How do phosphorus, potassium and sulphur affect plant growth and biological nitrogen fixation in crop and pasture legumes? A meta-analysis

•Review of phosphorus, potassium and sulphur effect on symbiotic N2 fixation.•Nodule mass and number are more responsive to nutrient deficit than shoot biomass.•N2 fixation is more responsive to P and K deficit than shoot and nodule mass.•The ratios N/K, N/P and N/S in shoots increased under nutrient deficiency.•Patterns of growth, N2 fixation and nutrient concentration were similar for P, K and S deficiency. Worldwide, legumes are grown on approximately 250Mha and fix about 90Tg of N2 per year. Plants involved in biological nitrogen fixation (BNF) are particularly sensitive to deficiencies of phosphorus (P), potassium (K) and sulphur (S). These nutrients can affect BNF directly; this is modulating growth of rhizobia, nodule formation and functioning, or indirectly by affecting the growth of the host plant. However, several process and mechanisms remain unclear. We compiled a data set (63 studies) on the effects of P, K, and S deficiency on shoot mass, nodule mass and number, nitrogenase activity (estimated by the acetylene reduction activity test, ARA) and the concentration of N, P, K and S in shoots and nodules. Our aims were (1) to compare the relative sensitivity of these traits to nutrient deficiency and (2) to probe for nutrient-specific patterns in trait responses. Our quantitative analysis confirms that nodule growth and number are more sensitive than shoot mass in response to deficiency of P, K and S. In addition, nodule activity decreases more than both shoot and nodule mass, which indicates a reduction in nodule productivity; this is likely related with direct effects of these nutrients on physiological and metabolic processes of nodules. The conserved shoot N concentration, in comparison to concentration of P, K and S indicates a relatively greater accumulation of N that matches the proposed N-feedback mechanism down-regulating BNF in nutrient-deficient systems. Despite some nutrient-specific differences, i.e. smaller nodules and higher N/K ratio with shortage of P and K, respectively, the patterns of growth, nodule activity and nutrient concentration were similar for all three nutrients P, K and S. This indicates that a unique mechanism could be depressing BNF (N-feedback) in conjunction with direct effects of the nutrients on nodule activity. Scarcity of data related to N, K and S concentration in nodules is a major constrain for deep analysis of the deficiencies of the nutrients. Critical concentrations of P, K and S in plant and nodule tiss