Plant litter composition and stable isotope signatures vary during decomposition in blue carbon ecosystems

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.

Restoring blue carbon ecosystems unlocks fisheries' potential

“…One consideration that could affect model outputs is the amount of detrital matter, which may be an aggregate of various primary producers, that is consumed by Eastern school prawns. Decomposition progressively changes the stable isotope values of detritus for δ 15 N and δ 13 C (Kelleway et al 2022), and this species of prawn does present with relatively large (approximately 50%) amounts of detrital matter in their digestive system (Ruello 1973). However, decomposition effects are minor relative to the isoscape positions of the major sources in this system (approximately 2‰ compared to values of −28 and −14‰ for mangroves and seagrasses, respectively), and detrital matter is likely to be a mix of fresh and partially decayed plant matter.…”

“…For instance, the United Nations (UN) General Assembly declared 2021-2030 the "UN Decade of Ecosystem Restoration" as well as "The Decade of Ocean Science for Sustainable Development", both of which strongly support largescale restoration of coastal wetlands (Waltham et al 2020). Further, projects that protect or restore blue carbon ecosystems are eligible to receive carbon credits through the globally applicable VERRA accreditation (Sapkota & White 2020), United Nations Framework Convention on Climate Chante market mechanisms (de Sépibus et al 2013), as well as in Australia through its Emissions Reduction Fund (Kelleway et al 2017).…”

Substantial blue carbon sequestration in the world’s largest seagrass meadow

“…A total of 15 large seagrass fragment (leaves and rhizomes) samples were collected from the C org cores and analyzed for δ 13 C values, C org and TN concentrations. We did so to account for the significant decomposition effects on δ 13 C signature and C/N ratio 66,67 . The fragment samples were washed with Milli-Q water, fumed with HCl to remove carbonates and then dried at 60 °C68 .…”

The roles of seagrass litter decomposition in determining blue carbon sink capacity

“…The double-component exponential decay model that can be described as a summation of two single-component models provides a reasonable approach for modeling the decomposition dynamics of litter materials over time (Fourqurean and Schrlau 2003). Relatively easily decomposed compounds (labile components) such as sugars, starches, and proteins are rapidly utilized by decomposers, whereas recalcitrant substrates such as cellulose, fats, waxes, tannins, and lignins are lost at slower rates (Fourqurean and Schrlau 2003;Trevathan-Tackett et al 2017b;Kelleway et al 2022). The doublecomponent exponential decay model can be represented as follows:…”