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

Abstract

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-Pi and NaHCO3-Po), 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 Po, 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 Po and soluble Pi pools and of the soil retained Po that may have originated from a reduction of the soil retained Pi pool. In contrast, in the low-P field, the decrease of the soil retained Pi pool coincided with a decrease of the soluble Po pool, while the soil retained Po did not show any significant change. In low-P soils, the soil retained Po pool appeared to be the main source of soluble Pi. Changes in the soil retained Pi 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/Po 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.