Mechanism of the trade-off between biological nitrogen fixation and phosphorus acquisition strategies of herbaceous legumes under nitrogen and phosphorus addition

Abstract

Aims Nitrogen fixation of herbaceous legumes is not only an important natural nitrogen input to terrestrial ecosystems, but also determines the economy and sustainability of grassland production.This study aimed to determine the underlying physiological and ecological mechanisms of the interaction between nitrogen (N) and phosphorus (P) on biological N fixation rate of legumes. MethodsIn a pot experiment, eight species of herbaceous legumes were separately grown in soils with four treatments including no fertiliser, N addition, P addition, and both N and P (NP) addition.Plant biomass and nutrients concentrations, root carbohydrate concentration, pH in rhizosphere, citric concentration in in rhizosphere, avaiable P concentration in rhizosphere, root nodule biomass, P concentration in root nodule, and N fixation rate of these legume plants were examined.Important findings Depending on legume species, N addition significantly increased relative rhizosphere P mobilization, but reduced investment in root biomass and the concentration of non-structural carbohydrate (NSC) in roots.Averaged results of N addition and NP addition treatments indicated that N addition caused 27%-36% decline in nodule biomass and 20%-33% decline in biological N fixation rate for the studied eight legume species.By contrast, P addition significantly promoted root development and NSC accumulation associated with decreasing relative rhizosphere P mobilization.Consequently, P addition increased the biological N fixation rate of the

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