The use of fertilizers with beneficial microorganisms has increased in recent years. In this study, the performance was assessed of two manures enriched with Azotobacter (BioF1 and BioF2), a non-enriched organic manure (Organ), an inorganic N fertilizer applied at a rate equivalent to the organic manures (MinR1) and applied at twice the rate (MinR2), and a control treatment. A field trial and a pot experiment were carried out both consisting of a sequence of three crops per year [lettuce (Lactuca sativa)-lettuce-turnip (Brassica rapa)] grown for two years. Above ground dry matter (DM) yield and N recovery were higher in the inorganic fertilized plots in comparison to the organic manured plots. Anion exchange membranes inserted into the soil in short periods during the growing seasons revealed higher soil nitrate levels in the inorganic fertilized treatments. Organic amendments improved performance over time, proving that their fertilizing effect, though modest in the short-term, lasts longer. The biofertilizers containing Azotobacter (BioF1, BioF2) increased the bioavailability of N over Organ, by an additional N-fixing value of 11.4 kg ha −1 estimated from the six crops of the field experiment (∼5.7 kg N per year). If compared on the basis of the same amount of N recovered, organic amendments produced an average increase of 720 kg DM ha −1 over the inorganic fertilizer (∼120 kg per crop) due to a general manuring effect. From the results of these experiments, no beneficial effects on crop growth could be attributed to biofertilizers other than the slight increase in N fixation.
