Abstract
The ratio of isotopes of carbon (13C:12C or δ13C) and nitrogen (15N:14N or δ15N) are common indicators of the flow and storage of organic matter in coastal wetland research. Effective use of these indicators requires quantification and understanding of: (1) the variability of isotope signatures of potential organic matter source materials; and (2) the influence of organic matter decomposition on isotopic signatures. While it is well-established that organic matter characteristics change during the decomposition process, there has been little direct quantification of any concurrent shifts in isotope signatures for coastal detritus. In this study, we addressed this by quantifying: (1) shifts in sample composition using solid-state 13C Nuclear Magnetic Resonance (NMR) spectroscopy; and (2) shifts in δ13C and δ15N signatures of coastal plant tissues from field litterbag experiments. We observed significant shifts in 13C NMR spectra across the course of deployment for all four plant tissues assessed (leaves of mangrove Avicennia marina; branchlets of supratidal tree Casuarina glauca; leaf wrack and roots/rhizomes of the seagrass Zostera muelleri), driven largely by the preferential loss of labile constituents and concentration of more resistant macromolecules, such as lignin and leaf waxes. While there were shifts in isotope ratios for all species, these varied in direction and magnitude among species, tissue type and isotopes. This included δ13C enrichments of up to 3.1‰ and 2.4‰ in leaves of A. marina, and branchlets of C. glauca, respectively, but δ13C depletions of up to 4.0‰ for Z. muelleri. Shifts in δ15N varied among species and tissue types, with few clear temporal patterns. Partial least squares regression analyses showed that some tissue isotope signatures can be reliably predicted on the basis of sample composition (13C NMR spectra), however, multiple inter- and intra-species variations preclude a simple explanation of isotopic signature shifts on the basis of plant-material molecular shifts alone. Further, we cannot preclude the potential influence of microbe-associated organic matter on sample composition or isotopic signatures. Our findings emphasise the importance of considering decomposition effects on stable isotope signatures in blue carbon ecosystems. Isotope approaches will remain a valuable tool in coastal ecosystem research, but require robust experimental approaches (including appropriate use of decomposed end-members or fractionation correction factors; quantification of microbial organic matter) and quantification of decomposition dynamics for specific plant tissues and environmental settings.
Highlights
Vegetated coastal ecosystems, including seagrass meadows, saltmarshes, mangroves and other tidal forests, are recognised among the most productive ecosystems on earth (Duarte et al 2013; Krauss et al 2018; Serrano et al 2019)
Critchley Department of Biological Sciences, Macquarie University, Sydney, NSW, Australia process, there has been little direct quantification of any concurrent shifts in isotope signatures for coastal detritus. We addressed this by quantifying: (1) shifts in sample composition using solidstate 13C Nuclear Magnetic Resonance (NMR) spectroscopy; and (2) shifts in δ13C and δ15N signatures of coastal plant tissues from field litterbag experiments
For Casuarina glauca branchlets, shifts in 13C Nuclear Magnetic Resonance (13C NMR) spectra over the course of deployment included increases in spectral intensity at 142–147 ppm and 150–155 ppm, but opposing trends to A. marina of decreasing intensity at 56 ppm and in the range 20–40 ppm (Fig. 2b)
Summary
Vegetated coastal ecosystems, including seagrass meadows, saltmarshes, mangroves and other tidal forests, are recognised among the most productive ecosystems on earth (Duarte et al 2013; Krauss et al 2018; Serrano et al 2019). Measuring the flow and/or storage of organic matter (OM) is of importance to our understanding of biochemical connectivity among terrestrial, coastal and marine ecosystems (Abrantes et al 2015; Alongi et al 1989; Guest & Connolly 2004; Hemminga et al 1994) It is becoming increasingly important in the valuation and accounting of ecosystem services, including the implementation and monitoring of carbon trading and offset projects under blue carbon accounting frameworks (Needelman et al 2018). These processes include the decomposition of freshly deposited OM from plants and other autotrophs (e.g. benthic algae)— typically over timescales of hours to months—and
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