Wet Tropical Soils Research Articles

Representing methane emissions from wet tropical forest soils using microbial functional groups constrained by soil diffusivity

Abstract. Tropical ecosystems contribute significantly to global emissions of methane (CH4), and landscape topography influences the rate of CH4 emissions from wet tropical forest soils. However, extreme events such as drought can alter normal topographic patterns of emissions. Here we explain the dynamics of CH4 emissions during normal and drought conditions across a catena in the Luquillo Experimental Forest, Puerto Rico. Valley soils served as the major source of CH4 emissions in a normal precipitation year (2016), but drought recovery in 2015 resulted in dramatic pulses in CH4 emissions from all topographic positions. Geochemical parameters including (i) dissolved organic carbon (C), acetate, and soil pH and (ii) hydrological parameters like soil moisture and oxygen (O2) concentrations varied across the catena. During the drought, soil moisture decreased in the slope and ridge, and O2 concentrations increased in the valley. We simulated the dynamics of CH4 emissions with the Microbial Model for Methane Dynamics-Dual Arrhenius and Michaelis–Menten (M3D-DAMM), which couples a microbial functional group CH4 model with a diffusivity module for solute and gas transport within soil microsites. Contrasting patterns of soil moisture, O2, acetate, and associated changes in soil pH with topography regulated simulated CH4 emissions, but emissions were also altered by rate-limited diffusion in soil microsites. Changes in simulated available substrate for CH4 production (acetate, CO2, and H2) and oxidation (O2 and CH4) increased the predicted biomass of methanotrophs during the drought event and methanogens during drought recovery, which in turn affected net emissions of CH4. A variance-based sensitivity analysis suggested that parameters related to aceticlastic methanogenesis and methanotrophy were most critical to simulate net CH4 emissions. This study enhanced the predictive capability for CH4 emissions associated with complex topography and drought in wet tropical forest soils.

Redox Fluctuations Control the Coupled Cycling of Iron and Carbon in Tropical Forest Soils.

Oscillating redox conditions are a common feature of humid tropical forest soils, driven by an ample supply and dynamics of reductants, high moisture, microbial oxygen consumption, and finely textured clays that limit diffusion. However, the net result of variable soil redox regimes on iron (Fe) mineral dynamics and associated carbon (C) forms and fluxes is poorly understood in tropical soils. Using a 44-day redox incubation experiment with humid tropical forest soils from Puerto Rico, we examined patterns in Fe and C transformations under four redox regimes: static anoxic, "flux 4-day" (4d oxic, 4d anoxic), "flux 8-day" (8d oxic, 4d anoxic) and static oxic. Prolonged anoxia promoted reductive dissolution of Fe-oxides, and led to an increase in soluble Fe(II) and amorphous Fe oxide pools. Preferential dissolution of the less-crystalline Fe pool was evident immediately following a shift in bulk redox status (oxic to anoxic), and coincided with increased dissolved organic C, presumably due to acidification or direct release of organic matter (OM) from dissolving Fe(III) mineral phases. The average nominal oxidation state of water-soluble C was lowest under persistent anoxic conditions, suggesting that more reduced organic compounds were metabolically unavailable for microbial consumption under reducing conditions. Anoxic soil compounds had high H/C values (and were similar to lignin-like compounds) whereas oxic soil compounds had higher O/C values, akin to tannin- and cellulose-like components. Cumulative respiration derived from native soil organic C was highest in static oxic soils. These results show how Fe minerals and Fe-OM interactions in tropical soils are highly sensitive to variable redox effects. Shifting soil oxygen availability rapidly impacted exchanges between mineral-sorbed and aqueous C pools, increased the dissolved organic C pool under anoxic conditions implying that the periodicity of low-redox events may control the fate of C in wet tropical soils.

Microscopic diagnosis of the leaf and stem of Piper solmsianum C.DC.

Piper solmsianum C.DC., which is popularly known as pariparoba, is a shrub that measures 1-3 m in height and it inhabits areas with wet tropical soils. The objective of this study was to analyze the leaf and stem anatomy using light microscopy, scanning electron micrographs, and energy-dispersive X-ray spectroscopy in order to provide information for species identification. The anatomical profile showed the following main microscopic markers: hypostomatic leaf; hypodermis layer on both sides; pearl glands; biconvex midrib shape; five collateral vascular bundles in open arc with the central bundle larger than the others; circular stem shape; collateral vascular bundles arranged in two rings; sinuous sclerenchymatic sheath in the pith; secretory idioblasts; and starch grains in the mesophyll, in the ground parenchyma of the midrib, petiole, and in the stem; and six morphotypes of calcium oxalate crystals (styloids, cuneiform, tabular crystal rosettes, cuneiform crystal rosettes, elongated square dipyramids, as well as very elongated square dipyramids).

Tropical soils cultivated with tomato: fractionation and speciation of Al.

Soil acidity and the associated problems of aluminum (Al) toxicity and scarce exchangeable bases are typically the most important limiting factors of agricultural yield in wet tropical regions. The goals of this study were to test how soil lime rates affect the forms and distribution of Al in the soil fractions and how different levels of bioavailable Al affect two tomato genotypes grown in wet tropical soils. The tomato genotypes CNPH 0082 and Calabash Rouge were grown in two wet tropical soils in a greenhouse. Soil lime rates of 0, 560, and 2240 mg kg(-1) soil (clay soil) and 0, 280, and 1120 mg kg(-1) soil (sandy soil) were applied to modify Al concentrations. Dry mass production and Al concentrations were determined in shoots and roots. Al was fractionated in the soil, and the soil solution was speciated after cultivation. The Calabash Rouge genotype possesses mechanisms to tolerate Al3+, absorbed less Al, exhibited smaller reduction in growth, and lower Al concentrations in plant parts than the CNPH 0082. Increased soil pH reduced the exchangeable Al fraction and increased the fraction mainly linked to organic matter. Al in the soil in the form of complexes with organic compounds and Al(SO4)+ (at the highest lime rate) did not affect plant development. Soil acidity can be easily neutralized by liming the soil, which transforms toxic Al3+ in the soil into forms that do not harm tomato plants, thereby avoiding oxidative stress in the plants. Al-induced stress in tomatoes varies with genotypes and soil type.

Draft Genome Sequence of the Lignin-Degrading Burkholderia sp. Strain LIG30, Isolated from Wet Tropical Forest Soil.

Burkholderia species are common soil Betaproteobacteria capable of degrading recalcitrant aromatic compounds and xenobiotics. Burkholderia sp. strain LIG30 was isolated from wet tropical forest soil and is capable of utilizing lignin as a sole carbon source. Here we report the draft genome sequence of Burkholderia sp. strain LIG30.

Patterns in understory woody diversity and soil nitrogen across native- and non-native-urban tropical forests

Urban expansion is accelerating in the tropics, and may promote the spread of introduced plant species into urban-proximate forests. For example, soil disturbance can deplete the naturally high soil nitrogen pools in wet tropical soils, favoring introduced species with nitrogen-fixing capabilities. Also, forest fragmentation and canopy disturbance are likely to favor high-light species over shade-adapted rainforest species. We measured understory woody diversity, the abundance of introduced species, and soil nitrogen and carbon in urban, suburban, and rural secondary forests in Puerto Rico with canopies dominated by (1) native species, (2) introduced Fabaceae (potential nitrogen-fixers), and (3) introduced non-Fabaceae species. We hypothesized that forest stands with introduced Fabaceae in the canopy have higher soil nitrogen levels than stands with other introduced canopy species, and that this higher nitrogen is linked to increased native woody species diversity in the understory. We also predicted that more open canopies and smaller fragment sizes would be positively related with introduced species in the understory, and negatively related with total understory diversity. We found that stands with introduced Fabaceae in the canopy had significantly higher soil nitrogen levels than stands with other non-nitrogen fixing introduced species, and understory woody diversity in Fabaceae stands approached similar diversity levels as stands with native-dominated canopies. As predicted, aboveground stand structure and fragment size were also significantly associated with understory woody diversity across stands. These results suggest that introduced nitrogen-fixing trees may improve recruitment of native woody species in degraded tropical sites where native soil nitrogen is naturally high, particularly as Fabaceae stands mature and canopies close.

Enzyme activities of aerobic lignocellulolytic bacteria isolated from wet tropical forest soils

Lignocellulolytic bacteria have promised to be a fruitful source of new enzymes for next-generation lignocellulosic biofuel production. Puerto Rican tropical forest soils were targeted because the resident microbes decompose biomass quickly and to near-completion. Isolates were initially screened based on growth on cellulose or lignin in minimal media. 75 Isolates were further tested for the following lignocellulolytic enzyme activities: phenol oxidase, peroxidase, β-d-glucosidase, cellobiohydrolase, β-xylopyranosidase, chitinase, CMCase, and xylanase. Cellulose-derived isolates possessed elevated β-d-glucosidase, CMCase, and cellobiohydrolase activity but depressed phenol oxidase and peroxidase activity, while the contrary was true of lignin isolates, suggesting that these bacteria are specialized to subsist on cellulose or lignin. Cellobiohydrolase and phenol oxidase activity rates could classify lignin and cellulose isolates with 61% accuracy, which demonstrates the utility of model degradation assays. Based on 16S rRNA gene sequencing, all isolates belonged to phyla dominant in the Puerto Rican soils, Proteobacteria, Firmicutes, and Actinobacteria, suggesting that many dominant taxa are capable of the rapid lignocellulose degradation characteristic of these soils. The isolated genera Aquitalea, Bacillus, Burkholderia, Cupriavidus, Gordonia, and Paenibacillus represent rarely or never before studied lignolytic or cellulolytic species and were undetected by metagenomic analysis of the soils. The study revealed a relationship between phylogeny and lignocellulose-degrading potential, supported by Kruskal–Wallis statistics which showed that enzyme activities of cultivated phyla and genera were different enough to be considered representatives of distinct populations. This can better inform future experiments and enzyme discovery efforts.

Changes in microbial dynamics during long-term decomposition in tropical forests

Humid tropical forest soils are characterized by low and fluctuating redox, conditions which are thought to inhibit organic matter degradation by microbes. However, evidence suggests that soil microbial communities are adapted to the redox conditions in these ecosystems. In this study we tested the hypothesis that soil oxygen (O2) availability as an index of redox conditions structures patterns in litter decomposition and associated microbial community dynamics over space and time in humid tropical forests. We conducted a two year decomposition experiment on a common litter substrate in four sites along an elevational gradient with well described climate and redox dynamics. Microbial community sequencing, potential enzyme activities, and litter chemistry measurements were made on litter and soil to determine the relationship between soil and litter communities and biogeochemistry. Decomposition was slowest in the upper elevation site, which was the wettest and had the lowest average soil O2 availability. However, soil hydrolytic and litter phenol oxidase activities were greatest at this site. Small subunit ribosomal RNA genes were sequenced with universal primers for bacteria, archaea and eukaryotes, yielding 40,850 unique taxa after quality filtering and clustering. Across all sites, microbial succession was observed as increasing litter richness, converging bacterial community profiles, and diverging fungal community profiles. Initial decomposers (1–4 weeks) included many r-selected bacteria, including Alpha-, Beta- and Gammaproteobacteria, Clostridia, Bacteroidetes. We also found evidence of anaerobic fungi such as Cryptococcus, as well as the plant-associated Phialocephala and Phyllachora species, suggesting that anaerobic and plant-associated fungi are prevalent later in decomposition in soils with low and fluctuating redox conditions. Because of the striking similarities between sites in functional potential despite differences in wet tropical soil decomposing communities and litter chemistry, we suggest that future climate-driven disruptions to redox fluctuations could significantly alter the terrestrial carbon (C) cycle in tropical forests.

Influence of oxic/anoxic fluctuations on ammonia oxidizers and nitrification potential in a wet tropical soil

Ammonia oxidation is a key process in the global nitrogen cycle. However, in tropical soils, little is known about ammonia-oxidizing microorganisms and how characteristically variable oxygen regimes affect their activity. We investigated the influence of brief anaerobic periods on ammonia oxidation along an elevation, moisture, and oxygen availability gradient in wet tropical soils. Soils from three forest types were incubated for up to 36 weeks in lab microcosms under three regimes: (1) static aerobic; (2) static anaerobic; and (3) fluctuating (aerobic/anaerobic). Nitrification potential was measured in field-fresh soils and incubated soils. The native ammonia-oxidizing community was also characterized, based on diversity assessments (clone libraries) and quantification of the ammonia monooxygenase α-subunit (amoA) gene. These relatively low pH soils appear to be dominated by ammonia-oxidizing archaea (AOA), and AOA communities in the three soil types differed significantly in their ability to oxidize ammonia. Soils from an intermediate elevation, and those incubated with fluctuating redox conditions, tended to have the highest nitrification potential following an influx of oxygen, although all soils retained the capacity to nitrify even after long anoxic periods. Together, these results suggest that wet tropical soil AOA are tolerant of extended periods of anoxia.

Anaerobic oxidation of methane in tropical and boreal soils: Ecological significance in terrestrial methane cycling

Anaerobic oxidation of methane (AOM) is a considerable sink for the greenhouse gas methane (CH4) in marine systems, but the importance of this process in terrestrial systems is less clear. Lowland boreal soils and wet tropical soils are two hot spots for CH4 cycling, yet AOM has been essentially uncharacterized in these systems. We investigated AOM in soils from sites in Alaska and Puerto Rico. Isotope tracers were utilized in vitro to enable the simultaneous quantification of CH4 production and consumption without use of biological inhibitors. Boreal peat soil and tropical mineral soil oxidized small but significant quantities of CH4 to CO2 under anoxic conditions (p < 0.001). Potential AOM rates were 21 ± 2 nmol gdw−1 d−1 and 2.9 ± 0.5 nmol gdw−1 d−1 for the boreal and tropical soils, respectively. The addition of terminal electron acceptors (NO3−, Fe(III), and SO42−) inhibited AOM and methanogenesis in both soils. In all incubations, CH4 production occurred simultaneously with AOM, and CH4 production rates were always greater than AOM rates. There was a strong correlation between the quantity of CH4 produced and the amount of CH4 oxidized under anoxic conditions (Alaska: r = 0.875, p < 0.0001; Puerto Rico: r = 0.817, p < 0.0001). CH4 oxidation under anoxic conditions was biological and likely mediated by methanogenic archaea. While only a small percentage of the total CH4 produced in these soils was oxidized under anoxic conditions (0.3% and 0.8% for Alaskan and Puerto Rican Soils), this process is important to understand since it could play a measurable role in controlling net CH4 flux.

Sample Pretreatment Affects the Distribution of Organic Carbon in Aggregates of Tropical Grassland Soils

Soil aggregation is an important mechanism for the stabilization of soil organic C (SOC). The distribution of SOC among aggregate classes depends on sample pretreatment and on the applied method of aggregate fractionation. We examined the effect of soil pretreatment (air‐dried vs. field‐moist soil) on the yield and SOC content of aggregate size fractions (8000–2000, 2000–250, 250–53, and <53 μm). Aggregate size fractions were separated by wet sieving tropical soils of different parent material and mineralogy (volcanic ashes dominated by short‐range‐order aluminosilicates and marine Tertiary sediments dominated by smectitic clays), which were used as pastures for 13 to 50 yr after deforestation. In addition, the proportion of pasture‐ and forest‐derived SOC in the aggregate fractions was determined using the 13C/12C isotope ratio. In volcanic ash soils, there was no clear effect of soil pretreatment on the distribution of aggregates into aggregate size classes. Furthermore, the SOC concentration and proportion of pasture‐derived SOC of aggregates within each size class did not differ across treatments. In smectitic clay soils, however, the two pretreatments resulted in distinct differences in the distribution of dry matter yield and also of SOC among the aggregate fractions. Wet sieving of dry soil led to a separation of macroaggregates rich in pasture‐derived SOC, whereas wet sieving of moist soil isolated microaggregates with high contents of pasture‐derived SOC. This implies that soil organic matter plays a major role in both the formation and the stabilization of macroaggregates and in the early stage of microaggregate formation in sedimentary soils but not in volcanic ash soils.

Suppression of methanogenesis by dissimilatory Fe(III)‐reducing bacteria in tropical rain forest soils: implications for ecosystem methane flux

AbstractTropical forests are an important source of atmospheric methane (CH4), and recent work suggests that CH4 fluxes from humid tropical environments are driven by variations in CH4 production, rather than by bacterial CH4 oxidation. Competition for acetate between methanogenic archaea and Fe(III)‐reducing bacteria is one of the principal controls on CH4 flux in many Fe‐rich anoxic environments. Upland humid tropical forests are also abundant in Fe and are characterized by high organic matter inputs, steep soil oxygen (O2) gradients, and fluctuating redox conditions, yielding concomitant methanogenesis and bacterial Fe(III) reduction. However, whether Fe(III)‐reducing bacteria coexist with methanogens or competitively suppress methanogenic acetate use in wet tropical soils is uncertain. To address this question, we conducted a process‐based laboratory experiment to determine if competition for acetate between methanogens and Fe(III)‐reducing bacteria influenced CH4 production and C isotope composition in humid tropical forest soils. We collected soils from a poor to moderately drained upland rain forest and incubated them with combinations of 13C‐bicarbonate, 13C‐methyl labeled acetate (13CH3COO−), poorly crystalline Fe(III), or fluoroacetate. CH4 production showed a greater proportional increase than Fe2+ production after competition for acetate was alleviated, suggesting that Fe(III)‐reducing bacteria were suppressing methanogenesis. Methanogenesis increased by approximately 67 times while Fe2+ production only doubled after the addition of 13CH3COO−. Large increases in both CH4 and Fe2+ production also indicate that the two process were acetate limited, suggesting that acetate may be a key substrate for anoxic carbon (C) metabolism in humid tropical forest soils. C isotope analysis suggests that competition for acetate was not the only factor driving CH4 production, as 13C partitioning did not vary significantly between 13CH3COO− and 13CH3COO−+Fe(III) treatments. This suggests that dissimilatory Fe(III)‐reduction suppressed both hydrogenotrophic and aceticlastic methanogenesis. These findings have implications for understanding the CH4 biogeochemistry of highly weathered wet tropical soils, where CH4 efflux is driven largely by CH4 production.

Antibacterial Activity of Extract, Fractions and Four Compounds Extracted from Piper solmsianum C. DC. var. solmsianum (Piperaceae)

Piper solmsianum C. DC. var. solmsianum (Piperaceae) is a shrub commonly found in areas with wet tropical soils. Other Piper species have been used in folk medicine as antitumoral and antiseptic agents. We studied the crude methanolic extract, some organic fractions and compounds isolated from this plant for possible antimicrobial activity against Gram-positive and Gram-negative bacteria. The bioautographic assays disclosed three inhibition zones. The minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) were determined showing excellent activity, particularly against the Gram-positive bacteria (Bacillus cereus, Staphylococcus aureus, Staphylococcus saprophyticus and Streptococcus agalactiae). It appears that the antimicrobial activity of Piper solmsianum is related mainly to the presence of conocarpan and eupomatenoid-5 (neolignans). However another, as yet unidentified, active compound could also be extracted from the plant.

Determining soil water status and other soil characteristics by spectral proximal sensing

Automated acquisition of soil information by remote sensing is required to improve soil evaluation. Water is one of the most important elements that interfere with spectral data, and its interaction with electromagnetic energy can help diagnose soil characteristics. The objectives of this study were 1) to evaluate spectral data of wet and dry tropical Brazilian soils with different hydration; 2) to determine a method to identify soil mineralogy; 3) to evaluate clay minerals at different moisture stages and their relationship with soil mineralogy; 4) to test remote sensing techniques for the identification and influence of moisture and, 5) to determine a model to estimate soil moisture using spectral data measured in the laboratory by a spectroradiometer (450–2500 nm). Spectral reflectance was capable of distinguishing different moisture levels. As the content of moisture increased, spectral reflectance in the soil and clay mineral samples decreased. The concavity feature of absorption bands 1400 and 1900 nm decreased during dehydration. The 2200 nm band characterized the presence of kaolinite, which was more evident under drier conditions. The spectral curves of hydrated montmorillonite and kaolinite samples presented more absorption features than those of dried samples. The evaluation of the absorption feature centered at 630 nm was sensitive to the amount of kaolinite. Our results suggest that soil mineralogy could be simultaneously evaluated with wet and dry samples. The absorption features observed for clay patterns were helpful to characterize soil mineralogy. Differences in moisture content could also be detected by the soil line technique. Moisture could be differentiated by analyzing reflectance intensity mainly at the 1550–1750 nm wavelengths. It was also possible to estimate soil moisture by a multiple regression model developed ( r 2 value of 0.98) with the intensity dimension of the absorption features centered at 1400, 1900 and 2200 nm. Spectral analysis gave specific soil information (moisture and mineralogy) by using unique basic information from spectral reflectance, thus providing a quick, non-destructive and non-polluting method of analysis.

Occurrence and simulation of nitrification in two contrasting sugarcane soils from the Australian wet tropics

The concentration of ammonium-nitrogen (NH4+-N) frequently exceeds that of nitrate-N (NO3−-N) in Australian wet tropical sugarcane soils. The amount of mineral N in soil is the net result of complex processes in the field, so the objective of this experiment was to investigate nitrification and ammonification in these soils under laboratory conditions. Aerobic and saturated incubations were performed for 1 week on 2 wet tropical soils. Net NO3−-N increased significantly in both soils during both types of incubation. A second series of aerobic incubations of these soils treated with NH4+-N and inoculated with subtropical nitrifying soils was conducted for 48 days. Nitrification in the wet tropical soils was not significantly affected by inoculation, and virtually all added N was nitrified during the incubation period. Mineral N behaviour of the 48-day incubations was captured with the APSIM-SoilN model. As nitrification proceeded under laboratory conditions and was able to be captured by the model, it was concluded that nitrification processes in the wet tropical soils studied were not different from those in the subtropical soils. Processes that remove NO3− from the soil, such as leaching and denitrification, may therefore be important factors affecting the proportions of NH4+-N and NO3−-N measured under field conditions.

Redox Fluctuation Structures Microbial Communities in a Wet Tropical Soil

Frequent high-amplitude redox fluctuation may be a strong selective force on the phylogenetic and physiological composition of soil bacterial communities and may promote metabolic plasticity or redox tolerance mechanisms. To determine effects of fluctuating oxygen regimens, we incubated tropical soils under four treatments: aerobic, anaerobic, 12-h oxic/anoxic fluctuation, and 4-day oxic/anoxic fluctuation. Changes in soil bacterial community structure and diversity were monitored with terminal restriction fragment length polymorphism (T-RFLP) fingerprints. These profiles were correlated with gross N cycling rates, and a Web-based phylogenetic assignment tool was used to infer putative community composition from multiple fragment patterns. T-RFLP ordinations indicated that bacterial communities from 4-day oxic/anoxic incubations were most similar to field communities, whereas those incubated under consistently aerobic or anaerobic regimens developed distinctly different molecular profiles. Terminal fragments found in field soils persisted either in 4-day fluctuation/aerobic conditions or in anaerobic/12-h treatments but rarely in both. Only 3 of 179 total fragments were ubiquitous in all soils. Soil bacterial communities inferred from in silico phylogenetic assignment appeared to be dominated by Actinobacteria (especially Micrococcus and Streptomycetes), "Bacilli," "Clostridia," and Burkholderia and lost significant diversity under consistently or frequently anoxic incubations. Community patterns correlated well with redox-sensitive processes such as nitrification, dissimilatory nitrate reduction to ammonium (DNRA), and denitrification but did not predict patterns of more general functions such as N mineralization and consumption. The results suggest that this soil's indigenous bacteria are highly adapted to fluctuating redox regimens and generally possess physiological tolerance mechanisms which allow them to withstand unfavorable redox periods.

Biological and Geochemical Sinks for Phosphorus in Soil from a Wet Tropical Forest

In many tropical and volcanic soils, phosphorus (P) availability is strongly influenced by geochemical sorption, which binds P to soil minerals. The aim of this study was to determine whether biological demand or soil sorption strength was the primary control over phosphate availability and retention in a wet tropical soil with high sorption capacity and low P availability. We added 32PO4 to soil from the upper two horizons and assessed the ability of soil microbes to immobilize the added phosphate in the presence of strong sorption. We added phosphate at two concentrations, one representing background turnover that adds low concentrations of P to the soil solution, and the other representing nutrient pulses that can add fairly high fluxes of P to the soil solution. Sorption and microbial immobilization were rapid for both concentrations, consuming most added P within 30 min. Thus, little P remained in the soil solution or extractable pools, which are considered more available to plants. Although soil sorption strength was almost identical for the two horizons, immobilization of tracer P was approximately three times greater in the upper horizon, where most microbial activity was located. This result suggests that microbial demand controlled how P was partitioned into biological versus geochemical sinks. Further evidence for microbial control is suggested by the movement of tracer P from the sorbed pool into the microbial pool when demand was stimulated by the addition of carbon (C). We also explored how increased nitrogen (N) and P availability changed P dynamics in this nutrient poor soil. In contrast to the unfertilized soil, long-term N and P fertilization substantially reduced biological control over inorganic P. P fertilization saturated the soils, overwhelming biological P demand, whereas N fertilization appeared to increase available P through reduced P sorption. Where biological demand for P is high and P becomes available in the soil solution, microbes may play an important role in controlling P partitioning into biological versus geochemical sinks even in soils that have high sorption capacity.

Soil biochemical and microbial indices in wet tropical forests: Effects of deforestation and cultivation

AbstractLittle information is available about the long‐term effects of deforestation and cultivation on biochemical and microbial properties in wet tropical forest soils. In this study, we evaluated the general and specific biochemical properties of soils under evergreen, semi‐evergreen, and moist deciduous forests and adjacent plantations of coconut, arecanut, and rubber, established by clear felling portions of these forests. We also examined the effects of change in land use on microbial indices and their interrelationships in soils.Significant differences between the sites occurred for the biochemical properties reflecting soil microbial activity. Microbial biomass C, biomass N, soil respiration, N mineralization capacity, ergosterol, levels of adenylates (ATP, AMP, ADP), and activities of dehydrogenase and catalase were, in general, significantly higher under the forests than under the plantations. Likewise, the activities of various hydrolytic enzymes such as acid phosphomonoesterase, phosphodiesterase, casein‐protease, BAA‐protease, β‐glucosidase, CM‐cellulase, invertase, urease, and arylsulfatase were significantly higher in the forest soils which suggested that deforestation and cultivation markedly reduced microbial activity, enzyme synthesis and accumulation due to decreased C turnover and nutrient availability. While the ratios of microbial biomass C : N and microbial biomass C : organic C did not vary significantly between the sites, the ratios of ergosterol : biomass C and ATP : biomass C, qCO2 and AEC (Adenylate Energy Charge) levels were significantly higher in the forest sites indicating high energy requirements of soil microbes at these sites.

Soil oxygen availability and biogeochemistry along rainfall and topographic gradients in upland wet tropical forest soils

We measured soil oxygen concentrations at 10 and 35 cm depths and indices of biogeochemical cycling in upland forest soils along a rainfall and elevation gradient (3500–5000 mm y−1; 350–1050 masl) and along topographic gradients (ridge to valley, ∼150 m) in the Luquillo Experimental Forest, Puerto Rico. Along the rainfall gradient, soil O2 availability decreased significantly with increasing annual rainfall, and reached very low levels (<3%) in individual chambers for up to 25 consecutive weeks over 82 weeks of study. Along localized topographic gradients, soil O2 concentrations were variable and decreased significantly from ridges to valleys. In the valleys, up to 35% of the observations at 10–35 cm depth were <3% soil O2. Cross correlation analyses showed that soil O2 concentrations were significantly positively correlated along the topographic gradient, and were sensitive to rainfall and hydrologic output. Soil O2 concentrations in valley soils were correlated with rainfall from the previous day, while ridge sites were correlated with cumulative rainfall inputs over 4 weeks. Soils at the wettest point along the rainfall gradient had very high soil methane concentrations (3–24%) indicating a strong influence of anaerobic processes. We measured net methane emission to the atmosphere at the wettest sites of the rainfall gradient, and in the valleys along topographic gradients. Other measures of biogeochemical function such as soil organic matter content and P availability were sensitive to chronic O2 depletion along the rainfall gradient, but less sensitive to the variable soil O2 environment exhibited at lower elevations along topographic gradients.

Mycorrhiza Inoculum Potentials in Tropical Secondary Succession

Vesicular-arbuscular mycorrhizae can be indispensable for establishment and growth of tree seedlings in infertile lowland wet tropical soils. Persistence of mycorrhizal fungi after disturbance, however, is problematic to assess. We used a greenhouse bioassay employing Psidium guajava L. and Allium cepa L. to estimate the most probable number of mycorrhizal fungus propagules in two Costa Rican soils (Oxic Dystropepts) with different vegetation histories. We collected soils at the La Selva Biological Station from sites in secondary forest, abandoned pasture, and plots kept bare of vegetation for four and six years. Both bioassay hosts yielded positively correlated estimates of mycorrhiza propagule numbers. Median propagule estimates per 100 gm dry soil for the pasture site are 57 and 63; these estimates significantly exceed those for the other sites which range from 0.2 to 10 for bare plots and were estimated to be 0.6 and 10 for secondary forest. These bioassay estimates are positively correlated (Pearson r = 0.66 and 0.72) with counts of whole spores in these soils, but not with counts of sporocarps, spore clusters, or the most numerous, spores apparently empty of cytoplasmic contents or parasitized. Growth of both bioassay hosts in pasture soil significantly exceeded growth in soils from the other three sites in accord with the bioassays and whole spore counts.