Five Decades of Wetland Soil Development of a Constructed Tidal Salt Marsh, North Carolina, USA Aaron Noll, Courtney Mobilian, Christopher Craft Ecological Restoration, Volume 37, Number 3, September 2019, pp. 163-170 (Article) Published by University of Wisconsin Press For additional information about this article https://muse.jhu.edu/article/731195 Access provided at 20 Aug 2019 15:22 GMT from Indiana University Libraries RESEARCH ARTICLE Five Decades of Wetland Soil Development of a Constructed Tidal Salt Marsh, North Carolina, USA Aaron Noll, Courtney Mobilian, and Christopher Craft ABSTRACT We periodically measured soil properties (bulk density, organic carbon, total nitrogen, and total phosphorus) in a tidal salt marsh constructed in 1970 over the past five decades (1984, 1995, 1998, 2017) to identify trajectories of soil formation over time. Bulk density was greater in surface (0–10 cm) than subsurface (10–30 cm) soils and decreased with time at a similar rate in both depths. Percent organic carbon (C), nitrogen (N), and total phosphorus (P) increased with time, with concentrations in surface soils increasing three times faster than subsurface soils. Surface C and N pools (g/m2, 0–10 cm) exhibited asymptotic trajectories with time, with rapid accumulation in the first two decades that began to equilibrate after 25 years, while subsurface pools of C and N continued to increase in a linear manner. Soil P pools did not exhibit a significant trend related to marsh age. Soil N:P increased in both depths over time, suggesting gradual relaxation of N-limitation of these systems. Our findings can help restoration ecologists identify soil properties, including bulk density, C, N, and C:N suitable for gauging restoration success and better estimate the time frame necessary for recovery of soil-based ecosystem services. Keywords: constructed tidal marsh, soil carbon, soil nitrogen, salt marsh restoration Restoration Recap • • Tidal salt marshes planted with Spartina alterniflora con- • Constructed and restored tidal salt marshes will likely tinue to sequester organic carbon and nitrogen nearly sequester C and N for the foreseeable future assuming five decades after establishment. marshes can accrete sufficient mineral sediment and soil • During the first 25 years, accumulation of C and N was organic matter to keep pace with sea level rise, and avoid confined to surface (0–10 cm) soils after which C and N human activities such as dredging and boat traffic that enrichment shifted to subsurface (10–30 cm) soils. may increase lateral erosion. idal marshes are valued for their ability to provide destroyed. However, it may take years for restored wetlands Timportant ecosystem services including shoreline sta- to re-establish some functions of natural systems, particu- bilization, water quality improvement, biodiversity, and larly those functions related to soils (Craft et al. 2003). Tidal carbon sequestration (Boesch and Turner 1984, Mcleod marsh and other wetland soils provide environmental con- et al. 2011, Shepard et al. 2011). However, they have been ditions that facilitate biogeochemical and nutrient cycling. subject to widespread degradation and loss stemming from Some unique functions of wetland soils that are distinct coastal development, hydrologic alterations, and sea level from terrestrial soils include high rates of denitrification, rise (Pendleton et al. 2012, Craft et al. 2009, Neubauer 2013, nutrient burial from sedimentation, and accelerated rates Vincent et al. 2013). of organic matter and carbon sequestration. Accumulation Restoration and creation of tidal marshes are used to of organic matter is essential to restoring other ecosystem replace ecosystem services lost when they are degraded or services such as water quality improvement (e.g., denitrifi- cation), providing organic C to support heterotrophic food webs, production of economically valuable wildlife (LaSalle Ecological Restoration Vol. 37, No. 3, 2019 et al. 1991, Craft et al. 2003) and building elevation in the ISSN 1522-4740 E-ISSN 1543-4079 ©2019 by the Board of Regents of the University of Wisconsin System. September 2019 ECOLOGICAL RESTORATION 37:3 • 163 face of rising sea level (Nyman et al. 2006, Chmura and Methods Hung 2004, Turner et al. 2002, Hatton et al. 1983). Wetland soil development is a slow but critical process Site Description to restoring wetland functions and re-establishing lost We measured soil properties in a Spartina alterniflora ecosystem services. In addition to the slow pace of soil Loisel tidal salt marsh (34°03'49.15" N, 77°55'40.25" W), development, successful restoration of wetland soils can Snow’s Cut, that was constructed in 1970 on a dredge spoil be challenging due to factors such as altered hydrology island on the Cape Fear River, NC. The U.S. Army Corps and degraded initial substrate, including dredged material of Engineers funded construction of the marsh to dem- that contains little organic matter or nitrogen (Cornell et al. onstrate the use of wetland vegetation to stabilize dredge 2007), terrestrial soils where surface (O, A) horizons are spoil material (Woodhouse and Seneca 1974). Marsh con- excavated to achieve target elevations (Craft et al. 2002), struction was accomplished by grading the site to intertidal and agricultural soils with little soil organic matter (Marton elevation, then planting Spartina alternifloraat elevations et al. 2014). In particular, sufficient N and C are needed between mean sea level and mean high water. The marsh to sustain plant productivity and support biogeochemical is 0.8 hectares in size, occupying a riverine geomorphic processes such as decomposition and denitrification in position with a tidal range of approximately 1.2 meters and these N limited systems. Craft et al. (2003) estimated that floodwater salinity ranging from 7–10 parts per thousand 100 g N/m2 and 1000 g C/m2 are needed to achieve full or (Craft et al. 1988). The site is located on the eastern side of complete restoration of these functions and accumulation the island and has served as the focus of numerous studies of such sizable pools may require decades. on ecosystem development of restored tidal marshes (Craft The chronosequence (space for time) approach has been et al. 1988, 1999, 2003, Craft and Sacco 2003, Zheng et al. widely used to assess successional trajectories in restored 2004, Cornell et al. 2007). wetlands and wetland soil, but its explanatory power can be limited by variation due to factors other than ecosystem Sample Collection and Analysis age. For example, it may be difficult to completely match Soils were collected in 1984, 1995, and 1998 and findings hydroperiod (depth, duration, and frequency of inunda- were published in Craft et al. (1988), (1999), and (2003), tion), salinity, vegetation type, or geomorphic position respectively. Soils were sampled again in 2017 using the among restored sites of differing ages; though for tidal identical sampling design, field and lab protocols, and wetlands, tidal hydroperiod and vegetation type (Spartina analyses that were used in previous work at the site. Ten alterniflora) can be readily matched when restoring salt soil cores were collected in total. Five cores were randomly marshes. However, the efficacy of the chronosequence collected from the streamside edge and five cores were col- approach (i.e., different aged sites can be sampled at one lected from the marsh interior. Soil cores were sectioned time) makes it the primary method for evaluating succes- into 0–10 cm and 10–30 cm subsamples in the field, then sion and ecosystem development over time for natural and returned to the lab and air-dried, weighed, ground, and restored ecosystems (Crocker and Major 1955, Craft et al. sieved through a 2-mm mesh diameter screen. Screening 2003, Ballantine and Schneider 2009, Yang and Guo 2018). removed large roots but included fine roots, detritus and An alternative approach is to sample the same site over soil organic matter. time but this approach is seldom used (Craft et al. 2002, Measured soil properties included bulk density, organic He et al. 2016) because of the long time intervals required carbon, total nitrogen, and total phosphorus. Bulk density between sampling. was calculated by weighing air-dried cores of a known The goal of this study is to evaluate ecosystem develop- volume and applying a moisture correction factor deter- ment of a tidal wetland constructed and planted nearly mined from subsamples oven-dried at 105°C (Blake and five decades ago. We tracked changes in soil properties at Hartge 1986). Total organic carbon (TOC) and nitrogen periodic intervals (1984, 1995, 1998, 2017) since its con- (N) were measured using a Perkin-Elmer CHN analyzer struction in 1970. The work is important because 1) soil (Perkin-Elmer, Norwalk, CT, USA). Total P concentration development is a slow process and decades may pass before was determined using the HNO3-HClO4 soil digestion soils and soil-based functions are completely restored, and method followed by colorimetric analysis (Sommers and 2) nearly all studies of soil change following wetland resto- Nelson 1972). Estuarine sediment (NIST standard no. ration are based on the chronosequence approach and its 1646a) was used as a baseline standard in P digestions, inherent variability in hydrology, salinity, nutrients, land yielding a recovery rate of 94%. An in-house salt marsh use history and other abiotic factors among sites, rather standard was used for C and N, returning recovery rates than the repeated measures approach used here. of 101% and 97% for C and N, respectively. Nutrient (C, N, P) pools (g/m2) in the 0–10 and 10–30 cm depths were calculated using bulk density and C, N, and P concentra- tions. Linear regressions of soil properties versus marsh 164 • September 2019 ECOLOGICAL RESTORATION 37:3 Figure 1. Linear regressions of (a) Bulk Density, (b) % Organic C, (c) % N, and (d) Total P (μg/g) as a function of marsh age. Each data point represents n = 10.