Phosphorus sorption and availability in an andosol after a decade of organic or mineral fertilizer applications: Importance of pH and organic carbon modifications in soil as compared to phosphorus accumulation.

The effect of organic fertilizers on soil phosphorus (P) availability is usually mainly associated with the rate and forms of P applied, while they also alter the soil physical-chemical properties, able to change P availability. We aimed to highlight the impact of pH and organic C modifications in soil on the inorganic P (Pi) sorption capacity and availability as compared to the effect of P accumulation after mineral or organic fertilizers. We conducted a 10-years-old field experiment on an andosol and compared fields that had been amended with mineral or organic (dairy slurry and manure compost) fertilizers against a non-fertilized control. Water and Olsen extractions and Pi sorption experiments were realized on soils sampled after 6 and 10 years of trial. We also realized an artificial and ex situ alkalization of the control soil to isolate the effect of pH on Pi sorption capacity. Organic fertilizer application increased total P, pH, and organic C in soil. Pi-Olsen increased mainly with soil total P (r2 adj = 0.79), while Pi-water increased jointly with soil total P and pH (r2 adj = 0.85). The Pi sorption capacity decreased with organic fertilizer application. Artificial and ex situ alkalization of the control soil showed that Pi sorption capacity decreased with increasing pH. Our study demonstrated that, beyond the P fertilization rate, the increase in organic C content and even more so in pH induced by a decade of organic fertilizer applications in soil decreased the Pi sorption capacity and consequently increased Pi-water in soil.

Phosphorus-acquisition strategies of canola, wheat and barley in soil amended with sewage sludges.

Crops have different strategies to acquire poorly-available soil phosphorus (P) which are dependent on their architectural, morphological, and physiological root traits, but their capacity to enhance P acquisition varies with the type of fertilizer applied. The objective of this study was to examine how P-acquisition strategies of three main crops are affected by the application of sewage sludges, compared with a mineral P fertilizer. We carried out a 3-months greenhouse pot experiment and compared the response of P-acquisition traits among wheat, barley and canola in a soil amended with three sludges or a mineral P fertilizer. Results showed that the P-acquisition strategy differed among crops. Compared with canola, wheat and barley had a higher specific root length and a greater root carboxylate release and they acquired as much P from sludge as from mineral P. By contrast, canola shoot P content was greater with sludge than with mineral P. This was attributed to a higher root-released acid phosphatase activity which promoted the mineralization of sludge-derived P-organic. This study showed that contrasted P-acquisition strategies of crops allows increased use of renewable P resources by optimizing combinations of crop and the type of P fertilizer applied within the cropping system.