Nutrients limit photosynthesis in seedlings of a lowland tropical forest tree species

We investigated how photosynthesis by understory seedlings of the lowland tropical tree species Alseis blackiana responded to 10 years of soil nutrient fertilization with N, P and K. We ask whether nutrients are limiting to light and CO2 acquisition in a low light understory environment. We measured foliar nutrient concentrations of N, P and K, isotopic composition of carbon (delta C-13) and nitrogen (delta N-15), and light response curves of photosynthesis and chlorophyll fluorescence. Canopy openness was measured above each study seedling and included in statistical analyses to account for variation in light availability. Foliar N concentration increased by 20% with N addition. Foliar P concentration increased by 78% with P addition and decreased by 14% with N addition. Foliar K increased by 8% with K addition. Foliar delta C-13 showed no significant responses, and foliar delta N-15 decreased strongly with N addition, matching the low delta N-15 values of applied fertilizer. Canopy openness ranged from 0.01 to 6.71% with a mean of 1.76 +/- 0.14 (+/- 1SE). Maximum photosynthetic CO2 assimilation rate increased by 9% with N addition. Stomatal conductance increased with P addition and with P and K in combination. Chlorophyll fluorescence measurements revealed that quantum yield of photosystem II increased with K addition, maximum electron transport rate trended 9% greater with N addition (p = 0.07), and saturating photosynthetically active radiation increased with N addition. The results demonstrate that nutrient addition can enhance photosynthetic processes, even under low light availability.