Modeling to discern nitrogen fertilization impacts on carbon sequestration in a Pacific Northwest Douglas-fir forest in the first-postfertilization year

Authors

  • BAOZHANG CHEN,

    1. LREIS Institute of Geographic Sciences & Nature Resources Research, Chinese Academy of Sciences, Beijing 100101, China
    2. Department of Forest Resources Management, Faculty of Forestry, University of British Columbia, Vancouver, BC, Canada V6T 1Z4
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  • NICHOLAS C. COOPS,

    1. Department of Forest Resources Management, Faculty of Forestry, University of British Columbia, Vancouver, BC, Canada V6T 1Z4
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  • T. ANDY BLACK,

    1. Biometeorology and Soil Physics Group, Faculty of Land and Food Systems, University of British Columbia, Vancouver, BC, Canada V6T 1Z4
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  • RACHHPAL S. JASSAL,

    1. Biometeorology and Soil Physics Group, Faculty of Land and Food Systems, University of British Columbia, Vancouver, BC, Canada V6T 1Z4
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  • JING M. CHEN,

    1. Department of Geography and Program in Planning, University of Toronto, 100 St. George St., Room 5047, Toronto, Ontario, Canada M5S 3G3
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  • MARK JOHNSON

    1. Department of Earth and Ocean Sciences, Institute for Resources, Environment and Sustainability, University of British Columbia, Vancouver, BC, Canada V6T 1Z4
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Baozhang Chen, LREIS Institute of Geographic Sciences & Nature Resources Research, Chinese Academy of Sciences, Beijing 100101, China, e-mail: baozhang.chen0808@gmailcom

Abstract

This study investigated how nitrogen (N) fertilization with 200 kg N ha−1 of urea affected ecosystem carbon (C) sequestration in the first-postfertilization year in a Pacific Northwest Douglas-fir (Pseudotsuga menziesii) stand on the basis of multiyear eddy-covariance (EC) and soil-chamber measurements before and after fertilization in combination with ecosystem modeling. The approach uses a data-model fusion technique which encompasses both model parameter optimization and data assimilation and minimizes the effects of interannual climatic perturbations and focuses on the biotic and abiotic factors controlling seasonal C fluxes using a prefertilization 9-year-long time series of EC data (1998–2006). A process-based ecosystem model was optimized using the half-hourly data measured during 1998–2005, and the optimized model was validated using measurements made in 2006 and further applied to predict C fluxes for 2007 assuming the stand was not fertilized. The N fertilization effects on C sequestration were then obtained as differences between modeled (unfertilized stand) and EC or soil-chamber measured (fertilized stand) C component fluxes. Results indicate that annual net ecosystem productivity in the first-post-N fertilization year increased by∼83%, from 302 ± 19 to 552 ± 36 g m−2 yr−1, which resulted primarily from an increase in annual gross primary productivity of∼8%, from 1938 ± 22 to 2095 ± 29 g m−2 yr−1 concurrent with a decrease in annual ecosystem respiration (Re) of∼5.7%, from 1636 ± 17 to 1543 ± 31 g m−2 yr−1. Moreover, with respect to respiration, model results showed that the fertilizer-induced reduction in Re (∼93 g m−2 yr−1) principally resulted from the decrease in soil respiration Rs (∼62 g m−2 yr−1).

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