Tropical Soil Samples Research Articles

Land Use Change from Natural Tropical Forests to Managed Ecosystems Reduces Gross Nitrogen Production Rates and Increases the Soil Microbial Nitrogen Limitation.

Understanding the underlying mechanisms of soil microbial nitrogen (N) utilization under land use change is critical to evaluating soil N availability or limitation and its environmental consequences. A combination of soil gross N production and ecoenzymatic stoichiometry provides a promising avenue for nutrient limitation assessment in soil microbial metabolism. Gross N production via 15N tracing and ecoenzymatic stoichiometry through the vector and threshold element ratio (Vector-TER) model were quantified to evaluate the soil microbial N limitation in response to land use changes. We used tropical soil samples from a natural forest ecosystem and three managed ecosystems (paddy, rubber, and eucalyptus sites). Soil extracellular enzyme activities were significantly lower in managed ecosystems than in a natural forest. The Vector-TER model results indicated microbial carbon (C) and N limitations in the natural forest soil, and land use change from the natural forest to managed ecosystems increased the soil microbial N limitation. The soil microbial N limitation was positively related to gross N mineralization (GNM) and nitrification (GN) rates. The decrease in microbial biomass C and N as well as hydrolyzable ammonium N in managed ecosystems led to the decrease in N-acquiring enzymes, inhibiting GNM and GN rates and ultimately increasing the microbial N limitation. Soil GNM was also positively correlated with leucine aminopeptidase and β-N-acetylglucosaminidase. The results highlight that converting tropical natural forests to managed ecosystems can increase the soil microbial N limitation through reducing the soil microbial biomass and gross N production.

Simple Shear Strength Analysis of Inherent Anisotropy for a Tropical Alluvial Soil

ABSTRACT Tropical alluvial soils have unusual engineering properties and their behavior is not typical of fine-grained soils. The literature regarding strength properties of alluvial soils is limited. This study presents an experimental evaluation of inherent anisotropy of undisturbed tropical alluvial soil samples by comparing the direct simple shear (DSS) test results of 52 specimens cut in horizontal and vertical orientations. The tests were carried out under consolidated undrained constant volume with excess pore water pressure measurements. The peak shear strength, effective stress paths, and secant shear modulus analyses showed that the difference in properties between horizontally and vertically cut specimens was negligible. Nonetheless, the results for vertically cut specimens presented a higher random error compared to the results for horizontal specimens in all aspects. A good relationship between the undrained shear strength and over-consolidation ratio was found but was not comparable to past DSS results for natural clays. Scanning electron microscopy images revealed flocculated edge-to-edge and edge-to-face particle associations. Directionality analysis of microfabric showed that the alluvial soils had low preferred orientation of particles. The general notion that all alluvial soils are highly anisotropic was not evident, based on the experiments reported in this study. The findings herein are useful for geotechnical engineers to better understand the strength behavior of tropical alluvial soils.

Automated detection and quantification of Enchytraeus crypticus (Oligochaeta: Enchytraeidae) in tropical artificial soil using image analysis.

The effects of toxic substance in soil matrices are evaluated by assessing adult worm survival and reproduction. Throughout the test, hundreds of juvenile potworms can be found. The current method for Enchytraeus crypticus quantification in soil samples is a laborious and time-consuming procedure that involves manual counting. The present work proposes a method for quick and reliable counting of E. crypticus by using an automated image analysis algorithm applied to soil images. Comparisons between automated and manual methods conducted in double-blind trials involving a large, routine batch of tropical artificial soil samples revealed no statistically significant differences for a wide range of worm densities. The proposed method overcomes time-consuming counts in manual methods and is suited to be deployed routinely for soil toxicity studies involving large batches of samples.

Seasonal Variation of Soil Quality in a Semi-deciduous Northern Tropical Forest of Nagaland, India.

To assess the seasonal changes in soil parameters and the soil quality of a semi-deciduous forest in Nagaland, tropical forest soil samples were tested for 10 physical and chemical variables. Apart from clay content, EC, and CEC, the rest of the parameters showed a seasonal mean significant difference at p < .05 level. Based on the principal component analysis, available nitrogen (Nav) and electrical conductivity (EC) were included in a minimum data set and are regarded to best represent the system attributes. In both additive and weighted soil quality method, maximum SQI was recorded in autumn season. The research summarized that seasonal variations can influence soil characteristics and soil quality through its aggregate effects. Considering the result obtained from the present study, the approach we have used in soil quality assessment would be suitable for primarily screening the tropical forest soil status. This would ultimately pave ways for future management and mitigation plans to facilitate the improvement of forest health and aid to biodiversity conservation.

Complete Genome Sequence of the Soil-Isolated Psychrobacillus sp. Strain AK 1817, Capable of Biotransforming the Ergostane Triterpenoid Antcin K.

ABSTRACTThe soil bacterium Psychrobacillus sp. strain AK 1817 was isolated from a tropical soil sample collected in Taiwan. Strain AK 1817 biotransforms the ergostane triterpenoid antcin K from the fungus Antrodia cinnamomea. The genome was sequenced using the PacBio RS II platform and consists of one chromosome of 4,096,020 bp, comprising 3,907 protein-coding genes, 75 tRNAs, 30 rRNAs, 5 noncoding RNAs (ncRNAs), and 100 pseudogenes.

Mapping the Available Water Capacity in tropical climate soils for soybean (Glycine max) cultivation in the state of Tocantins-Brazil

Brazilian soybean has undergone considerable economic growth. Its production depends on the demand for some inputs. One of these inputs is the soil water supply, which can be made artificially or obtained by natural rainfall. Knowledge of available water capacity (AWC), which depends on total water availability (TWA), is poorly accessible and difficult to measure in the field. This study aimed to map the AWC of the state of Tocantins, based on pedotransfer functions (PTFs), to evaluate the water availability of the soils of the microregions of that state. We used the Arya and Paris model, aided by a computer program, Qualisolo, made by Embrapa Instrumentação. One hundred fifty-seven tropical soil samples were extracted from the Embrapa Solos portal. Preliminarily, the soil water retention curve (SWRC) was obtained and, subsequently, the TWA and AWC for this oilseed were estimated. Multiple linear regressions show the correlation between TWA and clay (CL), Silt (ST) and total sand (TS) contents. The correlation established was TWA = 3.2993 – 0.0028TS – 0.0034CL. This main conclusion reflects a fruitful AWC for decision-making by the soybean agribusiness and exposes the regional weaknesses for this crop under a rainfed regime in some regions of Tocantins. We could observe that, in terms of water availability, agribusiness is a potential threat to the environment protection area (APA) of the Ilha do Bananal/Cantão, Formoso River microregion.

Carbon stability and biochar aging process after soil application

Abstract: During the pyrolysis process for biochar production there is the stabilization of C in aromatic compounds, which accredits biochar to function as an input for C sequestration in the soil. The aim of this study was assessing of C sequestration potential through pyrolyzed biochar in low temperature (350-400oC), checking the possible deleterious effect in soil organic matter (SOM) and assessing the cation exchange capacity (CEC) alteration, as the biochar is aging.The research strategy involved, besides the materials characterization, the installation and conduction of two incubations under controlled laboratory conditions.The evaluation of the biochar stability of C carbon, the effects on SOM and, therefore, the potential for the use of biochar in soil C sequestration was carried out in a respirometric experiment, with four typical tropical soil samples mixed with eucalyptus biochar dose (0.0, 2.5, 5.0, 10.0 and 20.0 g Ckg-1). After 120 days of incubation under non-limiting temperature and moisture conditions, the results showed C mineralization rates between 0,4 and 9,3%, which demonstrates the high carbon stability in the biochar. Through the 13C technique in soil analysis at the end of the incubation was observed small reductions of original C in SOM for two of the evaluated soils, although gains from biochar application represented more than double the original C content of soils.The 360-day incubation experiment to evaluate biochar aging, focusing on the generation of negative charges, considered the variation in the presence or absence of soil; moisture and temperature (25 or 32°C). Among the factors tested, there was only effect of time on CEC. The increase in CEC was practically 100 mmolc kg-1 in one year, which represented a 50% increase in surface charges due to the biochar aging process.The research led to the conclusion that biochar must be considered an input to soil C sequestration, in the reason of its high stability after application, recommending the use of the conservative value of 90% of the total C applied via biochar rate to environmental accounting related to C sequestration. In addition, increases in CEC in biochar treated soils are expected due to biochar surface charges as well as the aging process after application.

Biodegradation of squalene and n-hexadecane by Gordonia amicalis HS-11 with concomitant formation of biosurfactant and carotenoids

Gordonia amicalis HS-11 has been enriched from a hydrocarbon contaminated tropical soil sample. The ability of this organism to utilize a triterpenic polyunsaturated hydrocarbon, squalene (2,6,10,15,19,23-hexamethyl-6,6,10,14,18,20-tetracosahexane) and the model saturated hydrocarbon n-hexadecane is described here. The isolate degraded squalene and n-hexadecane (79 ± 3.02 and 96 ± 4.11%, respectively) after eight days of incubation. The isolate produced an extracellular biosurfactant that reduced surface tension from 69 ± 2.83 to 40 ± 1.63 and 35 ± 2.34 mN m−1 with squalene and n-hexadecane as carbon sources, respectively, after 6 days. The Actinomycete cleaved squalene to geranylacetone and farnesyl acetaldehyde that were further utilized for supporting growth. n-Hexadecane was degraded via monoterminal oxidation and activities of important enzymes (alkane hydroxylase and alcohol dehydrogenase) were highest (215 ± 8.76 and 169 ± 6.02 units mg−1 protein, respectively) after four days. Cells grown on squalene were short and with n-hexadecane there were clumps of longer cells. Squalene and n-hexadecane-grown cell surfaces were smooth possibly due to extracellular surface active compounds. While growing on hydrophobic substrates, some cells were seen adhering to droplets and others were in the free form. The culture was able to simultaneously degrade hydrocarbons and produce two commercially relevant value-added products. The yield of the extracellular biosurfactant on n-hexadecane was 480 mg l−1 and cells grown on squalene and n-hexadecane also yielded carotenoids (2.3 ± 0.14 and 2.9 ± 0.10 mg g−1 dry cell weight, respectively). This is the first report on the utilization of squalene by Gordonia.

Spectral fusion by Outer Product Analysis (OPA) to improve predictions of soil organic C

Soil organic carbon (C) is an important indicator of agricultural and environmental quality. It improves soil fertility and helps to mitigate greenhouse gas emissions. Soil spectroscopy with either vis–NIR (350–2500 nm) or mid-IR (4000–400 cm−1) spectra have been used successfully to predict organic C concentrations in soil. However, research to improve predictions of soil organic C by simply combining vis–NIR and mid-IR spectra to model them together has been unsuccessful. Here we use the Outer Product Analysis (OPA) to fuse vis–NIR and mid-IR spectra by bringing them into a common spectral domain. Using the fused data, we derived models to predict soil organic C and compared its predictions to those derived with vis–NIR and mid-IR models separately. We analyzed 1259 tropical soil samples from surface and subsurface layers across agricultural areas in Central Brazil. Soil organic C contents were determined by a modified Walkley-Black method, and vis–NIR and mid-IR reflectance spectra were obtained with a FieldSpec Pro and a Nicolet 6700 Fourier Transformed Infrared (FT-IR), respectively. Reflectances were log-transformed into absorbances. The mean content of soil organic C was 9.14 g kg−1 (SD = 5.64 g kg−1). The OPA algorithm was used to emphasize co-evolutions of each spectral domain into the same one by multiplying the absorbances from both sets of spectra to produce a matrix with all possible products between them. Support Vector Machine with linear kernel function was used for the spectroscopic modeling. Predictions of soil organic C using vis–NIR, mid-IR, and fused spectra were statistically compared by the Tukey's test using the coefficient of determination (R2), root mean squared error (RMSE), and ratio of performance to interquartile distance (RPIQ). Absorbances in vis–NIR and mid-IR were emphasized in the common spectral domain presenting stronger correlations with soil organic C than individual ranges. Soil organic C predictions with the OPA fused spectra were significantly better (R2 = 0.81, RMSE = 2.42 g kg−1, and RPIQ = 2.87) than those with vis–NIR (R2 = 0.69, RMSE = 3.38 g kg−1, and RPIQ = 2.08) or mid-IR spectra (R2 = 0.77, RMSE = 2.90 g kg−1, and RPIQ = 2.43). Fusing vis–NIR and mid-IR spectra by OPA improves predictions of soil organic C.

A Simple And Effective Tool To Extrude Tropical Residual Soil Sample

Soil Investigation is a vital part of a feasibility study of modern construction project and it helps to determine the types and characteristics of soils for better construction planning. In order to determine properties of soil, preparation for sampling of soil must be done. Sampling of tropical residual soil is vital and need a special care to extrude soil from sample casing or thin wall tube. Traditional soil extruder and trimmer take much longer time to be done for sampling but this project to propose a new soil extruder and trimmer that can changes the way to conducted for much more easier, faster and also ensure the quality of soil sample. This proposed equipment is a specially design to reduce and overcome disturbing of soil sample during extruding and trimming the soil sample from thin wall tube including piston sampler. A part from that, the procedures to handle the tools will becomes easier and faster, especially on tropical residual soil that notoriously known as non-homogenous. Related to time consuming to prepared sample and effort to extrude soil samples it can concluded that the new proposed soil extruder is better than existing soil extruder.

Optimized high-throughput methods for quantifying iron biogeochemical dynamics in soil

Iron (Fe) redox cycling and sorption/complexation reactions influence numerous soil biogeochemical processes, and the precise, rapid, and low-cost determination of reactive Fe pools is critical for understanding these dynamics. Colorimetric methods are often used to measure Fe, yet assay conditions vary widely among studies, and the robustness of these methods and their potential interferences remain poorly characterized. Here, we developed optimized ferrozine methods (modified from water and sediment protocols) to determine Fe concentrations in three common soil extractions: reduced (Fe(II)) and oxidized Fe (Fe(III)) in 0.5M HCl (FeHCl), and Fe extracted by citrate-ascorbate (Feca) and citrate-dithionite (Fecd). These methods were adapted for 96-well microplates by employing increased buffer concentrations and longer incubation times relative to published cuvette methods. Iron quantitation was sensitive to the final pH of the reaction mixture and duration of incubation period, factors that have varied widely in previous studies. We obtained consistent results with an assay pH near 7, half hour incubations for FeHCl, and 1h incubations for Feca and Fecd. These ferrozine methods compared favorably with inductively coupled plasma optical emission spectrometry (ICP-OES) across a broad range of soils, including Oxisols, Mollisols, and Inceptisols with as much as 18% organic C. Iron determination in HCl extractions from 158 tropical forest soil samples with widely varying C content was not influenced by dissolved organic carbon (DOC) or phenolics at lower Fe concentrations (<2.5mgg−1 soil), and showed only minor effects (<6% overestimation) at higher Fe concentrations. This was likely due to co-variation between Fe and DOC, which measured as high as 691mgCL−1 in a cloud forest soil extract. These microplate-based ferrozine methods can be applied to quantify several reactive soil Fe phases with high precision and throughput, minimal interference, and low cost relative to ICP-OES.

A Study of Naturally-occurring L-glutamic Acid Producing Bacteria From Tropical Soil and Aquatic Environments.

Microbial means of L-glutamic acid production haveadvantage of directly yielding optical and biological active form of the aminoacid. The production of L-glutamic acid was investigated using bacterialisolates obtained from tropical soil and water samples obtained from Lagos,Nigeria. L–glutamic acid producing bacterial strains were isolated andidentified using the Analytical Profile Index (API) Test 50CHB/20E kit. TheL-Glutamic acid production was tested on Luria Bertani Broth and determined byqualitative and quantitative approaches. Six (6) isolates were L-Glutamateproducers of which the best producing isolates were Paenibacillus alvei and Corynebacteriumglucuronolyticum – seminale giving a yield higher than 0.7 g/L. The best L-glutamic acid producing isolateswere further tested to determine the effects of a variety of carbon andnitrogen sources as well as varying concentration of biotin on the yield of theamino acid produced. Glucose gave yields of 1.19 g/L and 1.05 g/L of L-glutamicacid for Paenibacillus alvei and Corynebacterium glucuronolyticum–seminale respectively,while ammonium chloride and optimum biotin concentration at 2µg gave yields of1.15 g/L and 2.09 g/L as well as 1.59 g/L and 1.75 g/L respectively. Thesebacterial strains therefore offer tremendous promise as candidates for furthermanipulation for enhanced production of L-glutamic acid using indigenousbacteria from the tropical environment

Shear Strength of Tropical Soils and Bentonite Mixtures for Barrier Design

Compacted clayey tropical soils have great potential to be used as barriers in waste disposal facilities, considering that some technical requirements are fulfilled (e.g. hydraulic conductivity; compatibility after disposal; shear strength; swelling/cracking, etc). In turn, the bentonite addition is relevant for the cases where the hydraulic conductivity must be lowered, and therefore investigations on the changes of the mechanical parameters of tropical soils induced by the bentonite addition are of great interest. This paper presents the results of a laboratory investigation on the shear strength of different samples of tropical soils and their mixtures with bentonite in the proportions of 0, 3, 6, 9 and 12% (dry weight basis). The tropical soil samples were collected considering the lateritic, transitional and non-lateritic behavior according to the MCT-M (Modified-Miniature-Compacted-Tropical) classification. The laboratory tests consisted of CU (ConsolidatedUndrained) triaxial tests under the confining stresses of 100, 200 and 400 kPa. The results have showed that the addition of bentonite produced a significant increase in the plasticity of the tropical soil samples (PI increases 4 to 6 times), considerably reduced their friction angle (by as much as 11o ) and stiffness at peak (modulus reductions of 2.5 to 4 MPa) and gave rise to a slight increase in their cohesion (up to 12 kPa). These facts must be considered in the barrier stability analyses when heavy loads are applied. The important role of the shear strength on barrier design is highlighted.

Extracellular DNA extraction is a fast, cheap and reliable alternative for multi-taxa surveys based on soil DNA

DNA metabarcoding on soil samples is increasingly used for large-scale and multi-taxa biodiversity studies. However, DNA extraction may be a major bottleneck for such wide uses. It should be cost/time effective and allow dealing with large sample volumes so as to maximise the representativeness of both micro- and macro-organisms diversity. Here, we compared the performances of a fast and cheap extracellular DNA extraction protocol with a total DNA extraction method in retrieving bacterial, eukaryotic and plant diversity from tropical soil samples of ca. 10 g. The total DNA extraction protocol yielded more high-quality DNA. Yet, the extracellular DNA protocol provided similar diversity assessments although it presented some differences in clades relative abundance and undersampling biases. We argue that extracellular DNA is a good compromise between cost, labor, and accuracy for high-throughput DNA metabarcoding studies of soil biodiversity.

Spatial and Temporal Distribution of Polychlorinated Biphenyl Residues in Tropical Soils

Polychlorinated biphenyls (PCBs) in tropical soil samples from the Central Region of Ghana, and their variation with the soil characteristics were assessed. The soil characteristics of seventy eight samples were first determined. The PCBs was extracted with (1:1) hexane:acetone mixture, and then the extract treated with concentrated H2SO4, 5% KMnO4 and copper. The extract was finally eluted from a silica gel column with hexane:dichloromethane (97:3 v/v), and the PCBs analyzed using a gas chromatograph equipped with 63Ni electron capture detector (GC-ECD model CP 3800). The composition of PCBs homologous in the soils followed the order: hexa-PCBs (31.89%) > penta- PCBs (23.98%) > tri-PCBs (18.47%) > tetra-PCBs (13.67%) > hepta-PCBs (11.99%). The PCB congener profile in the study area was PCB 28 > PCB 153 > PCB 138 > PCB 52 > PCB 101 > PCB 180 > PCB 118. The mean concentration of the PCB congeners (dry matter basis) was PCB 28, 2.27 ± 2.436; PCB 52, 1.84 ± 2.27; PCB 101, 2.17 ± 2.52 μg/kg; PCB 118, 0.83 ± 0.89 μg/kg; PCB 138, 0.99 + 0.91 μg/kg; PCB 153, 1.18 ± 1.11 μg/kg and PCB 180, 1.18 ± 1.19 μg/kg. The mean concentrations of ΣPCBs were 9.15 ± 0.52 μg/kg for the surface soils (0 - 10 cm), 7.55 ± 0.56 μg/kg and 7.82 ± 0.55 μg/kg for the 10 - 20 cm and 20 - 30 cm sub-surface soils respectively. Significant but weak correlations were observed between PCBs and the soil pH, moisture, and exchangeable base. The coefficient of determination for all significant correlations was between 6.6% and 18.75%.

Characterization and hydraulic conductivity of tropical soils and bentonite mixtures for CCL purposes

Tropical soils have great potential to be used as compacted clay liners (CCLs) for waste disposal facilities, considering their wide distribution over the globe. However, to be used as liners, several technical requirements must be correctly addressed, including low hydraulic conductivity (k), high contaminant retention capacity and suitable long-term performance (compatibility). The use of bentonite as an amendment for reducing k of soils and increasing the cation exchange capacity, in turn, can be an alternative to fulfill these requirements. This paper involves a laboratory-based investigation on the characterization and k determination of compacted mixtures of three tropical soil samples and bentonite at different proportions (3, 6, 9 and 12%, dry weight basis). These tropical soils were classified as lateritic, transitional and non-lateritic. The experimental program consisted of mineralogical, chemical, physicochemical and geotechnical characterization and k determination using a triaxial cell. Additionally, Kozeny–Carman derived equations were proposed to be after correlated with the k measured in the laboratory. The bentonite addition considerably reduced k of the soil samples. For instance, under a confining stress of 80kPa, the addition of 12% (dry weight basis) resulted in k values of 1.5×10−8cm/s, 1.2×10−9cm/s and 6.3×10−10cm/s, for the lateritic, transitional and non-lateritic soil samples, respectively; these values correspond respectively to k reduction factors of 225.2, 661.5 and 290.0. Additionally, lateritic soil required greater bentonite content to reach the same k value when compared to transitional and non-lateritic soils. The mesoplastic point (MP) successfully replaced the specific surface (So) and the liquid limit (ωL) to represent the plasticity of fine grained soil in the “Kozeny–Carman” modified equation, which resulted in very good correlations with the k measured in the laboratory for these soils.

Sequential Injection Analysis with Square Wave Voltammetry (SI-SWV) Detection for Investigation of Adsorption of Picloram on a Clay Soil

This paper describes a sequential injection analysis method, with detection by square wave voltammetry using a hanging mercury drop electrode, to determine the herbicide picloram in soil extracts (0.010 mol L-1 CaCl2). The method was applied for determination of the Freundlich parameters related to adsorption of picloram on a tropical soil sample. The major advantage of the proposed method over the conventional chromatographic methods is the high sampling throughput of 190 analyses per hour, with detection and quantification limits of 0.036 and 0.12 mg L-1, respectively. The Kf (0.0016 ± 0.0005 µmol1-1/n L1/n kg-1) and 1/n (0.85 ± 0.06) obtained by the proposed method did not differ from those determined by liquid chromatography with UV detection.

Comparison of Soil Water Retention Functions for Humid Tropical Soils

The determination of soil hydraulic properties is of paramount importance as they are needed in many models of water and solute transport in soils, however conventional methods are quite difficult, expensive and sometimes cumbersome to use. Most studies of soil water retention functions are for temperate soils and their soil water retention curve (SWRC) cannot be extrapolated to tropical region, as such this study focused solemnly on SWRC of soils of tropical region (Malaysia and Indonesia to be specific). The analytical models of Brooks &amp; Corey, van Genuchten and Kosugi were applied to model the SWRC for humid tropical soils, parameters of the three models were optimised by fitting them to 191 soil samples, of 10 different classes (International society of soil science classification) using VBA &amp; MS excel solver add in. A comparison of the fitting capabilities and model quality was made using the sum of square of errors (SSQE) between observed and modelled values and, Akaike information criterion (AIC) respectively. The Kosugi model was found to describe the SWRC of the tropical soil samples better than the other models, as it has the lowest SSQE and AIC values.

Do we need to understand microbial communities to predict ecosystem function? A comparison of statistical models of nitrogen cycling processes

Despite the central role of microorganisms in biogeochemistry, process models rarely explicitly account for variation in communities. Here, we use statistical models to address a fundamental question in ecosystem ecology: do we need to better understand microbial communities to accurately predict ecosystem function? Nitrogen (N) cycle process rates and associated gene abundances were measured in tropical rainforest soil samples collected in May (early wet season) and October (late wet season). We used stepwise linear regressions to examine the explanatory power of edaphic factors and functional gene relative abundances alone and in combination for N-cycle processes, using both our full dataset and seasonal subsets of the data. In our full dataset, no models using gene abundance data explained more variation in process rates than models based on edaphic factors alone, and models that contained both edaphic factors and community data did not explain significantly more variation in process rates than edaphic factor models. However, when seasonal datasets were examined separately, microbial predictors enhanced the explanatory power of edaphic predictors on dissimilatory nitrate reduction to ammonium and N2O efflux rates during October. Because there was little variation in the explanatory power of microbial predictors alone between seasonal datasets, our results suggest that environmental factors we did not measure may be more important in structuring communities and regulating processes in October than in May. Thus, temporal dynamics are key to understanding the relationships between edaphic factors, microbial communities and ecosystem function in this system. The simple statistical method presented here can accommodate a variety of data types and should help prioritize what forms of data may be most useful in ecosystem model development.

The Role of Mineralogy in Organic Matter Stabilization in Tropical Soils

The association of organic matter (OM) with minerals in soils that leads to the stabilization of OM has been linked to specific surface area (SSA) and mineralogy. This study investigated the mineral properties and OM stabilization in 12 surface tropical soil samples. The results showed that stable carbon (stable C) was primarily preserved in silt and clay fractions. The clay fraction was mainly composed of kaolinite, with minor amounts of hematite. The SSA was highly correlated with both crystalline and poorly crystalline Fe oxides and clay content, but there was low correlation with stable and labile carbon. The correlation between SSA and Fe oxides was stronger with crystalline than with poorly crystalline minerals and clay. In general, the SSA of soils increased after removal of organic carbon. However, some samples, especially in the clay fraction, showed a decrease in SSA after removal of organic carbon because of the effect of coating or precipitation of OM on the weakly crystalline Fe oxides.