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Abstract: Rainfall, throughfall and stemflow were monitored on an event basis in an undisturbed open tropical rainforest with a large number of palm trees located in the southwestern Amazon basin of Brazil. Stemflow samples were collected from 24 trees with a diameter at breast height (DBH)>5cm, as well as eight young and four full-grown babassu palms (Attalea speciosa Mart.) for 5weeks during the peak of the wet season. We calculated rainfall, throughfall and stemflow concentrations and fluxes of , , , , , , , and and stemflow volume-weighted mean concentrations and fluxes for three size classes of broadleaf trees and three size classes of palms. The concentrations of most solutes were higher in stemflow than in rainfall and increased with increasing tree and palm size. Concentration enrichments from rainfall to stemflow and throughfall were particularly high (81-fold) for . Stemflow fluxes of and exceeded throughfall fluxes but stemflow fluxes of other solutes were less than throughfall fluxes. Stemflow solute fluxes to the forest soil were dominated by fluxes on babassu palms, which represented only 4% of total stem number and 10% of total basal area. For , stemflow contributed 51% of the total mass of nitrogen delivered to the forest floor (stemflow+throughfall) and represented more than a 2000-fold increase in flux compared what would have been delivered by rainfall alone on the equivalent area. Because these highly localized fluxes of both water and persist in time and space, they have the potential to affect patterns of soil moisture, microbial populations and other features of soil biogeochemistry conducive to the creation of hotspots for nitrogen leaching and denitrification, which could amount to an important fraction of total ecosystem fluxes. Because these hotspots occur over very small areas, they have likely gone undetected in previous studies and need to be considered as an important feature of the biogeochemistry of palm-rich tropical forest. [Copyright &y& Elsevier]

Abstract: Secondary forests are gaining increased importance in tropical landscapes and have recently been reported to act as potential belowground carbon sinks. While economic interest in the management of secondary forests to mitigate carbon emissions is rising, the dynamics of soil carbon stocks under these ecosystems remain poorly understood. Recent studies report conflicting results concerning soil carbon trends as well as multiple confounding factors (e.g. soil type, topography and land-use history) affecting these trends. In this study, organic carbon stocks were measured in the mineral soil up to 20cm depth of at 24 active pastures, 5–8-year-old, and 12–15-year-old secondary forest sites on former pastures. Additionally, we estimated carbon stocks under a 100-year-old secondary forest and compared them to those of nearby mature forests. Abiotic conditions in the study area were homogenous, enabling us to isolate the effect of land-use change on soil organic carbon stocks. Contrary to our expectations, soil carbon stocks in the top 10cm did not change with young secondary forest development. Pasture soils stored 24.8±2.9Mgha−1 carbon (mean±standard error) in the top 10cm, and no accumulation of soil carbon was apparent during the first 15 years of secondary succession. Soil carbon stocks under 100-year-old secondary forests, averaging 43.0±7.9Mgha−1 (mean±standard error), were clearly higher than those recorded at younger sites and approached levels of soil carbon stocks under mature forests. These data indicate that soil carbon stocks in this region of Panama are not affected by the land-use transition from pasture to young secondary regrowth. However, an increase of soil carbon storage might be possible over a longer period of time. Our results support trends observed in other tropical areas and highlight the importance of environmental conditions such as soil properties rather than land-use transitions on soil carbon dynamics. While our understanding of organic carbon dynamics in tropical soils remains limited, these results underscore the challenges of undertaking short-term reforestation projects with the expectation of increasing soil carbon sequestration. [Copyright &y& Elsevier]

Abstract: Landscapes in the humid tropics are undergoing a continuous change in land use. Deforestation is still taking its toll on forested areas, but at the same time more and more secondary forests emerge where formerly agricultural lands and pastures are being abandoned. Regarding soil hydrology, the extent to which secondary succession can recover soil hydrological properties disturbed by antecedent deforestation and pasture use is yet poorly understood. We investigated the effect of secondary succession on saturated hydraulic conductivity (Ks) at two soil depths (0–6 and 6–12cm) using a space-for-time approach in a landscape mosaic in central Panama. The following four land-use classes were studied: pasture (P), secondary forest of 5–8 years of age (SF5), secondary forest of 12–15 years of age (SF12) and secondary forest of more than 100 years of age (SF100), each replicated altogether four times in different micro-catchments across the study region. The hydrological implications of differences in Ks in response to land-use change with land use, especially regarding overland flow generation, were assessed via comparisons with rainfall intensities. Recovery of Ks could be detected in the 0–6cm depth after 12 years of secondary succession: P and SF5 held similar Ks values, but differed significantly (α =0.05) from SF12 and SF100 which in turn were indistinguishable. Variability within the land cover classes was large but, due to sufficient replication in the study, Ks recovery could be detected nonetheless. Ks in the 6–12cm depth did not show any differences between the land cover classes; only Ks of the uppermost soil layer was affected by land-use changes. Overland flow – as inferred from comparisons of Ks with rainfall intensities – is more likely on P and SF5 sites compared to SF12 and SF100 for the upper sample depth; however, generally low values at the 6–12cm depth are likely to impede vertical percolation during high rainfall intensities regardless of land use. We conclude that Ks can recover from pasture use under secondary succession up to pre-pasture levels, but the process may take more than 8 years. In order to gain comprehensive understanding of Ks change with land use and its hydrological implications, more studies with detailed land-use histories and combined measurements of Ks, overland flow, precipitation and throughfall are essential. [Copyright &y& Elsevier]

Abstract: Soils play a crucial role in biogeochemical cycles as spatially distributed sources and sinks of nutrients. Any spatial patterns depend on soil forming processes, our understanding of which is still limited, especially in regards to tropical rainforests. The objective of our study was to investigate the effects of landscape properties, with an emphasis on the geometry of the land surface, on the spatial heterogeneity of soil chemical properties, and to test the suitability of soil–landscape modeling as an appropriate technique to predict the spatial variability of exchangeable K and Mg in a humid tropical forest in Panama. We used a design-based, stratified sampling scheme to collect soil samples at 108 sites on Barro Colorado Island, Panama. Stratifying variables are lithology, vegetation and topography. Topographic variables were generated from high-resolution digital elevation models with a grid size of 5m. We took samples from five depths down to 1m, and analyzed for total and exchangeable K and Mg. We used simple explorative data analysis techniques to elucidate the importance of lithology for soil total and exchangeable K and Mg. Classification and Regression Trees (CART) were adopted to investigate importance of topography, lithology and vegetation for the spatial distribution of exchangeable K and Mg and with the intention to develop models that regionalize the point observations using digital terrain data as explanatory variables. Our results suggest that topography and vegetation do not control the spatial distribution of the selected soil chemical properties at a landscape scale and lithology is important to some degree. Exchangeable K is distributed equally across the study area indicating that other than landscape processes, e.g. biogeochemical processes, are responsible for its spatial distribution. Lithology contributes to the spatial variation of exchangeable Mg but controlling variables could not be detected. The spatial variation of soil total K and Mg is mainly influenced by lithology. [Copyright &y& Elsevier]

Abstract: Many modern landscapes in the humid tropics appear as a mosaic of primary forest, agricultural land and abandoned areas at various stages of regrowth. Whereas the hydrological functioning of primary forest and agricultural ecosystems is known to some degree, the impact of secondary forests on the hydrological cycle has yet to receive the same attention; this is particularly true for the effects of forest regrowth on soil hydrology. We investigated the effects of land-use and land-cover types representative of largely deforested areas of the Amazon basin on soil hydrological flowpaths by quantifying infiltrability and field-saturated hydraulic conductivity (Ksat) at two depths (12.5 and 20cm) under primary forest, recently cleared secondary forest (prepasture), teak (Tectona grandis), pasture and secondary forests originating from a former banana–cacao plantation (banana) and from pasture (capoeira). We further inferred potential changes in the hydrological flowpath regime by comparing our results with prevailing rainfall intensities. Infiltrability varied (α =0.05) among the cover types according to: {forest}>{banana, capoeira, teak}>{pasture}, with prepasture overlapping with forest, banana and capoeira. Although the infiltrability of pasture is lower by an order of magnitude in comparison to the other cover types, this decrease does not affect flowpath patterns as infiltrability still exceeds even the highest short-term, such as 5min, rainfall intensities. Ksat at the 12.5cm depth varies as follows: {forest, banana, capoeira}>{teak}>{pasture}, with prepasture being undistinguishable from the first group. Under teak and pasture, Ksat is lower than prevailing rainfall intensities, which implies the possible generation of a perched water table at a shallow depth and hence the generation of saturation overland flow. The impact of 13 years of cattle grazing prior to planting teak is still measurable even after 10 years of growth. At the 20cm depth, the differences in Ksat among the land uses are minor compared to prevailing rainfall intensities, i.e., the vertical flow of water is impeded regardless of land use. We conclude that there is a considerable memory effect; hence, any assessment of the hydrological functioning of secondary forests, quasi-natural or man-made, must take into account the kind, intensity and duration of land use prior to regrowth. [Copyright &y& Elsevier]