Seasonal drought and dry-season irrigation influence leaf-litter nutrients and soil enzymes in a moist, lowland forest in Panama

Climatic conditions should not hinder nutrient release from decomposing leaf-litter (mineralization) in the humid tropics, even though many tropical forests experience drought lasting from several weeks to months. We used a dry-season irrigation experiment to examine the effect of seasonal drought on nutrient concentrations in leaffall and in decomposing leaf-litter. In the experiment, soil in two 2.25-ha plots of old-growth lowland moist forest on Barro Colorado Island, Republic of Panama, was watered to maintain soil water potential at or above field capacity throughout the 4-month dry season. Wet-season leaf-fall had greater concentrations of nitrogen (N, 13.5 mg g –1 ) and calcium (Ca, 15.6 mg g –1 ) and lower concentrations of sulfur (S, 2.51 mg g –1 ) and potassium (K, 3.03 mg g –1 ) than dry-season leaf-fall (N = 11.6 mg g –1 , Ca = 13.6 mg g –1 , S = 2.98 mg g –1 , K = 5.70 mg g –1 ). Irrigation did not affect nutrient concentrations or nutrient return from forest trees to the forest floor annually (N = 18 g m –2 , phosphorus (P) = 1.06 g m –2 , S = 3.5 g m –2 , Ca = 18.9 g m –2 , magnesium = 6.5 g m –2 , K = 5.7 g m –2 ). Nutrient mineralization rates were much greater during the wet season than the dry season, except for K, which did not vary seasonally. Nutrient residence times in forest-floor material were longer in control plots than in irrigated plots, with values approximately equal to that for organic matter (210 in control plots vs 160 in irrigated plots). Calcium had the longest residence time. Forest-floor material collected at the transition between seasons and incubated with or without leaching in the laboratory did not display large pulses in nutrient availability. Rather, microorganisms immobilized nutrients primarily during the wet season, unlike observations in tropical forests with longer dry seasons. Large amounts of P moved among different pools in forest-floor material, apparently mediated by microorganisms. Arylsulfatase and phosphatase enzymes, which mineralize organically bound nutrients, had high activity throughout the dry season. Low soil moisture levels do not hinder nutrient cycling in this moist lowland forest.