Hydrological and biogeochemical controls on nitrogen losses from tropical forests: Responses to anthropogenic nitrogen additions

We examined the effects of anthropogenic N additions on the pattern and regulation of nitrogen (N) leaching losses from two wet tropical forests located at the extreme ends of a well-characterized soil age and fertility gradient in the Hawaiian Islands. We measured soil-water retention, hydraulic conductivity, and flow path characteristics to determine the effects of soil age on hydrological regulation of nutrient losses. We then evaluated the effects of first-time and long-term experimental N additions on N solution losses and the hydrological and biogeochemical processes controlling them.; Hydrological properties and flow paths varied substantially with soil age. At the young, 300 year old soil site, both near-surface and subsurface soil horizons drained rapidly resulting in vertical water movement. At the old, 4.1 million year old soil site, water moved rapidly as macropore flow through the near-surface horizons to an impeding subsurface clay horizon and then moved both laterally along the clay horizon contact and slowly downward through the horizon as piston flow. Across the soil age gradient, soil anisotropy and the probability of lateral flow increased due to declines in saturated hydraulic conductivity in subsurface horizons.; Both forests responded to first-time N additions with significant nitrate (NO₃−) leaching losses, with more immediate and larger losses from the N-limited forest on the young soils than the P-limited forest on old soils. Long-term fertilization at both forests resulted in even greater losses suggesting that their capacities to retain additional N inputs have been exceeded. Differences in hydrological and chemical properties due to soil age were more important than nutrient status in explaining differences in losses between sites. Tropical forests growing on highly weathered soils offered greater hydrological resistance to vertical losses of NO₃ − than those growing on poorly developed soils. Anion exchange capacity (AEC) in the older soils also appeared to retain first-time N inputs as NO₃− and delay losses. Long-term N additions increased the NO₃− retention capacity of the old soils because soil acidification increased AEC and nitrate adsorption. This process may delay the onset of large NO₃− losses in tropical forests experiencing elevated N inputs.