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 NH₄ and ¹⁵ 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 ⁺ 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 ⁺ , and were a greater sink for N than soil microbial biomass. Soil microbes were not a significant sink for added ¹⁵ NH₄ ⁺ or ¹⁵ NO₃ ^- during the first 24 hours, and only for ¹⁵ NH₄ ⁺ 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 N₂ O. Rates of dissimilatory NO₃ ^- reduction to NH₄ ⁺ (DNRA) were high in this forest, accounting for up to 25% of gross NH₄ ⁺ production and 35% of gross nitrification. DNRA was a major sink for NO₃ ^- , which may have contributed to the lower rates of N₂ O and leaching losses. Despite considerable N conservation via DNRA and plant NH₄ ⁺ uptake, the fate of ∼45% of the NO₃ ^- produced and 4% of the NH₄ ⁺ 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 ⁺ as a potential N-conserving mechanism within tropical forests.