Accumulation of atmospheric sulfur in some Costa Rican soils

Accumulation of atmospheric sulfur in some Costa Rican soils

Carleton R. Bern1,2,3 and Alan R. Townsend1,2

Received 18 January 2008; revised 4 April 2008; accepted 17 April 2008; published 1 July 2008.

[1] Sulfur is one of the macronutrient elements whose sources to terrestrial ecosystemsshould shift from dominance by rock-weathering to atmospheric deposition as soils andunderlying substrate undergo progressive weathering and leaching. However, the natureand timing of this transition is not well known. We investigated sources of sulfur totropical rain forests growing on basalt-derived soils in the Osa Peninsula region of CostaRica. Sulfur sources were examined using stable isotope ratios (d34S) and compared tochemical indices of soil development. The most weathered soils, and the forests theysupported, are dominated by atmospheric sulfur, while a less weathered soil type containsboth rock-derived and atmospheric sulfur. Patterns of increasing d34S with increasing soilsulfur concentration across the landscape suggest atmospheric sulfur is accumulating, andlittle rock-derived sulfur has been retained. Soil sulfur, minus adsorbed sulfate, iscorrelated with carbon and nitrogen, implying that sulfur accumulation occurs as plantsand microbes incorporate sulfur into organic matter. Only the lower depth increments ofthe more weathered soils contained significant adsorbed sulfate. The evidence suggests apattern of soil development in which sulfur-bearing minerals in rock, such as sulfides,weather early relative to other minerals, and the released sulfate is leached away. Sulfuradded via atmospheric deposition is retained as organic matter accumulates in the soilprofile. Adsorbed sulfate accumulates later, driven by changes in soil chemistry andmineralogy. These aspects of sulfur behavior during pedogenesis in this environment mayhasten the transition to dominance by atmospheric sources.

Citation: Bern, C. R., and A. R. Townsend (2008), Accumulation of atmospheric sulfur in some Costa Rican soils, J. Geophys. Res.,

113, G03001, doi:10.1029/2008JG000692.

1. Introduction

[2] The sources of nutrient elements to terrestrial ecosys-tems exert fundamental controls on how those ecosystemsfunction [Likens and Bormann, 1995; Schlesinger, 1997].Many recent studies have focused on calcium and sought todetermine its origin by using the isotopic tracer strontium asa proxy [Bern et al., 2005; Kennedy et al., 2002; Perakis etal., 2006; Porder et al., 2006]. Such tracer studies, andthose using mass balance techniques, show that sources ofnutrients undergo a predictable shift over geologic time-scales in many ecosystems [Chadwick et al., 1999]. Early insoil development, weathering supplies ample quantities ofrock-derived nutrients relative to plant requirements. Inenvironments where precipitation exceeds evapotranspira-tion, weathering and leaching progressively deplete themineral stocks of more soluble elements. Given sufficienttime, the small amounts of the same elements entering thesystem via atmospheric deposition become the dominantsource of formerly rock-derived nutrients.

[3] Sulfur is a macronutrient element that can be suppliedto ecosystems in appreciable quantities by both rock weath-ering and atmospheric deposition, yet the proportions ofthose source contributions have not been examined in manysettings. Instead, the large anthropogenic flux of sulfur intothe atmosphere from industrial sources has focused much ofthe study of sulfur on its role as a pollutant and componentof acid rain [Adriano and Havas, 1989]. Sulfur cycling inundisturbed ecosystems of the tropics has been particularlyunderstudied [Mitchell et al., 1992]. Sulfur research in thetropics has often focused on agro-ecosystems [Fritzsche,2005], or been incorporated into studies of all the majornutrients [Hedin et al., 2003; Hughes et al., 1999]. Certainareas of the tropics have been identified as having so littletotal [Acquaye and Beringer, 1989; Kang et al., 1981], orplant-available soil sulfur [Hue et al., 1990] that it may limitproductivity.[4] While sulfur deficiency in ecosystems is uncommon,

its causes can be numerous. Low atmospheric depositionrates contribute to deficiency in some locations [McGrathand Zhao, 1995]. Sulfate (SO4

2�), the dominant dissolvedinorganic form of sulfur, is poorly retained against leachinglosses by sandy soils and leaching of sulfate is the majorsulfur loss pathway from soil [Tabatabai, 1984]. In contrast,soils with low soil solution pH values, large reactive surfaceareas, and high in crystalline or amorphous Fe and Alsesquioxide contents can adsorb sulfate so efficiently that

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, G03001, doi:10.1029/2008JG000692, 2008

1INSTAAR: Earth and Environmental Systems Institute, University ofColorado, Boulder, Colorado, USA.

2Department of Ecology and Evolutionary Biology, University ofColorado, Boulder, Colorado, USA.

3U.S. Geological Survey, Denver, Colorado, USA.

Copyright 2008 by the American Geophysical Union.0148-0227/08/2008JG000692

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little remains in solution for plant uptake [Alves andLavorenti, 2004; Hasan et al., 1970]. Fire reduces sulfuravailability by volatilizing sulfur from biomass [Acquayeand Beringer, 1989; Ewel et al., 1981]. Harvest and exportof biomass can also remove significant quantities of sulfur[Soloman et al., 2001; Tabatabai, 1984]. With the advent ofpollution control technologies, anthropogenic sulfur emis-sions are declining in some parts of the world, even as theyincrease in others [Streets et al., 2003]. Sulfur deficiency isnow a risk in some areas where atmospheric depositioninputs have declined [McGrath and Zhao, 1995]. Moregenerally, an understanding of the sources and cycling ofecosystem sulfur is important as patterns of land-use,climate, and sulfur emissions shift in the coming decades.[5] Here we trace the origins of sulfur in tropical rain

forests on the Osa Peninsula of Costa Rica and comparethem to the degree of soil development. This paper exam-ines soil sulfur in detail at sites where ecosystem sulfursources were previously compared to those for strontium[Bern et al., 2007]. Hillslope and alluvial soils in theseforests receive similar atmospheric inputs and have acommon basalt parent-material, but have undergone differ-ent amounts of chemical weathering. Stable sulfur isotoperatios allow the origins of sulfur from weathering andatmospheric deposition to be traced directly. Comparisonto the degree of soil development, as indicated by chemistryand mineralogy, is used to infer how sources and retentionof sulfur change during pedogenesis.

2. Methods

2.1. Site Descriptions

[6] Study sites were lowland tropical rain forests locatedacross the Osa Peninsula of Costa Rica (Figure 1); thesehave a mean annual temperature of 26�C and receive annualprecipitation of �5 m, with a short but distinct dry season inJanuary through March [Cleveland and Townsend, 2006].The soils are well drained and lack strong redoxymorphicfeatures in the soil profiles sampled. All sites are underlainby the cataclastic Osa basaltic complex [Berrange andThorpe, 1988; Hauff et al., 2000], also referred to as theOsa melange due to the occasional presence of sandstone,limestone, and chert [Sak et al., 2004]. The Osa basaltcomplex is a Mid Ocean Ridge Basalt in composition, andis considered to be an extension of the Nicoya ophiolitecomplex that accreted onto the Caribbean plate [Berrangeand Thorpe, 1988]. Based upon isotopic and immobile traceelement ratios, basalt without significant quantities of sed-imentary rock is considered to be the parent material for thesoils sampled in this study [Bern et al., 2005].[7] The study focused on forests growing on alluvial soil

and hillslope soil at two sites at Rancho Mariposa (RM),near the town of Progresso. Hillslope soil (RMH) occurs onsteep slopes and ridges, and appears to have formed fromin-place basalt weathering. Alluvial soil (RMA) has devel-oped on basalt-derived alluvium at the mouth of a valley.The two sites are <1 km apart. The hillslope and alluvialsoils have been respectively described as Ultisols andMollisols [Perez et al., 1978]. Five additional sites withbasalt-derived hillslope soils were sampled across the OsaPeninsula. Fila Ganado (FG) is just up-valley (<1 km) fromRM. Agua Buena (AB), Suital Lodge (SL), Playa Cativo

(PC), and Punta Adelas (PA) are further away (Figure 1),but all have similar climate, vegetation, parent material, andtopography to the RMH site [Kappelle et al., 2003].

2.2. Sample Collection and Preparation

[8] Two soil profiles were collected from each of theprimary RM sites. Profiles were sampled in 10 cm incre-ments because soils lacked easily field-identifiable horizons.Additional samples of surface soil (0–10 cm) were collectedat both sites. An 8m tall vertical cut slope at RMwas sampledat 0.5 m intervals to assess deeper soil that grades intosaprolite with more rock-like texture at �5.5 m depth. The8 m profile was capped by a typical hillslope soil and createdby headward erosion of a spring into a slope. Hand samples ofbasalt were collected from outcrops exposed by streamerosion. A bulk precipitation collector with all internalsurfaces pre-washed with trace metal grade HNO3, wasinstalled in an open area at RM. The collector consisted ofa 71 mm diameter polypropylene funnel, 2 m off the ground,connected to a 6 L polypropylene reservoir by vinyl tubing.The tubing and reservoir were wrapped with aluminum foil toexclude sunlight and reduce algal growth. Leaves werecollected during the dry season from canopy emergentBrosimum utile, Schizolobium parahyba (legume), Caryocarcostaricense, and Hyeronima alchorneoides using slingshottechniques. Additional samples of B. utile and S. parahybawere collected during the wet season.[9] A 1 m soil profile was collected from a ridge top and a

mid-slope position at each of the five additional sites.Leaves of B. utile were collected from trees near the soilpits and basalt hand samples from erosion exposures.[10] Leaves, soil, and saprolite were oven-dried at 50�C.

Leaves were ground by Wiley Mill. Soil (sieved to <2 mm)and saprolite were ground by agate mortar and pestle. Basaltsamples were cut to remove weathered rinds and majorfractures containing oxidized material. Blocks of relativelyunweathered basalt were pulverized by steel mortar andpestle. Bulk precipitation samples were evaporated on a hot-plate in a laminar flow hood to yield a solid residue.

2.3. Sulfur Concentration and Isotopic Analysis

[11] Sulfur isotopic measurements were made at the U.S.Geological Survey Stable Isotope Laboratory in Denver,Colorado. An ECS4010 Elemental Analyzer (CostechAnalytical Technologies Inc.) was interfaced to a FinniganGas Bench II and then to a Delta XP (ThermoFinnigan,Bremen, Germany) isotope ratio mass spectrometer operatedin continuous flow configuration. The use of trade names isfor descriptive purposes only and does not constitute en-dorsement by the U.S. government. SO2 generated by samplecombustion was separated from CO2 and other combustiongases by a gas chromatographic column, then cryo-focused ina liquid nitrogen trap on the Gas Bench II, and subsequentlyinjected into a helium stream which delivered it to the massspectrometer [Fritzsche and Tichomirowa, 2006]. Samplesand standards were weighed into tin capsules along with 1–2 mg V2O5 as a combustion aid, dried overnight at 50�C, andstored in a desiccator prior to analysis. Sample sizes variedbetween 1.5 and 25 mg depending on sulfur content. Amemory effect was observed where the measured d34S of asample could be influenced by the d34S of the preceding

G03001 BERN AND TOWNSEND: ATMOSPHERIC SULFUR IN SOILS

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sample. Therefore samples expected to have similar isotoperatios were grouped in analytical sequences or re-analyzed.[12] Sample gases generated by combustion were mea-

sured against SO2 reference gas from one side of the dual-inlet system. Resulting data were expressed as d34S valuesand adjusted to the Vienna Canyon Diablo Troilite (VCDT)scale using international mineral standards NBS 127(+21.1%) and IAEA-SO-6 (�34.05%) and internal inor-ganic standards previously calibrated against these [Coplenet al., 2002]. Calibration against inorganic standards, andthe incorporation of organic matter structural oxygen intosample SO2 gas, could have shifted the d34S of organicsamples 0.3 to 2.9% higher as described by Yun et al.[2005] and discussed later. Sulfur concentrations werecalculated based on peak areas measured by the massspectrometer. NIST 2710 Montana soil (0.240% S) wasused as an elemental standard. Uncertainty for unknowns is±10% of the measured sulfur concentration, and ±0.6% ford34S values (1s), based upon repeated analyses in multipleanalytical sequences. The d34S isotopic value produced forNIST 2710 from these analyses was +2.7 ± 0.5%.

2.4. Soil and Rock Analysis

[13] Adsorbed and water soluble sulfate were removedfrom some soil samples by bicarbonate extraction using0.04 M NaHCO3 [van Stempvoort et al., 1990]. Five gramsof soil were shaken for 30 min with 25 mL of extractant,centrifuged to remove the supernatant, and freeze-dried. Sixsamples were extracted using a scaled-up procedure and theextracted sulfate was precipitated as BaSO4 for d34S anal-ysis. The proportion of sulfides relative to total sulfur in

basalt samples was determined by wet chemical extraction[Tuttle et al., 1986]. Chemical composition of soils andbasalts were determined by WDXRF and EDXRF by SGSMineral Services, Activation Laboratories, or USGS, Den-ver, Colorado. Al, Fe, and Si were extracted using dithionite-citrate and acid ammonium oxalate [Ross and Wang, 1993]and measured by ICP-OES. Soil pH was measured on 1:1mixtures of soil and deionized water. Soil mineralogy wasassessed using standard techniques and a Siemens D500x–ray diffraction system with Cu K-alpha radiation and agraphite monitor at USGS, Boulder, Colorado, and thedata were analyzed using the ROCKJOCK software[Eberl, 2002]. Soil particle size was determined by thepipette method after treatment with H2O2 [Gee andBauder, 1986]. Weight percent carbon and nitrogen inair-dried soil were measured on a Thermo Quest EA1110 CHN analyzer at the University of Colorado withan accuracy of ±0.03 weight percent for nitrogen and ±0.4weight percent for carbon.

2.5. Basalt Microscopic Analysis

[14] Chips of relatively unweathered basalt were exam-ined using a JEOL 6460-LV scanning electron microscope(SEM). Operating conditions were 15 keV, 1–2 nA beamcurrent, and 10 mm working distance. Secondary andbackscattered electron images were collected. Energy dis-persive spectrometry (EDS) was used to chemically char-acterize phases of interest. X–ray mapping was used toidentify the extent of sulfur-containing minerals within thesample.

2.6. Calculations and Statistical Analysis

[15] Because the chemistry of the parent material of thesesoils is relatively well constrained, the extent of weatheringand development can be examined from the perspective ofdepletion or accumulation of different elements. An Eluvi-ation/Illuviation Coefficient (EIC) can be calculated forweathered material from elemental data by the equation(rearranged for simplicity):

EIC ¼ Sh=Xhð ÞSp=Xp

� � ð1Þ

where Sh and Sp represent the concentration of the elementof interest in the depth increment and parent-materialrespectively, and Xh and Xp represent concentration of animmobile index element in those same materials [Muir andLogan, 1982]. EIC describes the net change of the contentof a given element in soil relative to the index element. Netaccumulation is indicated by values >1, and depletion byvalues <1. A value of zero indicates complete removal ofthe element of interest. The results of EIC calculation areequivalent [Vidic, 1994] to those produced by the tau (t)calculation [Brimhall et al., 1992] and account for changesin bulk density associated with soil dilation or collapse.Statistical analyses were conducted using StatView [SASInstitute Inc., 1998].

3. Results

3.1. Soil Development

[16] The alluvial and hillslope soils at Rancho Marposashare similar parent material, climate, and vegetation, yet

Figure 1. Map of the Osa Peninsula showing samplinglocations and geology. Inset shows the location of the OsaPeninsula on the Central American Isthmus. Principalsampling location is Rancho Mariposa (RM). Supplementalsites are Fila Ganado (FG), Agua Buena (AB), Suital Lodge(SL), Playa Cativo (PC), and Punta Adelas (PA). Geologicformations are the basalt-dominated, Cretaceous/PaleoceneOsa Melange (Kom), undifferentiated Tertiary/QuaternarySediments (TQs), PleistoceneMarenco Formation (Qm), andQuaternary alluvium (Qal). Map based on Sak et al. [2004].


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numerous indices indicate that the hillslope soil has under-gone more weathering and leaching. The hue of the hill-slope soil is much redder than the alluvial soil (Table 1).The index Fed-Feo indicates this is because the hillslope soilhas more Fe is sequestered as the secondary mineralshematite and goethite. The ratio of dithionite extractableFe to total Fe in soil (Fed/FeT) is a measure of the proportionof Fe sequestered in secondary soil minerals, and is alsohigher in the hillslope soil. In contrast, the alluvial soil has ahigher proportion of amorphous Si (Sio/SiT), a metric thatshould decline during maturation of a basalt-derived soil[Chorover et al., 2004].[17] The hillslope soil has a finer, clay texture (4 ± 1%

sand, 16 ± 3% silt, 80 ± 4% clay) while the alluvial soil hasa clay loam texture (24 ± 8% sand, 36 ± 3% silt, and 40 ±8% clay). Soil texture does not trend or vary significantlywith depth to 1 m in either soil.[18] Quantitative mineralogy is also indicative of greater

weathering and leaching in hillslope versus alluvial soil(Table S1, available as auxiliary material).1 Hillslope soil atFG contained more kaolinite, gibbsite, goethite, and maghe-mite compared to the alluvial soil at RM. In contrast, thealluvial soil contains more smectite, x–ray amorphousminerals, and feldspars. Quartz is present in the alluvialsoil and likely derived from Osa basalt, which also containstrace quantities (S. Sutley, USGS, unpublished data).[19] EIC calculations to determine element gains and

losses require the selection of an immobile element, towhich gains or losses of other elements are indexed.Comparison of results using Ti, Zr, and Nb as indexelements [Kurtz et al., 2000] for Rancho Mariposa soilsfound all to be quite similar. Choice of parent materialcomposition in EIC calculations can be challenging due torock heterogeneity. In addition to calculations using themean composition for basalts collected at Rancho Mariposa,the full range of sample compositions for basalts were usedto calculate minimum and maximum EIC values. Using Tias the index element produced the narrowest ranges ofvalues and those results are presented, along with the valuecalculated using mean basalt composition (Figures 2 and 3and Table S2).[20] Greater depletion of more mobile elements demon-

strates more extensive weathering of the hillslope soil. Ca,

Mg, Na, and K depletion from the hillslope soil is nearcomplete, while the alluvial soil EIC values are generally0.3 or greater (Figure 2). The net increase for K in thealluvial soil is likely a function of low basalt K contentcombined with alluvial sorting of resistant K-feldspargrains. Support for this idea is provided by the presenceof measurable anorthoclase in the alluvial soil, and high EICvalues for Ba, an element concentrated in K-feldspars [Deeret al., 2001]. Depletion of silicon is a classic indicator ofweathering status because it can be lost by degrees throughthe sometimes continuous weathering sequence from pri-mary minerals, to 2:1 clays, to 1:1 clays, to sesquioxides[Chadwick and Chorover, 2001]. The hillslope soil appearsto have lost half or more of the original basalt silicon. Incontrast, the alluvial soil has undergone little to no desili-cation, although alluvial sorting of quartz grains could beinfluencing this pattern. Depletion patterns for the hillslopesoil profiles extend into the deeper saprolite. EIC values forFe and Al indicate no depletion in the alluvial soil and slightto negligible depletion in the hillslope soil, suggesting littleredox-driven mobilization of these elements despite highannual rainfall (Figure 3).[21] EIC values for sulfur have a particularly wide range

due to variability of sulfur concentrations in the basaltparent-material. Despite this, it is apparent that both thealluvial and hillslope soils have lost substantial sulfurrelative to the parent-material. Results for sulfur can becompared to those for P, which can also occur as acomplexed anion in solution and be retained by adsorption.Total P is depleted by less than half, with little differencebetween the soil types. Mobility of the minor element Ba isalso deserving of consideration, because Ba mobility can bestrongly linked to sulfur by precipitation of the sparinglysoluble mineral barite (BaSO4) [McBride, 1994]. Precipita-tion of barite may explain why Ba has not been lost from thehillslope soil or saprolite, where losses of K argue againstK-feldspar presence. While barite precipitation may driveBa retention in soil, low concentrations limit Ba to a minorrole in sulfur retention here.[22] All indices examined show that the Ultisols are more

weathered than the alluvial soil. Topographic distributionsof the soils obviously differ [Perez et al., 1978], implyingthat drainage could explain differences in weathering.Another possibility is that the alluvial soil has had less timefor development than the hillslope soil. Tectonic uplift rateson the Osa Peninsula abruptly increased from near zero to

Table 1. Selected Soil Chemical and Color Data for Profiles From Rancho Mariposaa

Soil

Acid Amm. Oxalate Extractable Dithionite Citrate Extractable

% Al % Si % Fe % Al % Si % Fe

Hillslope 0 cm 0.4 0.1 1.3 1.2 0.5 10.7Hillslope 50 cm 0.5 0.2 0.8 1.4 0.5 11.5Alluvial 0 cm 0.5 0.3 0.9 0.5 0.7 5.5Alluvial 50 cm 0.6 0.9 0.7 0.4 0.8 4.5

Soil pH H2O Moist Color Fed-Feo Fed/FeT Sio/SiT

Hillslope 0 cm 5.2 5 YR 4/6 9.4 0.76 0.01Hillslope 50 cm 4.9 2.5 YR 4/6 10.6 0.75 0.01Alluvial 0 cm 6.1 7.5 YR 3/3 4.7 0.52 0.02Alluvial 50 cm 5.7 10 YR 4/4 3.9 0.37 0.05

aExtract percentages are relative to air dry soil. Fed = dithionite extractable Fe; Feo = oxalate-extractable Fe; FeT = total iron measured by XRF; Sio =oxalate-extractable Si; SiT = total Si measured by XRF.

1Auxiliary materials are available in the HTML. doi:10.1029/2008JG000692.


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relatively rapid (6.5 mm yr�1) at 32 ka [Sak et al., 2004]. Ithas been suggested that such a history may have placed thecurrent landscape in a transient state and therefore thealluvial soil parent-material may be relatively recent depos-its produced as rejuvenated streams cut aggressively into theregolith and bedrock [Bern et al., 2007].

3.2. Atmospheric Sulfur Deposition

[23] Based on concentrations of sulfur in bulk depositioncollected at RM (Table 2), it is estimated that 4.5 kg S ha�1

yr�1 is deposited there. This is about half the deposition rateof 7.5–12.5 kg S ha�1 yr�1 at La Selva, Costa Rica[Eklund et al., 1997; Johnson et al., 1979] or the 8.9–9.3 kg S ha�1 yr�1at slightly higher elevation Turrialba,Costa Rica [Hendry et al., 1984; Johnson et al., 1979].Much of the sulfate deposition at La Selva was linked tovolcanic emissions, and Turrialba is located in a valleybetween volcanoes [Eklund et al., 1997]. Thus, the lowersulfur deposition rate on the Osa Peninsula could be

Figure 2. Elluviation/illuviation coefficient (EIC) profiles for Si, Ca, Mg, Na, and K calculated using Tias the index element. The shaded area represents values generated using the full range of basalt chemistry,and the solid line represents mean basalt chemistry. The dashed line marks where no net loss or gain hasoccurred (EIC = 1).

Figure 3. Elluviation/illuviation coefficient (EIC) profiles for Al, Fe, P, Ba, and S calculated using Ti asthe index element. The shaded area represents values generated using the full range of basalt chemistry,and the solid line represents mean basalt chemistry. The dashed line marks where no net loss or gain hasoccurred (EIC = 1).


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attributed to greater distance from volcanic sources and lessvolcanic activity directly upwind. However, the excess ofsulfur deposition relative to Mg in sea-salt ratios [Keene etal., 1986] is similar at all three sites (La Selva non-seasalt-sulfur = 72%, Turrialba = 76%, and RM = 80%) suggesting alower deposition rate at RM for both elements.[24] Sulfur in bulk atmospheric deposition at RM has a

mean d34S of 10.4%. The sea-salt component of that sulfurshould have a d34S value of +21.0% [Rees et al., 1978] andby mass balance the excess sulfur is�+7.8%. Likely sourcesof excess sulfur include volcanic emissions of SO2 from theCentral American volcanic section. A global mean d34S valueof +5.6% has been estimated for such emissions [Nielsen etal., 1991]. Reduced sulfur gases emitted by biological marineand terrestrial sources are oxidized to sulfate in the atmo-sphere and contribute to deposition, but their d34S values areless known. In 1977, a mean d34S value for Pacific andAtlantic Ocean precipitation was calculated to be +13.3%[Chukrov et al., 1980], a value not far removed from thatmeasured in the relatively pristine coastal environment atRM.

3.3. Parent-Material Sulfur

[25] The range of sulfur concentrations expected forunaltered, sub-aqueously extruded, mantle-derived basaltare 600–1700 ppm, with d34S values of +0.3 ± 0.5% [Sakaiet al., 1982; Sakai et al., 1984]. Results of whole-rockanalysis of basalts collected on the Osa Peninsula show amuch greater range of both sulfur concentration and d34S,with the majority of isotope ratios being quite low (Table 2).Chemical extraction found the vast majority of sulfur in Osabasalts to be present as sulfides, as would be expected[Sakai et al., 1984]. However, SEM/EDS analysis of sawcut chips of sample RFR 4, with extremely negative d34S,revealed that sulfur was concentrated in a distinct mineral

phase in and around abundant small fractures. The mineralwas tentatively identified by EDS as pyrite (FeS2), andoccurred as larger crystals within what can be recognized aszones of possible silicon alteration in SEM backscatteredelectron images (Figures 4a and 4b). In places, the mineralalso occurs as smaller crystals outside the zone of visiblealteration (Figure 4b). Some fractures also contained a Ca-Smineral tentatively identified as gypsum (Figure 4b).

Table 2. Total Sulfur Concentrations, and d34S VCDT Values of

Basalt Whole-Rock Analyses and Bulk Precipitation Samples

Collected for the Study

SiteBasaltSample

S(mg/kg)

d34SVCDT (%)

Rancho Mariposa RFR 1A 880 �29.9RFR 2 1000 �23.5RFR 4 2460 �31.9RFR 5 270 �18.7RPR 960 �12.2

Rancho Mariposa Average 1100 ± 800 �23.3 ± 8.1

Agua Buena ABR 1 260 �15.5ABR 3 350 +0.3

Suital Lodge SRW-04 560 �24.9SRS-04 1160 �2.3

Playa Cativo CR-04 40 �16.6Punta Adelas AR-04 1560 �5.7

Osa Peninsula Average 900 ± 700 �16.4 ± 10.8

Rancho Mariposa Bulk PrecipitationMar ’04 – Jul ’04 0.12 +9.6Jul ’04 – Feb ’05 0.06 +10.6Mar ’04 – Feb ’05 0.07 +10.7Sep ’05 – Jan ’06 0.10 +10.7

Bulk Precipitation Average 0.08 ± 0.03 10.4 ± 0.5

Figure 4. (a, b) Backscattered electron SEM images ofchips from basalt sample RFR4. High atomic number (Z)phases are brighter than those containing lower Z. Tentativemineral identifications are pyrite (Py) and gypsum (Gp).

Figure 5. Scatterplot of sulfur and carbon concentrationsin Rancho Mariposa soil profiles and surface samples afterextraction with 0.04 NaHCO3.


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[26] Concentration of sulfur along the abundant fracturesin these basalts suggests open-system behavior subsequentto solidification. The d34S of pyrite associated with oceanfloor hydrothermal systems is expected to be 0 to 10%[Shanks, 2001]. Highly negative d34S values are bestexplained by the involvement of microbial sulfate reduction,which is associated with fractionations as large as �46%

[Thode, 1991]. A potential source of low d34S sulfur wouldhave been the reduced marine sediments overlaying thebasalts while part of the ocean floor and after uplift, and insitu microbial reduction of sulfate is also possible. Investi-gating the complex alteration history of the Osa basalts wasbeyond the scope of this study, though we note that the d34Sof sulfur released during parent-material weathering in this

Figure 6. Soil sulfur concentrations and d34S VCDT values plotted by depth for soil profiles fromRancho Mariposa. Diamonds designate data for unextracted soil (total sulfur), open squares representsulfate extracted by 0.04 M NaHCO3, closed squares represent sulfur remaining in soil after theextraction. Error bars reflect ±10% of the measured value uncertainty for concentration, and ±0.6% forsulfur isotope values.


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system is a markedly light isotopic end-member relative toother recognized sources.

3.4. Sulfur in Soil and Vegetation

[27] Total inventories of carbon, nitrogen, and sulfur weregreater in the hillslope soil (21, 2.1, and 0.63 kg m�2

respectively) compared to the alluvial soil (11, 1.2, 0.18 kgm�2) in the 1 m profiles sampled. In addition to total sulfur,concentrations and d34S were measured in soil subjected to abicarbonate extraction [van Stempvoort et al., 1990]. Theextraction primarily removed soluble and non-specificallyadsorbed sulfur [Mitchell et al., 1992] and the low bicarbon-ate concentration should have extracted minimal organicsulfur [Zhao and McGrath, 1994]. Sulfur forms not extractedby this method include organic, mineral, and specificallyadsorbed [Prietzel and Hirsch, 1998].[28] Non-extractable sulfur in surface soils and through-

out the 1 m profiles was linearly correlated with both soilcarbon (r2 = 0.78, p < 0.0001, n = 45) (Figure 5) andnitrogen (r2 = 0.84, p < 0.0001, n = 45). Carbon andnitrogen are also strongly correlated (r2 = 0.97, p <

0.0001, n = 45), and the association with sulfur suggeststhat the majority of non-extractable sulfur is organicallybound. Changes in slope between surface and depth sampleslikely reflect greater litter and/or fine root content at thesurface and C:S stoichiometry. Some non-extractable sulfurcould also be held in minerals, though the extent ofweathering indicated by the chemistry and mineralogy ofboth soil types makes the presence of primary sulfur-bearingminerals unlikely. However, sulfate in inner sphere surfacecomplexes with secondary clay minerals and sesquioxides isa possibility. Significant differences in soil sulfur concen-tration, composition, and d34S were observed between thealluvial and hillslope soil at Rancho Mariposa (Figure 6).[29] Concentration of sulfur was significantly greater in

the surface layer of the hillslope soil than the alluvial soil(Scheffe post-hoc, p = 0.04), as was d34S (p < 0.0001)(Table 3). Differences in sulfur concentrations were greater atdepth, with alluvial soil total sulfur at 50 cm and below beingless than a third of that at the surface. Total sulfur concen-tration in the hillslope soil declined only slightly with depth,but the proportion of extractable sulfate increased.

Table 3. Individual Values or Means ± 1 SD for Sulfur Concentrations and d34S VCDT for Soil and Vegetation Samples Collected at

Rancho Mariposaa

Soil Type Species/Sample Season d34S VCDT (%) S (mg/kg) n

Hillslope B. utile Dry +11.9 ± 1.0 1500 ± 150 3B. utile Wet +13.3 ± 1.2 1700 ± 450 6S. parahyba Dry +13.0 ± 0.6 4900 ± 1100 3S. parahyba Wet +12.0 ± 0.2 3200 ± 200 6H. alchorneoides Dry +12.5 ± 0.9 2500 ± 500 5C. costaricense Dry +13.3 ± 0.5 2000 ± 450 4Surface soil Dry +12.8 ± 0.7 620 ± 80 11Profile A, 60 cm Dry +13.0 610 1Profile B, 60 cm Dry +13.4 410 1

Alluvial S. parahyba Dry +8.7 ± 2.1 4600 ± 600 4S. parahyba Wet +7.6 ± 1.3 2700 ± 300 6B. utile Dry +10.7 ± 1.5 1400 ± 100 5B. utile Wet +10.5 ± 1.7 1600 ± 400 6Surface soil Dry +10.9 ± 0.6 530 ± 120 10Profile A, 60 cm Dry +10.6 90 1Profile B, 60 cm Dry +11.7 70 1

aSeason refers to the season when leaves were collected; n is number of samples analyzed.

Figure 7. Concentrations and d34S VCDT values through an 8 m deep profile of Ultisol and underlyingsaprolitic material. Diamonds designate data for unextracted soil (total sulfur), open squares representsulfate extracted by 0.04 M NaHCO3, closed squares represent sulfur remaining in soil after theextraction. Error bars reflect uncertainty as described in the text. Error bars reflect ±10% of the measuredvalue uncertainty for concentration, and ±0.6% for sulfur isotope values.


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[30] All soil d34S values in hillslope soil profiles weregreater than those in alluvial soil profiles, and d34S in eachprofile generally varied little with depth. Differences in d34Sbetween non-extractable and total soil sulfur were subtle andgenerally within the range of measurement error. The d34S ofextracted sulfate, where measured, was consistently lowerthan that of total or non-extractable sulfur. Concentrations oftotal sulfur in the deep hillslope soil profile generally declineto 5.5 m depth, and then increase in the more saproliticmaterial below (Figure 7). Sulfur isotope ratios increase withdepth to 5.5 m to values greater than in any 1m profile. At 5.5m, d34S abruptly declines and then increases again below.Low carbon and nitrogen content indicate little organicmatter in the saprolite, but significant non-extractable sulfuris present. The pH range measured in the saprolite (4.7–5.1)could permit the formation of secondary aluminum hydroxysulfate minerals [Delfosse et al., 2005; Nordstrom, 1982].Assuming a bulk density of 1.4 g cm�3 across the entire 8 mprofile, the total sulfur content is 5 kg m�2.[31] The d34S of the canopy emergent vegetation gener-

ally matched that of the underlying soil (Table 3). Nosignificant differences between soil, species or season werefound for the hillslope soil. The leaves of S. parahybacollected on the alluvial soil during the wet season wereisotopically lighter than alluvial surface soil (p = 0.0002) andleaves of B. utile from the wet (p = 0.0048) and dryseason (p = 0.0038). Those differences were not significantfor dry season S. parahyba leaves. No such differencesappear in hillslope S. parahyba, arguing against biologicalsulfur isotope fractionation. Rather, low d34S in alluvialS. parahyba might reflect uptake of sulfur from depth. Ageneral agreement between d34S of canopy vegetation andsoil with all other species-soil combinations suggests thatlittle fractionation of sulfur isotopes occurs during uptake andtranslocation in the species sampled. Leaf sulfur concentra-tions generally corresponded to differences between species.

3.5. Additional Sites

[32] Sulfur isotope ratios and concentrations in basalt-derived hillslope soils from the additional sites around theOsa Peninsula region were similar to those measured in thehillslope soil at Rancho Mariposa (Table 4). The d34S valuesin leaves of B. utile again generally matched those in surfacesoil. To overcome limited parent-material sampling at thesesites, EIC values for silicon and sulfur were calculated usingan average basalt composition from samples collectedacross the region. The resulting EIC values are comparableto the primary hillslope soil site in showing depletion ofboth sulfur and silicon.

4. Discussion

4.1. Atmospheric Sulfur Accumulation

[33] Large isotopic differences between potential sourcesof an element are ideal for distinguishing their relativeproportions in a mixed system. This is the situation forsulfur at Rancho Mariposa, where mean parent-materiald34S (�23.3%) differs greatly from mean bulk atmosphericdeposition (+10.4%). All soil and ecosystem components atRancho Mariposa match the atmospheric d34S signaturemuch more closely than parent-material and suggest themajority of sulfur in all ecosystem components is derivedfrom atmospheric deposition (Figure 8).[34] The greater soil sulfur content in the more weathered

hillslope soil, relative to the alluvial soil, speaks to anaccumulation of atmospheric sulfur with increased soildevelopment. Total content of sulfur to 1 m in the alluvialsoil is equivalent to approximately 400 years of atmosphericdeposition, at the estimated rate, and 1,400 years for thehillslope soil. Those are relatively short periods of time onpedogenic time-scales. Even total sulfur in the 8 m deephillslope profile is equivalent to only 11,000 years ofatmospheric deposition. At the same time, EIC values(Figure 3) show that all regolith material has lost sulfurrelative to the basalt parent-material, suggesting that removal

Table 4. Data for Samples Collected at Additional Hillslope Soil Sites Around the Osa Peninsulaa

Site Sample d34S VCDT (%) S (mg/kg) N EIC Silicon EIC Sulfur

Agua Buena B. utile leaves +11.7 ± 0.6 1300 ± 200 70–10 cm soil (ridge) +11.2 390 1 0.8 0.40–10 cm soil (slope) +10.7 370 1 0.8 0.4

Punta Adelas B. utile leaves +13.6 ± 0.7 2400 ± 900 80–10 cm soil (ridge) +13.3 960 1 0.4 0.40–10 cm soil (slope) +13.6 690 1 0.6 0.660–70 cm soil (ridge) +13.2 380 1 0.5 0.360–70 cm soil (slope) +10.6 80 1 0.8 0.1

Playa Cativo B. utile leaves +13.6 ± 1.8 2400 ± 1300 70–10 cm soil (ridge) +12.5 860 1 0.1 0.20–10 cm soil (slope) +11.3 700 1 0.2 0.260–70 cm soil (ridge) +11.5 130 1 0.3 0.160–70 cm soil (slope) +12.0 390 1 0.3 0.2

Suital Lodge B. utile leaves +12.7 ± 0.2 2400 ± 900 60–10 cm soil (ridge) +12.4 620 1 0.3 0.40–10 cm soil (slope) +12.9 780 1 0.3 0.560–70 cm soil (ridge) +13.2 540 1 0.4 0.360–70 cm soil (slope) +13.5 580 1 0.4 0.4

Fila Ganado 0–10 cm soil (ridge) +13.3 960 1 0.4 1.00–10 cm soil (slope) +13.6 690 1 0.5 0.660–70 cm soil (ridge) +14.1 1050 1 0.4 0.960–70 cm soil (slope) +14.4 590 1 0.5 0.4

aIncluded are individual values, or means ± 1 SD, for d34S VCDT, sulfur concentrations, and EIC values.


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of parent-material sulfur has been significant at some point insoil development.[35] A broad trend of higher d34S values in samples with

greater sulfur content also points toward atmospheric sulfuraccumulation in soil. This pattern is strong in samples frombelow 10 cm depth (Figure 9). Regression analysis of thisdata shows a significant positive relationship (r2 = 0.66, p =< 0.0001, n = 44). The zero concentration intercept and 95%confidence interval (2.2 ± 2.4%) are above almost all themeasured basalt parent-material values. A more selectiveanalysis can be done using a single data point that repre-sents the 60 cm depth increment from each 1 m profile(Figure 10). Regression of this data also finds a significantpositive relationship (r2 = 0.82, p = < 0.0001, n = 12) with ahigh zero concentration intercept of 5.5 ± 2.3 %. Therelationship is little changed when analysis is restricted to

only the hillslope soils (r2 = 0.83, p = 0.0003, n = 10), oruses extracted soil values (r2 = 0.75, p = 0.0003, n = 12).Logarithmic relationships and high d34S values of the zeroconcentration intercepts of the regressions argue againstlinear two-source mixing of parent-material and atmosphericsulfur. The data are more consistent with losses of sulfur fromthe ecosystem during cycling, even as a net accumulation ofatmospheric sulfur occurs. The relatively high d34S of thezero concentration intercepts also suggest that there is norecalcitrant pool of parent-material sulfur that remains pro-tected from loss.

4.2. Sulfur Isotope Fractionations

[36] While soil sulfur concentration and isotope datapoint toward an accumulation of atmospheric sulfur, it isimportant to note that most of the d34S values measured insoil and vegetation are greater than that of atmosphericdeposition. It is likely that d34S values of atmosphericdeposition fluctuate over pedogenic time-scales, particularlydownwind from a volcanic source. The d34S measured inshort-term collections of atmospheric deposition thereforemay not represent the long-term average, and may havebeen lower or higher in the past.[37] Another possible explanation for ecosystem d34S

values greater than atmospheric deposition is fractionationof sulfur isotope ratios during cycling. Where fractionationsare linked with processes that retain or export sulfur the netd34S value of the ecosystem can be changed, potentially tovalues outside the range of sources. Fractionations associ-ated with plant uptake [Trust and Fry, 1992] and dissimi-latory sulfate reduction [Thode, 1991] would be expected toproduce a net effect of lowering ecosystem d34S, not the netincrease observed.[38] Three distinct fractionating processes seem like po-

tential drivers for a net increase in ecosystem d34S values assuggested by the data. First, precipitation of the mineralalunite favors incorporation, and therefore retention, of 34Sby a fractionation of 0.8% [Prietzel and Mayer, 2005];other aluminum sulfate minerals may generate similarfractionations. Such fractionations could be responsiblefor the observed increase in d34S values in the saprolite

Figure 8. Sulfur isotope values for Rancho Mariposaecosystem components and potential sources of sulfur asdescribed in the text. Error bars represent 1 SD of the meanfor values measured in this study.

Figure 9. Scatterplot and regression of total sulfurconcentration versus d34S VCDT value of all soil andsaprolite samples below 10 cm depth.

Figure 10. Scatterplot and regression of sulfur concentra-tion versus d34S VCDT value at 60 cm depth for each of the1 m profiles sampled. Each data point represents one soilprofile.


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(Figure 7). Another fractionation occurs in plants, as theycan emit H2S with d34S values decreased by 9.7 to 16.8%relative to the sulfur taken up by roots [Winner et al., 1981].Calculations using literature emission rates of 0.2 –0.5 kg S ha�1 yr�1 [Andreae et al., 1990], and the RMsulfur deposition flux rate, suggest that with persistentlosses of low d34S sulfur by this pathway the overall forestd34S could be elevated over inputs by 0.4–2.1% understeady state conditions [Bern et al., 2007]. A third possi-bility is mineralization reactions that can discriminateagainst 34S and yield products with d34S values 5.1% lowerthan sources [Kaplan and Rittenberg, 1964]. Mineralizationrelease of sulfate, with lower d34S values relative to organicforms, could increase the d34S of soil and the forest as awhole as released sulfate is leached away.[39] Another possible explanation for the discrepancy

between ecosystem and atmospheric d34S is that the d34Svalues for the organic sulfur measured here in soil and plantsamples, and calibrated against mineral standards, areincreased due to contribution of structural 18O from organicmatter [Yun et al., 2005]. Lower d34S values measured in thesulfate extracted from hillslope soils and precipitated asBaSO4 (Figures 6 and 7) suggest that organic d34S valuescould be elevated by such an effect. It is worth noting thatextracted sulfate from 10 to 20 cm in Hillslope Profile B(+10.9%) is a good match for atmospheric deposition(+10.4%). However, cycling-related isotopic fractionations,such as that for mineralization described above, could alsocause soil organic sulfur to be enriched in 34S relative tosulfate.

4.3. Sulfur Behavior During Pedogenesis

[40] The data presented suggest that the early stages ofrock weathering and leaching rapidly release and removeparent-material sulfur, while relatively small stocks oforganic soil sulfur accumulate during early soil develop-ment. A relatively rapid decline of weathering inputs, andsmall stocks of total soil sulfur, act to increase the signif-icance of persistent inputs from atmospheric deposition. Assoil conditions develop to promote sulfur accumulation,total concentrations of sulfur rise and ecosystem poolsbecome dominated by atmospheric deposition.[41] Geochemistry and mineralogy play large roles in

driving this pattern. Sulfates and sulfides are among themost easily weathered minerals contained in rocks [Allenand Hajek, 1989] and bacterial oxidation can acceleratesulfide weathering [Banfield et al., 1999]. Under aerobicconditions, parent-material sulfur should be released assulfate into solution before other minerals, particularlysilicates, have been extensively weathered [Likens et al.,2002]. This is significant, because weathering of silicatesproduces the secondary soil minerals necessary to promotesulfate retention by anion adsorption and surface complex-ation reactions. Soil capacity for these reactions is correlatedwith both amorphous and crystalline iron and aluminumsesquioxides, acidic soil pH, anion exchange capacity, andpossibly clay content [Alves and Lavorenti, 2004; Mitchellet al., 1992]. Soil pH declines as those same reactionsrelease and deplete the base cations that buffer soil solution[Chadwick and Chorover, 2001]. Precipitation of low sol-ubility, aluminum hydroxy sulfate minerals, such as aluniteand basaluminite, is another means of sulfate retention in

soil [Delfosse et al., 2005]. Precipitation of these mineralsrequires free aluminum, as well as low pH [Nordstrom,1982], and should, therefore, also be associated with moreadvanced weathering. Although anion exchange capacitymay seldom be a large factor in sulfate retention, it is alsocorrelated with decreasing pH and therefore the progressionof soil weathering.[42] Early soil development is unlikely to promote the

inorganic retention mechanisms above, and thus sulfurretention will occur primarily by immobilization in organicmatter. Retention of parent-material sulfur may therefore belimited in two important ways. First, organic matter takestime to accumulate in young soils. If weathering release ofsulfur proceeds faster than immobilization, net sulfur con-tent will decline, as may have occurred on the alluvial soil.Second, nutrient uptake and immobilization are greatestnear the soil surface and may do little to counter sulfurlosses at depth.[43] Evidence supporting each aspect of the broad pattern

outlined above can be found in the soils examined. Sulfurhas been leached from the lower sections of the alluvial soil,although the removal of other highly soluble elements isless extensive. Upper sections of the alluvial soil retainlarger stocks of sulfur that are immobilized in organicforms. The d34S values throughout the alluvial soil are moreclosely aligned with atmospheric inputs than basalt parent-material. The hillslope soil has accumulated more organicmatter throughout the soil profile, immobilizing more sulfurthan the alluvial soil.[44] All metrics examined suggest more extensive weath-

ering in the hillslope soil, and it contains significantadsorbed sulfate while the alluvial soil does not. Thepresence of significant non-extractable sulfur in the sapro-lite, despite extremely low carbon and nitrogen content,may indicate sulfur retention there by precipitation ofaluminum hydroxy sulfate minerals.[45] The pattern of sulfur behavior outlined here will not

apply to all soils. One example is soils with high initialsulfur content, such as acid sulfate soils, that would beexpected to undergo persistent sulfur loss during pedogen-esis [Wagner et al., 1982]. Another example is soils withgreater non-crystalline mineral content than Osa soils. Suchsoils stabilize significantly more organic matter duringintermediate development than in later stages [Torn et al.,1997]. While exceptions will exist, the processes describedhere are likely to be at work in many other soils and thesame may be true for the overall pattern.

5. Conclusions

[46] Stable sulfur isotope ratios indicate that sulfur insoils and vegetation of the Osa Peninsula is largely derivedfrom atmospheric deposition. Positive correlations betweensoil sulfur concentrations and d34S below the surfacesuggest atmospheric sulfur is accumulating, and that par-ent-material sulfur is rapidly lost early in soil development.Conditions that promote sulfate adsorption are slower todevelop, and strong correlations between carbon, nitrogen,and sulfur concentrations suggest that retention of sulfurduring early soil development occurs primarily via immo-bilization in organic matter. The less-weathered alluvial soilstudied here appears to be in such a weathering stage. In


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contrast, the more-weathered hillslope soil has accumulatedmore organic sulfur, but has also developed the lower pHand mineralogy necessary to accumulate adsorbed sulfatebelow the surface. Atmospheric deposition rates for sulfurare often significant relative to ecosystem stocks, and incombination with sulfur behavior during pedogenesis coulddrive a rapid transition from rock-weathering to atmosphericsources in many settings.

[47] Acknowledgments. This manuscript benefited from thoughtfulreviews by George Breit and two anonymous reviewers. We thank CraigStricker, Robert Rye, Craig Johnson, Dennis Eberl, Tammy Hannah, andNataly Ascarrunz for laboratory access and analyses. Amy Bern conductedSEM and EDS analysis of basalts. Cory Cleveland, Sasha Reed, ErikaEngelhaupt, and Diana Nemergut assisted with field collections. We thankHerbert and Marleny Michaud as well as Rainbow Adventures for access,logistical support, and use of field sites. The Organization for TropicalStudies (OTS) and the Ministerio de Ambiente y Energia (MINAE) inCosta Rica facilitated all aspects of the field research. This work wassupported by the Andrew W. Mellon Foundation, NSF grant DEB-0089447, and a University of Colorado, Ecology and Evolutionary BiologyDepartment research grant.

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��C. R. Bern, U.S. Geological Survey, DFC, MS 973, Denver, CO 80225,

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University of Colorado, Campus Box 450, Boulder, CO 80309-0450, USA.([email protected])


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Accumulation of atmospheric sulfur in some Costa Rican soils