Document: KAT-3-59-24

Interactions of soil hydrologic and biotic processes in regulating nitrate retention in wet tropical forests: Comparisons using isotope tracers.

LOHSE, K.A.* ¹, H.FARRINGTON ², J.MOEN ², P.MATSON ² and G.ASNER ³

University of California, Berkeley, Berkeley, CA 94720 ¹
Stanford University, Stanford, CA 94305 ²
University of Colorado, Boulder, Boulder, CO 80309 ³

Abstract:
The consequences of anthropogenic nitrogen (N) additions for nitrate soil solution losses and the processes controlling them in wet tropical forests are poorly understood. Studies in many N-limited temperate forest ecosystems suggest that forests experience a temporal delay between initiation of anthropogenic N inputs and nitrate losses. In the Hawaiian Islands, we found that wet tropical forests respond to first-time N additions with substantial nitrate losses, and that, surprisingly, N limited systems have higher losses than P-limited systems. Nitrate losses following first-time fertilization accounted for 32-73% of the N added as fertilizer (50kg/ha) in the N-limited site, whereas total nitrate losses only accounted for 8-17% in the P-limited site. We investigated the interaction of hydrologic flow processes and soil biotic processes in controlling nitrate loss in these N-limited and P- limited wet tropical forests by tracing the fate of isotopically enriched nitrate and water in a simulated rainfall experiment. In the N-limited site, 80% of the initial input concentration of nitrate was recovered at 30cm depth in the lysimeter after 41 hrs at a 0.25 cm/hr tracer rate application. Relatively small proportions of N were recovered in microbial biomass in both sites, although microbial uptake was greater in the N-limited site. A range of evidence suggests that diffusion of solute into areas of stagnant water or intra-aggregates and adsorption of solute onto the solid phase may be reducing nutrient loss in the older soils. Differences in soil hydrologic properties are apparently more important than biological properties in controlling variation in nitrate solution losses in these tropical ecosystems.

Keywords: tropical forest ecosystems, nitrogen, isotope tracers

This abstract is being presented at: 1:45 PM in session:
Oral Session #28: Effects of N Deposition in Ecosystems.