Geoderma Regional 14 (2018) e00186

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Salt-affected soils of the coastal plains in Rio Grande do Sul, Brazil

Luiz Gustavo deOliveira Denardin a,⁎,1, Felipe de Campos Carmona b,1, Tales Tiecher a, Amanda PosseltMartins a,Lucas Aquino Alves a, Eliseu José Weber a, Ibanor Anghinoni a

a Federal University of Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazilb Integrar, Capivari do Sul, Rio Grande do Sul, Brazil

⁎ Corresponding author at: Av. Bento Gonçalves 7712,ZIP Code 91540-000, Brazil.

E-mail addresses: luizgdenardin@gmail.com (L.G.O. Defelipecarmona@integrarcampo.com.br (F.C. Carmona), tal(T. Tiecher), amanda.posselt@ufrgs.br (A.P. Martins), luca(L.A. Alves), eliseu.weber@ufrgs.br (E.J. Weber), ibanghi@

1 These two authors contributed equally to this paper.

https://doi.org/10.1016/j.geodrs.2018.e001862352-0094/© 2018 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 April 2018Received in revised form 26 July 2018Accepted 30 July 2018

In Rio Grande do Sul (RS) State, Brazil, the soils are not prone to salinization due to the large amounts of rainfall,which are sufficient to leach the soluble salts. However, flooded rice crops grown in the coastal plains of RS maybe affected by soil salinity, once the soils in this region are formed by marine, fluvial-lacustrine sediments. Theuse of water for irrigation of crops from sources connected with the Atlantic Ocean, especially Patos Lagoon,can lead to deposition of excessive amounts of sodium in the soil, especially in southern portion, by the proximityto channel linking the lagoon to the sea. This study aimed to map the occurrence of salt-affected soils in thecoastal plains of RS State, Brazil. The study was performed in three regions: Internal Coastal Plain (ICP), ExternalCoastal Plain (ECP) to Patos Lagoon andNorth Shore. 766 georreferenced soil sampleswere collected and levels ofexchangeable sodium, exchangeable sodium percentage (ESP) and electrical conductivity (EC) of saturatedextract (ECse)were analyzed. The resulting analytical data of the sampling pointswere submitted to a descriptivestatistic and used to generate maps aiming to depict the continuous spatial variation of each measured variable.Most of soil samples showed indicators of low soil salinity, especially inmost of the ICP andNorth Shore. The soilsmost affected are concentrated in regions overlapping the paleochannels sub-surface previously identified in thesub-surface of the ECP and in areas closer to links between sources of freshwater and sea. In the ICP, the salinityproblem was restricted to only some areas due to the use of saline water from Patos Lagoon.

© 2018 Elsevier B.V. All rights reserved.

1. Introduction

According to the FAO, the yearly global rice production is currently750 million tonnes. The Asian continent is responsible for 90% of thisproduction, followed by 5% from the American continents. In America,Brazil is highlighted as one of the major producers, with 1.3% of globalrice production, generating approximately 12 million tonnes per year(FAOSTAT, 2013). Brazilian rice production is mostly irrigated (86.3%),and RS is the state with the highest production, exceeding 8 milliontonnes (Embrapa Arroz e Feijão, 2015). This production is concentratedin five regions of the state that represent an area of 1.1 million ha. TheICP and ECP of RS are responsible for 25% of Brazilian rice productionand are the regions with the greatest water availability (IRGA, 2018).These regions are situated on the southern coast of Brazil, which is dom-inated by lagoons separated from the Atlantic Ocean by only a thin

Porto Alegre, Rio Grande do Sul

nardin),es.tiecher@gmail.comsalves.la@hotmail.comufrgs.br (I. Anghinoni).

sandy barrier that acts as an obstacle to the superficialflux of freshwater(Machado et al., 2008).

In general, the soils of RS State are not prone to salinization due to thelarge amounts of rainfall, which are sufficient to leach the soluble salts,such as sodium chloride (NaCl). However, in irrigated rice fields on thecoastal plains of RS State, where the soils are formed by marine andfluvial-lacustrine sediments (Villwock and Tomazelli, 1995), soil salinityproblems frequently occurbecause irrigation is performedwith lowqual-ity water (i.e., water with a high content of salts). The soil salinity andsodicity of this region is attributedmainly to the irrigation of ricewith sa-line water from coastal rivers and from Patos Lagoon. Patos Lagoon is themain water source for irrigation of rice fields and is a body of shallowwater connected to the adjacent ocean (Atlantic Ocean) through a chan-nel 22-km-long, 2-km-wide and 12-m-deep, on average (Kjerfve, 1986).

In the winter, the passage of frontal systems causes an increase inthe frequency of south quadrant winds (Moller Jr. et al., 1996), whichgenerates a water gradient between Patos Lagoon and the AtlanticOcean, with a lower water level in Patos Lagoon. The established gradi-ent is responsible for “pumping” the water of the Atlantic Ocean to theinterior of Patos Lagoon, increasing its salinity (Castelão et al., 2003)mainly in its southern and estuarine portions, which occupy approxi-mately 10% of the total surface of the lagoon. Salinization of Patos La-goon frequently occurs in January and February, which corresponds to

2 L.G.O. Denardin et al. / Geoderma Regional 14 (2018) e00186

the reproductive stage of the rice crop (Marcolin et al., 2005; SOSBAI,2016). Salt accumulation in the soil can damage the rice yield due tothe high sensitivity of rice to salt stress (Khatun and Flowers, 1995;Gao et al., 2007; Fraga et al., 2010). When monitoring different localwater catchments from Patos Lagoon for rice irrigation, Fraga et al.(2007) verified a large range of EC values: from 0.5 to N6 dS m−1.Many of the measured values were considerably above the thresholdvalues for irrigated rice (Ayers and Westcot, 1985).

Salt transport to the subsurface is a frequent phenomenon when anaquifer connects with surface water through permeable sediments(Burnett and Dulaiova, 2003), as occurs in the case of the ECP in RS,Brazil. Windom and Niencheski (2003) suggested that a significantamount of such water goes to the sea through an aquifer, mixing thefresh water from Patos Lagoon with saline water from the AtlanticOcean. This undergroundmixture tends to be discharged into the Atlan-tic Ocean, especially after significant rain events that increase the levelof Patos Lagoon (Machado et al., 2008). However, the quantity ofreverseflow of thismixture, returning to Patos Lagoon, is still unknown.This reverse flow event may be observed in seasons with low rainfall inthe headwaters of the main tributary rivers of Patos Lagoon, as in yearsof La Niña phenomena. In thisway, Patos Lagoon can become a potentialsource of salt intrusion in coastal soils and many irrigated rice fields.

The dimension of this area and the salinity level of salt-affected soilsare still unknown (Marcolin et al., 2005). However, it is known thatsalinity tends to occur in lowland soils adjacent to the southern portionof Patos Lagoon. The proximity of this region to the Atlantic Oceanand to the mouths of rivers that connect to the Atlantic Ocean(e.g., Tramandaí River and São Gonçalo Channel) confirms the occur-rence of soils with sodic or solodic character (Pinto et al., 2004).

In this context, the survey of areas with salt-affected soils is a realand important demand to identify the scale of the problem in this indis-pensable Brazilian rice-producing region. The identification of suchareas is also important to establish action strategies to minimize or cor-rect the problem. Thus, the objective of the present study was to map

Fig. 1. Location of the study area in the Rio Grande do Sul State, with the indication of themainmsmall black spots represent the site of soil sampling in the Internal and External Coastal Plains

the occurrence of salt-affected soils in the coastal plains of RS State,Brazil.

2. Materials and methods

The study was performed in three regions of RS State, Brazil: (i) ICP(Camaquã, Rio Grande, Arambaré, Tapes and Pelotas municipalities);(ii) ECP (Mostardas, Viamão and Palmares do Sul municipalities); and(iii) North Shore (Santo Antônio da Patrulha, Osório and Torres munic-ipalities) (Fig. 1). The region of the coastal plains and the North Shore isformed by sedimentary deposits. The soils commonly found inthe region were formed from sediments of marine and fluvial-lacustrine origin (Villwock and Tomazelli, 1995) and, therefore, are al-most all characterized as sandy soils, being mainly represented by pla-nosols, gleysols, arenosols and plinthosols (Streck et al., 2018). Inaddition to the sandy texture, most of the soils have a low organic mat-ter content, resulting in a low cation exchange capacity (CEC) (Pintoet al., 2004).

The evaluation period was from July 2008 to August 2009. In total,766 soil samples were collected from the 0–20 cm layer, with anauger, in areas with current soybean or pasture cultivation but with arecent history of rice cropping with irrigation from Patos Lagoon. Theareaswere chosen based on previous information from field techniciansof Riograndense Rice Institute (IRGA) and rice farmers. The samplingdensity was 0.08 and 10 observations km−2 (low intensity recognitionsurvey) in areas without and with historic knowledge of a soil salinityand sodicity problem, respectively. All the soil samplings weregeo-referenced with GPS devices and composed of three sub-samples.

The following analyses were performed on all soil samples: ECse,levels of exchangeable Ca2+, Mg2+ and Al3+ extracted by 1.0 M KCl(soil:extractant ratio 1:20) and levels of exchangeable Na+ and K+ ex-tracted by Mehlich 1 (Tedesco et al., 1995). Exchangeable Al3+ in KClextract was determined by titrating with 0.0125 M NaOH solution; Ca2+ and Mg2+ were determined in an atomic absorption

unicipalities around the region of Costal Plains and rivers that flow into Patos Lagoon. Theand North Shore of Rio Grande do Sul State, Brazil.

Fig. 2. Frequency histograms of the variables soil Exchangeable sodium (Na+) (A),Exchangeable Sodium Percentage (ESP) (B) and Electrical Conductivity (EC) insaturation extract (ECse) (C), of 766 soil samples collected from the 0–20 cm layer onthe coastal plains of Rio Grande do Sul State, Brazil.

3L.G.O. Denardin et al. / Geoderma Regional 14 (2018) e00186

spectrophotometer, and Na+ and K+ were determined in a flame pho-tometer. The potential acidity (H+ + Al3+) was calculated usingEq. (1) proposed by Kaminski et al. (2001) and adopted by CQFS-RS/SC (2016):

Hþ þ Al3þ ¼ e10:665–1:1483�pH SMP=10 ð1Þ

where H++Al3+ is estimated by the pH balance of the soil with 1.78 MSMP solution (triethanolamine, paranitrofenol, K2CrO4, Ca(CH3COO)2and CaCl2.2H2O) buffered at pH 7.5 (Shoemaker et al., 1961). The CECat pH 7.0 (CECpH 7.0) was calculated as the sum of H+ + Al3+ + Ca2+

+ Mg2+ + K+ (CQFS-RS/SC, 2016). The ESP was calculated using theEq. (2).

ESP %ð Þ ¼ Naþ=CECpH 7:0� �� 100 ð2Þ

The mean, median, standard deviation, coefficient of variation,asymmetry and kurtosis, minimum and maximum values fromexchangeable Na+, ESP, ECse and CEC were determined. The resultinganalytical data of the sampling points were used as input to generate aset of raster maps aiming to depict the continuous spatial variation ofeach measured variable. Given the topographic homogeneity of thestudy area, a simple linear interpolation approach weighted by thesquared inverse distance was chosen. GIS software IDRISI 15.0 wasused both for the interpolation process and for making the final maps.

3. Results and discussion

In general, the average values of soil exchangeable Na+, ESP andECse (Table 1) show that the soil sodicity and salinity in ECP and ICPdo not reach sufficient levels to cause damage to irrigated rice or thesoil physical structure. The asymmetry and kurtosis values do not followa normal distribution, as these values are above 0 and 3, respectively(Snedecor and Cochran, 1967). The data distribution is lognormal(Table 1). This asymmetrical distribution is the result of the discrepancybetween the mean and median observed in the three evaluated soilproperties. The observed asymmetry was always positive, indicating ahigh frequency of values above the mean. The evaluated soil measuresof salinity and sodicity also presented a high coefficient of variation,above 100% (Table 1). These results were expected due to the compre-hensiveness of sampling, which was performed over a distance N400km. Theminimum andmaximumvalues obtained for each soil attributeconfirmed the high coefficient of variation observed. The standard devi-ation was also high (Table 1), demonstrating a great variation of thevalues around the mean.

From the soil samples evaluated, 46% presented exchangeable Na+

contents up to 2 mmolc kg−1 (Fig. 2a). The samples with contents be-tween 2.1 and 4 mmolc kg−1 and 4.1 and 8 mmolc kg−1 represented,

Table 1Descriptive statistics parameters of the variables Exchangeable sodium (Na+), Exchange-able Sodium Percentage (ESP), Electrical Conductivity (EC) in saturation extract (ECse)and Cation Exchange Capacity (CEC) of 766 soil samples collected from the 0–20 cm layeron the coastal plains of the Rio Grande do Sul State, Brazil.

Descriptive statisticsparameters

ExchangeableNa+

(mmolc kg−1)

ESP(%)

EC(dSm−1)

CEC(mmolkg−1)

n 766 766 766 766Mean 5.6 4.2 2.3 120.8Median 2.3 2.2 1.2 104Standard deviation 10.7 5.7 3.9 59.8Cefficient of variationa 1.9 1.4 1.7 0.5Kurtosisa 24.5 13.6 28.7 9.0Skewnessa 4.6 3.3 5.0 2.4Minimum 0.1 0.1 0.3 44.0Maximum 86.1 44.3 33.9 569.5

a Non-dimensional parameters.

together, 39%. The soil samples with exchangeable Na + contentsabove 8.1 mmolc kg−1 were in the minority (15%). Three-quarters ofthe soil samples presented a low ESP (b5%) (Fig. 2b), and the soilswith medium ESP (5.1–15%) represented 20%. The soils with ESP N

15%, characterized by a high ESP, totalled only 5% of the samples. How-ever, in sandy soils, which have lowwater-holding capacity, thepotassicfertilization in the sowing line can cause damage and affect the initialestablishment of rice plants in conditions of low soil moisture(Carmona et al., 2009), even with medium sodicity level. Soil ESPabove 15% is considered a risk because it potentially alters soil physicalproperties (Ayers and Westcot, 1985), and excess sodium in relation tothe other cations causes clay dispersion, reducing the soil hydraulic con-ductivity (Luo et al., 2015). Despite these risks, this physical damage be-comes much more important for rainfed crops. In irrigated ricecultivation, the flooding by itself already promotes clay dispersion andimpairs the soil structure.

Although these two measures of sodicity (exchangeable Na+ andESP) are not the main factors liable for rice crop damage, they are di-rectly related to the EC, the main measure of salinity. The majority ofthe soil samples (75%) showed a low ECse (≤2.0 dSm−1) (Fig. 2c), indi-cating that soil salinity is a problemwith moderate comprehensivenesson the coastal plains of RS State, Brazil. Even so, 25% of the soils pre-sented ECse values above the critical level for irrigated rice (N3.0 dSm−1) (SOSBAI, 2016), representing a limitation to this crop due to ex-cess salt. In soils with high ECse, there is a low water uptake by theplant roots, where the natural process of osmosis is impaired (Chuet al., 2016). When the salt concentration in soil is above that insidethe root cells, the water movement is inverted, which may cause desic-cation and even death of the plant (Tester and Davenport, 2003).

Although soil flooding, performed on irrigated rice, increases theECse due to ion mobilization (Dedatta, 1981), the utilization of water

Fig. 3. Spatial distribution of Exchangeable sodium (Na+) at 0–20 cm soil layer on the coastal plains of Rio Grande do Sul State, Brazil, interpolated from 766 field samples.

Fig. 4. Spatial distribution of Exchangeable Sodium Percentage (ESP), at 0–20 cm soil layer on the coastal plains of Rio Grande do Sul State, Brazil, interpolated from 766 field samples.

4 L.G.O. Denardin et al. / Geoderma Regional 14 (2018) e00186

Fig. 5. Spatial distribution of Electrical Conductivity (EC) in saturation extract (ECse), at 0–20 cmsoil layer on the coastal plains of RioGrande doSul State, Brazil, interpolated from766fieldsamples.

5L.G.O. Denardin et al. / Geoderma Regional 14 (2018) e00186

with low amounts of salts in rice cropping tends to facilitate ionleaching, especially Na+ andK+, to soil layers that the plant root systemdoes not reach (Carmona et al., 2010). The ionic strength of soil solutionis diminished and, consequently, the EC also declines.

Higher levels of soil exchangeable Na+, ESP and ECwere observed inthe ECP (Figs. 3, 4 and 5). In the proximity of Casamento Lagoon, nearthe Viamão, Palmares do Sul and Mostardas municipalities, the waterutilized for irrigation is practically free of salt and/or sodium(Carmona, 2011). Therefore, the salinization of such soils cannot be at-tributed to the utilization of saline water for the irrigation of rice fields.The salt presented in these areas may be related to the soil genesis.

The current geographic position of the coastal barriers of RS State is aresult of migration of pre-existent barriers towards the continent(Tomazelli and Villwock, 2005; Travessas et al., 2005). The stratigraphyand dating of 14C in the Holocene barrier in Tramandaí municipalitydemonstrate that the sedimentary deposits of the lagoon background,superimposed by deposits of the transgressive phase of the barrier,began to be formed under conditions of intense salinity in the interiorof Patos Lagoon due to the pre-existence of a barrier situated to thesoutheast of its current position (Medeanic and Dillemburg, 2001).This barriermigrated towards the continent, overlapping the lagoon de-posits, in response to themigration of sea level (Travessas, 2003). Strat-igraphic records collected by Dillemburg et al. (2005) also demonstratea migration of a barrier towards the continent. This displacement is stillin progress, due to an erosive process of the coastline, arising to a sedi-ment loss (Dillemburg et al., 2000). These sediments, which may bemore or less permeable, favour the flow of groundwater, providing a re-action zonewhere the freshwater of Patos Lagoon and salt seawater aremixed (Charette and Sholkovitz, 2006). The freshwater–seawater frontmigrates in response to seasonal and tidal water level and groundwaterhydraulic gradients (Niencheski et al., 2007). This movement indicates

that, although these barriers prevent contact between the lagoon andthe sea, they can also be very permeable, due to groundwater transportpathways.

The subterranean connection of the water in this region occursthrough a series of paleochannels. Structural elements of the continen-tal shelf of RS State indicate that paleochannels developed because of re-gressive marine events during the Quaternary. Subsequenttransgressive events filled these channels (Abreu and Calliari, 2005).Toldo Jr. et al. (2000) studied the occurrence of paleochannels in the vi-cinity of Patos Lagoon and identified a Pleistocene-aged channel of theCamaquã River on the western shore of the lagoon. The youngerpaleochannel system indicates that its sedimentary fill occurred duringthe last big transgressive event (end of the Pleistocene) and during aphase of high sea level (Holocene),when the coastal channelswere pro-gressively filled by fluvial, estuarine and marine sediments(Weschenfelder et al., 2008). Among the younger paleochannels isBarra Falsa (Bujuru municipality), the old link channel between the At-lantic Ocean and Patos Lagoon (Toldo et al., 1991). Martins et al. (1996)linked the paleodrainages to the courses of themain rivers (Jacuí, Sinos,Taquari and Camaquã Rivers) that currently drain the highlands of thenorth and centre of RS State. The continental drains explain thepaleocanal recorded in the north of Patos Lagoon, correlated to theJacuí River. Just like this canal, the prolongation of the Camaquã Rivercoincides with the current Barra Falsa. In general, the paleochannelsare relatively deep and up to 3700 m in length (Abreu and Calliari,2005).

In the central region of ECP, near the Mostardas municipality, highsoil salinity levels were found (Figs. 3 and 4). In the same region,Weschenfelder et al. (2008) found evidence for the existence ofpaleochannels. The high salinity levels found in ECP (Figs. 3 and 4) areprecisely superimposed on the paleochannels of the Jacuí and Camaquã

6 L.G.O. Denardin et al. / Geoderma Regional 14 (2018) e00186

Rivers. This pattern indicates that the soil salinity in the arable layer insuch areas may be related to the subterranean saline water flux in thepaleochannels, filled by permeable sediments. In areas where thewater table is closer to the surface, salt may rise up to the soil arablelayer. Corroborating these results, in a study evaluating the water qual-ity of Patos Lagoon in municipalities near Mostardas, Carmona (2011)showed a mean ECse of 3.3 dS m−1, demonstrating the low probabilityof soil salinization due to the poorwater quality used in irrigation. How-ever, the high Na+ levels observed in soils of the southern region of ECP,in São José doNorte and Bujurumunicipality,may be linked to theuse ofsalt water from Patos Lagoon. In these regions, the water EC levels arenot suitable for irrigated rice cropping (SOSBAI, 2016).

In the North Shore, salt-affected soils occurred in a small area(Figs. 3, 4 and 5) in fields near the riverbed of Tramandaí River. How-ever, the soils utilized for rice cropping in Torres municipality differfrom those near the Tramandaí River (as in Imbé municipality). Thehigher clay content found in Torres municipality results in higher CECand, thus, possibly lower availability of this ion in the soil solution,due to the greater amount of charges present in the soil, adsorbing theNa+ ions that enter into the system (Sposito, 2008).

The occurrence of soils potentially harmful to rice cropping in ICPwere restricted to the southern portion of that region, in some areas lo-cated in the Pelotas and Rio Grande municipalities (Figs. 3, 4 and 5). Inthese cases, the soil salinity may be exclusively due to the addition ofsalt water from Patos Lagoon. The greater proximity to the AtlanticOcean leads that portion of Patos Lagoon to be more susceptible to theintrusion of salt water from the sea. Thus, the surrounding irrigatedareas are exposed to possible salt deposition. According to Carmona(2011), who evaluated the water quality in different places of thePatos Lagoon, the exchangeable Na+ and the ECse increase with prox-imity to the Rio Grande channel. The water-collection sites of thePatos Lagoon that presented the highest water salinity were in Pelotasand Rio Grande, with ECse of 7.49 and 10.5 dS m−1, respectively(Carmona, 2011), demonstrating that irrigation with saline water isthe main form of soil salinization in these places. Pinto et al. (2004) al-ready reported the predominant occurrence of saline, sodic and solodicsoils through the São Gonçalo Channel. Despite the adequate local to-pography and water availability for irrigated rice cropping, such soilsare currently utilized only as pastures due to the harmful effects ofsalt, making rice farming impractical.

4. Conclusions

The high occurrence of salt-affected soils was verified in the ExternalCoastal Plain of Patos Lagoon, due to the rise of salts from the subsoil inareas under paleochannels that link the lagoon with the Atlantic Ocean.In theNorth Shore, the occurrence of salt-affected soilswas verifiedonlyin Imbé municipality, due to irrigation with saline water fromTramandaí River. In the Internal Coastal Plain, the salinity problemwas restricted to only some areas of the Rio Grande and Pelotas munic-ipalities, possibly due to the use of saline water from Patos Lagoon.

Acknowledgements

We would like to thank the staff of Rio-Grandense Rice Institute(IRGA) forfield support.We also thank the Brazilian institutions the Na-tional Council for the Development of Science and Technology (CNPq)and the Coordination for the Improvement of Higher Education Person-nel (CAPES) for scholarship support.

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