Document: DAR-3-69-41

A modeled analysis of N and P limitation in disturbed tropical forests.

HERBERT, D.A.*, M.WILLIAMS and E.B.RASTETTER

The Ecosystems Center, Woods Hole, MA 02543 USA 1

Abstract:
We used the Multiple-Element-Limitation (MEL) model to examine N and P limitation on C stocks at a tropical P-limited forest subsequently cleared of trees by harvest and fire. In mature tropical forest, vegetation demands for both N and P are supplied largely through the recycling of these elements within the ecosystem. Accumulation of N and P during forest aggradation depends on supply rates from external sources (weathering, deposition, and for N, biological fixation), the rates at which they are lost (occlusion, leaching, and volatilization), and the capacity of the ecosystem to entrain N and P into internal cycles. If the mature forest remains undisturbed the rates of N and P fluxes from the system will be proportional to their rates of supply from external sources. With disturbance, N and P losses accompany biomass death or removal, and proportional N losses often exceed those of P. Our simulations illustrated a recovery to pre-disturbance C stocks in the model system that is constrained early by N and later by P availability. Mechanisms controlling this shift between N and P limitation included (1) differential losses of N and P through fire with early enhancements on P availability, (2) buffering of immediately available P pools (resin P) by labile P pools (bicarbonate, NaOH and dilute HCl extractable P), and (3) biological N fixation. Differential losses of dissolved organic N and P may also influence shifts between limiting elements in a recovering system. When the model site was subjected to agricultural practices without fertilizer enhancements the early N limitation was absent. In this case the return to pre-disturbance C stocks was constrained largely by P due to depletion of the labile P buffer which supplies the pool of immediately available P. In this scenario the long-term recovery was controlled by the limiting rate at which P could be supplied from outside the system.

Keywords: forest, disturbance, recovery, nutrients

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This abstract is being presented at: 2:15 PM in session:
Oral Session #13: N Fixation and Biochemical Patterns.