A dual isotopic (32P and 18O) incubation study to disentangle mechanisms controlling phosphorus cycling in soils from a climatic gradient (Kohala, Hawaii)

Changes in the isotopic composition of oxygen associated with phosphate can provide information on the impact of phosphatase activity on soil P dynamics, whereas the use of radioactive P delivers information on P fluxes within soil systems. Although these two tracers may provide complementary data, they have rarely been used together to study soil P cycling. We conducted a dual isotopic soil incubation study of one month with soils originating from four sites of a climatic gradient (Kohala, Hawaii), which provides well-controlled geochemical and biological variations on soils derived from the same parent material. Three groups of soils were incubated in parallel, the first group labelled with P-32 radioisotopes, the second group labelled with O-18 enriched water and the third group not labelled and used for CO2 emission measurements. The dual labelling study informed about three processes controlling P dynamics in soils: those that maintain the bond between P and O and transfer phosphate from one pool to another (category I processes), those that involve the cleavage of the P-O bond and transfer phosphate from one pool to another (category Ita processes), and those that involve the cleavage of the P-O bond but do not transfer phosphate from one pool to another (category IIb processes). The use of P-32 showed that the studied soils contained a large amount of P that was isotopically exchangeable with the resin P pool (category I process) and that microorganisms had taken up P, but in much lower amounts, from the resin P pool (category I process). O-18 added with water was incorporated into microbial and resin P, but not into the other pools obtained from the modified Hedley extraction. Thus, the turnover of O associated with P within microbial cells (category IIb process) and/or enzymatic hydrolysis of organic P (category Ita process) had occurred and had affected active microbes, which passed the O-18 labelled phosphate to the resin pool (category I process). The dual isotopic approach thus provided complementary insights on P cycling processes.