Effects of Nutrient Availability on Root Nutrient Uptake and Carbon Exudation of Mature Tetragastris panamensis in a Lowland Humid Tropical Forest

Ecosystem processes are fundamentally limited by nutrient availability, but the effects of nutrient limitation or co-limitation on key processes like root nutrient uptake and carbon (C) exudation into soils is relatively unknown. New paradigms of nutrient limitation suggest that colimitation by multiple nutrients is more common than single nutrient limitation, therefore the established view that ecosystem processes are limited by phosphosus (P) in lowland tropical forests on highly weathered soils ecosystems should be revisited. Rates of nutrient uptake are of particular importance because these govern downstream plant physiological processes like photosynthesis and basic metabolism. Also, nutrient uptake is directly linked to root C exudates, which are increasingly recognized as an important plant contribution to soil C pools. Here, we explored the effect of changes in nutrient availability on root nutrient uptake in a well-studied Panamanian forest, and linked nutrient uptake to C exudates. We used a long-term factorial nutrient fertilization experiment, and focused our study on a relatively abundant canopy tree species (Tegtragastris panamensis). We found a significant interacting effect of nitrogen (N) and potassium (K) on root uptake rates for ammonium, nitrate, and phosphate. Nitrogen and K added in combination produced uptake rates indistinguishable from control plots, however, N or K added individually increased the uptake rates of all nutrients compared to control and NK plots. No significant effects of N or P addition were observed on root C exudation rates. These results suggest that increased N or K availability may accelerate N and P uptake rates by roots. These results support the growing literature indicating that most ecosystem processes in tropical forests are governed by the availability of multiple nutrients, with implications for understanding plant productivity in this biome.