 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com 818 DOI: 10.1002/jpln.201000445 J. Plant Nutr. Soil Sci. 2011, 174, 818–826 Effects of lignin-modified Populus tremuloides on soil organic carbon Raysa Roque-Rivera 1 , Alan F. Talhelm 1,3 , Dale W. Johnson 1 *, Vincent L. Chiang 2 , and Kurt S. Pregitzer 1,3 1 Department of Natural Resources and Environmental Science, University of Nevada, Reno, Nevada 89557, USA 2 Forest Biotechnology Group, North Carolina State University, Raleigh, North Carolina, 27695–7247, USA 3 Present Address: College of Natural Resources, University of Idaho, Moscow, Idaho 83844, USA Abstract Several genes in the aspen genome have been modified to generate stem wood with lower lignin content and an altered lignin composition. Lower lignin in wood reduces the time and energy required for pulping. Further, this modification can also increase the allocation of photosynthate to cellulose and total biomass production, potentially increasing CO 2 -sequestration capacity. However, widespread planting of trees with altered lignin content and composition could alter soil organic-C dynamics in complex ways. To further examine the effects of altered lignin bio- synthesis on plant growth and accrual of soil organic C (SOC), we conducted a repeated green- house study with four lines of transgenic aspen (Populus tremuloides Michx.) and one wild-type (control) aspen. Accrual of aspen-derived SOC was quantified by growing aspen trees (C3 plants) in C4 soil and measuring changes in the natural abundance of d 13 C. We measured plant growth, biomass, and C content and combined these data with SOC measurements to create C budgets for the plant mesocosms. Lignin modifications resulted in differences in the accrual of aspen-derived SOC and total mesocosm C, primarily due to differences in biomass between genetically modified lines of aspen. One genetic alteration (low lignin, line 23) was able to per- form similarly or better than the wild-type aspen (control, line 271) without altering SOC. Altera- tions in lignin structure (S : G ratios) had negative effects on biomass production and SOC for- mation. The addition of new (aspen-derived) SOC was proportional to the loss of existing SOC, evidence for a priming effect. The pool of new SOC was related to total plant biomass, suggest- ing that the effects of lignin modification on SOC are driven by changes in plant growth. Key words: 13 C / carbon storage / soil C formation / transgenic aspen / lignin modifications Accepted May 1, 2011 1 Introduction In recent years, there has been increased interest in engineer- ing genetically modified (GM) trees because of the potential for these modifications to lead to economic and environmental ben- efits for the paper-making industry (Hu et al., 1999; Chen et al., 2001; Pilate et al., 2002; Herrera, 2005; Hancock et al., 2007, 2008). Lignin is one of the primary molecular components of wood, but must be removed from the wood before pulping for use in paper products or before wood products can be refined for the production of cellulosic ethanol. The great energetic and chemical expense of the pulping process (Biermann, 1996; Hu et al., 1999; Chen et al., 2001) has inspired the development of GM trees with reduced or modified lignin structures that could reduce the business and environmental costs associated with pulping (Hu et al., 1999; Chen et al., 2001; Pilate et al., 2002; Herrera, 2005; Hancock et al., 2007, 2008). Greenhouse and field research on GM quaking aspen (Populus tremuloides Michx.) has compared trees with low and altered lignin to unmo- dified trees for characteristics such as plant growth, biomass partitioning, structural integrity, interactions with insects, soil microbial interactions, decomposition rates, soil organic C (SOC) formation, and pulping performance (Hu et al., 1999; Pilate et al., 2002; Tilston et al., 2004; Bradley et al., 2007; Hancock et al., 2007, 2008). These studies provide some evi- dence that in addition to the direct benefits to the paper-pro- ducts industry, the planting of GM trees could also increase ecosystem C sequestration in soil and plant biomass. Although it is clear that GM trees can increase pulping effi- ciency (Pilate et al., 2002), more research is needed to deter- mine the full impact of these trees on ecosystem processes. Aspen (and other Populus species) is ideal for transgenic studies because it is one of the most common and wide- spread trees in N America, it is commercially important, it is easily propagated, and much is known about the Populus genome (Barnes and Wagner , 2002; Herrera, 2005). Several genes in the aspen genome can be manipulated to encode for lower lignin content and an altered lignin composition (Hu et al., 1999; Chen et al., 2001; Pilate et al., 2002). Typically, the reduction in lignin is balanced by increased production of cellulose, a molecule that requires less energy for the plant to produce (Amthor , 2003). The increase in the energy effi- ciency for wood production allows for more biomass produc- tion, increasing C storage in plants (Hu et al., 1999; Chen et al., 2001; Pilate et al., 2002). Increased pulping efficiency can also be achieved by altering the molecular composition of lignin. Lignin in hardwoods is made up primarily from syrin- * Correspondence: Dr. D. W. Johnson; e-mail: dwj@cabnr.unr.edu