The available pool of phosphorus (P) in most soils is rather small, while a large part is considered unavailable or poorly available to plants, even in the context of heavily-fertilized agroecosystems. Plant nutrition models assume that available P is depleted in the rhizosphere, due to root uptake and restricted mobility of soil P. These models implicitly assume that P uptake is the driving process of P acquisition by plants, and hence P bioavailability. However, as a consequence of a number of root functions, the rhizosphere is a hotspot of soil biogeochemical processes that can result in significant increase of soil P availability in the vicinity of roots, in spite of their uptake activity. We aimed at challenging the hypothesis of P depletion being the driving force of P acquisition in the rhizosphere and addressed this in several, contrasting soils and plant species. The rhizosphere of various crop species, either grown alone or intercropped, was sampled in field experiments, in order to measure changes of P availability relative to bulk soil. This was restricted to topsoils in field trials conducted in Southern France. We also sampled the rhizosphere of eucalypt trees down to 4-meter depth in a deeply-weathered soil in Brazil.
Results/Conclusions
Phosphorus availability (as assessed through water-, bicarbonate- and/or resin-extraction) seldom decreased and frequently increased in the rhizosphere, for P-poor and P-rich soils. In Brazil, whatever the depth, available P in the rhizosphere of eucalypt trees remained almost identical to bulk soil P, while significant acidification and increase of available potassium and organic carbon occurred concurrently. In Southern France, available P most frequently increased in the rhizosphere of a range of maize hybrids grown in a long-term fertilizer trial, especially in the calcareous soil and at the earlier stages of growth. At flowering however, a slight depletion of available P was found in the rhizosphere. In the same field trial and in another site, a systematic increase of available P was reported in the rhizosphere of durum wheat, and even more so in that of legumes. Collectively these results point to the occurrence of P-mobilizing processes of root- or microbial-origin, resulting in such increase of P availability in the rhizosphere. The bioavailability of soil P thus only partly depends on P availability and P uptake as it varies with plant species or genotypes, depending on their capacity to alter biogeochemical properties around roots, and ultimately to increase soil P availability.