Effects of Forest Management on Deep Soil Carbon and Nitrogen in a Highly Productive Pacific Northwest Andisol.

Soil is the most important long-term sink for carbon (C) in terrestrial ecosystems, containing more carbon than plant biomass and the atmosphere combined. However, soil has historically been under-represented both forest management and carbon cycling literature, especially in regards to information about subsurface (>1.0 m) layers and processes. Research on the effects of silvicultural treatments on deep soil C and nitrogen (N) has been particularly lacking, even in soils that are known to be many meters deep. This project will build upon previous research by Erika Knight at the Fall River Long-term Soil Productivity (LTSP) Site located in western Washington to examine the effects of silvicultural treatments on deep soil C and N. The Douglas-fir (Pseudotsuga menziesii) stand at Fall River was established in 1999 with four blocks of 12 plots. This site has a deep, well-drained soil with few rocks, which developed from weathered basalt and is classified as an Andisol of the Boistfort Series. Our project is focused on the following three silvicultural treatments implemented at the Fall River site: commercial bole only removal with vegetation control by annual herbicide application (BO+VC), commercial bole only removal without vegetation control (BO-VC), and total-tree plus coarse woody debris removal with vegetation control (TTP+VC). In the most recent re-sampling at Fall River, Knight found that differences in soil carbon and nitrogen between treatments were greatest deep in the soil profile, primarily below 0.6 m. This study aims to investigate whether different treatments continue to have an important effect in layers deeper than 1.0 m. Samples were taken at 9 depth intervals to a depth of 3 meters. Bulk density and carbon and nitrogen concentrations were determined for each sample. These data will indicate how C and N concentration and content change with depth and serve to estimate soil carbon and nitrogen pools at Fall River on an areal basis. These estimates will allow for the comparison of deep C and N pools at Fall River relative to more commonly measured shallow pools. We hypothesize that the C and N concentrations will depend on both the treatment and the depth of a given observation. Samples will also be analyzed to determine cation exchange capacity, anion exchange capacity, and short-range order mineral content, which previous studies have suggested may play an important role in soil C and N retention, particularly in Andisols. We will use this data to investigate soil mineralogy as a driver for carbon and nitrogen storage at Fall River. The Fall River LTSP Site represents the most productive soils in the Pacific Northwest, and results from this study can potentially be extrapolated to millions of hectares of industrial forestland. This project will provide valuable information about how silvicultural treatments affect soil C and N cycling in deep layers of highly productive Douglas-fir plantations in the Pacific Northwest.