Resumen de Soil and vegetation carbon dynamics (stocks and fluxes) and litter decomposition in a mediterranean non-tidal salt marsh
Salt marshes are important ecosystems for global climate regulation because of their high plant primary production and low rates of organic matter decomposition, which would promote carbon accumulation into the soil. Literature concerning the carbon sequestration capacity of salt marshes has been mainly focused on the estimation of carbon stocks from vegetation and soil, and their changes over time (net primary production and soil carbon burial). However, few of these studies incorporate measurements of the structure and composition of plant communities, being also scarce the seasonal characterization of carbon fluxes from soil, and especially, from vegetation. Moreover, although litter decomposition rates in these ecosystems have been broadly estimated, other parameters related with the litter decomposition process, such as the litter chemical quality and microbial activity, remain poorly studied and thus understood. On the other hand, most studies on carbon sequestration capacity and litter decomposition have been performed in tidal salt marshes, especially from North America, being studies from non-tidal salt marshes, such as those of the Mediterranean Basin, really scarce.
In this context, the specific objectives of this thesis were: (1) to analyse the changes over one year in the structure and composition of plant communities, and also in the amount of carbon stored in vegetation and soil, of three non-tidal Mediterranean salt marsh habitats, as well as of a disturbed (and later restored) zone; (2) to assess daily and seasonally CO2 fluxes from vegetation and soil, as well as soil CH4 fluxes, in these three salt marsh habitats throughout one year; and (3) to study the litter decomposition of the dominant species of these salt marsh habitats considering the decomposition rate, the chemical quality and the microbial community dynamics of the litter.
The study was performed at La Pletera salt marsh (NE of the Iberian Peninsula), being the three well-preserved salt marsh habitats studied (halophilous scrub, salt meadow, and glasswort sward) habitats of community interest (HCI). The more mature plant communities were those of the halophilous scrub, dominated by Sarcocornia fruticosa and the salt meadow, dominated by Elymus pycnanthus and Atriplex portulacoides, which presented a higher amount of carbon stored in both vegetation and soil compared with the glasswort sward. This latter habitat was formed by pioneer vegetation, concretely it was dominated by Salicornia patula, and results indicate that it will probably evolve towards a plant community dominated by late-successional species similar to that of the halophilous scrub. The disturbed zone, dominated by ruderal vegetation, regenerated naturally after the restoration, changing the plant community composition with the colonization by halophytic species, mainly Suaeda maritima, but also with some individuals of Sarcocornia fruticosa.
Standing biomass at La Pletera salt marsh was lower than that from tidal salt marshes, but it was similar to that from other Mediterranean non-tidal salt marshes. Conversely, soil organic carbon was generally lower than previous reported values for both types of salt marshes. The study of CO2 and CH4 fluxes revealed that E. pycnanthus was the species with the highest photosynthetic rates during the entire year, being also remarkable those of S. patula in summer. On the other hand, the four species studied (S. fruticosa, E. pycnanthus, A. portulacoides and S. patula) showed daily net CO2 uptake from the green fraction during most of the year, except in December when S. fruticosa and A. portulacoides showed net CO2 emission. The woody fraction of S. fruticosa and A. portulacoides showed net CO2 uptake in winter and spring and in winter spring and summer, respectively. The halophilous scrub and the salt meadow showed higher soil CO2 emissions than the glasswort sward, and, in general, these values were higher than those reported for other tidal salt marshes. At La Pletera salt marsh, CH4 absorption and emission were also detected, being CH4 emissions remarkably high, and, in general, higher than those of other salt marshes with high water table salinity, but similar to those of salt marshes with low salinity. The soils of the halophilous scrub and the salt meadow presented lower mineralization quotients than those of the glasswort sward, which would suggest a higher soil carbon sequestration potential.
The litter of S. fruticosa and A. portulacoides, likely due to their low lignin content, decomposed faster than that of E. pycnanthus. Regarding the microbial activity, bacteria dominated along all the litter decomposition process, although fungi became especially important in the later stages, when the relative lignin litter content increased. Litter decomposition of all the species was affected by the spatial heterogeneity, suggesting that flooding (in the halophilous scrub) or soil texture (in the salt meadow) could have greatly modified the process of decomposition.
Results from this thesis contribute to increase the knowledge on carbon dynamics and litter decomposition in Mediterranean Basin salt marshes. The conservation of these ecosystem and the recovery of their degraded areas, jointly with the reestablishment of their natural hydrologic fluxes, will contribute to increase the amount of carbon stored, avoiding its release to the atmosphere, which will guarantee the continuation of their valuable ecosystem services, including climate regulation.
