Soil organic matter (SOM) contains more carbon than plant biomass and the atmosphere combined. Despite its important role in the global life support system, interactions among biological, chemical, and physical processes regulating SOM accumulation, stabilization, and turnover are poorly understood. The DIRT (Detrital Inputs and Removal Treatments) experiment, started at the H.J. Andrews Forest in 1997, is a multi-decade experiment that manipulates plant-litter inputs to large plots in a western coniferous forest. This experiment is designed to address such questions as: what controls the long-term storage of carbon in forest soils? What chemical and physical fractions of SOM are most stable? Can added inputs of detritus increase soil carbon storage, or is maximum storage determined exclusively by climate and soil mineralogy? By following changes in soil carbon in the DIRT plots over the next few decades, we will address these long-term questions directly.

The multi-decade, experimental nature of the DIRT experiment is uniquely suited to explore the role of plant detrital quality and quantity in determining SOM accumulation, which is a significant unknown factor in current ecosystem models. In addition, this research focuses on research training of undergraduate Honors students, and this experiment will be incorporated into an Honors Ecology course required of all Honors Environmental Science students at Oregon State University. Both data and soils from this experiment will be made available to researchers across the globe through the Andrews LTER web site. This experiment will be available to researchers, students and climate modelers for decades to come.

Project Report

Globally, soils contain more than three times more carbon than the atmosphere, and four and a half times more carbon than the world’s biota. Despite their importance, however, soil carbon stocks have been degraded through land use change and unsustainable forest management practices. It has been proposed that management efforts to increase forest productivity can result in increased carbon (C) storage within living forest biomass and thereby slow the rate of atmospheric CO2 increase. Elevated plant inputs to soils associated with higher primary productivity should lead to increased C inputs to soils, which could, in turn, could lead to increased C storage in soil organic matter (SOM). As such, forest managers are increasingly pressed to manage existing forests in ways that will increase soil carbon storage. However, the extent to which forests can be manipulated to enhance C sequestration soil remains a question. Controls on the amounts of carbon that may potentially be sequestered in soils, and the long-term controls of carbon sequestration in soils are poorly understood. Models of ecosystem C balance generally assume a strong relationship among net primary productivity (NPP), litter inputs, and soil C accumulation, but there is little direct experimental evidence for such a coupled relationship. The Detrital Input and Removal Treatment (DIRT) Project assesses the role of plant detritus input amounts and quality on the accumulation and dynamics of organic matter in forest soils. DIRT uses an experimental approach of chronically adding aboveground litter, excluding litter, and preventing root ingrowth to long-term experimental to assess the importance of plant detrital sources and loading rates on SOM formation and accumulation or loss. The DIRT network includes eight forested and two shrub sites across a broad geographic gradient. Across the DIRT network and in contrast to standard model predictions, SOM pools decreased slightly in response to chronic exclusion of aboveground litter, but responded only slightly, or not at all, to chronic doubling of aboveground litter inputs. Explanations for the slow or even negative response of SOM to litter additions include increased decomposition resulting in greater soil respiration, priming (increased microbial utilization of old soil organic matter following inputs of new organic matter inputs), and increased dissolved organic carbon (DOC) output. Partitioning of belowground contributions to total soil respiration were predictable based on site-level soil C and N as estimates of site fertility; contributions to soil respiration from root respiration were negatively related to soil fertility and contributions from decomposing aboveground litter in soil were positively related to site fertility. Taken together our results suggest that management to increase C storage in forested soils will have little effect, at least on decadal scales.

Agency
National Science Foundation (NSF)
Institute
Division of Environmental Biology (DEB)
Type
Standard Grant (Standard)
Application #
0817064
Program Officer
Saran Twombly
Project Start
Project End
Budget Start
2008-09-01
Budget End
2014-08-31
Support Year
Fiscal Year
2008
Total Cost
$431,979
Indirect Cost
Name
Oregon State University
Department
Type
DUNS #
City
Corvallis
State
OR
Country
United States
Zip Code
97331