Fertilizing a tropical rainforest stimulates cycling of phosphorus but not nitrogen via increased foliar leaching

Palani Akana and Duncan Menge, Ecology, Evolution, and Environmental Biology, Columbia University, New York, NY, S Joseph Wright, Smithsonian Tropical Research Institute, Panama, Panama

Background/Question/Methods
While litterfall is usually considered the predominant pathway of nutrient recycling in forests, recycling driven by foliar leaching of nutrients in rainwater is also an important pathway, especially in tropical rainforests where precipitation is ample. Tropical rainforests exhibit large variation in soil fertility, and this has been linked to a variety of adaptations in nutrient acquisition, retention, usage, and recycling by trees. However, the effect of soil fertility on nutrient recycling via foliar leaching is mostly uncharacterized in tropical rainforests. We asked: “How does soil fertility affect (a) the amount of foliar leaching of canopy nitrogen and phosphorus, and (b) the contribution of foliar leaching relative to litterfall in rainforest nutrient recycling?” We sampled precipitation from the Gigante Fertilization Project in Panama, where the forest has been fertilized with nitrogen, phosphorus, and potassium in a factorial design since 1998. Rainfall and throughfall were collected from four rain events during the rainy season in 2018 and analyzed for concentrations of inorganic (plant-available) nitrogen and phosphorus. Each of the nearly 400 samples were paired with a canopy photo to determine the effect of leaf area index on nutrient concentrations in throughfall.
Results/Conclusions
Inorganic nitrogen deposition via throughfall was mostly insensitive to fertilization treatment, and foliar leaching contributed a very small fraction to total nitrogen recycling. In contrast, inorganic phosphorus (phosphate; PO4) deposition in throughfall increased 500% with phosphorus fertilization, from 1.4 to 8.5 kg ha-1 y-1, suggesting that increased soil phosphorus supply stimulates leaching of phosphorus from canopy leaves during rain events. Previous data from the Gigante Fertilization Project shows that litterfall phosphorus increased 100% with phosphorus fertilization, from 5.4 to 11.1 kg ha-1 y-1. When both litterfall and throughfall are considered, phosphorus fertilization increased the rate of phosphorus cycling between the canopy and forest floor by 200%. Foliar leaching of phosphate scaled positively with leaf area index (the number of leaf layers with which precipitation interacts while passing through the canopy), while inorganic nitrogen showed an increase in foliar leaching of ammonium (NH4+) with increasing leaf area index being offset by increased canopy retention of nitrate (NO3-). While our results agree with previous research where phosphorus was more easily leached from leaves than was nitrogen, the magnitude of the effect of phosphorus fertilization on foliar phosphorus leaching was unexpected and highlights an important area for further research on tropical phosphorus cycling.