Microbial community phosphorus immobilization is inhibited by overabundance of labile phosphorus

Background/Question/Methods

When soil microorganisms decompose organic matter, homeostasis is maintained with growth and can dictate the availability of soil nutrients. This can affect ecosystem productivity and function because nutrients that exceed microbial metabolic demand will be released whereas when deficient will be retained. Ecological stoichiometry can be a useful research tool to describe this microbial nutrient economy. Since many modern ecosystems are accumulating N via anthropogenic means, other nutrients such as P may be having a stronger influence on microbial growth and elemental stoichiometry. This work investigated microbial community P uptake strategy. We hypothesized that microbial P immobilization should be relatively lower in microbial communities that have had chronic access to more labile P compared to those deficient by P. Microbial P immobilization and release was measured through microbial C:P composition in native soils, and soils annually amended with P for five years. Treatments were incubated for five months to cause labile C limitation, followed by a 35 mg P kg soil^-1pulse to saturate P and induce potential P immobilization. Microbial community composition and biomass were measured through total fatty acid methyl ester analysis.

Results/Conclusions

There was no significant change in the microbial community composition, biomass, or soil pH caused by the pulse between treatments over the course of the experiment. Microbial C:P nearly doubled (P<0.03) in the elevated P treatment following the P pulse, which appeared to occur above an available P level of ~13 mg P kg soil^-1. Results also suggest that soil pH may be an important driver of microbial C:P content, because pH explained nearly 50% of the variation in microbial P (P≤0.02). This indicates an overabundance of labile P caused an increase in microbial C:P, which is inconsistent with our hypothesis - suggesting that the pulse may have actually inhibited microbial P immobilization instead of stimulating it. In reality, there may be selection factors that call for communities to generally take up as much P as possible, with any upper stoichiometric constraints generally never being reached in natural systems due to overall bioavailable P scarcity or physiologic limitation. Surpluses in the microbial P economy may result in decreased P hoarding.