Contribution of beneficial rhizosphere micro-organisms to growth and nitrogen uptake of lettuce plants under field conditions.

Improvement of crop yield with the use of beneficial rhizosphere micro-organisms could be considered a promising strategy to reduce the input. Several mechanisms have been proposed to better explain plant growth stimulation, but the information is often contradictory in relation to the influence of a wide range of biological and agronomic factors. A research was carried out to provide more knowledge for the use of these microorganisms, through the evaluation of their influence on lettuce growth and nitrogen uptake in relation to different nitrogen supply. Lettuce plants (Lactuca saliva L. var. longifolia Lam., 'Passpartout' F(1)) were grown on sandy soil during a spring-summer cycle. Four micro-organism treatments (Alcaligenes piechaudii, Azospirillum brasiliense, Pseudomonas mendocina, Glomus intraradices) and an untreated control were combined with two nitrogen levels (33 and 110 kg ha(-1)). The results showed a positive effect of micro-organism inoculation on lettuce response which was in some cases dependent by nitrogen applied. At the end of cycle (56 days after transplanting), all the studied micro-organisms caused an increase in yield level and total dry biomass (about +40% compared to the control). In the lowest N availability, all micro-organisms determined a similar (A. brasiliense) or a significantly greater dry matter production than the untreated plants grown at high N supply (+23% for A. piechaudii and P mendocina and +28% for G intraradices). The most evident effect of micro-organism inoculation was an increase of root length, especially in sub-optimal nitrogen conditions, that consequently had a positive influence on mineral absorption. The results evidence the utility of these micro-organisms to improve the sustainability of agro system by reducing the supply of fertilizers.