Document: MAR-3-87-12

Modeling nitrogen and carbon dynamics during organic matter decomposition: Incorporating nonsymbiotic N₂-fixation and wet deposition of inorganic nitrogen.

CARMONA, M.R.*, C.PÉREZ and J.J.ARMESTO

Universidad de Chile, Santiago, CHILE ¹

Abstract:
Current models of organic matter decomposition do not incorporate explicitly the process of non-symbiotic nitrogen (N) fixation, even though it may represent a significant input of new N to decomposing organic detritus. In this work we incorporate both non-symbiotic N fixation and wet deposition of dissolved inorganic N as variables in a simulation model of N and carbon (C) dynamics of decomposing organic matter in forests, considering microclimatic effects, in order to answer the following questions: i) What is the time course of N₂-fixation in relation to N and C dynamics during decomposition?, and ii) what is the effect of enhanced wet deposition of inorganic N (typical of northern hemisphere forests) on N₂-fixation rates? The model was built in STELLA 5.1.1 software (run-time version, High Performance Systems, Inc.) and is based on a previous mathematical model of N and C dynamics for homogeneous substrates developed by gren G. I. & E. Bosatta (Theoretical Ecosystems Ecology. Cambridge University Press,). N₂-fixation rate was modeled as a function of inorganic N concentration in precipitation (negative exponential), temperature (bell-shaped) and total amount of detrital C (positive linear). The model was calibrated with parameters taken from literature for northern hemisphere forests and from our own measurements of potential N₂-fixation (acetylene reduction) in litter samples for unpolluted old-growth forests in southern Chile. We assumed C:N ratio (mass) of fresh detritus of 100, and used average concentrations of inorganic N in precipitation of southern Chile and Hubbard Brook, USA (3.3 x 10^-5 and 1.7 x 10^-3 g liter^-1, respectively). The model was run for 96 months using climatic data for both regions. Simulation results showed that N₂-fixation was limited to the initial phase of decomposition (first 20 months), associated with microbial N immobilization, and net N accumulation in substrate. N₂-fixation input was negligible when dissolved organic N in precipitation increased by two orders of magnitude.

Keywords: nitrogen fixation, organic matter decomposition, N and C dynamics, wet inorganic nitrogen deposition, temperate forests, modelling

This abstract is being presented at: 3:30 PM in session:
Poster Session #15: Nutrient Cycling.