Consequences of nitrogen additions for soil losses from wet tropical forests

Wet tropical forests growing on highly weathered soil, depleted in rock-derived nutrients, yet rich in nitrogen (N), may respond quite differently to anthropogenic N inputs than those growing on younger soils low in N. We evaluated the effects of first-time and long-term N additions on the pattern and regulation of hydrologic N losses from wet tropical forests located at the extreme ends of a soil age and fertility gradient in the Hawaiian Islands. In contrast to our expectations that a N-limited forest on 300-year-old soils would initially retain N inputs, both forests, regardless of soil age or fertility, responded to first-time N additions with immediate and significantly elevated nitrate (NO3-) solution losses. However, patterns of NO3- loss differed markedly between sites and largely reflected differences in hydrological processes due to soil age. In the N-limited forest on young soils, N additions to previously unfertilized soils resulted in significant microbial immobilization of ammonium (NH4+) and small losses as NH4+, whereas N added as NO3- appeared to be free to move in solution. Nitrogen additions to long-term N-fertilized forests (13 years) on young soils significantly increased rates of nitrification, and losses were similar to the total N added during that time period. Poor soil development, and therefore low hydraulic resistance, was a critical factor determining the low NO3--retention capacity in the young soils. In contrast, first-time N inputs to a N-rich forest on 4.1-million-year-old soils resulted in significantly elevated rates of nitrification, but NO, losses were more delayed and lower than those from the young soils. The old, highly developed soils offered greater hydraulic resistance to leaching losses as nitrate than the young soils. High anion exchange capacity (AEC) in the subsurface clay horizon of the old soils also appeared to delay NO3- losses. Our findings suggest that responses to N additions in the tropics will vary as a function of soil age, nutrient status, and the form of N added, and that chemical and physical mechanisms may be more important than biological ones in' controlling losses. While retention of added N will be determined by the strength of biotic demand (and the ability of plants and microbes to retain these inputs) and the relative strength of other pathways of retention (cation and anion exchange) and loss (denitrification), hydrological properties and flow paths may be the dominant controls determining the residence time and routing of water and nutrients. Nitrate adsorption on anion exchange sites may also serve as an important abiotic mechanism delaying the onset of large NO3- losses from tropical forests receiving elevated anthropogenic N inputs.