For mangrove ecosystems an C/N approach has been applied, but in combination with for instance stable isotope ratios etc. See:
Ranjan, R.K., Routh, J., Ramanathan, A.L. & Klump, J.V. (2011) Elemental and stable isotope records of organic matter input and its fate in the Pichavaram mangrove-estuarine sediments (Tamil Nadu, India). Marine Chemistry, 126, 163–172.
Ono, K., Hiradate, S., Morita, S., Hiraide, M., Hirata, Y., Fujimoto, K., Tabuchi, R. & Lihpai, S. (2015) Assessing the carbon compositions and sources of mangrove peat in a tropical mangrove forest on Pohnpei Island, Federated States of Micronesia. Geoderma, 245-246, 11–20.
Saintilan, N., Rogers, K., Mazumder, D. & Woodroffe, C. (2013) Allochthonous and autochthonous contributions to carbon accumulation and carbon store in southeastern Australian coastal wetlands. Estuarine, Coastal and Shelf Science, 128, 84–92.