Use of ¹³C- and phosphate ¹⁸O-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 ¹³C-labeled glucose backbone attached to a phosphate group with known δ¹⁸O_P 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 δ¹³C values of respired CO₂ and the δ¹⁸O_P values of soil bioavailable P were measured. Results: The δ¹⁸O_P 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 ¹³C glucose respiration in the higher P soil also triggered a stronger priming effect than in the lower P soil. Conclusions: Our approach demonstrates that ¹³C- and P-¹⁸O-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.