Nutrient dynamics in Amazonian floodplains: Decomposition of herbaceous plants in aquatic and terrestrial environments

Summary

In the floodplain of the Amazon river (Várzea) decomposition of herbaceous plants (Sphenoclea zeylanica, Cyperus imbricatus, and Ludwigia densiflora) was studied in litter bag experiments both under aquatic (lake water) and under terrestrial conditions (dry sediment bar under plant cover). Weight loss and change in nutrient content of decomposing material were determined.

Under aquatic conditions plant biomass had lost up to 75% of its initial weight, and up to 92% of its initial K amount within a period of 2 weeks (Sphenoclea zeylanica). Loss rates of the other elements studied were somewhat slower. At the end of the experiments (after 4 weeks) weight losses varied between 67% and 90%, the remaining detritus was strongly impoverished in K and partly in N, Ca and P.

Under terrestrial conditions rates of dry weight and nutrient losses were lower. Within 2 weeks a dry weight loss of up to 30% and a K loss of up to 25% were observed in Sphenoclea zeylanica. In contrast to aquatic conditions the content of the nutrients K, Mg, Ca and N remained high within the 4 weeks study period, indicating a high mineral quality of the detritus.

Leaching is considered to be responsible for high decomposition rates and selective element losses under aquatic conditions. It is suggested, that the regular change between terrestrial and aquatic phases leads to a complete decomposition of all organic material inhibiting an accumulation in spite of the very high productivity of herbaceous plants in the floodplain of the Amazon river.