Salinity affects microbial activity and soil organic matter content in tidal wetlands EMBER M. MORRISSEY, JAIMIE L. GILLESPIE, JOSEPH C. MORINA andRIMA B. FRANKLIN Department of Biology, Virginia Commonwealth University, 1000 W Cary Street, Richmond, VA 23284, USA Abstract Climate change-associated sea level rise is expected to cause saltwater intrusion into many historically freshwater ecosystems. Of particular concern are tidal freshwater wetlands, which perform several important ecological func- tions including carbon sequestration. To predict the impact of saltwater intrusion in these environments, we must first gain a better understanding of how salinity regulates decomposition in natural systems. This study sampled eight tidal wetlands ranging from freshwater to oligohaline (0–2 ppt) in four rivers near the Chesapeake Bay (Vir- ginia). To help isolate salinity effects, sites were selected to be highly similar in terms of plant community composi- tion and tidal influence. Overall, salinity was found to be strongly negatively correlated with soil organic matter content (OM%) and C : N, but unrelated to the other studied environmental parameters (pH, redox, and above- and below-ground plant biomass). Partial correlation analysis, controlling for these environmental covariates, supported direct effects of salinity on the activity of carbon-degrading extracellular enzymes (b-1, 4-glucosidase, 1, 4-b-cellobios- idase, b-D-xylosidase, and phenol oxidase) as well as alkaline phosphatase, using a per unit OM basis. As enzyme activity is the putative rate-limiting step in decomposition, enhanced activity due to salinity increases could dramati- cally affect soil OM accumulation. Salinity was also found to be positively related to bacterial abundance (qPCR of the 16S rRNA gene) and tightly linked with community composition (T-RFLP). Furthermore, strong relationships were found between bacterial abundance and/or composition with the activity of specific enzymes (1, 4-b-cellobiosi- dase, arylsulfatase, alkaline phosphatase, and phenol oxidase) suggesting salinity’s impact on decomposition could be due, at least in part, to its effect on the bacterial community. Together, these results indicate that salinity increases microbial decomposition rates in low salinity wetlands, and suggests that these ecosystems may experience decreased soil OM accumulation, accretion, and carbon sequestration rates even with modest levels of saltwater intrusion. Keywords: carbon cycling, decomposition, extracellular enzyme activity, marsh, microbial community structure, saltwater intrusion, sea level rise Received 13 August 2013 and accepted 26 September 2013 Introduction Climate change is predicted to alter the global hydro- logical cycle in many ways. For example, rising sea lev- els (Nakada & Inoue, 2005; Wigley, 2005; Church & White, 2006), reduced precipitation in watersheds (Smith et al., 2005) with resulting declines in stream flow (Milley et al., 2005), and global increases in water consumption (Gleick, 2003) may result in widespread saltwater intrusion into freshwater coastal ecosystems. Of particular concern is the impact of increased salinity on tidal freshwater wetlands, where it has been shown to drive changes in microbial metabolism (Weston et al., 2011; Neubauer et al., 2013), nutrient cycling (Weston et al., 2006; Marton et al., 2012), plant commu- nity composition (Sharpe & Baldwin, 2012), and pri- mary production (Baldwin & Mendelssohn, 1998). Taken together, these changes may significantly alter the carbon (C) biogeochemistry and organic matter (OM) storage capacity of freshwater wetlands (Craft, 2007; Loomis & Craft, 2010). Wetlands store an esti- mated 45–70% of all terrestrial C (Mitra et al., 2005), making them important targets for conservation and major players in the global C cycle (Mcleod et al., 2011). One of the reasons for the high C sequestration rate of wetlands is that decomposition slows in water-satu- rated anaerobic soils (Reddy & DeLaune, 2008). Micro- bial decomposition of soil organic C and plant detritus begins with extracellular enzyme-mediated hydrolysis of complex substrates into monomers and oligomers that can be directly used for metabolism (Shi, 2011). This enzymatic hydrolysis has been proposed by many researchers to regulate decomposition rates (Sinsab- augh et al., 1991; Sinsabaugh & Moorhead, 1994; Schi- mel & Weintraub, 2003; Freeman et al., 2004; Allison & Vitousek, 2005) and has been tied to rates of soil respi- ration in multiple ecosystems including wetlands (Sinsabaugh & Findlay, 1995; Freeman et al., 1998; Margesin et al., 2000). Correspondence: Rima B. Franklin, tel. + 804 828 6753, fax + 804 828 0503, e-mail: rbfranklin@vcu.edu © 2013 John Wiley & Sons Ltd 1351 Global Change Biology (2014) 20, 1351–1362, doi: 10.1111/gcb.12431 Global Change Biology