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Abstract. The strong phosphorus (P) sorption capacity of iron (Fe)and aluminum (Al) minerals in highly weathered, acidic soils of humidtropical forests is generally assumed to be an important driver of Plimitation to plants and microbial activity in these ecosystems. Humidtropical forest soils often experience fluctuating redox conditions thatreduce Fe and raise pH. It is commonly thought that Fe reduction generallydecreases the capacity and strength of P sorption. Here we examined theeffects of 14 d oxic and anoxic incubations on soil P sorption dynamics inhumid tropical forest soils from Puerto Rico. Contrary to the conventionalbelief, soil P sorption capacity did not decrease under anoxic conditions,suggesting that soil minerals remain strong P sinks even under reducingconditions. Sorption of P occurred very rapidly in these soils, with atleast 60 % of the added P disappearing from the solution within 6 h.Estimated P sorption capacities were much higher, often by an order ofmagnitude, than the soil total P contents. However, the strength of Psorption under reducing conditions was weaker, as indicated by the increasedsolubility of sorbed P in NaHCO3 solution. Our results show that highlyweathered soil minerals can retain P even under anoxic conditions, where itmight otherwise be susceptible to leaching. Anoxic events can alsopotentially increase P bioavailability by decreasing the strength, ratherthan the capacity, of P sorption. These results improve our understanding ofthe redox effects on biogeochemical cycling in tropical forests. 

Humid tropical forests are among the most productive ecosystems globally, yet they often occur on soils with high phosphorus (P) sorption capacity, lowering P availability to biota. Short‐term anoxic events are thought to release sorbed P and enhance its acquisition by soil microbes. However, the actual effects of anoxic conditions on microbial P acquisition in humid tropical forest soils are surprisingly poorly studied. We used laboratory incubations of bulk soils, NanoSIMS analysis of single microbial cells, and landscape‐scale measurements in the Luquillo Experimental Forest (LEF), Puerto Rico to test the hypothesis that anoxic conditions increase microbial P acquisition in humid tropical forests. In laboratory and field experiments, we found that microbial P uptake generally decreased under anoxic conditions, leading to high microbial carbon (C) to P ratios in anoxic soils. The decreased P acquisition under anoxic conditions was correlated with lower microbial C use efficiency (CUE), an index of microbial energy transfer in ecosystems. Phosphorus amendments to anoxic soils led to increased microbial P uptake and higher CUE suggesting that microbes were less able to access and utilize P under natural low redox conditions. Under oxic conditions, microbial C:P ratios and CUE did not respond to changes in substrate stoichiometry. These results challenge the existing paradigm by showing that anoxic conditions can decrease microbial P uptake and ultimately constrain microbial CUE. Our findings indicate that soil redox conditions tightly couple soil P and C cycles and advance our understanding of controls on P cycling in humid tropical forest ecosystems.