Controls on Forest Soil Organic Matter Development and Dynamics: Chronic Litter Manipulation as a Potential International LTER Activity.

Nadelhoffer, Knute

The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, USA

Abstract

Organic matter (or humus) content strongly influences key soil properties such as moisture holding capacity, aeration, and cation retention in terrestrial ecosystems. It also constitutes large reservoirs of nutrients and reduced carbon that fuel microbial processes and support complex communities of soil and forest floor organisms. Because nutrient cycles are relatively closed in most forests, trees and understory plants depend mainly on the nutrients released from decomposing organic matter to meet their nutritional requirements. As such, the amount and the quality of soil organic matter greatly influence primary production and ecosystem biogeochemical cycles.

Despite the critical roles of soil organic matter, the degree to which above- and belowground plant inputs influence its formation and dynamics is not well understood. To address this gap, my collaborators and I established a long-term study of controls on soil organic matter formation: the DIRT (Detritus Input Removal and Transfer) project. The goal of the DIRT project is to assess how rates and sources of plant litter inputs control the accumulation and dynamics of organic matter and nutrients in forest soils over decadal time scales.

DIRT treatments consist of chronically altering above- and belowground plant inputs to permanent plots in a mid-successional oak-maple-birch forest at Harvard Forest in the northeastern US.

Treatment	Manipulation
CONTROL	(normal litter inputs)
NO LITTER	(aboveground litter excluded from plots)
DOUBLE LITTER	(twice aboveground litter inputs)
NO ROOTS	(roots excluded from plots by lined trenches)
NO INPUTS	(no aboveground litter and no roots)
O/A‑LESS	(organic and A horizons replaced with B horizon soil, normal inputs thereafter)

The DIRT project has the potential to become a long-term, intersite experiment. To that end we have forged linkagess with similar and experiments at sites in a nutrient-rich maple forest in Pennsylvania (Allegheny College Bousson Environmental Research Reserve) and an old-growth forest in Oregon (H. J. Andrews Experimental Forest USFS). We hope to develop additional linkages to similar experiments located across climate and soil texture gradients. This will allow an assessment of the importance of physical as well as biological factors in controlling soil organic matter accumulation. At the international LTER meeting to be held in Korea in 1999, I will present selected results from the first decade of DIRT manipulations at the Harvard Forest site to illustrate how chronic alterations of leaf and root inputs to soils can provide valuable information about short- and long-term processes in ecosystems.