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FOREST, RANGE & WILDLAND SOILS

Plantation Management Intensity Affects Belowground Carbon and Nitrogen Storage in Northern California

^a Dep. of Forest Eng., Resources and Management, Oregon State Univ., Corvallis, OR 97330
^b currently at: Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, P.O. Box 808, L-397, Livermore, CA 94551
^c Virginia Water Resources Research Center, 210 Cheatham Hall (0444), Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061-0002
^d USDA Forest Service, Pacific Southwest Research Station, 3644 Avtech Parkway, Redding, CA 96002

^* Corresponding author (mcfarlane3{at}llnl.gov).

Belowground C and N storage is important in maintaining forest productivity and to CO₂ sequestration. How these pools respond to management is poorly understood. We investigated effects of repeated applications of complete fertilizer and competing vegetation control with herbicides on C and N storage in forest-floor, fine-root, and mineral-soil C and N pools to 1-m depth at three Pinus ponderosa P. Lawson & C. Lawson var. ponderosa plantations across a site quality gradient in northern California. Belowground C pools without treatment were 66, 153, and 199 Mg C ha^–1 for the low-, intermediate-, and high-quality sites, respectively, and N pools were 5.1, 6.7, and 6.5 Mg N ha^–1, respectively. Treatments increased tree-bole volume at 20 yr as much as 400%, while changes in C and N pools belowground were less dramatic. Herbicide treatment increased forest-floor C pools 35% at the poorer quality site. Fertilization increased forest-floor C and N storage 46 to 106% at all sites. Fertilization decreased fine-root C pools at 0 to 0.3 m at the most productive site 43% and increased this N pool 43% at the least productive site, but did not influence fine-root pools to 1 m. Fertilization increased mineral-soil C pools on lower quality sites, resulting in 12 to 57% more belowground C storage. At the intermediate site, fertilization increased total belowground N storage 12%. Results of this study suggest that the major sequestration mechanism up to this point in stand development is through gains in tree biomass rather than storage in fine roots and soil belowground.

Abbreviations: SOM, soil organic matter