PARENT SESSION
Contributed Oral Session 133: Biogeochemistry of Streams and Wetlands
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 516 C, Level 5, Palais des congrès de Montréal

Decomposition of celluloses and lignin of belowgroung litter in Tagus Salt Marshes (Portugal).

Caçador, Isabel¹, Pereira, Patrícia², Vale, Carlos², Costa, Ana Luísa¹, Caetano, Miguel², ¹ Institute of Oceanography, Faculty of Sciences, University of Lisbon, Campo Grande, Lisbon, Portugal² National Institute for Agronomy and Fishery Research, Av. Brasília, Lisbon, Portugal

ABSTRACT- Salt marshes typically exhibit high rates of productivity and excellent carbon sinks as they take CO₂ from the atmosphere and store it in living plant tissue. High belowground production means that a large amount of litter is produced annually, buried in sediments, and a part of the decomposed mass of plant litter remains in the marsh. After clipping H. portulacoides at ground level a lump of sediment was collected at Pancas and Corroios marshes. Belowground parts were separated from soil particles. Approximately 5 g of root material were placed in 24 mesh bags, and the contents of each bag were individually weighed. The bags were buried at 10 cm depth in their respective environments. A set of three bags was collected monthly between February 1999 and September 1999. In the laboratory the plant material was removed from the litterbags, rinsed with distilled water, dried to constant, weighted, homogenized and analyzed for C, N, cellulose, hemicellulose and lignin. Before burring, sub-samples were separated to determine initial concentrations of these components. Decay rates (k) were calculated through this first-order function, Xt=Xoe-kt. The litterbag experiments revealed that litter from belowground material of H. portulacoides decomposed rapidly in the two salt marshes. Between February and March, 47% of litter were decomposed in Pancas and 41% in Corroios. During subsequent periods the loss of weight was smaller. C and N concentrations remained relatively constant during the 7-month decomposition, meaning that weight loss is proportional to C and N mobilisation from litter. However, the C/N elemental ratio was higher in Corroios than in Pancas, indicating a preferential loss of nitrogen in Corroios. Decay rates of weight loss, cellulose, hemicellulose and lignin fit first-order functions. The decay rates of weight varied from 0.023 d^-1 in the first month of decomposition to 0.0026 d^-1 in subsequent periods. Decay rates of cellulose and hemicellulose were higher in Pancas (0.037-0.0057 d^-1 and 0.029-0.0054 d^-1, respectively) than in Corroios (0.013-0.0035 d^-1 and 0.012-0.0027 d^-1), a more acidic sediment environment. On the contrary, the decay rate of lignin during the first was higher in Corroios (0.021 d^-1) than in Pancas (0.014 d^-1), and rates in subsequent periods are the same for the two marshes.

Key words: saltmarshes, cellulose, lignin