Mass and nutrient loss of fresh plant biomass in a small black-water tributary of Caura river, Venezuelan Guayana

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Decomposition rates and nutrient dynamic (N, P, K, Ca and Mg) were determined for green leaves and fine branches immersed in the water of a small tributary of Caura river (SE-Venezuela). 16% of the original dry weight of leaves and 11% of branches were lost at the end of the first sampling period: first month for leaves and second month for branches. This dry weight reduction was probably due to leaching of soluble material. After a 9-month period, the mass loss was 60% for leaves and 20% for fine branches. The pattern of dry weight and nutrient losses are in general agreement with previous studies of decomposition of leaf litter in both terrestrial and aquatic ecosystems. Potassium and magnesium are the elements most rapidly lost, showing the dominance of leaching processes; at the end of the first month 7% of the initial amount of K and 18% of the initial amount of Mg remained in leaves. The loss of calcium and phosphorus was much slower: 61% of Ca and 47% of P remained in the leaf material after the first sampling period. In contrast to K, Mg, Ca and P, the initial amount of nitrogen in leaves remained relatively unchanged during the first month of decomposition; in the subsequent sampling period, the amount of N decreased. The elements K and Mg in branches behaved similar to leaves: 4% of K and 22% of Mg were left at the end of the first sampling period. The initial amount of Ca and P in branches decreased slightly: 88% of Ca and 83% of P remained in branches at the end of this first sampling. Nitrogen behaved differently in branches than that in leaves. In branches the amount of N remained relatively unchanged during the first 5 months of decomposition; afterwards, N showed gradual increases, probably due to immobilization. At the end of the experiment the amount of N in branches was 16% higher than the initial amount.

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