Restoration of declining soil carbon stocks and lost surface elevations in degraded mangroves on the northern coast of Java, Indonesia

Trialaksita Sari Priska Ardhani^1,2*Cecep Kusmana³Dietriech Geoffrey Bengen⁴Joeni Setijo Rahajoe⁵Phidju Marrin Sagala²Bayu Budi Hanggara⁶Yohanes Risky Shellen Ginting⁷Milkah Royna²Daniel Murdiyarso^1,2

• ¹Department of Geophysics and Meteorology, Faculty of Mathematics and Natural Sciences, IPB University, Bogor, Indonesia
• ²Center for International Forestry Research - World Agroforestry (CIFOR-ICRAF), Bogor, Jakarta, Indonesia
• ³Department of Silviculture, Faculty of Forestry and Environment, IPB University, Bogor, West Java, Indonesia
• ⁴Department of Marine Science and Technology, Faculty of Fisheries and Marine Sciences, IPB University, Bogor, Jakarta, Indonesia
• ⁵Research Center for Ecology and Ethnobiology, National Research and Innovation Agency (BRIN), Bogor, Jakarta, Indonesia
• ⁶Department of Biogeochemical Integration, Max Planck Institute for Biogeochemistry, Jena, Thuringia, Germany
• ⁷Department of Meteorology, Faculty of Medicine, University of Bonn, Bonn, North Rhine-Westphalia, Germany

Restoring degraded coastal zones could help to stem the loss of carbon and soil surface elevations due to mangrove conversion and sea-level rise. In mangroves, total ecosystem carbon stocks are primarily stored in soil, however the presence of vegetation remains crucial. When we assessed soil carbon across contrasting land uses in Wedung and Sayung Districts in Demak Regency, Central Java, we found that converting mangrove to aquaculture increased soil carbon by 17.68 Mg C ha-1 in Wedung but decreased it as much as 62.58 Mg C ha-1 in Sayung. Meanwhile, abandonment of ponds resulted in a loss of 34.24 Mg C ha-1 in Wedung but a gain of 8.81 Mg C ha-1 in Sayung. The estimated local sea-level rise of 0.45 cm yr-1 led to relative sea-level rises of 5.56, 6.82, and 1.36 cm yr-1 in mangroves, working and abandoned ponds respectively in Wedung, and 8.00, 10.39, and 10.46 cm yr-1 respectively in the same environments in Sayung. This indicates that Sayung is more severely inundated. After considering land subsidence levels, only mangroves in Wedung experienced an elevation surplus of 2.02 cm yr-1. The remaining land uses suffered elevation deficits, hence being inundated by 1.86 and 15.00 cm yr-1 in abandoned ponds in Wedung and Sayung respectively. Despite variation in this study’s findings, some scenarios could be considered useful to support relevant coastal areas to enhance soil carbon stocks or surface elevation.