Document: CHR-3-87-17

The effects of spatial resolution of plant-soil heterogeneity on the dynamics of nitrogen transformation in two model forest systems .

TONITTO, C.* and T.M.POWELL

University of California, Berkeley, CA 94702 USA ¹

Abstract:
This modeling study examines the influence of local heterogeneity in plant and soil properties on the aggregate behavior of biogeochemical cycles. Biotic controls on biogeochemical processes lead to non-linear model systems. In non-linear systems, the scale of process representation significantly influences whole-system dynamics. This is especially relevant in biogeochemical systems, where processes interact on spatial and temporal scales spanning several orders of magnitude. Model non-linearity and scale diversity complicates the application of empirical results to regional and global model systems. In order to address biogeochemical scaling relationships, this work develops two strategies for defining nitrogen movement between plant-soil-microbe phases. In one model, nitrogen transformations are described mechanistically based on microbial processes. In an alternative paradigm, nitrogen transformations are governed by both hydrologic dynamics as well as microbial processes. Equations for the homogeneous model systems are developed spatially to include horizontal heterogeneity in plant-soil properties and to model horizontal fluxes in solute transport. These spatial models are parameterized to study nitrogen movement in Douglas-fir forests of western Oregon. We explore the influence of spatial scale on nutrient distribution through perturbation analysis at different spatial resolutions. Perturbation scenarios correspond to biomass harvest, soil degradation and fertilization. The results show that in the microbially-driven model, the time-scale of the return to equilibrium is on the order of a generation of an unharvested forest, and the spatial differences in plant-soil nutrient distributions diminish during the return to steady state. In contrast, the hydrologic model system remains at the perturbed levels, and the heterogeneity in nutrient distribution remains a distinct feature of the system. These results demonstrate that biogeochemical predictions differ widely based on assumptions regarding both model development and spatial scale.

Keywords: spatial heterogeneity, nitrogen cycling

This abstract is being presented at: 8:45 AM in session:
Oral Session #41: N Dynamics: Additions, Retention and Transformations.