Effects of inorganic and organic P availability on N fixing capacity of Vicia villosa

Cover crops offer various ecosystem services that contribute to the sustainability of intensive cropping systems. In particular, leguminous cover crops may represent an important source of nitrogen (N) for the following income crop. Through the symbiotic association between plant roots and N2 fixing bacteria, biological N fixation (BNF) may contribute as much as 65-95% of the total plant N. However, BNF requires considerable amounts of energy in the form of ATP, and its efficiency could therefore depend on soil phosphorus (P) availability. Apart from the uptake of readily available inorganic P (Pi), leguminous plants can favour the hydrolysis and P availability from organic P sources (Po), through the production of phosphatase enzymes in order to overcome P deficiencies. However, there is scant information on the possible limiting effect of low P availability on BNF, and it is still unclear if, and to what extent, the dominant form of P in soil (Po vs Pi) can affect BNF. The aim of this work was to understand how the availability and forms of soil P can affect the production of biomass and BNF efficiency of a typical cover crop (Vicia villosa). Vetch plants were cultivated in mesocosms utilizing a P-poor agricultural soil at three P addition levels (no P: control; low P: +50 mg P kg-1; high P: +200 mg P kg-1) applied in form of inorganic (KH2PO4) or organic P (inositol hexaphosphate). Soils and plants were destructively sampled after 30, 50 and 70 days. Above and below ground biomass production, the total amount of nodules, as well as C, N and P contents were quantified. BNF efficiency was evaluated by isotope dilution after application of 15NO3 ‒. Soil inorganic N forms and available P were also determined and the microbial activity in the rhizosphere has been studied. Results evidenced that above ground biomass was proportional to the amounts of Pi applied and, hence, to Pi availability. With adequate Pi supply, the shoot/root ratio is the highest, suggesting that the plant allocated less C and other nutritional resources below ground for soil exploration for nutrient acquisition by developing an extended rooting system. Plants receiving organic P, conversely, needed a longer time to increase P availability with the progressive hydrolysis of organic P sources, requiring a greater investment of plant resources.