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Nutrient storage and turnover in organic layers under tropical montane rain forest in Ecuador

Abstract:

In tropical montane forests nutrients released from the organic layers of the soil can supply a large part of the vegetation's requirements. We have examined concentrations, storage, and turnover times of nutrients in the organic layer and the fluxes of nutrients by the fall of small litter (leaves, seeds, flowers, small twigs, and plant debris that passed an opening of 0.3 m × 0.3 m) in such a forest in Ecuador. The times taken for litter to turn over were estimated by relating nutrient storage in the organic layer to rate of litterfall and by incubating samples in the laboratory. The organic layer had a thickness of 2-43 cm, a mass of 30-713 t ha-1, and a nutrient storage of 0.87-21 t N, 0.03-0.70 t P, 0.12-2.5 t K, 0.09-3.2 t Ca, and 0.07-1.0t Mg ha-1. The pH (in H2O) ranged between 3.1 and 7.4 and was correlated with the concentrations of Ca and Mg (r=0.83 and 0.84, respectively). The quantity of small litter (8.5-9.7 t year-1) and mean concentrations of nutrients in litter (19-22 g N, 0.9-1.6 g P, 6.1-9.1 g K, 12-18 g Ca, and 3.5-5.8 g Mg kg-1) were larger than in many other tropical montane forests. The mean turnover times of elements in the organic layer increased in the order, Mg (7.0 years) < Ca (7.9) < K (8.5) < P (11) < N (14) < S (15) when calculated as the quotient of storage in the organic layer to flux by litterfall; they were < 12 years for N, P, and S in the incubation experiment. Under optimum conditions in the laboratory, the mineralization of S was just as large as the S deposition by litterfall. In weakly acid soils Mn and Zn and in strongly acid soils Ca added in a nutrient solution were immobilized during incubation. Thus, lack of S, Mn, Zn, and Ca might limit plant growth on some soils.