Use of 13 C- and phosphate 18 O-labeled substrate for studying phosphorus and carbon cycling in soils: a proof of concept.

RATIONALE: Stable isotope tracers are commonly used to track the transformations of organic carbon (C) and nitrogen (N) in soils but they have not been used to follow coupled cycles of phosphorus (P) and C because P has only one stable isotope.
METHODS: A novel dually labeled substrate, composed of a 13 C-labeled glucose backbone attached to a phosphate group with known δ18 OP value, was used to follow the fate of C and P derived from the same compound. The substrate was amended to soils from two natural oak forests, differing in their P levels, and the δ13 C values of respired CO2 and the δ18 OP values of soil bioavailable P were measured.
RESULTS: The δ18 OP values of the phosphate accumulated in the soil deviated from the expected value of P that is released from phosphomonoesters by enzymatic reactions (3.7 ‰) and were driven towards isotopic equilibrium with soil water (21.1 ‰), a process which is produced as part of microbial metabolism. This suggests that the substrate's P passed through the microbial biomass before accumulating in the soil, reflecting the rapid microbial turnover rate of the added P. A higher fraction of the substrate-C was respired in the higher P soil by the end of the incubation (20.3 % in comparison with 9.1 % in the lower P soil), indicating stronger energy limitation in the higher P soils. The higher 13 C glucose respiration in the higher P soil also triggered a stronger priming effect than in the lower P soil.
CONCLUSIONS: Our approach demonstrates that 13 C- and P-18 O-dually labeled tracers provide invaluable data on the fate of organic P and C in soils. We encourage biogeochemists and soil scientists to develop similar tracers of this kind.