Summary
1Ratios of nitrogen to phosphorus (N:P) in plant foliage have been used to assess nutrient limitation in wetland ecosystems and to indicate nitrogen saturation. Extension of this application to ecosystems other than wetlands remains to be evaluated.
2We compared published N:P ratios as thresholds of nutrient limitation with published accounts from nutrient-addition experiments and the N:P ratios of understorey vegetation (Acer spp., Dryopteris intermedia, Erythronium americanum, Lycopodium lucidulum, Oxalis acetosella and Viola macloskeyi) from the Catskill Mountains of New York State, USA. We also performed a nutrient-addition experiment to test the response of these understorey plant species to inputs of N and P.
3N:P ratios of Catskill understorey species indicated they were at or near P limitation relative to N:P ratios from other upland ecosystems. Our experiment supported this finding in that none of the species responded to N addition but all increased in P concentration and one increased in biomass with added P. Collectively, these results suggest that the understorey vegetation of the Catskill Mountains is not nitrogen limited, providing further evidence that hardwood forests in this area are nitrogen-saturated.
4Synthesis and applications. This study demonstrates that N:P ratios can be effective predictors of nutrient limitation in upland ecosystems. Therefore N:P ratios can be used for management and monitoring purposes in considering the nutrient status of upland ecosystems with particular relevance to the continued deposition of elevated atmospheric N and to the diagnosis of nitrogen saturation.

Use of nitrogen to phosphorus ratios in plant tissue as an indicator of nutrient limitation and nitrogen saturation

C, N and P are three of the most important elements used to build living beings, and their uptake from the environment is consequently essential for all organisms. We have reviewed the available studies on water, soils and organism elemental content ratios (stoichiometry) with the aim of identifying the general links between stoichiometry and the structure and function of organisms and ecosystems, in both aquatic and terrestrial contexts. Oceans have variable C:N:P ratios in coastal areas and a narrow range approximating the Redfield ratio in deep water and inner oceanic areas. Terrestrial ecosystems have a general trend towards an increase in soil and plant N:P ratios from cool and temperate to tropical ecosystems, but with great variation within each climatic area. The C:N:P content ratio (from now on C:N:P ratio) is more constrained in organisms than in the water and soil environments they inhabit. The capacity to adjust this ratio involves several mechanisms, from leaf re-absorption in plants to the control of excretion in animals. Several differences in C:N:P ratios are observed when comparing different taxa and ecosystems. For freshwater ecosystems, the growth rate hypothesis (GRH), which has consistent experimental support, states that low N:P supply determines trophic web structures by favoring organisms with a high growth rate. For terrestrial organisms, however, evidence not yet conclusive on the relevance of the GRH. Recent studies suggest that the N:P ratio could play a role, even in the evolution of the genomes of organisms. Further research is warranted to study the stoichiometry of different trophic levels under different C:N:P environment ratios in long-term ecosystem-scale studies. Other nutrients such as K or Fe should also be taken into account. Further assessment of the GRH requires more studies on the effects of C:N:P ratios on anabolic (growth), catabolic (respiration), storage and/or defensive allocation. Combining elemental stoichiometry with metabolomics and/or genomics should improve our understanding of the coupling of different levels of biological organization, from elemental composition to the structure and evolution of ecosystems, via cellular metabolism and nutrient cycling.

The elemental stoichiometry of aquatic and terrestrial ecosystems and its relationships with organismic lifestyle and ecosystem structure and function: a review and perspectives

Phosphorus (P) and nitrogen (N) effects on nodulation, nitrogenase activity and plant growth were studied in the root-hair-infected actinorhizal plant Alnus incana (L.) Moench. A split-root experiment, as well as a short-term experiment with entire root systems and a broader range of P concentrations, showed that P effects were specific on nodulation and not a general stimulation via a plant growth effect. These results indicate that nodule initiation and nodule growth have a high P demand. The split-root assay, comprising seven combinations of two N and two P levels, showed that P could counteract systemic N inhibition of nodulation, but did not counteract N inhibition of nitrogenase activity.

Local and systemic effects of phosphorus and nitrogen on nodulation and nodule function in Alnus incana

Understanding the interactions between endophytic bacteria, rhizobia, free living root associated bacteria and their host plants under stressed conditions remains a significant challenge for proposing strategies to improve the efficacy of PGPR In this study we analyzed the role of the endophytic bacterium Stenotrophomonas rhizophila in alleviating salinity stress in plants The nodulation efficiency, plant growth, nitrogen and phosphorus uptake of soybean under hydroponic salt stress conditions were determined Soybean seedlings were inoculated with Bradyrhizobium japonicum BDYD1 and S rhizophila ep-17 were grown in hydroponic plastic pots containing 2 l of Hoagland solution for 42 days Salinity conditions were established by adding 50 and 75 mM NaCl to the nutrient solution The results showed that the salinity decreased the colonization of B japonicum BDYD1 in the rhizosphere of soybean, inhibited shoot, root growth, and nodulation compared with those of unstressed plants We found synergistic interactions between compatible salt tolerant S rhizophila ep-17 and B japonicum BDYD1 strains which were manifested themselves as improved root, shoot length, dry weight, N and P uptake and number of nodules compared with the uninoculated plants grown under 75 mM NaCl condition S rhizophila and Bradyrhizobium build beneficial association in the rhizosphere and can act synergistically on promoting plant growth, nutrient uptake and fitness of hydroponically grown soybean under salt stress condition

Synergistic interactions between Bradyrhizobium japonicum and the endophyte Stenotrophomonas rhizophila and their effects on growth, and nodulation of soybean under salt stress

Frankia is a genus of soil actinomycetes famous for its ability to form N2-fixing root nodule symbioses with actinorhizal plants. Although Frankia strains diplay a high diversity in terms of ecological niches in soil, current knowledge about Frankia is dominated by its life as an endophyte in root nodules. Increased use of molecular methods has refined and expanded insights into endophyte-host specificities and Frankia phylogeny. This review has focus on Frankia as a soil organism, including its part of microbial consortia, and how to study Frankia in soil. We highlight the use of nodulation tests and
molecular methods to reveal population size and genetic diversity of Frankia in soil and discuss how
autoregulation of nodulation and interactions with other soil microorganisms may influence the results. A
comprehensive record of published interactions between Frankia and other soil microbes is summarized.

Life in soil by the actinorhizal root nodule endophyte Frankia

Red clover (Trifolium pratense L.) is one of the most important plants in forage production, especially in northern areas. Fertilisation practices are focused on high yield and forage quality but effects of nutrients on nodulation and N2 fixation are poorly understood. The aim of this work was to study how nitrogen (N) and phosphorus (P) separately as well as in combination affected nodulation. Red clover plants were grown in pots with gravel in a greenhouse for 11 weeks. To resemble field conditions the root temperature was kept lower than the shoot temperature. Plants were given five different combinations of N and P concentrations during growth. The result showed that at high N concentrations P had a counteracting effect on the N inhibition. The N2-fixation parameters, nodule number, nodule dry matter and specific nitrogenase activity, were six times higher in plants grown with high N and high P than in plants with high N and low P. When the N2-fixation parameters and the dry matter of roots and shoots...

Interaction Effects of Nitrogen and Phosphorus on Nodulation in Red Clover (Trifolium pratense L.)

Nodulation of Alnus incana by Frankia was studied at three levels of N (ammonium nitrate, 0.071-7.1 mM N) combined with three levels of P (0.01-1.0 mM) in a factorial design. Nodulation of Trifolium pratense by Rhizobium leguminosarum bv. trifolii was studied in a partial factorial design. Plants were in growth pouches for 10.5 weeks. In general, the degree of N inhibition depended on the P level. In A. incana, high P level stimulated nodule number and nodule dry matter per plant and per plant dry matter or per root dry matter. High P also stimulated nodule size and nitrogenase activity. Effects on nodule number seemed to be largely explained by plant growth whereas P had more of a specific effect on nodule dry matter. The N/P ratio was important, and increased N levels inhibited nodulation at N/P ratios >7 but not at N/P ratios <7. In T. pratense, high P level counteracted the inhibition of high N on nodule number and nitrogenase activity. The fact that N effects on nodulation and nitrogenase activity depend on P level should encourage more detailed work on effects of nutrient interactions on nodulation, both in actinorhizal plants and in legumes.

Anna Hellsten

Papers

Interaction Effects of Nitrogen and Phosphorus on Nodulation in Red Clover (Trifolium pratense L.)

Nitrogen, Phosphorus, and the Ratio Between them Affect Nodulation in Alnus incana and Trifolium pratense