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Introduction: Human activities have increased the nitrogen (N) and phosphorus (P) supply ratio of the natural ecosystem, which affects the growth of plants and the circulation of soil nutrients. However, the effect of the N and P supply ratio and the effect of plant on the soil microbial community are still unclear. Methods: In this study, 16s rRNA sequencing was used to characterize the response of bacterial communities in Phragmites communis (P.communis) rhizosphere and non-rhizosphere soil to N and P addition ratio. Results: The results showed that the a-diversity of the P.communis rhizosphere soil bacterial community increased with increasing N and P addition ratio, which was caused by the increased salt and microbially available C content by the N and P ratio. N and P addition ratio decreased the pH of non-rhizosphere soil, which consequently decreased the a-diversity of the bacterial community. With increasing N and P addition ratio, the relative abundance of Proteobacteria and Bacteroidetes increased, while that of Actinobacteria and Acidobacteria decreased, which reflected the trophic strategy of the bacterial community. The bacterial community composition of the non-rhizosphere soil was significantly affected by salt, pH and total carbon (TC) content. Salt limited the relative abundance of Actinobacteria, and increased the relative abundance of Bacteroidetes. The symbiotic network of the rhizosphere soil bacterial community had lower robustness. This is attributed to the greater selective effect of plants on the bacterial community influenced by nutrient addition. Discussion: Plants played a regulatory role in the process of N and P addition affecting the bacterial community, and nutrient uptake by the root system reduced the negative impact of N and P addition on the bacterial community. The variations in the rhizosphere soil bacterial community were mainly caused by the response of the plant to the N and P addition ratio.

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Clarifying the pattern of relative nitrogen (N)-to-phosphorus (P) requirements for phytoplankton growth is of great significance for eutrophication mitigation and aquatic system management. The relative N-to-P requirement for phytoplankton growth is considered an essential trait determining species dominance within ecosystems and explaining phytoplankton response to nutrient availability. These requirements vary with environmental trophic status, though this variation remains unclear. Here, we evaluated the relative N-to-P requirements under different absolute nutrient levels using previous and current experimental data on eight phytoplankton species (three studied by us and five extrapolated from previous studies). Results showed that relative N-to-P requirements for phytoplankton growth decreased as absolute nutrient levels increased. Thus, N may be crucial for enhancing phytoplankton growth under low nutrient conditions, whereas P may be the primary limiting factor of phytoplankton growth under sufficient nutrient conditions. This result applies to single species as well as species assemblages, which are independent of species shifts occurring along water N:P gradients. The response observed in our large trophic status gradient may help elucidate the relative importance of N and P reductions in mitigating the impact of eutrophication on ecosystems.

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Urban freshwater lakes play an indispensable role in maintaining the urban environment and are suffering great threats of eutrophication. Until now, little has been known about the seasonal bacterial communities of the surface water of adjacent freshwater urban lakes. This study reported the bacterial communities of three adjacent freshwater lakes (i.e., Tangxun Lake, Yezhi Lake and Nan Lake) during the alternation of seasons. Nan Lake had the best water quality among the three lakes as reflected by the bacterial eutrophic index (BEI), bacterial indicator (Luteolibacter) and functional prediction analysis. It was found that Alphaproteobacteria had the lowest abundance in summer and the highest abundance in winter. Bacteroidetes had the lowest abundance in winter, while Planctomycetes had the highest abundance in summer. N/P ratio appeared to have some relationships with eutrophication. Tangxun Lake and Nan Lake with higher average N/P ratios (e.g., N/P = 20) tended to have a higher BEI in summer at a water temperature of 27 °C, while Yezhi Lake with a relatively lower average N/P ratio (e.g., N/P = 14) tended to have a higher BEI in spring and autumn at a water temperature of 9-20 °C. BEI and water temperature were identified as the key parameters in determining the bacterial communities of lake water. Phosphorus seemed to have slightly more impact on the bacterial communities than nitrogen. It is expected that this study will help to gain more knowledge on urban lake eutrophication.

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Species specific nitrogen-to-phosphorus molar ratio (NPR) has been suggested for green microalgae. Algae can store nitrogen and phosphorus, suggesting that the optimum feed concentration dynamically changes as function of the nutrient storage. We assessed the effect of varying influent NPR on microalgal cultivation in terms of microbial community stability, effluent quality and biokinetics. Mixed green microalgae (Chlorella sorokiniana and Scenedesmus sp.) and a monoculture of Chlorella sp. were cultivated in continuous laboratory-scale reactors treating used water. An innovative image analysis tool, developed in this study, was used to track microbial community changes. Diatoms proliferated as influent NPR decreased, and were outcompeted once cultivation conditions were restored to the optimal NPR range. Low NPR operation resulted in decrease in phosphorus removal, biomass concentration and effluent nitrogen concentration. ASM-A kinetic model simulation results agreed well with operational data in the absence of diatoms. The failure to predict operational data in the presence of diatoms suggest differences in microbial activity that can significantly influence nutrient recovery in photobioreactors (PBR). No contamination occurred during Chlorella sp. monoculture cultivation with varying NPRs. Low NPR operation resulted in decrease in biomass concentration, effluent nitrogen concentration and nitrogen quota. The ASM-A model was calibrated for the monoculture and the simulations could predict the experimental data in continuous operation using a single parameter subset, suggesting stable biokinetics under the different NPR conditions. Results show that controlling the influent NPR is effective to maintain the algal community composition in PBR, thereby ensuring effective nutrients uptake.

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Ecological stoichiometry is concerned with the ratios of different elements, particularly carbon, nitrogen, and phosphorus. Ratios by their nature do not respond symmetrically to changes in the numerator and denominator and do not follow normal distributions; however, researchers frequently fail to consider these properties in their analyses, which has biased reported results. Calculating means, variance, or linear slopes based on untransformed ratios results in biased results. I demonstrate the consequences of these errors for inferences from stoichiometric analyses using simple examples and several large monitoring data sets. I then review 100 studies in ecological stoichiometry and find that misuse of ratio data is common, with 93% of studies containing at least one error. These errors may be problematic, particularly in large-scale meta-analyses summarizing data over large ranges. Fortunately, most of these mistakes can be easily avoided by first log transforming elemental ratios. I therefore recommend that, to ensure robust and reproducible results, researchers in ecological stoichiometry should adopt a convention of presenting stoichiometric ratio data as the logarithm of molar ratios in the future. The widespread use of untransformed nitrogen to phosphorus ratio as an indicator of nutrient limitation has likely exaggerated the importance of phosphorus limitation, particularly in freshwater systems.

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Temperature, primary productivity, plant functional traits, and herbivore abundances are considered key predictors of leaf herbivory but their direct and indirect contributions to community-level herbivory are not well understood along broad climatic gradients. Here, we determined elevational herbivory patterns and used a path analytical approach to disentangle the direct and indirect effects of climate, land use, net primary productivity (NPP), herbivore abundance, and plant functional traits on community-level invertebrate herbivory along the extensive elevational and land use gradients at Mt. Kilimanjaro, Tanzania. We recorded standing leaf herbivory caused by leaf chewers, leaf miners and leaf gallers on 55 study sites distributed in natural and anthropogenic habitats along a 3,060 m elevation gradient. We related the total community-level herbivory to climate (temperature and precipitation), NPP, plant functional traits (specific leaf area, leaf carbon-to-nitrogen [CN] ratio and leaf nitrogen-to-phosphorus [NP] ratio) and herbivore abundances. Leaf herbivory ranged from 5% to 11% along the elevation gradient. Total leaf herbivory showed unimodal pattern in natural habitats but a strongly contrasting bimodal pattern in anthropogenic habitats. We also detected some variation in the patterns of leaf herbivory along environmental gradients across feeding guilds with leaf chewers being responsible for a disproportionally large part of herbivory. Path analyses indicated that the variation in leaf herbivory was mainly driven by changes in leaf CN and NP ratios which were closely linked to changes in NPP in natural habitats. Similarly, patterns of leaf herbivory in anthropogenic habitats were best explained by variation in leaf CN ratios and a negative effect of land use. Our study elucidates the strong role of leaf nutrient stoichiometry and its linkages to climate and NPP for explaining the variation in leaf herbivory along broad climatic gradients. Furthermore, the study suggests that climatic changes and nutrient inputs in the course of land use change may alter leaf herbivory and consequently energy and nutrient fluxes in terrestrial habitats.

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Considered to play an important role in plant mineral nutrition, arbuscular mycorrhizal (AM) symbiosis is a common relationship between the roots of a great majority of plant species and glomeromycotan fungi. Its effects on the plant host are highly context dependent, with the greatest benefits often observed in phosphorus (P)-limited environments. Mycorrhizal contribution to plant nitrogen (N) nutrition is probably less important under most conditions. Moreover, inasmuch as both plant and fungi require substantial quantities of N for their growth, competition for N could potentially reduce net mycorrhizal benefits to the plant under conditions of limited N supply. Further compounded by increased belowground carbon (C) drain, the mycorrhizal costs could outweigh the benefits under severe N limitation. Using a field AM fungal community or a laboratory culture of Rhizophagus irregularis as mycorrhizal inoculants, we tested the contribution of mycorrhizal symbiosis to the growth, C allocation, and mineral nutrition of Andropogon gerardii growing in a nutrient-poor substrate under variable N and P supplies. The plants unambiguously competed with the fungi for N when its supply was low, resulting in no or negative mycorrhizal growth and N-uptake responses under such conditions. The field AM fungal communities manifested their potential to improve plant P nutrition only upon N fertilization, whereas the R. irregularis slightly yet significantly increased P uptake of its plant host (but not the host's growth) even without N supply. Coincident with increasing levels of root colonization by the AM fungal structures, both inoculants invariably increased nutritional and growth benefits to the host with increasing N supply. This, in turn, resulted in relieving plant P deficiency, which was persistent in non-mycorrhizal plants across the entire range of nutrient supplies.