ABSTRACT:

Humid tropical forests on highly weathered soils are often characterized by low bioavailable phosphorus (P) concentrations. These ecosystems also often experience low and fluctuating redox conditions. Little is known about how soil redox conditions affect P availability and how this might feed back on biogeochemical cycling. Here we used soils from a wet tropical rainfall gradient in Puerto Rico to explore the effects of redox on P bioavailability and associated biogeochemical processes. Concentrations of soil carbon (C) and poorly crystalline iron (Fe) and aluminum (Al) minerals increased at least two-fold with increasing rainfall, reflecting stronger anaerobic conditions at wetter sites and associated declines in decomposition. The fraction of the total P pool in the NaOH-extractable organic form also generally increased with increasing rainfall. In a laboratory incubation experiment using three sites along the gradient, P amendment increased aerobic CO2 production. However, anaerobic processes, including anaerobic respiration, Fe reduction, and methanogenesis, increased with P amendment at the driest site only. Microbial C:P ratios decreased with P amendment under anoxic conditions at the driest site, an indicator of possible microbial P limitation at this site. Both microbial biomass C and P concentrations were lower under anoxic conditions than under oxic conditions across all soils, suggesting that anoxic conditions could be a more limiting factor to microbes than P concentrations. Overall, our results demonstrate that redox conditions regulate the extent of P limitation to biogeochemical processes in tropical forest soils. Phosphorus limitation was pronounced in aerated environments with low mean annual rainfall, whereas low redox conditions or associated factors under high rainfall conditions may have a stronger impact on biogeochemical cycling than P availability.

ABSTRACT:

This dataset includes the CO2 flux and the FTICR-MS data reported in https://doi.org/10.1007/s10533-021-00790-y.

CO2 flux data
CO2 fluxes were measured in a laboratory incubation. Approximately 20 g (oven dry weight equivalent, ODE) of soil was weighed into each of 44 glass microcosms (487 ml). Twenty ‘trace gas’ microcosms (5 per treatment) were repeatedly monitored for CO2 production and destructively harvested at the end of the experiment. For the initial 16 days, all microcosms were exposed to a 4-day oxic (compressed medical grade air)/4-day anoxic (flushing with N2) pre-incubation period to allow soil respiration to stabilize. After the pre-incubation period, all microcosms were amended with 180 mg 13C-labeled ground ryegrass litter (97 atom%, Isolife, Wageningen, Netherlands; ~6% of the soil’s native C content) and incubated for 44 days. Microcosms were split into four redox treatments, managed via headspace manipulation: (1) static anoxic (N¬2 gas), (2) static oxic (medical air), (3) 4 days oxic/4 days anoxic (high frequency), (4) 8 days oxic/4 days anoxic (low frequency). Both high and low frequency treatments started and ended with oxic phases. Headspace samples were collected approximately every 4 days from the 20 trace gas microcosms (4 redox regimes × 5 replicates) to assess fluxes of CO2 and 13C-CO2 concentrations. For fluctuating treatments, microcosms were sampled immediately before the redox conditions were altered. To measure CO2 fluxes, microcosms were temporarily sealed for 2 hours, and gas samples were collected at the beginning and end of the sealed period by removing 30 ml of the headspace via septa into pre-evacuated 20 ml glass vials. Microcosms were sealed for 3 hours when CO2 production rates decreased towards the end of the experiment. CO2 concentrations were measured on a gas chromatograph (GC-14A, Shimadzu, Columbia, MD), equipped with a thermal conductivity detector. CO2 fluxes were determined by calculating the concentration difference during the sealed period, assuming a linear flux rate. An extra set of gas samples collected from each microcosm after the sealed period was analyzed for the 13C/12C CO2 isotope ratio with an isotope ratio mass spectrometer (IRMS; IsoPrime 100, Elementar, Hanau, Germany).

FTICR-MS data
Soil microcosms were destructively harvested at three timepoints during the experiment, on days 20 and 36 (n = 3 per treatment each day) and day 44 (n = 5 per treatment, the trace gas microcosms).
Samples exposed to an oxic headspace preceding the harvest were processed on the benchtop; those finishing an anoxic period were processed in an anoxic glove box (Coy Laboratory Products, Grass Lake, MI). 100 mg of soil (ODE) was shaken with water (2 ml) for 2 hours on an Eppendorf Thermomixer and centrifuged before collecting the supernatant. Water extracts were then desalted by solid phase extraction using Varian PPL cartridges according to Dittmar et al. (2008). A 12 T Bruker SolariX FTICR mass spectrometer was used to collect high resolution mass spectra of the organic compounds in the extracts. Refer to the GRE_13C_FTICR.R for instructions on processing the data in R.

ABSTRACT:

The strong phosphorus (P) sorption capacity of iron (Fe) and aluminium (Al) minerals in highly weathered, acidic soils of humid tropical forests is generally assumed to be an important driver of P limitation to plants and microbial activity in these ecosystems. Humid tropical forest soils often experience fluctuating redox conditions that reduce Fe and raise pH. It is commonly thought that Fe reduction generally decreases the capacity and strength of P sorption. Here we examined the effects of 14-day oxic and anoxic incubations on soil P sorption dynamics in humid tropical forest soils from Puerto Rico. Contrary to the conventional belief, soil P sorption capacity did not decrease under anoxic conditions, suggesting that soil minerals remain strong P sinks even under reducing conditions. Sorption of P occurred very rapidly in these soils, with at least 60% of the added P disappearing from the solution within six hours. Estimated P sorption capacities were much higher, often by an order of magnitude, than the soil total P contents. However, the strength of P sorption under reducing conditions was weaker, as indicated by the increased solubility of sorbed P in NaHCO3 solution. Our results show that highly weathered soil minerals can retain P even under anoxic conditions, where it might otherwise be susceptible to leaching. Anoxic events can also potentially increase P bioavailability by decreasing the strength, rather than the capacity, of P sorption. These results improve our understanding of the redox effects on biogeochemical cycling in tropical forests.