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USING RADIOCARBON TO ASSESS SOIL ORGANIC MATTER STABILIZATION IN A TRANSECT OF MATURE FORESTS IN THE PACIFIC NORTHWEST USA
Citation:
JOHNSON, M. G. AND C. W. Swanston. USING RADIOCARBON TO ASSESS SOIL ORGANIC MATTER STABILIZATION IN A TRANSECT OF MATURE FORESTS IN THE PACIFIC NORTHWEST USA. Presented at AGU, San Francisco, CA, December 05 - 09, 2011.
Impact/Purpose:
The accumulation and storage of C in soils is a major factor in the global C cycle and is crucial for sustaining ecosystem health and function, yet gaps remain in our understanding of the processes that lead to the accumulation and stabilization of soil organic matter (SOM). This information is essential for ascertaining ecosystem health and the trajectory of carbon sequestration.
Description:
Soils influence the cycling of nutrients, movement and storage of water, and serve as an important global reservoir of carbon (C). The accumulation and storage of C in soils is a major factor in the global C cycle and is crucial for sustaining ecosystem health and function, yet gaps remain in our understanding of the processes that lead to the accumulation and stabilization of soil organic matter (SOM). This information is essential for ascertaining ecosystem health and the trajectory of carbon sequestration. Because vegetation, clay mineralogy, and environmental conditions play important roles in the production, stabilization, and sequestration of SOM, we developed a study to investigate their role in the accumulation of SOM across a range of forested soils in the Pacific Northwest USA. We selected 8 mature (≥ 150 years old) forest stands in the Oregon Coast Range Mountains and Cascade Mountains. These forests cover a range of forest types, environments and soil parent materials. Annual precipitation values range from less than 30 cm for the dry Juniper forest to more than 300 cm for the wet coastal Douglas-fir and Sitka spruce forests. Parent materials include volcanic ash, other volcanics, marine sediments and basalts. Soil chemical and physical properties were quantified. Soil particle size distribution and clay mineralogy was determined. We hypothesized that particle density is directly proportional to SOM stability (i.e., residence time), and separated SOM by density using sodium polytungstate. Total C and N and δ13C and δ15N in whole soil and in 4 density fractions were determined for each soil horizon. Accelerator mass spectrometry (AMS) was used to measure the 14C in the whole soil from each horizon for the purpose of determining radiocarbon-based mean residence times of C. Infrared spectroscopy was used to characterize C chemistry. We found a 5-fold difference between the amount of C in the soil with the lowest soil C and the soil with the greatest soil C. Clay mineralogy of the sites is quite diverse, reflecting the soil parent material, age and weathering environment. The amount of heavy-density fraction associated organic matter seems to be related to the amount and kind of clay present in the soil. Radiocarbon abundance decreased with increasing depth, indicating higher mean residence times in deep soil. Soil C at depth was much older in the wet forest soils and the most recent C was found in the dry forest soils. However, the strongest relationship appears to be between mean residence times and the amount of clay, which is indicative of the protective and stabilizing nature of clay on SOM. These data along with environmental data and forest site history provide a unique way to evaluate the interacting factors that affect the accumulation and stabilization of SOM in forested soils in the Pacific Northwest USA.