Soil phosphorus (P) is commonly thought to limit primary productivity and microbial activities in humid tropical forests. Many soils in these forests experience frequent low oxygen conditions. Dr. Yang Lin, assistant professor of soil health in the UF/IFAS soil and water sciences department, wanted to examine the little-known interactive effects of P and redox on soil biogeochemistry. Redox, or reduction-oxidation reactions, is the transfer of electrons between chemicals.
He and his colleagues at the University of California-Berkley and Ben Gurion University of the Negev combined field observations with laboratory incubation for the project. They picked the Luquillo Mountains of Puerto Rico for their soil redox study. The location experiences a mean annual rainfall increase as elevation increases. For example, rainfall is 2000 mm near the coast but 4800 mm near the mountain peak (1000 m in elevation). At higher elevations, Lin said soils are wetter with more frequent low redox events.
“We studied soil properties along this redox/elevation gradient and soil organic carbon at 1000 meters was more than double that from 300 meters,” he explained. “Soil organic P also accounted for a larger proportion of soil total P at wetter sites.”
These results indicate that low redox conditions at wetter sites suppressed organic matter decomposition and organic P mineralization.
The team then took three soils from this gradient (at 400, 800, and 1000 m elevation) and exposed them to a factorial experiment of redox (oxic vs anoxic) and P availability (with vs. without P addition).
“We found that redox conditions regulated how P amendment influenced soil biogeochemical processes,” Lin said. “Under oxic conditions, P amendment increased soil carbon dioxide production in all three soils, indicative of P limitation of microbial decomposition. Surprisingly, soil phosphorus amendment only increased anaerobic biogeochemical processes. That includes carbon dioxide production, methane production, and iron reduction, in one of the three soils.”
Results show anoxic conditions could be a more limiting factor for microbial processes than low P availability. In all three soils, anoxic treatment strongly reduced microbial biomass carbon and P relative to oxic conditions.
“Overall, our results demonstrate that redox conditions regulate the extent of P limitation to biogeochemical processes in tropical forest soils,” Lin said.