Environmental factors such as rainfall and redox are known to influence soil phosphorus (P) status via regulating the leaching loss of P and the retention of P by minerals and organic matter. However, in ecosystems experiencing extremely high rainfall (e.g., wet tropical forests), it is unclear whether changes in rainfall can still cast a meaningful impact on soil P speciation and on soil microbial P cycling. We studied soil P speciation and microbial biomass C:P ratios along a well-defined elevation gradient with 3200-4800 mm/yr rainfall in the Luquillo Experimental Forest in Puerto Rico. We analyzed soils from parallel transects of mixed forests and monodominant palm forests to examine the relative importance of rainfall and vegetation cover in affecting soil P dynamics. We predicted that rainfall variability would have a minimal impact on soil P speciation, as intensive weathering depleted soil P and left it mostly in the organic and residual forms. We also predicted that palm forest soils would contain more soil P and support microbes with low biomass C:P ratios, because palm trees prefer recently disturbed and unstable soils with more unweathered material.
Results/Conclusions
Along both transects, more soil P was found in the NaOH-extractable organic fraction with increasing mean annual rainfall, and less soil P was in the NaOH-extractable inorganic fraction. The accumulation of organic P was accompanied by the increases of soil C concentration and poorly crystalline iron and aluminum minerals with increasing rainfall. These results indicate that the stronger reducing conditions at wetter sites promoted the build-up of soil organic matter and increased its importance in retaining soil P. Neither the residual P pool nor soil total P changed linearly along the rainfall gradient. The mean value and variability of soil microbial biomass C:P ratios generally increased with increasing elevation except at the wettest site. This trend was stronger in mixed forest transect than in palm forest transect, as the latter contained more soil total P. In a laboratory experiment, we observed higher microbial biomass C:P ratios after anoxic incubation relative to oxic incubation in soils with different redox history. Overall, our results demonstrate that redox remains an important control of soil P speciation and microbial P cycling even in sites receiving extremely high rainfall. Compared to redox, vegetation cover likely imposes a secondary impact on soil P status.