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The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m year), phosphorus (P as superphosphate, 5 g P m year) and potassium (K as KCl, 5 g K m year) within 38 ha of old-growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images. We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg), low concentrations of Bray-extractable phosphate (PO = 2.2 mg kg), and modest concentrations of KCl-extractable nitrate (NO = 5.0 mg kg) and KCl-extractable ammonium (NH = 15.5 mg kg). Added N increased concentrations of KCl-extractable NO and acidified the soil by one pH unit. Added P increased concentrations of Bray-extractable PO and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m. The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species-rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study. [ABSTRACT FROM AUTHOR]

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 leaf-fall 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. [ABSTRACT FROM AUTHOR]