Analysis of Organic Matter Decomposition in the Salt Marshes of the Venice Lagoon (Italy) Using Standard Litter Bags

Tidal salt marshes are widespread along the World's coasts, and are ecologically and economically important as they provide several valuable ecosystem services. In particular, their significant primary production, coupled with sustained vertical accretion rates, enables marshes to sequester and store large amounts of organic carbon and makes them one of the most carbon-rich ecosystems on Earth. Organic carbon accumulation results from the balance between inputs, that is, organic matter produced by local plants or imported, and outputs through decomposition and erosion. Additionally, organic matter deposition actively contributes to marsh vertical accretion, thus critically affecting the resilience of marsh ecosystems to rising relative sea levels. A better understanding of organic-matter dynamics in salt marshes is key to address salt-marsh conservation issues and to elucidate marsh importance within the global carbon cycle. Toward this goal, we empirically derived rates of organic matter decomposition by burying 712 commercially available tea bags at different marshes in the microtidal Venice Lagoon (Italy), and by analyzing them following the Tea Bag Index protocol. We find values of the decomposition rate (k) and stabilization factor (S) equal to 0.012 +/- 0.003 days(-1) and 0.15 +/- 0.063, respectively. Water temperature critically affects organic matter decomposition, enhancing decomposition rates by 8% per degrees C on average. We argue that, at least in the short term, the amount of undecomposed organic matter that actively contributes to carbon sequestration and marsh vertical accretion strongly depends on the initial organic matter quality, which is a function of marsh and vegetation characteristics.Plain Language Summary Salt marshes are important coastal environments regularly flooded by the tide and dominated by herbaceous plants, providing several valuable ecosystem services. They are, however, threatened by the effects of climate changes and human interferences. As organic matter accumulated in salt-marsh soil importantly contribute to surface elevation necessary for marshes to keep up with sea level rise and to store atmospheric carbon, this project aims to improve our understanding of decomposition processes affecting organic matter preservation and their controls in salt-marsh environment. Toward this goal, following the so-called Tea Bag Index protocol, we buried 712 commercially available tea bags in salt-marsh soils of the Venice Lagoon (Italy) measuring the reduction of their organic content due to decomposition processes after 3 months. Our results confirm that salt marshes are among biomes with the slowest decomposition rates. However, we observed a loss of about two-thirds of the initial labile organic mass after 90 days and that initial litter quality, depending on litter and vegetation characteristics, exerts a primary control on the amount of preserved organic matter contributing to carbon sequestration and marsh accretion.