Fluxes and fates of different forms of N in surface soils of wet tropical forests varying in nutrient status and substrate age in Hawaii

The fluxes and fate of different forms of anthropogenic nitrogen (N) inputs in tropical forest soils are poorly understood. I applied ¹⁵N pool dilution and tracer techniques to understand the flows of inorganic N in the surface soils of three tropical forests varying in substrate age and fertility in the Hawaiian Islands. Nitrogen limits tree growth at the 300 year old substrate site, neither N nor P limit tree growth at the 20,000 year old site, and P limits tree growth at the 4.1 million year old site. Leachates collected after 24hr showed that less that 2% of the ¹⁵N added as NH4⁺ at tracer levels of N (0.05g/m²) was lost as solution loss at the 300 and 20,000 year old sites; losses were higher but more variable at the 4.1 million year old site. Losses as NO3^- as a fraction of the ¹⁵N added as NH4⁺ were very low at the N-limited site whereas they represent 42% of the ¹⁵N added at the 20,000 year old site and 4% at the 4.1 million year old site. Additions of ¹⁵NH4⁺ at elevated levels (0.5 g/m²) resulted in higher losses as NH4⁺ (11-28% of N added) and NO3^- (10-20% of N added). Solution losses as nitrate under tracer and elevated levels of N were relatively high at all sites representing 33- 70% of the N added as ¹⁵NO3 . Findings from this study suggest that leaching loss responses to N additions in the tropics will vary as a function of soil age, nutrient status, and the form of N added.