Phosphorus mobilization in low-P arable soils may involve soil organic C depletion

Organic farming systems often show negative nutrient balances that may compromise the availability of phosphorus over the mid-term. Calcareous soils with low organic matter generally show low or very low phosphorus availability. Under these conditions, P retained in soil organo-mineral complexes -either in organic or inorganic forms-after its solubilization and -in the case of organic P compounds-subsequent mineralization may be a source of P to plants and soil organisms. By studying the changes in soil retained and soluble P pooloccurring during the growth of two legumes in two scenarios of contrasting P availability we aimed to describe the release of soil retained P, organic and inorganic, into the soluble pools and to relate it to changes in organic C during crop growth. The experimental design was a split-plot of two nearby fields with contrasting P availability. In each field, two species of legumes, chickpea and bitter vetch, were planted and manure was added in alternating plots. During the legume growth we monitored soil P by analyzing soluble inorganic and organic P (NaHCO3-P-i and NaHCO3-P-o), the soil retained inorganic and organic P pools (H2SO4-P before and after ashing), and acid and alkaline phosphatase activities in each plot at sowing time and at the late flowering stage. All P forms were higher in the high-P field than in the low-P field, except for soluble P-o, which was higher in the low-P field at sowing time. In the high-P field during legume growth we detected an increase of the soluble P-o and soluble P-i pools and of the soil retained P-o that may have originated from a reduction of the soil retained P-i pool. In contrast, in the low-P field, the decrease of the soil retained P-i pool coincided with a decrease of the soluble P-o pool, while the soil retained P-o did not show any significant change. In low-P soils, the soil retained P-o pool appeared to be the main source of soluble P-i. Changes in the soil retained P-i pool during legume growth occurred in all soils and were much larger than the amount of P required by plants. Likewise, the most likely P transformations in our soils involved changes between inorganic and organic forms suggesting that these changes were mainly mediated by soil microbiota. P transformations in low-P soils reduced soil organic C and the C/P-o ratio, thus suggesting that crops growing in low-P soils may deplete organic matter from protected mineral-organic associations in low organic matter arable soils likely by promoting organic acid exudation by roots and soil microbiota