Controls Over Leaf Litter and Soil Nitrogen Fixation in Two Lowland Tropical Rain Forests

Global comparisons suggest that rates of N fixation in tropical rain forests may be among the highest on earth. However, data supporting this contention are rare, and the factors that regulate N fixation within the biome remain largely unknown. We conducted a full-factorial (N × P) fertilization experiment in two lowland tropical rain forests in Costa Rica to explore the effects of nutrient availability on rates of free-living N fixation in leaf litter and soil. P fertilization significantly increased N fixation rates in both leaf litter and soil, and the effect was dependent on sampling date. Fertilization with N did not affect rates of N fixation at any time. In addition, variation in N fixation rates measured in unfertilized plots at four sampling time points suggested seasonal variability in N fixation: leaf litter N fixation ranged from 0.36 kg/ha/yr in the dry season to 5.48 kg/ha/yr in the wet season. Soil N fixation showed similar patterns ranging from a dry season low of 0.26 kg/ha/yr to a wet season high of 2.71 kg/ha/yr. While the observed temporal variability suggests potential climatic control over free-living N fixation in these forests, data suggest that neither soil nor leaf litter moisture alone regulate N fixation rates. Instead, we hypothesize that a combination of ample C availability, low leaf litter N:P ratios, and high rainfall coincide during the latter portions of the rainy season and drive the highest free-living N fixation rates of the year.     

Nutrient limitation in rainforests and cloud forests along a 3,000-m elevation gradient in the Peruvian Andes

We report results from a large-scale nutrient fertilization experiment along a “megadiverse” (154 unique species were included in the study) 3,000-m elevation transect in the Peruvian Andes and adjacent lowland Amazonia. Our objectives were to test if nitrogen (N) and phosphorus (P) limitation shift along this elevation gradient, and to determine how an alleviation of nutrient limitation would manifest in ecosystem changes. Tree height decreased with increasing elevation, but leaf area index (LAI) and diameter at breast height (DBH) did not vary with elevation. Leaf N:P decreased with increasing elevation (from 24 at 200 m to 11 at 3,000 m), suggesting increased N limitation and decreased P limitation with increasing elevation. After 4 years of fertilization (N, P, N + P), plots at the lowland site (200 m) fertilized with N + P showed greater relative growth rates in DBH than did the control plots; no significant differences were evident at the 1,000 m site, and plots fertilized with N at the highest elevation sites (1,500, 3,000 m) showed greater relative growth rates in DBH than did the control plots, again suggesting increased N constraint with elevation. Across elevations in general N fertilization led to an increase in microbial respiration, while P and N + P addition led to an increase in root respiration and corresponding decrease in hyphal respiration. There was no significant canopy response (LAI, leaf nutrients) to fertilization, suggesting that photosynthetic capacity was not N or P limited in these ecosystems. In sum, our study significantly advances ecological understanding of nutrient cycling and ecosystem response in a region where our collective knowledge and data are sparse: we demonstrate N limitation in high elevation tropical montane forests, N and P co-limitation in lowland Amazonia, and a nutrient limitation response manifested not in canopy changes, but rather in stem and belowground changes.     

Soil nutrient dissimilarity and litter nutrient limitation as major drivers of home field advantage in riparian tropical forests

Decomposition is a key process driving carbon and nutrient cycling in ecosystems worldwide. The home field advantage effect (HFA) has been found to accelerate decomposition rates when litter originates from “home” when compared to other (“away”) sites. It is still poorly known how HFA plays out in tropical, riparian forests, particularly in forests under restoration. We carried out three independent reciprocal litter transplant experiments to test how litter quality, soil nutrient concentrations, and successional stage (age) influenced HFA in tropical riparian forests. These experimental areas formed a wide gradient of soil and litter nutrients, which we used to evaluate the more general hypothesis that HFA varies with dissimilarity in soil nutrients and litter quality. We found that HFA increased with soil nutrient dissimilarity, suggesting that litter translocation uncouples relationships between decomposers and litter characteristics; and with litter N:P, indicating P limitation in this system. We also found negative HFA effects at a site under restoration that presented low decomposer ability, suggesting that forest restoration does not necessarily recover decomposer communities and nutrient cycling. Within each of the independent experiments, the occurrence of HFA effects was limited and their magnitude was not related to forest age, nor soil and litter quality. Our results imply that HFA effects in tropical ecosystems are influenced by litter nutrient limitation and soil nutrient dissimilarity between home and away sites, but to further disentangle major HFA drivers in tropical areas, a gradient of dissimilarity between litter and soil properties must be implemented in future experimental designs.     

Microbial community shifts influence patterns in tropical forest nitrogen fixation

The role of biodiversity in ecosystem function receives substantial attention, yet despite the diversity and functional relevance of microorganisms, relationships between microbial community structure and ecosystem processes remain largely unknown. We used tropical rain forest fertilization plots to directly compare the relative abundance, composition and diversity of free-living nitrogen (N)-fixer communities to in situ leaf litter N fixation rates. N fixation rates varied greatly within the landscape, and ‘hotspots’ of high N fixation activity were observed in both control and phosphorus (P)-fertilized plots. Compared with zones of average activity, the N fixation ‘hotspots’ in unfertilized plots were characterized by marked differences in N-fixer community composition and had substantially higher overall diversity. P additions increased the efficiency of N-fixer communities, resulting in elevated rates of fixation per nifH gene. Furthermore, P fertilization increased N fixation rates and N-fixer abundance, eliminated a highly novel group of N-fixers, and increased N-fixer diversity. Yet the relationships between diversity and function were not simple, and coupling rate measurements to indicators of community structure revealed a biological dynamism not apparent from process measurements alone. Taken together, these data suggest that the rain forest litter layer maintains high N fixation rates and unique N-fixing organisms and that, as observed in plant community ecology, structural shifts in N-fixing communities may partially explain significant differences in system-scale N fixation rates.     

Biological nitrogen fixation in a post-volcanic chronosequence from south-central Chile

Biological nitrogen fixation is a key ecosystem function incorporating new nitrogen (N) during primary successions. Increasing evidence from tropical and northern temperate forests shows that phosphorus (P) and molybdenum (Mo) either alone or in combination limit the activity of free-living diazotrophs. In this study, we evaluated the effects of Mo, P, and carbon (C) addition, either singly or in combination, and moisture, on diazotrophic activity in a post-volcanic forest chronosequence in south-fentral Chile. Diazotrophic activity, both free-living (associated with fine litter) and symbiotic (associated with the moss Racomitrium lanuginosum and the cyanolichens Pseudocyphellaria berberina and P. coriifolia), was evaluated by incubation of samples and subsequent acetylene reduction assays conducted in the field and laboratory, in winter, spring and autumn of two consecutive years. Results showed that diazotrophic activity varied with the season of the year (lowest during the drier spring season), successional stage (highest in the maximal stage), and N-fixer community type (highest in symbiotic diazotrophs). In general, C+P+Mo limitation was documented for heterotrophic diazotrophs and P+Mo limitation for symbiotic diazotrophs. Limitation of diazotrophic activity was not associated with soil nutrient and C status in the chronosequence. Strong inhibition of diazotrophic activity by high N addition and by low moisture suggests that reductions in precipitation expected for south-central Chile under climate change, as well as increasing inputs of reactive N from atmospheric deposition due to increasing use of N fertilizers, may drastically alter the composition and functional role of cryptogamic assemblages in native forests.     

Litter Nutrient Dynamics During Succession in Dry Tropical Forests of the Yucatan: Regional and Seasonal Effects

Land-use change in the tropics is creating secondary forest at an unprecedented rate. In the tropical Americas, mature dry tropical forest is rapidly being converted to secondary forest during the fallow period of shifting cultivation. We investigated litter phosphorus (P) and nitrogen (N) dynamics in forests recovering from shifting cultivation of maize (corn) in three regions of the Southern Yucatan Peninsula, Mexico. Our goal was to understand how nutrient and water availability affect forest recovery following conversion of mature forest to agricultural land. To investigate such changes at a regional scale, newly fallen litter was collected monthly along a seasonal, a successional, and a precipitation gradient. Reflecting possible P limitation, litter P concentration declined with forest age, while litter N concentration did not differ between age classes. Average litter P concentration from the southern, wettest region was 0.87 mg/g, almost twice the litter P concentration in the drier central and northern regions (0.44 and 0.45 mg/g, respectively). Average N concentrations of litter from the three regions ranged from 1.1% to 1.2%, with no regional differences. However, minima in both P and N concentration from all regions were pronouncedly timed with peak litterfall, suggesting nutrient retranslocation during periods of water stress. Additionally, successional differences in litter P were clearest during wetter months. P nutrient-use efficiency was lowest in the southern region and highest in the central and northern study regions. N nutrient-use efficiency was up to 40 times lower than P nutrient-use efficiency and showed no regional differences. Overall, our results suggest that litter nutrient dynamics in secondary dry tropical forests of the Southern Yucatan are strongly influenced by water and nutrient availability, especially P, as well as land-use history.     

Nutrient addition affects net and gross mineralization of phosphorus in the organic layer of a tropical montane forest

In tropical ecosystems with highly weathered soils, transformation of organic phosphorus (P) to bioavailable inorganic P plays a crucial role for the nutrition of organisms. In these ecosystems, P is suspected to be growth-limiting and might therefore be affected by atmospheric nutrient deposition occurring even in remote areas such as montane rainforests. We assessed effects of P and nitrogen (N) addition on net and gross P mineralization rates, and microbial P immobilization in the organic layer along an altitudinal gradient of a tropical montane rainforest in Ecuador. Net P mineralization rates amounted to 1.8 ± 0.9 (at 1000 m a.s.l.), 3.7 ± 0.6 (at 2000 m) and 2.0 ± 0.4 (at 3000 m) mg P kg^?1 day^?1. Altitudinal differences led to an increased microbial P immobilization at 1000 m compensating for a higher gross P mineralization. P addition increased net P mineralization rates at 2000 and 3000 m, suggesting a higher P demand at 1000 m. At higher altitudes, P likely was released as a by-product during organic matter decomposition. Gross P mineralization, determined by means of the isotopic dilution approach, could not be calculated for control and N treatments due to rapid microbial P immobilization. For P (+N) treatments, gross P mineralization rates were lowest at the 1000 m site towards the end of the long-term incubation period. Atmospheric P deposition in the tropics might lead to P fertilization effects through direct input as well as through acceleration of P release from organic matter, thereby increasing P availability for organisms.     

Leaf litter inputs reinforce islands of nitrogen fertility in a lowland tropical forest

The role of lowland tropical forest tree communities in shaping soil nutrient cycling has been challenging to elucidate in the face of high species diversity. Previously, we showed that differences in tree species composition and canopy foliar nitrogen (N) concentrations correlated with differences in soil N availability in a mature Costa Rican rainforest. Here, we investigate potential mechanisms explaining this correlation. We used imaging spectroscopy to identify study plots containing 10–20 canopy trees with either high or low mean canopy N relative to the landscape mean. Plots were restricted to an uplifted terrace with relatively uniform parent material and climate. In order to assess whether canopy and soil N could be linked by litterfall inputs, we tracked litter production in the plots and measured rates of litter decay and the carbon and N content of leaf litter and leaf litter leachate. We also compared the abundance of putative N fixing trees and rates of free-living N fixation as well as soil pH, texture, cation exchange capacity, and topographic curvature to assess whether biological N fixation and/or soil properties could account for differences in soil N that were, in turn, imprinted on the canopy. We found no evidence of differences in legume communities, free-living N fixation, or abiotic properties. However, soils beneath high canopy N assemblages received ~ 60% more N via leaf litterfall due to variability in litter N content between plot types. The correlation of N in canopy leaves, leaf litter, and soil suggests that, under similar abiotic conditions, litterfall-mediated feedbacks can help maintain soil N differences among tropical tree assemblages in this diverse tropical forest.

     

Regional patterns and controls of leaf decomposition in Australian tropical rainforests

Future climates have the potential to alter decomposition rates in tropical forest with implications for carbon emissions, nutrient cycling and retention of standing litter. However, our ability to predict impacts, particularly for seasonally wet forests in the old world, is limited by a paucity of data, a limited understanding of the relative importance of different aspects of climate and the extent to which decomposition rates are constrained by factors other than climate (e.g. soil, vegetation composition). We used the litterbag method to determine leaf litter decay rates at 18 sites distributed throughout the Australian wet tropics bioregion over a 14‐month period. Specifically, we investigated regional controls on litter decay including climate, soil and litter chemical quality. We used both in situ litter collected from litterfall on site and a standardized control leaf litter substrate. The control litter removed the effect of litter chemical quality and the in situ study quantified decomposition specific to the site. Decomposition was generally slower than for other tropical rainforests globally except in our wet and nutrient‐richer sites. This is most likely attributable to the higher latitude, often highly seasonal rainfall and very poor soils in our system. Decomposition rates were best explained by a combination of climate, soil and litter quality. For in situ litter (native to the site) this included: average leaf wetness in the dry season (LWDS; i.e. moisture condensation) and the initial P content of the leaves, or LWDS and initial C. For control litter (no litter quality effect) this included: rainfall seasonality (% dry season days with 0‐mm rainfall), soil P and mean annual temperature. These results suggest that the impact of climate change on decomposition rates within Australian tropical rainforests will be critically dependent on the trajectory of dry season moisture inputs over the coming decades.     

Nutrient fluxes via litterfall and leaf litter decomposition vary across a gradient of soil nutrient supply in a lowland tropical rain forest

The extent to which plant communities are determined by resource availability is a central theme in ecosystem science, but patterns of small-scale variation in resource availability are poorly known. Studies of carbon (C) and nutrient cycling provide insights into factors limiting tree growth and forest productivity. To investigate rates of tropical forest litter production and decomposition in relation to nutrient availability and topography in the absence of confounding large-scale variation in climate and altitude we quantified nutrient fluxes via litterfall and leaf litter decomposition within three distinct floristic associations of tropical rain forest growing along a soil fertility gradient at the Sepilok Forest Reserve (SFR), Sabah, Malaysia. The quantity and nutrient content of small litter decreased along a gradient of soil nutrient availability from alluvial forest (most fertile) through sandstone forest to heath forest (least fertile). Temporal variation in litterfall was greatest in the sandstone forest, where the amount of litter was correlated negatively with rainfall in the previous month. Mass loss and N and P release were fastest from alluvial forest litter, and slowest from heath forest litter. All litter types decomposed most rapidly in the alluvial forest. Stand-level N and P use efficiencies (ratios of litter dry mass to nutrient content) were greatest for the heath forest followed by the sandstone ridge, sandstone valley and alluvial forests, respectively. We conclude that nutrient supply limits productivity most in the heath forest and least in the alluvial forest. Nutrient supply limited productivity in sandstone forest, especially on ridge and hill top sites where nutrient limitation may be exacerbated by reduced rates of litter decomposition during dry periods. The fluxes of N and P varied significantly between the different floristic communities at SFR and these differences may contribute to small-scale variation in species composition.     

Coordination of aboveground and belowground responses to local‐scale soil fertility differences between two contrasting Jamaican rain forest types

There is growing interest in understanding how declining soil fertility in the prolonged absence of major disturbance drives ecological processes, or ‘ecosystem retrogression’. However, there are few well characterized study systems for exploring this phenomenon in the tropics, despite tropics occupying over 40% of the Earth's terrestrial surface. We studied two types of montane rain forest in the Blue Mountains of Jamaica that represent distinct stages in ecosystem development, i.e. an earlier stage with shallow organic matter and a late stage with deep organic matter (hereafter ‘mull’ and ‘mor’ stages). We characterized responses of soil fertility and plant, soil microbial and nematode communities to the transition from mull to mor and whether these responses were coupled. For soil abiotic properties, we found this transition led to lower amounts of both nitrogen (N) and phosphorus (P) and an enhanced N to P ratio. This led to shorter‐statured and less diverse forest, and convergence of tree species composition among plots. At the whole community (but not individual species) level foliar and litter N and P diminished from mull to mor, while foliar N to P and resorption efficiency of P relative to N increased, indicating increasing P relative to N limitation. We also found impairment of soil microbes (but not nematodes) and an increasing role of fungi relative to bacteria during the transition. Our results show that retrogression phenomena involving increasing nutrient (notably P) limitation can be important drivers in tropical systems, and are likely to involve aboveground–belowground feedbacks whereby plants produce litter of diminishing quality, impairing soil microbial processes and thus reducing the supply of nutrients from the soil for plant growth. Such feedbacks between plants and the soil, mediated by plant litter and organic matter quality, may serve as major though often overlooked drivers of long term environmental change.     

Molybdenum and phosphorus interact to constrain asymbiotic nitrogen fixation in tropical forests

Biological di-nitrogen fixation (N(2)) is the dominant natural source of new nitrogen to land ecosystems. Phosphorus (P) is thought to limit N(2) fixation in many tropical soils, yet both molybdenum (Mo) and P are crucial for the nitrogenase reaction (which catalyzes N(2) conversion to ammonia) and cell growth. We have limited understanding of how and when fixation is constrained by these nutrients in nature. Here we show in tropical forests of lowland Panama that the limiting element on asymbiotic N(2) fixation shifts along a broad landscape gradient in soil P, where Mo limits fixation in P-rich soils while Mo and P co-limit in P-poor soils. In no circumstance did P alone limit fixation. We provide and experimentally test a mechanism that explains how Mo and P can interact to constrain asymbiotic N(2) fixation. Fixation is uniformly favored in surface organic soil horizons--a niche characterized by exceedingly low levels of available Mo relative to P. We show that soil organic matter acts to reduce molybdate over phosphate bioavailability, which, in turn, promotes Mo limitation in sites where P is sufficient. Our findings show that asymbiotic N(2) fixation is constrained by the relative availability and dynamics of Mo and P in soils. This conceptual framework can explain shifts in limitation status across broad landscape gradients in soil fertility and implies that fixation depends on Mo and P in ways that are more complex than previously thought.     

Nutrient feedbacks to soil heterotrophic nitrogen fixation in forests

Multiple nutrient cycles regulate biological nitrogen (N) fixation in forests, yet long-term feedbacks between N-fixation and coupled element cycles remain largely unexplored. We examined soil nutrients and heterotrophic N-fixation across a gradient of 24 temperate conifer forests shaped by legacies of symbiotic N-fixing trees. We observed positive relationships among mineral soil pools of N, carbon (C), organic molybdenum (Mo), and organic phosphorus (P) across sites, evidence that legacies of symbiotic N-fixing trees can increase the abundance of multiple elements important to heterotrophic N-fixation. Soil N accumulation lowered rates of heterotrophic N-fixation in organic horizons due to both N inhibition of nitrogenase enzymes and declines in soil organic matter quality. Experimental fertilization of organic horizon soil revealed widespread Mo limitation of heterotrophic N-fixation, especially at sites where soil Mo was scarce relative to C. Fertilization also revealed widespread absence of P limitation, consistent with high soil P:Mo ratios. Responses of heterotrophic N-fixation to added Mo (positive) and N (negative) were correlated across sites, evidence that multiple nutrient controls of heterotrophic N-fixation were more common than single-nutrient effects. We propose a conceptual model where symbiotic N-fixation promotes coupled N, C, P, and Mo accumulation in soil, leading to positive feedback that relaxes nutrient limitation of overall N-fixation, though heterotrophic N-fixation is primarily suppressed by strong negative feedback from long-term soil N accumulation.     

Nitrogen and Phosphorus Limitation over Long-Term Ecosystem Development in Terrestrial Ecosystems

Nutrient limitation to net primary production (NPP) displays a diversity of patterns as ecosystems develop over a range of timescales. For example, some ecosystems transition from N limitation on young soils to P limitation on geologically old soils, whereas others appear to remain N limited. Under what conditions should N limitation and P limitation prevail? When do transitions between N and P limitation occur? We analyzed transient dynamics of multiple timescales in an ecosystem model to investigate these questions. Post-disturbance dynamics in our model are controlled by a cascade of rates, from plant uptake (very fast) to litter turnover (fast) to plant mortality (intermediate) to plant-unavailable nutrient loss (slow) to weathering (very slow). Young ecosystems are N limited when symbiotic N fixation (SNF) is constrained and P weathering inputs are high relative to atmospheric N deposition and plant N:P demand, but P limited under opposite conditions. In the absence of SNF, N limitation is likely to worsen through succession (decades to centuries) because P is mineralized faster than N. Over long timescales (centuries and longer) this preferential P mineralization increases the N:P ratio of soil organic matter, leading to greater losses of plant-unavailable N versus P relative to plant N:P demand. These loss dynamics favor N limitation on older soils despite the rising organic matter N:P ratio. However, weathering depletion favors P limitation on older soils when continual P inputs (e.g., dust deposition) are low, so nutrient limitation at the terminal equilibrium depends on the balance of these input and loss effects. If NPP switches from N to P limitation over long time periods, the transition time depends most strongly on the P weathering rate. At all timescales SNF has the capacity to overcome N limitation, so nutrient limitation depends critically on limits to SNF.     

The response of microbial biomass and hydrolytic enzymes to a decade of nitrogen, phosphorus, and potassium addition in a lowland tropical rain forest

Nutrient availability is widely considered to constrain primary productivity in lowland tropical forests, yet there is little comparable information for the soil microbial biomass. We assessed microbial nutrient limitation by quantifying soil microbial biomass and hydrolytic enzyme activities in a long-term nutrient addition experiment in lowland tropical rain forest in central Panama. Multiple measurements were made over an annual cycle in plots that had received a decade of nitrogen, phosphorus, potassium, and micronutrient addition. Phosphorus addition increased soil microbial carbon (13 %), nitrogen (21 %), and phosphorus (49 %), decreased phosphatase activity by ~65 % and N-acetyl β-glucosaminidase activity by 24 %, but did not affect β-glucosidase activity. In contrast, addition of nitrogen, potassium, or micronutrients did not significantly affect microbial biomass or the activity of any enzyme. Microbial nutrients and hydrolytic enzyme activities all declined markedly in the dry season, with the change in microbial biomass equivalent to or greater than the annual nutrient flux in fine litter fall. Although multiple nutrients limit tree productivity at this site, we conclude that phosphorus limits microbial biomass in this strongly-weathered lowland tropical forest soil. This finding indicates that efforts to include enzymes in biogeochemical models must account for the disproportionate microbial investment in phosphorus acquisition in strongly-weathered soils.     

Relationships among precipitation regime, nutrient availability, and carbon turnover in tropical rain forests

The effect of high precipitation regime in tropical forests is poorly known despite indications of its potentially negative effects on nutrient availability and carbon (C) cycling. Our goal was to determine if there was an effect of high rainfall on nitrogen (N) and phosphorous (P) availability and indexes of C cycling in lowland tropical rain forests exposed to a broad range of mean annual precipitation (MAP). We predicted that C turnover time would increase with MAP while the availability of N and P would decrease. We studied seven Neotropical lowland forests covering a MAP range between 2,700 and 9,500 mm. We used radiocarbon (?¹⁴C) from the atmosphere and respired from soil organic matter to estimate residence time of C in plants and soils. We also used C, N, and P concentrations and the stable isotope ratio of N (δ¹⁵N) in live and dead plant tissues and in soils as proxies for nutrient availability. Negative δ¹⁵N values indicated that the wettest forests had N cycles that did not exhibit isotope-fractionating losses and were potentially N-limited. Element ratios (N:P and C:P) in senescent leaves, litter, and live roots showed that P resorption increased considerably with MAP, which points towards increasing P-limitation under high MAP regimes. Soil C content increased with MAP but C turnover time only showed a weak relationship with MAP, probably due to variations in soil parent material and age along the MAP gradient. In contrast, comparing C turnover directly to nutrient availability showed strong relationships between C turnover time, N availability (δ¹⁵N), and P availability (N:P) in senescent leaves and litter. Thus, an effect of MAP on carbon cycling appeared to be indirectly mediated by nutrient availability. Our results suggest that soil nutrient availability plays a central role in the dynamic of C cycling in tropical rain forests.     

Nutrient limitation of plant productivity in scrubby flatwoods: does fire shift nitrogen versus phosphorus limitation?

Differences in the biogeochemistry of nitrogen (N) and phosphorus (P) lead to differential losses and inputs during and over time after fire such that fire may affect nutrient limitation of primary productivity. We conducted a nutrient addition experiment in scrubby flatwoods, a Florida scrub community type, to test the hypothesis that nutrient limitation of primary productivity shifts from N limitation in recently burned sites to P limitation in longer unburned sites. We added three levels of N, P, and N and P together to sites 6 weeks, 8 years, and 20 years postfire and assessed the effects of nutrient addition on above- and belowground productivity and nutrient concentrations. At the community level, nutrient addition did not affect aboveground biomass, but root productivity increased with high N?+?P addition in sites 8 and 20 years after fire. At the species level, N addition increased leaf biomass of saw palmetto (Serenoa repens) in sites 6 weeks and 20 years postfire, while P addition increased foliar %P and apical shoot growth of scrub oak (Quercus inopina) in sites 8 and 20 years postfire, respectively. Contrary to our hypothesis, nutrient limitation does not appear to shift with time after fire; recently burned sites show little evidence of nutrient limitation, while increased belowground productivity indicates that scrubby flatwoods are co-limited by N and P at intermediate and longer times after fire.     

Variation in leaf litter nutrients of a Costa Rican rain forest is related to precipitation

By assessing current leaf litter nutrient dynamics, we may be able to predict responses of nutrient cycling in tropical ecosystems to future environmental change. The goal of this study was to assess whether nutrient cycling is related to seasonal variation in rainfall in a wet tropical forest. We examined leaf litter of an old-growth tropical rain forest in N.E. Costa Rica over a 4-year period to explore seasonal and inter-annual changes in leaf litter nutrient concentrations, and to evaluate potential short- and long-term drivers of variation in litter nutrient concentration, particularly that of phosphorus (P) and nitrogen (N). We also examined the temporal dynamics of calcium, potassium, and magnesium in the leaf litter. Leaf litter [P] and %N changed significantly with time, both seasonally and inter-annually. Seasonal changes in leaf litter [P] were strongly positively correlated with rainfall from the previous 2 weeks; cations, however, were inversely related to this measure of current rainfall, while %N was not related to rainfall. We propose that the positive relationship between current rainfall and leaf litter [P] is due to a response by the vegetation to an increase in nutrient availability and uptake. In contrast, given the negative relationship between current rainfall and cation concentrations, leaching from live leaf tissue is a more likely driver of short-term changes in cations. Should global climate change include altered rainfall patterns in this biome, one class of ecosystem-level responses could be significant changes in P and cation cycling.     

Effects of Nutrient Limitation on Aboveground Carbon Dynamics during Tropical Dry Forest Regeneration in Yucatán,Mexico

Tree growth (as diameter increment), litterfall production, and litter biomass were studied in two secondary tropical dry forests of the Yucatán Peninsula under four treatments of nutrient addition. The studys objective was to assess how variations in the nutrient supply affect aboveground net primary production and carbon (C) accumulation on the floor of two forests in different stages of regeneration. The study included an area of young forest (10 years old) with phosphorus (P)-poor soils and an area of old forest (around 60 years old) where soil P was comparatively less limiting. Four replicate plots (12 × 12 m) at each forest were either left intact (controls) or fertilized with nitrogen (N), P, or N plus P during 3 consecutive years. After 3 years of fertilization, relaxation of the constraints on nutrient limitation resulted in increased trunk growth rates at both the young and old forests. This effect was more pronounced with the addition of P or N plus P (trunk growth doubled with respect to controls), whereas N addition increased tree growth by 60% in comparison to trees in plots without nutrient supplements. In both forests, there were no significant differences in litterfall production among treatments during the first 2 years after fertilization. In the 3rd year of nutrient addition, litterfall production was significantly higher in plots fertilized with N plus P compared to control plots at both forest sites; however, changes in litterfall were not accompanied by litter accumulation in the floor of the two forests. The results of this study support the hypothesis that there is nutrient limitation during tropical dry forest regeneration. They further show that it may be maintained in the long term during secondary succession.     

Litter Decomposition and Nutrient Release in a Tropical Seasonal Rain Forest of Xishuangbanna,Southwest China1

We studied litter decomposition and nutrient release in a tropical seasonal rain forest of Xishuangbanna, Southwest China. The monthly decay rates (k) of leaf litter ranged from 0.02 to 0.21/mo, and correlated with rainfall and soil moisture. Annual k values for leaf litter (1.79/yr) averaged 4.2 times of those for coarse wood (2.5–3.5 cm in diameter). The turnover coefficients of forest floor mass (annual litterfall input/mean floor mass) were: 4.11/yr for flowers and fruits, 2.07/yr for leaves, and 1.17/yr for fine wood (≤2 cm in diameter), with resident time decreasing from fine woods (0.85 yr) to leaves (0.48 yr) and to flower and fruits (0.24 yr). Nutrient residence times in the forest floor mass were ranked as: Ca (1.0 yr) > P (0.92 yr) > Mg (0.64 yr) > N (0.36 yr) > K (0.31 yr). Our data suggest that rates of litter decomposition and nutrient release in the seasonal rain forest of Xishuangbanna are slower than those in typical lowland rain forests, but similar to those in tropical semideciduous forests.