Exploring the drivers that affect nitrate in soil solution at Luquillo Experimental Forest, Puerto Rico

Humid tropical forests are important at the global scale because of their large biomass and high rates of carbon fixation and nutrient cycling. In many tropical regions, hurricanes play an important role in structuring forests and altering biogeochemical cycles. Hurricanes transfer green leaves with high nutrient content to the forest floor, with potentially large effects on soil solution chemistry. Here we synthesize data (e.g., soil solution chemistry, temperature, litterfall etc.) from the Canopy Trimming Experiment in the Luquillo Mountains in Puerto Rico to address the biogeochemical controls on nitrate concentrations following experimental manipulations that simulate various aspects of hurricane damage. We conducted analyses based on the four treatments (control without debris, control with debris, trim removing debris and trim with debris) using two different statistical approaches- a machine learning approach in the form of a random forest model, and partial least square regression (PLSR). Across treatments, dissolved organic nitrogen was identified as one of the most important factors controlling nitrate concentration, but within treatments, we found that different factors also play an important role in regulating nitrate dynamics. In the control with debris treatment, ammonium is the most important factor in regulating nitrate, likely due to ammonium conversion to nitrate through nitrification. In the trim with debris treatment, temperature is the most important driver regulating nitrate in soil solution, possibly due to with accelerated plant regrowth in the understory. The other two treatments - control without debris and trim with removing debris didn't show important drivers of nitrate concentrations. Understanding the mechanistic nature of the response to disturbance provides insights into controls on soil solution chemistry that can be used to better constrain community land models that represent N dynamics and ultimately N loss from these montane tropical watersheds.