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 32P radioisotopes, the second group labelled with 18O 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 IIa 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 32P 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). 18O 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 IIa process) had occurred and had affected active microbes, which passed the 18O labelled phosphate to the resin pool (category I process). The dual isotopic approach thus provided complementary insights on P cycling processes.