Summary• After nitrogen (N), phosphorus (P) is the nutrient that most limits plant productivity. The role of P on growth and root nodulation was studied in the actinorhizal symbiosis between Discaria trinervis and Frankia , an intercellular infected N 2 -fixing association.• Growth, nodulation and nutrient content (N and P) were analysed in symbiotic plants receiving different supplies of P in nutrient solutions. The relative requirement of P for nodulation was analysed in P-deficient plants.• Nodule initiation was less impaired than general plant growth by low P. However, low P impaired nodule growth to a greater extent than plant growth. The proportion of nodule biomass, although not the number of nodules per plant, was stimulated by P supply. Autoregulation of nodulation was not affected by P. Use of N was limited by availability of P. Reserves of P in seeds were enough for the seedling to establish nodules. However nodule (and plant) growth was limited in the absence of exogenous P.• It is possible that P interacts with the feedback control of nodule growth that is associated with the plant demand for N. Leaf N : P ratio is negatively correlated with the proportion of nodule tissue.

Analysis of phospholipids (PLFA) and neutral lipids fatty acids (NLFA) was used to characterize no‐till productive agricultural soils associated with different crop rotation levels, replicated across a 400 km transect in the Argentinean pampas, during two sampling seasons, summer and winter. High rotation (HR) management consisted in maize–wheat–soybean intense rotation including cover crops. Low rotation (LR) management trend to soybean monocultures. Soils from nearby natural environments (NEs) were used as references. Fatty acids concentration in soils (nmol/g) decreased c.a. 50% from summer to winter differentially according to soil treatment being the smallest decrease in HR management 35%. Both PLFA and NLFA profiles showed strong potential to discriminate between different land uses. In winter samples, some rare or unknown fatty acids were relevant for the discrimination of agricultural practices while NLFA 20:0 appears to be a good marker of HR soils despite season or location. The PLFA‐based taxonomic biomarkers for total bacteria, Gram‐negative bacteria and arbuscular mycorrhiza showed a significant trend NE>HR>LR in the winter sampling. HR management was also characterized by high levels of NLFA in winter samples as if high crop rotation improves lipids reserves in soil during winter more than in monocropping soil management. In conclusion, PLFA and particularly NLFA profiles appear to provide useful and complementary information to obtain a footprint of different soil use and managements, improving soil biochemistry characterization tools.

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