Determining the Spatial Variability of Wetland Soil Bulk Density, Organic Matter, and the Conversion Factor between Organic Matter and Organic Carbon across Coastal Louisiana, U.S.A. Hongqing Wang † *, Sarai C. Piazza † , Leigh A. Sharp ‡ , Camille L. Stagg † , Brady R. Couvillion † , Gregory D. Steyer † , and Thomas E. McGinnis ‡ † U.S. Geological Survey, Wetland and Aquatic Research Center Baton Rouge, LA 70803, U.S.A. ‡ Coastal Protection and Restoration Authority of Louisiana Lafayette, LA 70506, U.S.A. ABSTRACT Wang, H.; Piazza, S.C.; Sharp, L.A.; Stagg, C.L.; Couvillion, B.R.; Steyer, G.D., and McGinnis, T.E., 2017. Determining the spatial variability of wetland soil bulk density, organic matter, and the conversion factor between organic matter and organic carbon across coastal Louisiana, U.S.A. Journal of Coastal Research, 33(3), 507–517. Coconut Creek (Florida), ISSN 0749-0208. Soil bulk density (BD), soil organic matter (SOM) content, and a conversion factor between SOM and soil organic carbon (SOC) are often used in estimating SOC sequestration and storage. Spatial variability in BD, SOM, and the SOM–SOC conversion factor affects the ability to accurately estimate SOC sequestration, storage, and the benefits (e.g., land building area and vertical accretion) associated with wetland restoration efforts, such as marsh creation and sediment diversions. There are, however, only a few studies that have examined large-scale spatial variability in BD, SOM, and SOM–SOC conversion factors in coastal wetlands. In this study, soil cores, distributed across the entire coastal Louisiana (approximately 14,667 km 2 ) were used to examine the regional-scale spatial variability in BD, SOM, and the SOM–SOC conversion factor. Soil cores for BD and SOM analyses were collected during 2006–09 from 331 spatially well-distributed sites in the Coastwide Reference Monitoring System network. Soil cores for the SOM–SOC conversion factor analysis were collected from 15 sites across coastal Louisiana during 2006–07. Results of a split-plot analysis of variance with incomplete block design indicated that BD and SOM varied significantly at a landscape level, defined by both hydrologic basins and vegetation types. Vertically, BD and SOM varied significantly among different vegetation types. The SOM– SOC conversion factor also varied significantly at the landscape level. This study provides critical information for the assessment of the role of coastal wetlands in large regional carbon budgets and the estimation of carbon credits from coastal restoration. ADDITIONAL INDEX WORDS: Soil organic carbon sequestration, Coastwide Reference Monitoring System, hydrological basins, vegetation types, van Bemmelen factor. INTRODUCTION Soil bulk density (BD) and soil organic matter (SOM) content are two important descriptors of soil physical and biological structures in terrestrial and wetland ecosystems (Gosselink, Hatton, and Hopkinson, 1984; Mitsch and Gosselink, 2000). BD is an indicator of pore space and solid particles within the soil profile, which determine soil water-holding capacity (McKee and Cherry, 2009; Mitsch and Gosselink, 2000). SOM is an indicator of soil development and an important source of nitrogen and micronutrients required for plant growth (Bru- land and Richardson, 2006). These two soil parameters are often used in estimating soil organic carbon (SOC) stocks and sequestration capacity (Hansen and Nestlerode, 2013; Marke- wich et al., 2007; Zhong and Xu, 2009), which, in turn, are used to assess contributions of ecosystems to global and regional carbon budgets and mitigation of greenhouse gas emissions (e.g., Crooks et al., 2011; DeLaune and White, 2012). To reduce chemical analysis costs, SOM is often used as a predictor of SOC, and the conversion factor of 1.724 from SOC to SOM (the van Bemmelen factor), which assumes organic matter is 58% organic carbon, has been widely used in not only terrestrial ecosystems but also wetland soils (DeLaune and White, 2012; Hatton, DeLaune, and Patrick, 1983; Zhong and Xu, 2009). Ecosystem restoration efforts have increased worldwide to mitigate the loss of wetlands, which provide critical ecosystem services, including carbon sequestration (e.g., Couvillion et al., 2013; Crooks et al., 2011). In coastal wetlands, BD and SOM are also used in estimating vertical accretion and surface elevation change (Couvillion et al., 2013; Day et al., 2011; DeLaune, Patrick, and van Breemen, 1990; Hatton, DeLaune, and Patrick, 1983; Nyman et al., 1993, 2006; Wang et al., 2014). Often, BD and SOM are required to assess restoration benefits, such as sustained or new land-building areas and carbon sequestration of sediments and nutrients at a scale equal or larger than project boundaries (Boustany, 2010; Couvillion et al., 2013; Crooks et al., 2011; DeLaune and White, 2012; Wamsley, 2013). The American Carbon Registry has recently approved a standard wetlands restoration methodology for the Mississippi Delta in which SOM and BD data in different stratums are required to estimate carbon sequestration capacity (http://americancarbonregistry.org/). Therefore, DOI: 10.2112/JCOASTRES-D-16-00014.1 received 26 January 2016; accepted in revision 21 June 2016; corrected proofs received 17 August 2016; published pre-print online 21 October 2016. *Corresponding author: wangh@usgs.gov Ó Coastal Education and Research Foundation, Inc. 2017 Journal of Coastal Research 33 3 507–517 Coconut Creek, Florida May 2017