil degradation (Li et al., 2019). Additionally, the changes in fungi community structure also supported our previous results that P addition could mitigate the negative impact of N addition on plant community structure (Zhao et al., 2019). The current results indicate that the changes in the community composition of soil fungi might be a good indicator that could be used to explain the reason that P addition affects plant community structure. However, how these increased or decreased soil fungi affect the growth of different functional groups needs further studies.