Oxygen Isotopes of Phosphate in Semi-Arid and Tropical Soils, and Their Usage for Dust Tracing.

Phosphorus (P) availability limits productivity in many ecosystems worldwide. As a result, improved understanding of P cycling through soil and plants is much desirable. The possible use of the stable oxygen isotopes of soil phosphate (d¹⁸O_p) as a tracer for P cycling has gained attention lately. This use is based on the stability of the P-O bond, which under natural soil conditions is broken only by enzyme-mediated reactions. While the basic enzymatic reactions that control variation in d¹⁸O_p are known, the combined effect of the processes occurring in natural soil systems is not well understood. We have performed a large set of incubation and field experiments with natural soils, from both semi-arid (Israel) and tropical forest (Panama) ecosystems. We found that in semi-arid systems, any phosphate added to the soils equilibrates isotopically with the soil water. Addition of organic phosphate caused a transient shift to lower isotopic values, which agrees well with enzyme experiments. However, the isotopic value quickly returns to the values expected from equilibration with water. Since release of phosphate from organic compounds is accompanied by a shift from equilibration values, the general trend toward equilibrium values cannot be explained by the death of microbiota, which release also organic phosphate. A more likely explanation is uptake and release of inorganic phosphate by live microbiota. The existence of this process was shown by previous studies, and indicates that the microbiota have strong control over phosphate availability for plants. In the tropical forest soil, in contrast to the equilibrium d¹⁸O_p values usually found in natural semi-arid and temperate soils, we found d¹⁸O_p values which are ~4 permil above equilibrium. However, we have also found fast isotopic exchange with the water, in experiments with labeled water.  These results may indicate a different pathway for phosphate cycling in tropical forests soils. Incubations of tropical forest soils from long-term factorial NPK fertilization treatments indicated variability in phosphate turnover, captured by temporal variation in d¹⁸O_p. The d¹⁸O_p signature was also used to trace the sources of P in dust arriving to Lake Kinneret (Israel) and to the Barro-Colarado Island (Panama).