Increasingly conservative N cycling in a wet tropical forest: Litter and stream N concentrations decline over 29 years despite surges from hurricanes

Abstract Long‐term litterfall trends are unexplored in hurricane‐prone regions but are needed to understand the consequences of altered hurricane regimes. In a wet tropical forest in northeastern Puerto Rico, we monitored litterfall mass (leaf fall, <2.5 cm diameter wood fall and miscellaneous), litterfall carbon (C) and nitrogen (N) and stream nitrate every 2 weeks for 29 years (1989–2017). Litterfall from the nine observed hurricanes averaged 500 g m −2 hurricane −1 (CV = 90%), nearly half of the long‐term mean of 951 g m −2 year −1 . Hurricane‐generated leaf fall and wood fall increased with increasing disturbance intensity (peak wind speed) and time since a previous hurricane. Wood fall from the two most intense hurricanes exceeded corresponding leaf fall and mean annual wood fall. Litterfall increased over the 29 years due to increases in wood fall and miscellaneous materials but not leaf fall. Recovery of litterfall after individual hurricanes was distinctive and illustrated the varying effects of hurricane intensity and history on litterfall dynamics. Leaf fall N concentrations [N] surged immediately after hurricanes yet decreased by around 33% over the 29 years. Similar changes were observed at the watershed scale, with increasing nitrate in streams following major hurricanes but then a long‐term decline. Mean annual leaf fall [N] was negatively related to the previous year's fine wood fall but positively related to that wood fall's [N], suggesting a link between fine wood dynamics and plant N uptake. Stream nitrate‐[N] showed a strong positive correlation with leaf fall [N] after hurricanes and across years, demonstrating tight coupling between terrestrial and aquatic N cycling throughout periods of cyclonic disturbances and recovery. The decadal decreases in litterfall [N] and stream nitrate‐[N] and the increase in wood fall suggest a trend towards more conservative cycling of N. Microbial immobilization of N in wood fall from hurricanes and during recovery likely contributed to the lower N availability. Synthesis . More conservative cycling of N can be expected under the current hurricane disturbance regime. However, with increased hurricane frequencies and intensities, the lack of high wood fall typical of mature canopies may alter present N cycling trends.