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

Soil phosphatase assay to evaluate arsenic toxicity should be performed at the soil's actual pH

Soil phosphatase is considered an indicator to assess soil arsenic (As) pollution. In the phosphatase activity determination, a fixed buffer value (pH 5-10) is commonly used for all soils, ignoring the soil's actual pH. Here, we determined the soil phosphatase activity of 20 soils under As stress at the soils' pH, and the As inhibition mechanism was also explored by the enzyme kinetics. Our results show that soil phosphatase activity was significantly inhibited under As stress. The inhibition rate in acid soils (39.2 %) was considerably higher than in alkaline soils (25.4 %) when As concentration was 600 mg kg^-1. For alkaline soils, As inhibited phosphatase by competitive inhibition or linear mixed inhibition, while for acid soils, it was more complex, including linear mixed inhibition, non-competitive inhibition, and anti-competitive inhibition. Simultaneously, our results showed that the ecological dose (ED10) described by the partial inhibition model was far below than the complete inhibition model. According to the partial inhibition model, the ED₁₀ of As ranged from 2.66 to 164.07 mg kg^-1 for alkaline soils and 0.11 to 89.95 mg kg^-1 for acid soils. Moreover, V_max/K_m of phosphatase is a more sensitive index for evaluating As contamination than V_max in partial inhibition models. The ED₁₀ obtained based on the relationship between V_max/K_m and As concentration was 0.64-34.75 mg kg^-1 for acid soils and 8.48 to 20.16 mg kg^-1 for alkaline soils. This also confirms V_max/K_m as a sensitive and ideal index for assessing As pollution under soils' actual pH. Furthermore, soil pH and cation exchange capacity are dominant factors affecting As inhibition on soil phosphatase. The above kinetic studies indicate that performing the assay by adjusting the buffer pH to the soil pH is essential for more accurately evaluating arsenic toxicity.

Shifting seasonal patterns of water availability: ecosystem responses to an unappreciated dimension of climate change

Seasonal patterns of water availability can differ dramatically among ecosystems, with well-known consequences for ecosystem structure and functioning. Less appreciated is that climate change can shift the seasonality of water availability (e.g. to wetter springs, drier summers), resulting in both subtle and profound ecological impacts. Here we (1) review evidence that the seasonal availability of water is being altered in ecosystems worldwide, (2) explore several mechanisms potentially driving these changes, and (3) highlight the breadth of ecological consequences resulting from shifts in the seasonality of water availability. We conclude that seasonal patterns of water availability are changing globally, but in regionally specific ways requiring more rigorous and nuanced assessments of ecosystem vulnerability as well as the ecological consequences.

Nutrient limitation of plant reproduction in a tropical moist forest

Nutrient addition experiments indicate that nitrogen and phosphorus limit plant processes in many tropical forests. However, the long-term consequences for forest structure and species composition remain unexplored. We are positioned to evaluate potential long-term consequences of nutrient addition in central Panama where we have maintained a factorial nitrogen-phosphorus-potassium fertilization experiment for 21 yr and an independent study quantified the species-specific nutrient requirements of 550 local tree species. Here, we ask whether nutrients limit reproduction at the species and community levels. We also ask whether species-specific reproductive responses to nutrient addition are stronger among species associated with naturally fertile soils, which could contribute to a shift in species composition. We quantified species-level reproductive responses for 38 focal species in the 21st year of the experiment and community-level reproductive litter production for the first 20 yr. Species-level reproductive responses to nitrogen and potassium addition were weak, inconsistent across species, and insignificant across the 38 focal species. In contrast, species-level responses to phosphorus addition were consistently and significantly positive across the 38 focal species but were unrelated to species-specific phosphorus requirements documented independently for the same species. Community-level reproductive litter production was unaffected by nutrient addition, possibly because spatial and temporal variation is large. We conclude that phosphorus limits reproduction by trees in our experiment but find no evidence that reproductive responses to phosphorus addition favor species associated with naturally phosphorus-rich soils.

Asymbiotic nitrogen fixation in the phyllosphere of the Amazon forest: Changing nitrogen cycle paradigms

Biological nitrogen fixation is a key process for the maintenance of natural ecosystems productivity. In tropical forests, the contribution of asymbiotic nitrogen fixation (ANF) to the nitrogen (N) input has been underestimated, even though few studies have shown that ANF may be as important as symbiotic nitrogen fixation in such environments. The inputs and abiotic modulators of ANF in the Amazon forest are not completely understood. Here, we determined ANF rates and estimated the N inputs from ANF in the phyllosphere, litter and rhizospheric soil of nine tree species in the Amazon forest over time, including an extreme drought period induced by the El Niño-Southern Oscillation. Our data showed that ANF rates in the phyllosphere were 2.8- and 17.6-fold higher than in the litter and rhizospheric soil, respectively, and was highly dependent on tree taxon. Sampling time was the major factor modulating ANF in all forest compartments. At the driest period, ANF rates were approximately 1.8-fold and 13.1-fold higher than at periods with higher rainfall, before and after the extreme drought period, respectively. Tree species was a key modulator of ANF in the phyllosphere, as well as N and Vanadium concentrations. Carbon, molybdenum and vanadium concentrations were significant modulators of ANF in the litter. Based on ANF rates at the three sampling times, we estimated that the N input in the Amazon forest through ANF in the phyllosphere, litter and rhizospheric soil, was between 0.459 and 0.714 kg N ha^-1 yr^-1. Our results highlight the importance of ANF in the phyllosphere for the N input in the Amazon forest, and suggest that changes in the patterns of ANF driven by large scale climatic events may impact total N inputs and likely alter forest productivity.

Indication of Natural Boreo-Continental Pine Sites Through Discrimination Analysis of the Soil Biochemical and Water-Holding Properties

Natural pine site differentiation is instrumental in the modification of Scots pine cultivation to environmental change. The aim of this study was to distinguish azonal pine sites in prevailing beechwood conditions by the means of soil property interrelationships. The study aimed at verifying assumptions (i) that intrinsic soil properties suggest differences at naturalness among various communities in the same mesoclimate, relief or on same soil group and (ii) whether pines differ from beechwoods uniformly or unevenly among different regional population areas. The verification was carried out by discrimination analysis of the H- and A-horizon forest soil properties at selected pine and beech stands in the Czech Republic between 2006 and 2015. Homogeneous pines were confirmed either on poorly developed or very infertile soils. Mixed pines were found on Cambisols. Complete separability was found between pines and beechwoods on Podzols due to inverse proportions of correlations among acid phosphomonoesterase (APMEA) and urease (UA) activities, Corg, Cmic, base saturation, bulk density and aeration. The inverse proportions among UA, Ntot, Cmic and soil hydrophysical properties conditioned the separability of pines on different soil groups than beechwoods. Soil indications of natural pines are related to phosphorus release by APMEA and site resistance to drought due to soil organic matter and water-holding capacity.

Carbon flux and forest dynamics: Increased deadwood decomposition in tropical rainforest tree-fall canopy gaps

Tree mortality rates are increasing within tropical rainforests as a result of global environmental change. When trees die, gaps are created in forest canopies and carbon is transferred from the living to deadwood pools. However, little is known about the effect of tree-fall canopy gaps on the activity of decomposer communities and the rate of deadwood decay in forests. This means that the accuracy of regional and global carbon budgets is uncertain, especially given ongoing changes to the structure of rainforest ecosystems. Therefore, to determine the effect of canopy openings on wood decay rates and regional carbon flux, we carried out the first assessment of deadwood mass loss within canopy gaps in old-growth rainforest. We used replicated canopy gaps paired with closed canopy sites in combination with macroinvertebrate accessible and inaccessible woodblocks to experimentally partition the relative contribution of microbes vs. termites to decomposition within contrasting understorey conditions. We show that over a 12 month period, wood mass loss increased by 63% in canopy gaps compared with closed canopy sites and that this increase was driven by termites. Using LiDAR data to quantify the proportion of canopy openings in the study region, we modelled the effect of observed changes in decomposition within gaps on regional carbon flux. Overall, we estimate that this accelerated decomposition increases regional wood decay rate by up to 18.2%, corresponding to a flux increase of 0.27 Mg C ha^-1  year^-1 that is not currently accounted for in regional carbon budgets. These results provide the first insights into how small-scale disturbances in rainforests can generate hotspots for decomposer activity and carbon fluxes. In doing so, we show that including canopy gap dynamics and their impacts on wood decomposition in forest ecosystems can help improve the predictive accuracy of the carbon cycle in land surface models.

High Recovery of Saplings after Severe Drought in Temperate Deciduous Forests

Drought episodes are predicted to increase their intensity and frequency globally, which will have a particular impact on forest vitality, productivity, and species distribution. However, the impact of tree species interaction on forest vulnerability to drought is not yet clear. This study aims to assess how deciduous saplings react to drought and whether tree species diversity can buffer the impact of drought stress on tree saplings. Based on field measurements of crown defoliation and species diversity, vulnerability, drought recovery, and species interaction were analyzed. Fieldwork was carried out in Central Eastern Germany in 2018 during the vegetation season and repeated in 2019. Ten random saplings were measured in each of the 218 plots (15 × 15 m) with 2051 saplings in total out of 41 tree species. We found that 65% of the saplings experienced defoliation during the drought of 2018, of which up to 13% showed complete defoliation. At the species level, Fagus sylvatica L. and Betula pendula Roth. saplings were less affected (<55%), whereas Carpinus betulus L., Sorbus aucuparia L., and Frangula alnus Mill. saplings were the most affected (≥85%). One year later, in 2019, C. betulus and S. aucuparia had a faster recovery rate than F. sylvatica, B. pendula, Quercus spp., and Crataegus spp. (p < 0.001). Furthermore, we showed that forest stands with high sapling species diversity had a reduced vitality under drought stress (p < 0.001), indicating a higher competition for resources. The study provides evidence that F. sylvatica saplings can withstand and survive to persistent drought. Species-specific responses to drought are essential to be considered for implementing adaptive forest management strategies to mitigate the impact of climate change.

Soil biochemical properties and stabilisation of soil organic matter in relation to deadwood of different species

Despite the increasing number of studies on deadwood, we still have limited knowledge of its dynamics. The aim of this study was to examine the effect of deadwood on the biochemical properties of soil and stabilisation of soil organic matter (SOM). The investigation was carried out in the Czarna Rózga Reserve in Central Poland. The logs of four tree species at different stages of decomposition (III, IV and V) were selected for the analysis. Three replicate logs were sampled for each combination of decay classes, and the soil samples were collected from directly under the logs and from 1 m away from the logs. In this way, changes to the chemical and biochemical properties of the wood were determined. The SOM was physically fractioned. As the rate of deadwood decomposition increases, its biochemical activity increases and its chemical properties change. The biochemical activity, especially the soil's enzyme activity, was stimulated under highly decayed deadwood. The effects of deadwood are visible in SOM fractions, particularly in the content of the light fraction of SOM.

Study of functional and physiological response of co-occurring shrub species to the Mediterranean climate

The Mediterranean basin is characterised by increasingly dry summers and the study of the adaptive traits developed by plants living in this stressful environment is of great interest, also in relation to climate projections for this area. Cistus monspeliensis, Myrtus communis and Phillyrea angustifolia are three co-occurring shrubs typical of the Mediterranean maquis. Their functional and physiological parameters were studied in spring, summer and autumn in order to highlight adjustments of these traits and to test eventual different adaptive strategies. Soil and leaf chemical characteristics were determined in the different seasons. Leaf area, specific leaf area, leaf dry matter content, succulence index, pigment contents hydric status and main markers of oxidative stress and antioxidant response were detected. The stressful summer season induced disturbance in hydric balance, decrease in succulence index and chlorophyll content and high contents of hydrogen peroxide. Thanks to higher enzymatic activities and total glutathione content, in the two evergreen species M. communis and P. angustifolia oxidative damage remained at levels equal to or lower than the other seasons. Only in the semideciduous C. monspeliensis both functional and biochemical traits showed a higher stress condition in summer. The higher stability of functional traits in the two evergreen species may be explained by the sclerophyllous nature of their leaves. Four environmental variables - Tmax, Tmin, soil conductivity and organic matter - mostly influenced NMDS segregation of these species.

Termites mitigate the effects of drought in tropical rainforest

Termites perform key ecological functions in tropical ecosystems, are strongly affected by variation in rainfall, and respond negatively to habitat disturbance. However, it is not known how the projected increase in frequency and severity of droughts in tropical rainforests will alter termite communities and the maintenance of ecosystem processes. Using a large-scale termite suppression experiment, we found that termite activity and abundance increased during drought in a Bornean forest. This increase resulted in accelerated litter decomposition, elevated soil moisture, greater soil nutrient heterogeneity, and higher seedling survival rates during the extreme El Niño drought of 2015–2016. Our work shows how an invertebrate group enhances ecosystem resistance to drought, providing evidence that the dual stressors of climate change and anthropogenic shifts in biotic communities will have various negative consequences for the maintenance of rainforest ecosystems.

Plant responses to fertilization experiments in lowland, species-rich, tropical forests

We present a meta-analysis of plant responses to fertilization experiments conducted in lowland, species-rich, tropical forests. We also update a key result and present the first species-level analyses of tree growth rates for a 15-yr factorial nitrogen (N), phosphorus (P), and potassium (K) experiment conducted in central Panama. The update concerns community-level tree growth rates, which responded significantly to the addition of N and K together after 10 yr of fertilization but not after 15 yr. Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora. Under these circumstances, we expect muted responses to fertilization, and we predicted species associated with low-P soils would respond most slowly. The data did not support this prediction, species-level tree growth responses to P addition were unrelated to species-level soil P associations. The meta-analysis demonstrated that nutrient limitation is widespread in lowland tropical forests and evaluated two directional hypotheses concerning plant responses to N addition and to P addition. The meta-analysis supported the hypothesis that tree (or biomass) growth rate responses to fertilization are weaker in old growth forests and stronger in secondary forests, where rapid biomass accumulation provides a nutrient sink. The meta-analysis found no support for the long-standing hypothesis that plant responses are stronger for P addition and weaker for N addition. We do not advocate discarding the latter hypothesis. There are only 14 fertilization experiments from lowland, species-rich, tropical forests, 13 of the 14 experiments added nutrients for five or fewer years, and responses vary widely among experiments. Potential fertilization responses should be muted when the species present are well adapted to nutrient-poor soils, as is the case in our experiment, and when pest pressure increases with fertilization, as it does in our experiment. The statistical power and especially the duration of fertilization experiments conducted in old growth, tropical forests might be insufficient to detect the slow, modest growth responses that are to be expected.

Responses of soil nutrient concentrations and stoichiometry to different human land uses in a subtropical tidal wetland

We studied the impacts of anthropogenic changes in land use on the stoichiometric imbalance of soil carbon (C), nitrogen (N), phosphorus (P) and potassium (K) in Phragmites australis wetlands in the Minjiang River estuary. We compared five areas with different land uses: P. australis wetland (control), grassland, a mudskipper breeding flat, pond aquaculture and rice cropland. Human activity has affected the elemental and stoichiometric compositions of soils through changes in land use. In general, soil C and N concentrations were lower and total soil K concentrations were higher at the sites under human land uses relative to the control site, and total soil P concentrations were generally not significantly different. The close relationship between total soil C and N concentrations in all cases, including fertilization with N, suggested that N was the most limiting nutrient in these wetlands. Lower soil N concentrations and similar soil P concentrations and higher soil K concentrations under human land-use activities suggest that human activity has increased the role of N limitation in these wetlands. Only grassland use increases soil N contents (only in the 0-10 cm of soil). Despite N fertilization, lower soil N concentrations were also observed in the rice cropland, indicating the difficulty of avoiding N limitation in these wetlands. The observed lower soil N:P ratio, together with higher soil P and K availabilities in rice croplands, is consistent with the tendency of human activity to change the competitive relationships of plants, in this case favoring species adapted to high rates of growth (low N:P ratio) and/or favoring plants with high demands for P and K. Both, soil C storage and respiration were higher in grasslands, likely due to the introduction of grasses, which led to a high density of plants, increased grazing activity and soil compaction. Soil C storage and respiration were lower under human land uses, except in the rice cropland, with respect to natural wetland. Using overall data, soil C storage and respiration were correlated, indicating that soil respiration was correlated with plant productivity. In this wetland area the impacts of different human land-uses on soil stoichiometry and C-cycle can be very different depending on the activity. Further regeneration of natural communities can be determined by the previous type of land-use.

Evidence of current impact of climate change on life: a walk from genes to the biosphere

We review the evidence of how organisms and populations are currently responding to climate change through phenotypic plasticity, genotypic evolution, changes in distribution and, in some cases, local extinction. Organisms alter their gene expression and metabolism to increase the concentrations of several antistress compounds and to change their physiology, phenology, growth and reproduction in response to climate change. Rapid adaptation and microevolution occur at the population level. Together with these phenotypic and genotypic adaptations, the movement of organisms and the turnover of populations can lead to migration toward habitats with better conditions unless hindered by barriers. Both migration and local extinction of populations have occurred. However, many unknowns for all these processes remain. The roles of phenotypic plasticity and genotypic evolution and their possible trade-offs and links with population structure warrant further research. The application of omic techniques to ecological studies will greatly favor this research. It remains poorly understood how climate change will result in asymmetrical responses of species and how it will interact with other increasing global impacts, such as N eutrophication, changes in environmental N : P ratios and species invasion, among many others. The biogeochemical and biophysical feedbacks on climate of all these changes in vegetation are also poorly understood. We here review the evidence of responses to climate change and discuss the perspectives for increasing our knowledge of the interactions between climate change and life.

Experimental drought in a tropical rain forest increases soil carbon dioxide losses to the atmosphere

Climate models predict precipitation changes for much of the humid tropics, yet few studies have investigated the potential consequences of drought on soil carbon (C) cycling in this important biome. In wet tropical forests, drought could stimulate soil respiration via overall reductions in soil anoxia, but previous research suggests that litter decomposition is positively correlated with high rainfall fluxes that move large quantities of dissolved organic matter (DOM) from the litter layer to the soil surface. Thus, reduced rainfall could also limit C delivery to the soil surface, reducing respiration rates. We conducted a throughfall manipulation experiment to investigate how 25% and 50% reductions in rainfall altered both C movement into soils and the effects of those DOM fluxes on soil respiration rates. In response to the experimental drought, soil respiration rates increased in both the -25% and -50% treatments. Throughfall fluxes were reduced by 26% and 55% in the -25% and -50% treatments, respectively. However, total DOM fluxes leached from the litter did not vary between treatments, because the concentrations of leached DOM reaching the soil surface increased in response to the simulated drought. Annual DOM concentrations averaged 7.7 +/- 0.8, 11.2 +/- 0.9, and 15.8 +/- 1.2 mg C/L in the control, -25%, and -50% plots, respectively, and DOM concentrations were positively correlated with soil respiration rates. A laboratory incubation experiment confirmed the potential importance of DOM concentration on soil respiration rates, suggesting that this mechanism could contribute to the increase in CO2 fluxes observed in the reduced rainfall plots. Across all plots, the data suggested that soil CO2 fluxes were partially regulated by the magnitude and concentration of soluble C delivered to the soil, but also by soil moisture and soil oxygen availability. Together, our data suggest that declines in precipitation in tropical rain forests could drive higher CO2 fluxes to the atmosphere both via increased soil 02 availability and through responses to elevated DOM concentrations.

Variation in pH optima of hydrolytic enzyme activities in tropical rain forest soils

Extracellular enzymes synthesized by soil microbes play a central role in the biogeochemical cycling of nutrients in the environment. The pH optima of eight hydrolytic enzymes involved in the cycles of carbon, nitrogen, phosphorus, and sulfur, were assessed in a series of tropical forest soils of contrasting pH values from the Republic of Panama. Assays were conducted using 4-methylumbelliferone-linked fluorogenic substrates in modified universal buffer. Optimum pH values differed markedly among enzymes and soils. Enzymes were grouped into three classes based on their pH optima: (i) enzymes with acidic pH optima that were consistent among soils (cellobiohydrolase, β-xylanase, and arylsulfatase), (ii) enzymes with acidic pH optima that varied systematically with soil pH, with the most acidic pH optima in the most acidic soils (α-glucosidase, β-glucosidase, and N-acetyl-β-glucosaminidase), and (iii) enzymes with an optimum pH in either the acid range or the alkaline range depending on soil pH (phosphomonoesterase and phosphodiesterase). The optimum pH values of phosphomonoesterase were consistent among soils, being 4 to 5 for acid phosphomonoesterase and 10 to 11 for alkaline phosphomonoesterase. In contrast, the optimum pH for phosphodiesterase activity varied systematically with soil pH, with the most acidic pH optima (3.0) in the most acidic soils and the most alkaline pH optima (pH 10) in near-neutral soils. Arylsulfatase activity had a very acidic optimum pH in all soils (pH ≤3.0) irrespective of soil pH. The differences in pH optima may be linked to the origins of the enzymes and/or the degree of stabilization on solid surfaces. The results have important implications for the interpretation of hydrolytic enzyme assays using fluorogenic substrates.

Effects of seasonality, litter removal and dry-season irrigation on litterfall quantity and quality in eastern Amazonian forest regrowth, Brazil

Litterfall quantity and quality may respond to alterations in resource availability expected with ongoing land-use and climate changes. Here, we quantify the effects of altered resource availability on non-woody litterfall quantity and quality (nitrogen and phosphorus concentrations) in eastern Amazonian forest regrowth (Brazil) through two multi-year experimental manipulations: (1) daily irrigation (5 mm d−1) during the dry season; and (2) fortnightly litter removal. Consistent with other tropical forest data litterfall exhibited seasonal patterns, increasing with the onset of the dry season and declining with the onset of the rainy season. Irrigation did not affect litterfall mass and had little impact on nitrogen (N) or phosphorus (P) concentrations and return, except for decreasing litter P concentration at the end of two irrigation periods. Litter removal did not alter litterfall mass or P concentration, but progressively reduced litterfall N during the course of the experiment. Overall, these results suggest significant resistance to altered resource availability within the bounds of our experimental treatments; our findings may help to constrain carbon and nutrient cycling predictions for tropical forests in response to land-use and climate changes.

Seedling growth responses to water and nutrient augmentation in the understorey of a lowland moist forest, Panama

We irrigated and fertilized (with nutrients) seedlings of Doliocarpus olivaceus (Dilleniaceae, a shade-tolerant liana), Faramea occidentalis (Rubiaceae, a shade-tolerant understorey tree) and Tetragastris panamensis (Burseraceae, a shade-tolerant canopy tree) growing in the understorey of an old-growth tropical moist forest in Panama to assess the impact of seasonal water availability and nutrient-poor soils on seedling growth rates. In control plots, height growth rates were greater in the dry season than in the wet season for Doliocarpus (21%) and for Faramea (89%), whereas Tetragastris had similar seasonal rates. For numbers of leaves, Faramea had 3.5-fold greater relative growth rates in the dry season than in the wet season, while Doliocarpus and Tetragastris lost leaves (semi-deciduous) during the same period. Irrigation and nutrient augmentation increased height growth rates for all three species (45% to 272%). For Doliocarpus, irrigation and nutrient augmentation prevented leaf fall during the dry season. For Faramea in the dry season, irrigation and nutrient augmentation when applied independently reduced the growth of new leaves by 65% to 87%, but relative growth rates for number of leaves were the same as the control rates in the combined irrigation and nutrient augmentation treatment. The growth of new leaves for Tetragastris responded to dry-season irrigation but not nutrient augmentation. Although all measurements occurred beneath the forest canopy, during the dry season, Tetragastris had a negative relationship between canopy openness and relative growth rate for number of leaves, whereas the other two species had a positive relationship. Our results show that soil resources influence growth rates even in the deep shade of the forest understorey, and demonstrate different responses to soil resources among species that might contribute to niche differentiation and species coexistence.

Rainfall and labile carbon availability control litter nitrogen dynamics in a tropical dry forest

N cycling in tropical dry forests is driven by rainfall seasonality but the mechanisms involved are not well understood. We studied the seasonal variation in N dynamics and microbial biomass in the surface litter of a tropical dry forest ecosystem in Mexico over a 2-year period. Litter was collected at 4 different times of the year to determine changes in total, soluble, and microbial C and N concentrations. Additionally, litter from each sampling date was incubated under laboratory conditions to determine potential C mineralization rate, net N mineralization, net C and N microbial immobilization, and net nitrification. Litter C concentrations were highest in the early-dry season and lowest in the rainy season, while the seasonal changes in N concentrations varied between years. Litter P was higher in the rainy than in the early-dry season. Water-soluble organic C (WSOC) and water-soluble N concentrations were highest during the early- and late-dry seasons and represented up to 4.1 and 5.9% of the total C and N, respectively. NH (4) (+) and NO (3) (-) showed different seasonal and annual variations. They represented an average 23% of soluble N. Microbial C was generally higher in the dry than in the wet seasons, while microbial N was lowest in the late-dry and highest in the early-rainy seasons. Incubations showed that lowest potential C mineralization rates and C and N microbial immobilization occurred in rainy season litter, and were positively correlated to WSOC. Net nitrification was highest in rainy season litter. Our results showed that the seasonal pattern in N dynamics was influenced by rainfall seasonality and labile C availability, and not by microbial biomass. We propose a conceptual model to hypothesize how N dynamics in the litter layer of the Chamela tropical dry forest respond to the seasonal variation in rainfall.