Phosphorus mineralization can be driven by microbial need for carbon

Despite the importance of phosphorus (P) mineralization to maintain soil fertility, little is known about the mechanisms that regulate microbial P mineralization. We tested the hypothesis that microbial P mineralization can be driven by microbial need for carbon (C). For this purpose, net microbial uptake kinetics of 14C and 33P from glucose-6-phosphate were studied in a Leptosol depending on availability of C, nitrogen (N), and P. After 60 h of incubation, 16.4% of the 14C from glucose-6-phosphate was recovered in the microbial biomass, while 33P incorporation into the microbial biomass was a third less. The higher net uptake of 14C than of 33P from the glucose-6-phosphate indicates that soil microorganisms use the organic moiety of phosphorylated organic compounds as a C source, but only use a small proportion of the P. Hence, they mineralize P without incorporating it. Our finding that the net uptake of 14C and 33P in the soils amended with inorganic P did not differ from the control treatment indicates that P mineralization was not driven by microbial need for P but rather for C. In a second experiment with three temperate forest soils we found that the activity of 14C from glucose-6-phosphate in soil solution decreased faster than the activity of 33P from glucose-6-phosphate. This might suggest that higher net uptake of C than of P from glucose-6-phosphate can also be observed in other temperate forest soils differing in C, N, and P contents from the Leptosol of the main experiment. In conclusion, the experiments show that microbial P mineralization can be a side-effect of microbial C acquisition from which plants potentially can benefit.