Decomposition process of Rhynchospora asperula and nitrogen budget in floating soils of Iberá wetland, Argentina

A. TORREMORELL; P. GANTES

Merida, Yucatan, Mexico

Congreso; ESA conference "Ecology in an Era of Globalization"; 2006

Ecological Society of America

Decomposition is a key process in wetlands, which are ecosystems in which the environmental conditions limit the speed of the process and generate soils with abundant organic matter and nutrients not available for primary production.  The Iberá swamps are among the most important wetlands of Argentina, Ramsar Site since 2002. A big part of it is covered by floating soils (“embalsados”) originated from the interweaving of macrophyte’s roots and vegetal detritus accumulation.  The objectives of this study were: (1) to estimate changes in biomass and nitrogen content of Rhynchospora asperula throughout the year.  (2) to estimate the decomposition rate of this species, and the changes in nitrogen and lignin content during decomposition.  (3) to determinate the effect of water availability in the substrate on the  decomposition rate. Litterbags with fallen leaves of R. asperula were placed in two coastal floating soils of the Iberá lagoon and removing a portion of them was performed every two months, during 340 days.  Whenever bags were retired, a portion of the floating soils and aerial biomass were extracted. The samples were dried and weighted, and total nitrogen was determined on them. Also, lignin content of the removed litterbags was determined.  In order to consider the effect of water on litter decomposition an experiment was made, litterbags were exposed to three water level treatments. R. asperula reached its maximum biomass in summer with 989.05 g/m2 and the minimum was registered in winter, with 371.89 g/m2.  Annual production was 671.16 g of dry weight/m2.  The nitrogen concentration in the aerial biomass remained relatively constant throughout the year, with an average value of 1.29 %. Decomposition rate, k, was 0.0032 day-1.  The nitrogen concentration in litterbags increased from 0.854 % to 1.43 % after 340 days of incubation.  Lignin concentration found in litter was 18.3 % and this amount stayed relatively constant until the sampling was completed.  The experiment performed on the banks of the Iberá lagoon revealed that decomposition is affected by water availability for the microorganisms, with greater decomposition rates for the substrates that, at least for some time, were in contact with water. In summary, the organic matter accumulation in the floating soils of Iberá, should not  be a consequence of a low decomposition rate along the first year of deposition, but of the following years, when the dead material is covered by new litter. Although a permanent flood condition is not common in floating soils, the decomposition rate depends on the litter contact to water. An important percentage of nitrogen remains in plant biomass and is not released in senescence; another part stays as organic matter built-in to the floating soil, therefore not available for the primary production. Finally, a smaller percentage is mineralized, which is the portion subject to loss by leaching or denitrification