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Influence of Soil and Climate on Carbon Cycling andMicrobial Activity of a Heterogeneous Tropical SoilPatricia Österreicher-Cunha a , Eurípedes do Amaral Vargas Jr. a , Franklin dos SantosAntunes a , Georgia Peixoto Bechara Mothé a , Jean Rémy Davée Guimarães b & Heitor Luísda Costa Coutinho ca Pontifícia Universidade Católica do Rio de Janeiro, Departamento de Engenharia Civil,Brazilb Universidade Federal do Rio de Janeiro, Instituto de Biofísica Carlos Chagas Fo, Brazilc EMBRAPA-Solos, Centro Nacional de Pesquisa de Solos (CNPS), Brazil

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Geomicrobiology Journal, 29:399–412, 2012Copyright © Taylor & Francis Group, LLCISSN: 0149-0451 print / 1521-0529 onlineDOI: 10.1080/01490451.2011.575914

Influence of Soil and Climate on Carbon Cyclingand Microbial Activity of a Heterogeneous Tropical Soil

Patricia Osterreicher-Cunha,1 Eurıpedes do Amaral Vargas Jr.,1 Franklin dosSantos Antunes,1 Georgia Peixoto Bechara Mothe,1 Jean Remy Davee Guimaraes,2

and Heitor Luıs da Costa Coutinho3

1Pontifıcia Universidade Catolica do Rio de Janeiro, Departamento de Engenharia Civil, Brazil2Universidade Federal do Rio de Janeiro, Instituto de Biofısica Carlos Chagas Fo, Brazil3EMBRAPA-Solos, Centro Nacional de Pesquisa de Solos (CNPS), Brazil

This study presents differentiated behaviors of two facies of atropical soil. Soil characteristics and microbiota, field-monitoredfor two years, showed that different water velocities originated di-verse responses to rainfall; altered oxygenation and nutrient condi-tions resulted in differentiated carbon availability and cycling. Thefacies with lower carbon and water content supported a 35% lessactive but 3–4 times more efficient microbiota. Higher soil moisturein the silty facies (67%) led to increased carbon (12-fold) degrada-tion and availability (3-fold), but not higher biomass. Soil and envi-ronmental conditions apparently rule microbiota metabolic state,hence the need to evaluate them as microbial growth support.

Keywords carbon degradation, microbial activity, residual tropicalsoil, hydraulic properties, soil microbiota

INTRODUCTIONSoils represent one of earth’s most important carbon reservoir

and as such play a crucial role in the carbon cycle. Since carbonsequestration in soils reduces the amount of carbon availablein the environment, the Kyoto Protocols have enhanced inter-est in soil carbon pools and their effect on carbon fluxes (Coxet al. 2000; Intergovernmental Panel on Climate Change 2010).Anthropogenic activities have been increasing environmental

Received 8 September 2010; accepted 14 March 2011.The authors acknowledge Amanda F. B. Fernando, Amaury C.

Fraga and Priscilla L. S. Guimaraes for assistance with laboratory andfield work; Branca Delmonte for helping with the biomass protocol;Marlene Tapia Morales for helping with characteristic and permeabil-ity curves. Funding for this work was provided by CNPq and Faperj(PRONEX).

Address correspondence to Patricia Osterreicher-Cunha, Departa-mento de Engenharia Civil, Pontifıcia Universidade Catolica do Riode Janeiro, PUC-Rio, Rua Marques de Sao Vicente 225 – 301 L,CEP 22451-900, Rio de Janeiro, Brazil. E-mail: osterr@esp.puc-rio.br;osterr@laposte.net

contamination and atmospheric greenhouse gases (GHGs) con-centrations since the 19th century, and present ecosystems neverresided in such high concentrations of available carbon.

Therefore, understanding carbon fluxes and dynamicsthrough ecosystems is fundamental to understand the impacts ofanthropogenic C releases, to develop solutions to address them,to reduce uncertainty in climate models and to understand thelong-term sequestration capacity of natural C pools. Indeed, theeffectiveness of natural ecosystems to sequester anthropogeniccarbon into biomass presents an efficient and cost-effective man-ner to address environmental contamination by organic com-pounds (e.g., Blair et al. 1995; Carbon Sequestration Researchand Development 1999; Young and Ritz 2000).

Organic carbon (Corg) varies from 1% to almost 100% ofsoil organic matter, according to soil characteristics, to climate,topography and soil previous and present use and management;it thus assures several important environmental functions, likethe maintenance of soil fertility, protection against erosion, se-questering of environmental contaminants, while also havingan important role in the cycles of GHGs. Carbon stocks de-pend greatly on soil type and occupation, varying with changesintroduced in soil usage and agricultural practices.

Energy transfer and material processing by biological sys-tems have been transforming the earth through geological agesas extremely diverse populations evolved, presenting diverseroles in the carbon cycle; processes powered by microbial pop-ulations respond for the sequestration of carbon in oceanic andterrestrial environments, assuring its stability. Besides providingscientific basis for the development, assessment and implemen-tation of strategies for carbon sequestration and biodegrada-tion, understanding those processes allows predicting impactson ecosystems as well as their response. Global carbon man-agement, including GHG and environmental contamination, re-quires understanding microbial dynamics and their role in car-bon cycling, their capabilities to capture and sequester CO2 andto degrade xenobiotic compounds may be enhanced.

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Soil behavior and responses depend on the combined effectsof several interacting factors, both inherent (mineralogy, soilparticles, cations, water content) and environmental (water con-tent, density, texture, temperature). Compositional factors showthe potential range of soil properties, whereas environmentalfactors determine their actual values. However, most soil prop-erties are not intrinsic and depend on environmental conditions.Taking both aspects into consideration gives a more meaningfulevaluation of soil behavior and of the variation of soil properties,especially when addressing unsaturated soils and geoenviron-mental problems.

Microbial systems are also sensitive indicators of environ-mental change and evaluating their metabolic state and compo-sition allows monitoring biodegradation mechanisms. However,conditions of the soil environment are determinant for these pop-ulations’ distribution, composition and activity, hence the needto assess soil as a support for microbial growth and activities.

Inter-disciplinary and multi-parametric studies provide dif-ferent views for a better understanding of environmental pro-cesses (e.g., Allison and Martiny 2008; Brockman and Murray1997; Carson et al. 2009; Holden and Fierer 2005; Ladd et al.1996; Liu et al. 2006; Pascual et al. 2000; Waldrop et al. 2000).Although interactions between microorganisms and chemicalfactors have been largely addressed, microbial interactions withtheir physical environment have been less contemplated by re-search in environmental microbiology. Geology is a primaryfactor influencing microbial establishment, physiology andactivity.

Despite the presence of relevant biomass in the subsurface,soil microbiology has traditionally focused on superficial soils,due to their importance for the maintenance of soil fertility andhence life on our planet, which depends almost exclusively onmicrobial processes. The vadose zone has been less studied andprocesses are still poorly understood, especially in tropical soils.The need to elucidate important phenomena of geotechnical en-gineering demands a better understanding of the role of biologyand chemistry in soil properties and behavior.

Differently from superficial ones, subsurface soils presentgreater heterogeneity in the distribution of nutrients, carbon,water and oxygen, which leads to a greater variability in mi-crobiota composition and activity – which may prove of greatimportance when great amounts of anthropogenic carbon be-comes available (e.g. Balkwill et al. 1989; Bekins et al. 2001;Boivin-Jahns et al. 1996; Fredrickson et al. 1995; Fierer et al.2003). Besides, Brazilian residual soils present different char-acteristics from those of the Northern hemisphere, where mostof the studies have been conducted to date, as they frequentlypreserve the structure of the mother-rock, and display great het-erogeneity when not submitted to important weathering.

This research has been focusing a young residual soil fromRio de Janeiro, describing its main facies from a mineralogi-cal, chemical and geotechnical point of view and relating thesecharacteristics to those of the soil microbiota and its activity(see also Osterreicher-Cunha et al. 2004, 2007).

Besides their ubiquity in Brazil, residual soils are a commonfeature all over the world, being found mainly in tropical en-vironments of Africa, Asia and the America. It is well knownin Geotechnical Engineering that these soils do not respond assedimentary soils when transport/flow and mechanical proper-ties are concerned. The chemistry and mineralogy of these soilscan also be very different from sedimentary soils, but muchless is known about their microbiological aspects and how thosegeotechnical and biological characteristics are related, which isone of the focuses of the present paper.

The study described in this article assesses changes in soilphysical and chemical characteristics according to environmen-tal changes brought about by climatic conditions, as well asresponses from the different soil facies, trying to establish howthose changes may affect microbial activity and carbon cycling.Results link microbial parameters monitored along two years tosoil properties and show how the latter act on microbial popu-lations and vice-versa.

MATERIALS AND METHODS

Description of the Study AreaThis study was conducted in the metropolitan area of Rio de

Janeiro on a slope from a weathering profile of kinzigitic gneis-sic rocks, ubiquitous in the region. Residual soils are the endproducts of weathering processes in such rocks and they consti-tute the material being focused in the present work. Slopes inand around the city of Rio Janeiro and in south-eastern Brazil asa whole, are constituted mostly by such materials and thereforepresent great interest from the environmental point of view. Itis also noteworthy that little data is available in the literature onthe behavior of tropical soils and specifically on residual soils.Figure 1 is illustrative of the degree of heterogeneity found in aresidual soil profile, where there are no layers as are encounteredin sedimentary profiles. The distribution in volume (or area) ofsandy or silty facies is reminiscent of the mineral distribution inthe parent rock (gneiss in this case) and has therefore a randomlike distribution. Figure 1a is an example of how the sandy faciesis intertwined with the silty one, as the latter is predominant.

Meteorological DataInformation on climate, temperature and rainfall regime was

obtained on the Brazilian Meteorological Institute website (In-stituto de Meteorologia). Sampling was performed along twoyears during Summer, Autumn and Spring, covering rainy anddry seasons, in March and November 2006, and in January,March and October 2007.

Soil CharacterizationSoil Sampling. Soil samples of unsaturated soil were taken

from the slope at a depth of approximately 3 metres of theprofile. Surface soil was excavated to remove the top layer fromthe slope face. Composite samples (from five sub-samples) were

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FIG. 1. (a) Undisturbed soil sample showing silty facies, predominant, and sandy facies inclusions; (b) Soil facies as visible in a slope from the same area (colorfigure available online).

then taken horizontally from approximately 50 cm into the slope.Sampling was performed in both main facies found (describedbelow), one silty and the other sandy (Figure 1).

Geotechnical Characterization. Undisturbed and dis-turbed/remoulded samples were collected in the field to charac-terise chemical and physical characteristics of both facies.

Physical Indexes. Grain size curve, consistency indexes,porosity, gravimetric and volumetric water content, dry spe-cific weight and relative density of grains were determined ac-cording to the methodology described in the Brazilian Associa-tion of Technical Standards (ABNT – NBR, 1984, 1986. 1988,1990).

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Hydraulic Properties. Furthermore, tests were carried outin the field in order to determine hydraulic properties of thematerials characteristic of the slope being studied. The proper-ties determined were the characteristic or water retention curveand the saturated-unsaturated permeability curve. The field testscarried out consisted on an infiltration device kept under con-stant head (maintained by a Mariotte bottle) associated withthe measurement of the pressure head at a certain depth with atensiometer. A back analysis of the test using optimization pro-cedures and numerical analysis allows the determination of thepreviously mentioned properties (Morales 2008; Velloso et al.2006).

Chemical and Mineralogical Characterization. Chemicalanalyses of soil samples were performed at Embrapa—BrazilianAgricultural Research Coporation (Embrapa 1999). Mineralog-ical data were obtained through optical microscopy and X-raydiffraction analyses.

Available carbon was determined measuring contents of car-bon readily oxidised by potassium permanganate (KMnO4), ac-cording to the methodology described by Tirol-Padre and Ladha(2004), modified from the original Blair et al. (1995) method-ology. Soil samples were incubated in 33 mM KMnO4 solutionfor 60 minutes in an orbital shaker. After centrifugation anddilution, permanganate concentrations were measured against ablank containing no soil in a Merck Spectroquant spectropho-tometer at 565 nm, and carbon amounts calculated.

Microbial Characterization. Microbial activity—Total de-grading activity of soil microbiota was evaluated by quantify-ing the hydrolysis of fluorescein diacetate (FDA), followingan adaptation of the methodologies modified and describedby Adam and Duncan (2001) and Green et al. (2006). FDAis a molecule recognised and identified by most endo- andexo-enzymes present in soil; its hydrolysis releases fluoresceinwhich can be quantified by spectrophotometry. As this studycontemplates a subsurface soil, where neither plants nor faunaare present, most of the enzymatic activity measured originatesfrom soil microorganisms. Total microbial degrading activityprovides a general measure of organic matter turnover in nat-ural habitats, as about 90% of the energy in the soil environ-ment flows through microbial decomposers (Heal and McClean,1975).

In short: soil samples were incubated statically with FDAat 30◦C for 20 to 30 min right after collection in the field.The reaction was interrupted by addition of a 2:1 chloroform-methanol solution. After filtration, fluorescein present in thesupernatant was quantified by spectrophotometry at 490 nm(Spectroquant, Merck).

Protein synthesis was determined by measuring the incor-poration of tritium-labelled leucine (3H-leu) by soil microbialcells. Protein synthesis gives a good estimation of soil synthesisactivity, which may be related either to a biomass increase or toa shift in enzymatic activity, factors that can respond to changesin the environment (e.g. Anand et al. 2003; Badiane et al. 2001;Daubaras and Chakrabarty 1992; Holliger et al. 1997).

The analytical protocol used in this study was adapted byMiranda et al. (2007) from Baath et al. (2001). Bacterial cellsextracted from soil samples in water were incubated with 3H-leufor 2 h at 25◦C. The reaction was interrupted by adding 37%formaldehyde; after a cleanup step (trichloroacetic acid addi-tion, centrifugation and supernatant removal, twice; followedby ethanol and NaOH addition) and the pellets were incubatedfor 1 hour at 85◦C, after which the scintillation cocktail wasadded and radioactivity measured for calculation of leucine in-corporation by soil cells.

Microbial biomass—Shifts in biomass amounts may indicatemicrobiota adaptation to changes in its environment. Biomassis an efficient parameter to be evaluated against activity mea-surements as it allows determining whether activity increasesreverse into biomass production or if they are linked to enzymesynthesis.

Determination of biomass (microbial C) was performed ac-cording to a protocol developed at the Laboratory of Soil Mi-crobial Ecology, Brazilian Agricultural Research Corporation,based on the Jorgensen and Brookes (1990) methodology. Soilsamples were chloroform-fumigated to liberate microbial C;then ninhydrin reactive nitrogen (ninhydrin-N) was extractedfrom soil with 0.5 M K2SO4. Non-fumigated samples were simi-larly processed and constituted the controls, providing amountsof non-microbial C. Ninhydrin-N was then measured in soilsamples and carbon amounts were calculated from the amountsof N. Microbial C was obtained subtracting C amounts ofnon-fumigated from fumigated soil samples (Jenkinson 1988;Jorgensen and Brookes 1990).

Microbial quotient—It is represented by the ratio of biomassC to soil organic C (Cmic/Corg) and reflects the contribution ofmicrobial biomass to soil total organic carbon (Anderson andDomsch 1986). It also indicates the substrate availability to thesoil microflora or, in reverse, the fraction of recalcitrant organicmatter in the soil; in fact this ratio declines as the concentrationof available organic matter decreases.

Statistical AnalysisRelations between variables were analysed at 5% level

of significance by variance testing (multi-factor ANOVA).Tests were performed with OpenStat (http://statpages.org/miller/openstat/).

RESULTSClimatic variations during the study, linked essentially to

shifts in rainfall regimen and temperature, did not cause relevantchanges in soil physical and chemical characteristics. Temper-atures varied between 14 and 39◦C, according to season andrainfall. Rains were regular and higher during the first year,while much more irregular in 2007, alternating with very dryperiods (Figure 2); samplings were performed after periods ofheavy rains (March and November/06, January/07) and after alonger than usual dry period (October/07).

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FIG. 2. Accumulated rain and number of days with rain during 2006 and 2007 (adapted from INMET—National Meteorological Institute, Brazil). Arrowsindicate sampling dates.

Average soil water in the sandy facies was lower, varyingbetween 10 and 23% in volume, while oscillating between 19and 42% the silty soil (Figure 3). However, periods of irregularrainfall caused relevant shifts in soil water content, with anincrease of approximately 30% in water content in the siltyfacies and 50% in the sandy facies (Table 2). Variance analysisshowed significant (p < 0.05) differences among both facies aswell as between the sampling dates along the two-year study.

Alternate layers of each facies are found in the study site, re-maining from the structure of the mother rock, which is enrichedin biotite and feldspar (Figure 1a). Each facies covers large ar-eas of the slope surface, although the silty facies is predominantat the sampling depth (Figure 1b). Figure 4 shows grain size

distribution of both main facies: the sandy soil contains around60% sand while the silty one has approximately 72% of fineparticles (Table 1). Both soils present low clay content and totalorganic carbon (Corg) as well as differentiated porosity and per-meability properties; the void index is higher for the silty soil(Table 1), while plasticity is the most differentiated index. Thesandy facies presents a lighter colour with dark spots, whichindicate the presence of manganese oxide (Figure 1a). Besidesquartz, this facies comprises potassium feldspar and biotite inseveral alteration phases.

The silty facies is more homogeneous, presenting a reddishcolouration with whitish and greenish components, residues ofthe original rock. Differences in mineralogy between the soil

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FIG. 3. Soil degree of saturation throughout the two-year study (% volume).

FIG. 4. Grain size curves of sandy (left) and silty (right) facies (two samples of each, collected in different areas of the slope).

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MICROBIAL ACTIVITY OF HETEROGENEOUS TROPICAL SOIL 405

TABLE 1Chemical and physical characteristics of both soil facies.

Sandy Siltyfacies W facies R

Chemical characteristics (g/kg)pH 4.9 4.6Cationic Exchange Capacity 1.8 6.3Calcium Ca2+ 0 0Magnesium Mg2+ 2.833 2.5Potassium K+ 0.02 0.01Sodium Na2+ 0.05 0.073Sum: S 0.333 0.267Aluminium Al3+ 0.967 0.85H+ 1.433 2.467Sum: T 3.133 3.583Total organic carbon Corg 0.925 1.25Nitrogen N 0.15 0.2C / N 7 6Phosphorous P 1.5 1.167Available carbon Cav 0.158 0.334

Mineralogy (mg/kg)Silicates SiO2 164 259.8Aluminium oxides Al2O3 226.5 238.8Ferrous iron oxides Fe2O3 39.5 120Titanium oxides TiO2 2.1 14.33Potassium oxides K2O5 0.233 0.7Manganese oxides MnO 0.05 0.2Ki = SiO2/Al2O3 1.218 1.845Kr = SiO2/R2O3 1.098 1.395Al2O3/Fe2O3 9.688 3.13

Physical characteristicsGrain size distribution (%)

Coarse gravel 0.000 0.000Medium gravel 0.014 1.144Fine gravel 5.765 1.693Coarse sand 33.73 4.357Medium sand 12.69 5.327Fine sand 10.20 14.17Silt 16.83 31.38Porosity 44.9 48Grain density (g/cm3) 2.638 2.769Relative grain density (g/cm3) 2.642 2.774

Consistency limits (%)Liquid limit 53.75 69.98Plastic limit 32.91 44.07Plasticity index 20.85 25.91

Averages of three analyses.

facies do not seem significant except for contents of iron oxides(p = 0.0057), total organic carbon (Corg, p = 0.049) and avail-able carbon (Cav, p = 0.031), all higher in the silty facies. Shiftsin soil water content do not alter carbon availability in the sandy

TABLE 2Volumetric water content in both facies (%)

θ Sandy Silty

March 06 14.59 25.5212.44 29.74

November 06 17.72 41.4722.43 36.34

January 07 21.25 41.8722.81 37.14

March 07 10.56 27.9913.54 21.59

October 07 10.84 26.7313.45 19.56

soil as it happens in the silty one (Figure 5). Concerning soilavailable carbon, analysis of variance indicated significant dif-ferences between facies as well as between sampling dates (p <

0.01). Interactions between variables are also significant (p <

0.05), meaning that the sampling date influences the differencesin Cav in soil facies.

Water retention curves reflect the amount of water a soilstores at different times of the year. Permeability curves re-produce (under common gradients) the velocity of the waterinfiltrating or leaving the soil. Permeability curves of both fa-cies (Velloso et al. 2006; Figure 6a) show higher permeabilityand hence higher water velocities in the sandy soil when satu-rated, behavior that inverts, however, under unsaturated condi-tions when higher permeabilities occur for silty facies (hencehigher velocities); this is a characteristic frequently observedin residual soils that may lead to important changes in the soilenvironment according to the rain regimen.

As for water retention (Figure 6b), the silty facies tends toretain more water under unsaturated conditions than the sandyfacies, and, after rainy periods, water content in the sandy faciesis similar to the one in the silty facies in less humid periods.Analyses of variance show significant differences among soilfacies and among sampling dates (p < 0.001 in both cases).However, interactions are not significant (p = 0.431), indicatingthat sampling dates had no effect on differences of moisturecontent in soil facies.

Figure 7 shows the total degrading activity (FDA hydrolysis)of soil microbiota in both facies. During the first year of the studythe degrading activity was higher in the silty facies, while duringthe second year it increased in both soils, more so following rainyperiods and tending to drop at the end of the year. However, theactivity of protein synthesis (3H-leu incorporation—Figure 8)was higher in sandy soils, while microbial biomass measure-ments (Cmic—Figure 9) were also higher in the sandy facies ex-cept for March/07, a dry phase following a longer rainy one.

Biomass amounts remained stable in the silty facies, finerand more organic, showing an enhanced reaction to shifts insoil water content in the sandy soil. These results indicate a

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FIG. 5. Available to total carbon ratio on both facies along the study.

better assimilation of soil carbon in the sandy facies, despitethe lower total activity, as shown by the microbial quotient(Qmic—Table 3). The increase in soil water content duringthe second year reverted into higher degrading activity of soilmicrobiota in both facies, more accentuated though in the sandyfacies (6-fold increase) but longer-lasting in the silty one, stillhigher in March/07. Protein synthesis as well as biomass islower after the rainy periods, suggesting a lower efficiency incarbon assimilation by soil microorganisms, despite soil watercontent being similar in both periods.

Statistical analyses indicated significant differences in de-grading activities among soil facies and among sampling dates(p < 0.05); significant differences in protein synthesis amongsoil facies (p < 0.05) but not among sampling periods (p =0.134). Differences in biomass measurements between soil fa-cies are not significant (p = 0.167), but are significant amongsampling dates (p < 0.05); no significant interactions among

TABLE 3Microbial quotient (Qmic) in both facies (average of samples

from each facies)

FaciesDates Sandy Silty

March 06 9.20 2.22March 07 1.37 1.72October 07 4.39 1.57

variables for these three parameters were found (p > 0.1 in allcases), showing that time of sampling did not alter differencesamong facies concerning microbial activity.

DISCUSSIONThe data herein show how microbial biomass and activity

relate to amount, availability and assimilation of organic carbonas well as their distribution in both soil facies according to soilphysical and chemical characteristics.

Soil physical indexes show different potential behaviors inwater retention and circulation, which are of paramount impor-tance for microbial survival and communities’ selection.

A wide variety of soil properties have been correlated to theliquid and plastic limits, concerning mainly engineering issues.The plastic limit defines the moisture content where the soilchanges from a semi-solid to a plastic, flexible state, while theliquid limit is the moisture content where the soil changes froma plastic to a viscous fluid state. The moisture content of thesandy facies is lower than its plastic limit, characterising a stiffersoil, expected to be less responsive for microbial development.As for the silty facies, natural soil moisture regulates with theliquid limit, defining a softer soil in which more mobile watermolecules create a better environment for the microbiota. Freerwater molecules also allow better dislocation and circulation ofmicrobial cells and therefore more extensive colonization of soilenvironment, as well as more accessible carbon.

Relevant dissimilarities in grain size composition found inthe two main facies create different soil architectures, mainly

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MICROBIAL ACTIVITY OF HETEROGENEOUS TROPICAL SOIL 407

FIG. 6. (a) Permeability curves; (b) Characteristic curves, of both facies.

concerning pores size and distribution. Grain size and the ar-rangement of soil pores and particles have great effect on thebiological stability of Corg, given their regency over water andair availability, accessibility to organic matter by microorgan-isms and soil aggregation. Biodegradation processes occur in-side soil pores, hence the importance of their characteristics,

such as size and inter-connexions. Increasing water input andcontent inside soil pores accelerate microbial activities, becauseof water increased availability; however, oxygen may decreaseand become scarcer with this enhancement of microbial activity.

Higher rates of C mineralization are observed with largerpore sizes, as when progressing from clay to sandy soils, for

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FIG. 7. Total degrading activity in soil samples (mg of fluorescein formed. minute−1. soil gram−1). Two samples of each facies, average of three sub-samples.

equivalent levels of air-filled pores. However, each soil environ-ment presents an optimum balance between water and oxygenavailability where microbial mediated processes are optimised.Also, the presence of relevant amounts of clays increases or-ganic matter protection as smaller particles occupy pores and

prevent access of decomposers (Baldock and Skemjstad 2000).Differences between the two facies reflect on microbial param-eters because of a differentiated distribution and retention ofwater, air and of microorganisms, and thus on the protectionof organic matter. The higher proportion of fine particles in the

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FIG. 8. Activity of protein synthesis in soil facies from two samplings (March 2006 and March 2008). Two samples of each facies, average of three sub-samples.

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FIG. 9. Microbial biomass (Cmic) in soil samples during the study (µg Cmic. gram−1 soil).

silty facies may lead to a lower microbial activity during rainyperiods, because of lesser availability of oxygen in soil pores.

Likewise, mineralogy plays an important role in microbialprocesses. Soil minerals provide more than strictly the physicalsupport of an inert matrix but select for specific and differ-entiated populations according to their nature and distribution(Carson et al. 2009). Iron and available carbon (Cav) are bothhigher in the silty facies. Iron may be important for anaero-bic metabolism of carbon and for bacterial metabolism (Donget al. 2009; Margesin et al. 2000; Stucki et al. 2007), while theavailability of C is crucial for cell maintenance and biomassproduction (Feller and Beare 1997; Wang et al. 2003).

Water velocity and soil permeability have a crucial role onnutrients and microorganisms transport, and therefore on thelatter’s survival whether attached to the porous medium or insuspension in the liquid phase. This study contemplates unsat-urated soil as the soil profile did not saturate under the rainsnormally found in the region. As expected in finer or moreorganic soils, the silty facies contain most of the time higherwater contents than sandy soils; less oxygen is available thanin the sandy facies, which are therefore expected to be moreprone to aerobic biodegradation because of a more continuousavailability of oxygen existing in the air phase.

As shown by characteristic curves, the silty soils are underdifferent conditions of soil suction in drier periods from rainierones (less than 5,KPa and up to almost 25 KPa, respectively),while the sandy facies remain at all times under much higher soilsuction values. In phases of alternating dry/rainy conditions, thesilty soils retain more water after rain and are near saturation,meaning that the oxygen supply is limited to dissolved oxygen in

water. Moreover, in these situations (nov/06 and jan/07), giventhe inversion of hydraulic properties of this soil’s facies, thesandy soils present higher velocities and therefore an even bettercirculation of water and therefore of nutrients.

In phases of lower soil moisture, the silty soils have higherpermeabilities, hence better circulation of water and nutrients.Ongoing studies on modelling of water movements in this het-erogeneous soil confirms that under this rainfall regimen thesilty soil retains higher amounts of water, and that the sandyfacies tend to rapidly lose moisture when water input ceases.As an effect of this increase in soil moisture and changes insoil permeability, higher amounts of Cav are found only in thesilty soils, but microbial degrading activities increase only 2.5times in this facies, yet in the sandy soils a 4-fold increase wasmeasured. Decreased input of oxygen and nutrients impacts mi-crobial activity and, under these conditions, the sandy faciespresents a better condition for microbial survival and develop-ment with better water circulation and oxygen still obtainablefor aerobic metabolism.

Higher microbial activity in silty soils is not unexpected giventheir finer grain composition and organic content. Still, proteinsynthesis and microbial biomass are higher in the sandy facies,revealing a more efficient microbiota in this soil, although poorerin Corg and water, when C assimilation is concerned. Whenunder conditions of higher soil moisture and higher degradingactivity, synthesis of microbial carbon decreases (two orders ofmagnitude), as does microbial biomass, showing an impact onsoil microbiota.

The increase of microbial degrading activity observed in bothfacies with higher soil water content can be explained by nutrient

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410 P. OSTERREICHER-CUNHA ET AL.

input which can be better used in the sandy soil were oxygenis still available. But lower protein synthesis and biomass af-ter rainy periods suggest lesser carbon assimilation. Biomassremains constant in the silty facies, seeming a more sensitiveparameter to shifts in soil water content in the sandy facies.

Water regimen and temperature variations, as determinedduring the study, did not alter soil physical and chemical char-acteristics, demonstrating the buffering effect of the subsurfaceenvironment against external impacts in such a time span. Car-bon mobilised with higher water contents in the silty faciesmay be due either to biological activity or to physical-chemicalinteractions.

Soil organic matter can be protected from biological attack onseveral levels, according to soil properties as well as its structure(architecture of soil matrix and distribution of mineral particles,presence of multivalent cations, orientation of functional groups,high specific surface area of particles—Baldock and Skjemstad2000; Feller & Beare, 1997; Oades, 1988; Wang et al., 2003),and organic carbon in soil has complex and variable effectson water retention, because of its physical characteristics aswell as its influence on microbial activities (Holden and Fiere2005; Lavelle 2002; Mitchell and Santamarina 2005; Yang et al.2005). As both facies in this study present similar mineralogy,cation effects on organic matter may not be relevant. However,with a higher amount of clay, the silty facies would be expectedto better protect Corg from mobilization by the biota. Whenwater content increases and carbon is more available, degradingactivities are also higher. Yet, protein synthesis and microbialbiomass are, on the contrary, lower, showing that water contentdoes not revert into biomass production, not even in the siltyfacies where available carbon increases. In the sandy facies,poorer in organic carbon, Cav remains constant throughout shiftsin soil water content, suggesting either that it is not mobilisedor that it is drained.

Temperature did not seem to act on the microbial parame-ters measured in this study, which agrees with the subsurfaceorigin of the soil. Soil moisture rules microbial activity andcarbon cycling as it varies according to climatic conditions(Baker et al. 2009). Under normal temperature and moistureconditions, soil respiration rates seem to be controlled and lim-ited by substrate availability rather than the size of microbialbiomass, as the availability of substrate is determinant for soil re-sponse to environmental factors (de Nobili et al. 2001; Liu et al.2006).

The relation of microbial activity to total biomass dependson the status of equilibrium of the ecosystem (Odum 1983).Thus, this ratio decreases in soil as a state of equilibrium isattained with the growing efficiency of the microbiota that leadsto less C being lost as respiration-linked CO2 and more C be-ing sequestered into biomass. The ratio Cmic/Corg—as well asNmic/Norg—expresses the nutritional quality of the availableorganic matter. Soils with poor quality organic matter generallyhost stressed microbial communities, status in which they areunable to assimilate C and N or assimilate them with lesser

efficiency and the mentioned ratios tend to decrease, while insituations where the organic matter presents good nutritionalquality, microbial biomass (Cmic) can increase even if total Cremains constant in the environment (Gregorich et al. 2000;Lavelle 2002).

Our measurements show a higher Qmic in the sandy soils(although not significantly different), while alternance of rainyand dry episodes leads to a drop in Qmic values, more drastic inthe sandy facies, that takes both facies to a similar condition ofefficiency. Corg appears more allocated in the biomass of sandysoils during periods of more constant rainfall regime, whilethe decrease in Qmic suggests a stressed microbiota after rainyperiods. The silty facies seems to host a steadier microbiota, lesssensitive to changes in water regimen, showing little variationsin Qmic throughout the study.

Our data suggest that higher water retention stimulates micro-bial degrading activity, more so in the sandy facies were oxygenis still available, not reverting into biomass, which could alsobe due to poor soil organic matter quality (Blume et al. 2002;Jardine et al. 2006; Krumholz 2000; Ranjard and Richaume2001; van Gestel et al. 1996; Wang et al. 2003; Yang et al.2005). Dissolved C could also be more rapidly drained from thesandy facies and thus not be measured. Understanding whichprocesses are determinant here requires further studies.

Carbon cycling from natural organic matter may providegood information about the biodegrading potential of anthro-pogenic carbon, as it is linked to the microbiota’s capacity toadapt to the presence of xenobiotic molecules and therefore en-sure their degradation. Previous studies have shown the capacityof the studied soil to biodegrade several organic compounds ofxenobiotic origin. Ongoing research aims to evaluate the linkbetween the efficiency of this soil to use natural carbon and itspotential for the bioremediation of xenobiotic contamination.This knowledge will also help establish procedures and tech-nologies best suited to remediate contaminated tropical soilsunder their specific climatic conditions, taking into considera-tion their unique characteristics and properties.

Further investigations could help understanding how theseinteractions act on microbial C assimilation and biomass con-struction, and which conditions favours what processes.

CONCLUSIONSThe present article describes a study of a natural residual soil

from a tropical region that determines how climatic conditionsaffect soil characteristics thus acting on soil microbiological car-bon cycling. This research relates the characteristics of soil mainfacies to those of the microbiota, linking biomass and activityto carbon amounts, availability and use, as well as microbialdistribution to soil physical-chemical characteristics.

Data indicate that higher and more evenly distributed rain-fall did not cause important shifts in microbial activities, indi-cating an equilibrium status of the microbiota; while alternat-ing dry/wet periods, even with less water input, led to more

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MICROBIAL ACTIVITY OF HETEROGENEOUS TROPICAL SOIL 411

important impacts on soil microbiota, which reacts in order toadapt to changes in its environment.

The two main facies of the soil present differentiated re-sponses to shifts in water input, basically because of their differ-ences in structure and given the particular properties of the resid-ual soil that alter water permeability differently in each facies.

Differences in microbial parameters in both facies seem toindicate that soil structure is the main factor affecting microbialactivity in the time span of this study. Different responses tochanges in soil water content are probably due to the role of soilstructure on the distribution and movement of water and air, aswell as on the input of nutrients from the surface and carbonavailability.

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