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 NH4 and NO3 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 NH4 þ produced was rapidly converted to NO3 via the process of gross nitrification. During the first 24 hours, plant roots took up 28% of the inorganic N produced, dominantly as NH4 þ, and were a greater sink for N than soil microbial biomass. Soil microbes were not a significant sink for added NH4 þ or NO3 during the first 24 hours, and only for NH4 þ 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 N2O. Rates of dissimilatory NO3 reduction to NH4 þ (DNRA) were high in this forest, accounting for up to 25% of gross NH4 þ production and 35% of gross nitrification. DNRA was a major sink for NO3 , which may have contributed to the lower rates of N2O and leaching losses. Despite considerable N conservation via DNRA and plant NH4 þ uptake, the fate of ;45% of the NO3 produced and 4% of the NH4 þ produced were not measured in our fluxes, suggesting that other important pathways for N retention and loss (e.g., denitrification to N2) are important in this system. The high proportion of mineralized N that was rapidly nitrified and the fates of that NO3 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 NH4 þ as a potential Nconserving mechanism within tropical forests.