PLANT AND MICROBIAL CONTROLS ON NITROGEN RETENTION AND LOSS IN A HUMID TROPICAL FOREST

Humid tropical forests are generally characterized by the lack of nitrogen (N) limitation to net primary productivity, yet paradoxically have high potential for N loss. We conducted an intensive field experiment with 15 NH 4 and 15 NO 3 additions to highly weathered tropical forest soils in Puerto Rico to determine the relative importance of N retention and loss mechanisms. Over one‐half of all the NH 4 + produced was rapidly converted to NO 3 − via the process of gross nitrification. During the first 24 hours, plant roots took up 28% of the inorganic N produced, dominantly as NH 4 + , and were a greater sink for N than soil microbial biomass. Soil microbes were not a significant sink for added 15 NH 4 + or 15 NO 3 − during the first 24 hours, and only for 15 NH 4 + after 7 days. Patterns of microbial community composition, as determined by terminal restriction fragment length polymorphism analysis (TRFLP), were weakly but significantly correlated with nitrification and denitrification to N 2 O. Rates of dissimilatory NO 3 − reduction to NH 4 + (DNRA) were high in this forest, accounting for up to 25% of gross NH 4 + production and 35% of gross nitrification. DNRA was a major sink for NO 3 − , which may have contributed to the lower rates of N 2 O and leaching losses. Despite considerable N conservation via DNRA and plant NH 4 + uptake, the fate of ∼ 45% of the NO 3 − produced and 4% of the NH 4 + produced were not measured in our fluxes, suggesting that other important pathways for N retention and loss (e.g., denitrification to N 2 ) are important in this system. The high proportion of mineralized N that was rapidly nitrified and the fates of that NO 3 − highlight the key role of gross nitrification as a proximate control on N retention and loss in humid tropical forest soils. Furthermore, our results demonstrate the importance of the coupling between DNRA and plant uptake of NH 4 + as a potential N‐conserving mechanism within tropical forests.