Processi rizosferici coinvolti nella nutrizione fosfatica

ABSTRACT RHIZOSPHERE PROCESSES INVOLVED IN PHOSPHATIC NUTRITION Although the phosphorus reserves in the soil are relatively abundant, the available form of phosphorus, the ortho-phosphate ion, is often present at concentrations too low for plants nutrition. It is for this reason that, to ensure adequate production levels in terms of quantity and quality, in the last century the use of large doses of fertilizers based on this nutrient has been growing. As the reserves of phosphate rocks, raw material for the production of phosphate fertilizers, are a limited resource, it has become increasingly important to understand the rhizosphere processes that regulate phosphate nutrition in order to better exploit the phosphorus present in the soil. Understanding the mechanisms that regulate the availability of phosphorus in the soil and how plants can change these mechanisms in order to increase the P availability can help to improve the use of phosphorus already present in the soil for a more sustainable agriculture. Scientific researches carried out to date highlight how the shortage of phosphorus in the soil can alter substantially the amount and composition of root exudates. In fact, phosphorus-lacking plants are able to produce and release significant quantities of di- or tri-carboxylic organic acids, such as citric, oxalic, malic, which can reach millimolar concentrations in the immediate vicinity of the root. These organic acids can mobilize phosphorus through exchange, solubilization and competition mechanisms. The more efficient carboxylic acids for the mobilization of phosphorus appear to be citric and oxalic acids, because of the high stability constants of the complexes with iron, aluminum and calcium. In various plant species, the phosphorus deficiency also leads to the production and release of phenolic compounds from the roots. The role of these compounds can be found in their antibiotic properties; in fact, they are able to limit microbial growth and thus they limit the microbial degradation of the organic acids released from the root. In addition, some phenols are signal molecules for the growth of mycorrhizae. Others, such as the p-hydroxyphenyl-tartaric acid, mobilize the P from iron phosphates because they are Fe(III) chelators. Moreover, in conditions of deficiency of phosphorus, the rhizosphere can be enriched in enzymes. High secretion of acid phosphatase and phytase by the roots determines an elevated hydrolysis of phosphorus organic esters, contributing to a greater absorption of this nutrient. The increased productivity of multi-species agro-systems can be explained by two important processes that result in a better use of resources: the complementarity and facilitation. Both in natural ecosystems and in agro-systems, the distribution of soil nutrients between different species has been studied mainly for N and especially for grain/legume intercropping system. Few studies have focused on the interactions of cereals and legumes regarding soil P. Recently, this field of research has attracted new interest with the reported evidence of P greater acquisition for cereals in intercropping with legumes. Current evidences for improving the growth and nutrition in cereals/legumes intercropping systems are promising for the ecological intensification of agro-ecosystems. Unfortunately, the studies on P are still rare, and few of these make the distinction between complementarity and facilitation. Further research are needed to confirm whether the positive interactions occur in consociations of cereals and legumes, which can be manipulated to improve the efficiency of P acquisition. It is desired that research and testing continue because, ultimately, this will be the real possibility that agriculture will have to increase food production in a sustainable way.