Long-term effects of site preparation and fertilization on total soil CO 2 efflux and heterotrophic respiration in a 33-year-old Pinus taeda L. plantation on the wet flats of the Virginia Lower Coastal Plain Michael C. Tyree * , John R. Seiler, W. Michael Aust, David Arthur Sampson, Thomas R. Fox Virginia Polytechnic Institute and State University, Department of Forestry, 228 Cheatham Hall, Blacksburg, VA 24061, USA Received 22 March 2006; received in revised form 10 May 2006; accepted 24 July 2006 Abstract Total soil CO 2 efflux at the soil surface (F S ) is the dominant respiratory flux in forest ecosystems. Therefore, minor changes in F S would impact net ecosystem productivity (NEP) and, thus, short- and long-term carbon sequestration. Intensive site management (i.e., site preparation, competition control, fertilization amendments) impacts F S by influencing resource availability and site productivity. We evaluated the long-term (rotation-age) effects of site preparation and fertilization on F S and heterotrophic respiration (R H ) in a 33-year-old loblolly pine (Pinus taeda L.) plantation located in the wet flats on the Lower Coastal Plain of Virginia. We utilized an existing long-term study that consisted of a randomized complete block design split by fertilizer treatments that were replicated three times. Each of the three site preparation treatments: (i) chopped, (ii) windrow-bedded and (iii) ditched were split by four fertilizer treatments: (1) no nutrient additions (control), (2) 84 kg P ha À1 (P), (3) 112 kg N ha À1 and 84 kg P ha À1 (NP) and (4) 112 kg N ha À1 , 84 kg P ha À1 , and 2240 kg lime ha À1 (NPL). Total soil CO 2 efflux and heterotrophic respiration were measured using an infrared gas analyzer with a dynamic closed cuvette chamber. We found decreased F S (P < 0.05) in bedded and ditching treatments relative to controls 33 years following treatment imposition, and increased F S (P < 0.05), 24 years later between NPL and P treatments. Our results indicate that silvicultural treatments can exert long lasting impacts on F S in intensively managed loblolly pine stands. Differences in F S among treatments were correlated with treatment differences in total stand biomass measured in a previous study. High within- treatment variation in R H masked potential differences in soil microbial biomass and activity for the treatment examined. # 2006 Elsevier B.V. All rights reserved. Keywords: Soil respiration; Microbial respiration; Bedding; Ditching; Loblolly pine; Intensive management 1. Introduction Soil respiration is the dominant flux in total respiratory carbon emissions from ecosystems at local or global scales (Raich and Potter, 1995; Lavigne et al., 1997; Raich et al., 2002). Root (autotrophic) respiration (R R ) and microbial (heterotrophic) respiration (R H ), produced during the decom- position of organic matter (OM) by soil organisms, determine total soil CO 2 efflux at the soil surface (F S ). The difference between net primary production (NPP) and R H determines net ecosystem carbon balance. Intensive forest management influences F S and R H by influencing NPP and the processes that regulate soil respiration. Intensive management has been demonstrated to decrease the time necessary for stands to shift from a carbon source to a carbon sink (Maier and Kress, 2000; Lai et al., 2002; Maier et al., 2004; Sampson et al., 2006). The largest impact on NEP comes from increased productivity; however, an additional benefit of intensive silviculture may be a reduction in carbon evolution from these soils. Site preparation (i.e., ditching and bedding) can lead to improved soil aeration, seedling survival, and accelerate early growth (Burger and Pritchett, 1988; Andrews, 1993; Allen and Lein, 1998; Wheeler et al., 2002; Miwa et al., 2004). Short-term site preparation treatments, such as bedding, can provide an enhanced soil environment (aeration, temperature, and moisture) as well as incorporate OM into the mineral soil, leading to increased decomposition of soil OM, and, consequently, increased F S and R H in the years immediately following site preparation (Ewel et al., 1987; Trettin et al., 1996; Gough et al., 2005). www.elsevier.com/locate/foreco Forest Ecology and Management 234 (2006) 363–369 * Corresponding author. Tel.: +1 540 231 9191; fax: +1 540 231 3330. E-mail address: mtyree@vt.edu (M.C. Tyree). 0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2006.07.021