!"#$%&$'()Chinampas)*+$(,)+-) ./0$&+)1$#23)#4/$")5"+6/"#$/,) '78)*'($7$9'#$+7):'9'"8,
Suelos artificiales de chinampas de la Ciudad de México: propiedades y riesgos de salinización Solos artificiais da cidade do México - Chinampas: propriedades e riscos de salinização
AUTHORS Received: 24.03.2011 Revised: 02.06.2011 Accepted: 24.06.2011
1 Ramos Bello, R. ABSTRACT García Calderón, 1 N.E. !e chinampas agriculture is a traditional land use practice in the Valley of Mexico since Pre-Hispa- nic time. !e chinampas soils were constructed by excavation of lake sediments that resulted in the Ortega Escobar, 2 H.M. creation of a system of islands separated by channels. !e agricultural productivity of these artificial soils was high; also the land use practices included forestry, fish breeding and hunting. Nowadays, 3@ Krasilnikov, P. the chinampas soils are affected by excessive salinity. We studied 10 representative soil profiles in the pavel.krasilnikov@ gmail.com chinampas zone of Mexico City in order to characterize their properties and origin, to provide their classification, and to evaluate soil salinization, vertical distribution of the salts and their chemical composition. !e soils are characterized by a layered structure, uniform dark grey colour, irregular vertical distribution of organic carbon and clay, and high percentage of carbon. Some soils show an @ Corresponding Author increase in organic matter with depth, and other profiles have maximum organic matter content in the surficial layers and in the subsoil. !e dynamics of sedimentation resulted in the decrease in 1 Facultad de Ciencias, organic matter in the upper layers of lacustrine sediments, because of recent increase in erosion rate UNAM, México. and consequent increase in the proportion of mineral particles in the sediments. Most probably 2 COLPOS, Montecillo, high organic matter content in surficial layers of some soils is due to excavation and accumulation of Edo.de México. organic-rich subsoil material in the course of digging the channels. !e concentration of soluble salts 3 Institute of Biology in superficial horizons, expressed as electric conductivity, varies in a wide range from 5 to almost 50 KarRC RAS, 185910, E4tN-1. !e salts concentrate mainly in the superficial layers of soils. !e abundance of the cations of Pushkinskaya str., 11, soluble salts is Na+>Mg2+>Ca2+>K+ and that of the anions is SO 2->Cl->HCO ->CO 2-. !e alkaline Petrozavodsk, Russia. 4 3 3 reaction of soils is caused by exchangeable Na rather than by free sodium carbonates. !e restoration of chinampas requires a complex approach, combining soil, water and ecosystems remediation.
RESUMEN
La agricultura de chinampas es una práctica tradicional de uso del suelo en el Valle de México desde la época prehispánica. Los suelos de chinampas fueron construidas por la excavación de sedimentos del lago que resultó en la creación de un sistema de islas separadas por canales. La productividad agrícola de estos suelos arti!ciales fue alta; también las prácticas de uso del suelo incluyeron la silvicultura, la pesca y la caza. Actualmente, los suelos de chinampas están afectados por una salinidad excesiva. Se estudiaron 10 per!les de suelos representativos de la zona de chinampas de la Ciudad de México con el objeto de caracterizar sus propiedades y origen, obtener su clasi- !cación, evaluar la salinización del suelo, la distribución vertical de las sales y su composición química. Los suelos se caracte¬rizan por presentar una estructura en capas, de color gris oscuro uniforme, y una irregular distribución vertical de carbono orgánico y arcilla. Algunos suelos muestran un aumento de la materia orgánica con la profun- didad y otros per!les presentan un contenido alto de esta en las capas super!ciales y en el subsuelo. La dinámica de
DOI: 10.3232/SJSS.2011.V1.N1.05
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70 71 la sedimentación como resultado de la disminución de la materia orgánica en las capas superiores de los sedimentos KEY WORDS lacustres se debe al aumento reciente en la tasa de erosión y al consiguiente aumento en la proporción de partículas arti!cial soils, minerales en los sedimentos. El alto contenido de materia orgánica en las capas super!ciales de algunos suelos se Anthrosols, debe a la acumulación de material del subsuelo rico en materia orgánica obtenida del fondo de los canales. La con- soluble salts, centración de sales solubles en los horizontes super!ciales, expresada como conductividad eléctrica, varía de 5 a 50 lacustrine R'-1. Las sales se concentran principalmente en las capas super!ciales de los suelos. La abundancia de los cationes sediments, + 2+ 2+ + 2- - - 2- de las sales solubles es Na >Mg >Ca >K y la de los aniones es SO4 >Cl >HCO3 >CO3 . La reacción alcalina de pyroclastic los suelos se debe al Na+ intercambiable más que por carbonatos de sodio libres. La recuperación de las chinampas sediments requiere de un planteamiento complejo, con la combinación del suelo, agua y de la recuperación de los ecosistemas. PALABRAS RESUMO CLAVE A agricultura de Chinampas é uma prática tradicional do uso do solo no Vale do México, desde tempos pré-hispâ- suelos artificiales, nicos. Os solos de Chinampas foram construídos em consequência da escavação de sedimentos lacustres, resultando Anthrosoles, na criação de um sistema de ilhas separadas por canais. A produtividade agrícola desses solos arti!ciais era elevada sales solubles, bem como as práticas de uso do solo incluindo pesca, silvicultura e caça. Estes solos caracterizam-se por uma estru- sedimentos tura em camadas, de cor cinza escuro uniforme, e uma distribuição vertical irregular de carbono orgânico e argi- lacustres, la. Alguns solos apresentam um aumento da matéria orgânica com a profundidade enquanto outros apresentam sedimentos per!s com um alto teor de matéria orgânica nas camadas super!ciais e subsolo. A dinâmica de sedimentação como piroclásticos resultado da diminuição de matéria orgânica nas camadas superiores dos sedimentos lacustres deve-se ao recente aumento da taxa de erosão e ao consequente aumento da proporção de partículas minerais nos sedimentos. Este alto PALAVRAS" teor de matéria orgânica nas camadas super!ciais de alguns solos deve-se à acumulação de material do subsolo rico CHAVE em matéria orgânica com origem no fundo dos canais. A concentração de sais solúveis nos horizontes de superfície, Solos artificiais, expressa pela condutividade eléctrica varia de 5-50 dS m-1. Os sais concentram-se principalmente nas camadas Antrossolos, sais super!ciais do solo. A abundância de catiões de sais solúveis segue a seguinte ordem: Na+>Mg2+>Ca2+>K+ e a dos solúveis, sedimentos aniões a ordem: SO 2->Cl->HCO ->CO 2-. A reacção alcalina dos solos deve-se sobretudo ao sódio de troca e não 4 3 3 lacustres, sedimentos ao carbonato de sódio de troca livre. A recuperação das chinampas exige uma abordagem complexa, que combina a piroclásticos remediação do solo, água e ecossistemas.
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70 71 [ ARTIFICIAL CHINAMPAS)*;<=*);>).?@<1;)1 HI)<7#"+8J&#$+7 Chinampas are agroecosystems developed in An important component of the agroecosystem the lacustrine zone of the catchment of Mexico is the net of channels, which have been used as in periodically flooded or shallow areas. The a drainage system, as well as for irrigation and beginning of chinampas agriculture is referred maintaining soil humidity. Excessive use of the to the Classical period of PreHispanic culture water of the channels resulted in the lowering of in Mesoamerica, to the beginning of 11th cen water level, drying and almost disappearance of tury P.C. (RojasRabiela 1983). These areas were the lake. Recently, since 1959, the water level used by small ethnic groups such as chichime has been improved with the use of wastewater. ca, acolhua, tepaneca, otomi, some groups in The wastewater has high concentration of salts, fluenced by Toltec culture, and, later, dominated toxic metals and pathogenic organisms, which by Aztecs. These groups developed a specific affect the soil, animals, and vegetation; more culture and technology: irrigation by flooding, over, it is a serious problem for public health (Ro and formation of a series of elevated fields for sas et al. 1984; Vallejo and Aguilera 1994). The agricultural production (Ezcurra 1990). Chinam concentration of salts in the water is the most pas (a word of nahuatl origin: chinamitl – “straw dangerous issue: for example, sodium concen bed”, and pan – “over”) are portions of soil mate trations in the main channels are regularly found rial designed for capturing water. The fields were to be 0.5 g per liter which hurts aquatic plants made by accumulation of organic matter, loamy and causes soil salinization (Lot et al. 1979). lacustrine sediments, or any material which served to consolidate the islands, separated by Surprisingly, the soils of chinampas have not a system of channels, which served for commu received much attention in scientific literature. nication and drainage (Jiménez et al. 1995). The Though numerous studies have been conduct soil surface is about one meter over the water ed in Mexico, their results mostly appeared as level. In the course of chinampas use, additional thesis of various levels or conference abstracts, lake mud could be added, or excessive soils i.e. in a form practically unavailable for scien could be removed to construct new chinampas tific community. Few papers published in sci (Coe 1964). entific journals dealt with particular issues, such as chinampas cultural heritage (RojasRebela The system of chinampas is a traditional agrar 1983), organic matter composition (García et ian technique, which includes a series of agro al. 1994; ReyesOrtigoza and GarcíaCalderón nomic, fishery, and forestry activities (Gómez 2004; GonzálezSalgado et al. 2010), contamina Pompa 1978; Ezcurra 1990; Rosas et al. 1984). tion (Vallejo 1994; RamosBello 2001) or micro Crop production, at least of 30 species, includ biology (Rosas 1984). Until now the morphology ing maize, vegetables and other eatable crops, and properties of chinampas soils have not been provided high diversity of the agroecosystem reported in internationally available papers. The and balanced nutrition for the producers (West need for an adequate characterization of these and Armillas 1950; RojasRabiela 1990). The unique soils is redoubled because the whole chinampas zone of Xochimilco was character chinampas system is endangered due to increas ized by a combination of growing vegetables ing urbanization and contamination of the area. It and flowers that favored biological diversity and is important to know reference characteristics of economic sustainability of the region. Moreover, chinampas soils and to estimate the salinization most cultivated fields were combined with fruit risks in order to develop efficient management, trees and some forest trees, such as Salix bon protection and improvement techniques. plandiana and Schinus mole, which stabilizes the soil. Another activity developed in the sys The objective of our study was to characterize tem was fishery and collection of various organ the morphology and properties of chinampas isms: birds, water invertebrates, rabbits, frogs, soils of Mexico City, to evaluate their actual sa etc., which formed a significant part of the diet linity and to estimate the potential salinization of of the producers (RojasRabiela 1983). these soils. SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 72 73 K)C!.;*)L?==;M)CIM)N!C1O!)1!=E?CPDM)DI?IM);CA?N!)?*1;L!CM):I.I)Q)RC!*<=D TI)*#J82),$#/, The district of Xochimilco is located at 88°09’04’’ Sierra de Patlachique in the south, Sierra Ne and 87°99’00’’ West longitude and 19°17’35’’ vada in the east, and Sierra de Las Cruces in the and 19°08’57’’ North latitude; the medium alti west. According to the geological data, the zone tude of the site is 2,240 m above sea level. The is composed mainly of alluvial materials, derived community is formed by 14 villages, among from Sierra Nevada and Sierra de Las Cruces. which Santa María Nativitas, Santa Cruz Acal Also, there are materials originated from the last pixca, San Gregorio Atlapulco, San Luís Tlaxi periods of volcanic activities, such as basalts altemalco, Santiago Tulyehualco and Xochimilco and pyroclastic materials. are “chinamperos”, i.e. are constructed on exca vated lacustrine sediments, dissected by chan The lake of Xochimilco has an irregular form, nels. Xochimilco is situated in the hydrological stretching from east to west, and is more than region ChalcoXochimilco, which is a part of the 11 km in length and 2 km in breadth, with a total catchment of Mexico. This catchment occupies area of 2,200 ha. It consists of a system of chan an area of 21,000 ha, where about 800 ha are nels; the most important are Cuemanco, Nacio not used for agriculture at the present time. nal, Caltongo, San Cristobal, el Bordo, Japón, Apatlaco, Tlicuili, and Tlilac. The climatic conditions, according to the sys tem of Köppen, modified by García (1973), are We studied 10 soil profiles, concentrated in four classified as temperate subhumid with sum sites, corresponding to the main chinampas mer rainfalls, with a medium annual tempera zones in Mexico City (Figure 1; Table 1). The soil ture between 12 and 18ºC, the temperature of pits were located in the sites of Xochimilco (Pro the coldest month varying from –3 to 18ºC, and files 1 and 2), San Luís Tlaxialtemalco (Profiles the temperature of the hottest month less than 3, 4, and 5), Tláhuac (Profiles 6, 7, and 8), and 22ºC. Medium annual precipitation varies from Míxquic (Profiles 9 and 10). The depth of the pro 680 to 936 mm, with the most rainy months from files varied between 90 and 180 cm depending June to September, and the driest period from on the water table. The sampling was done dur November to March (INEGI 1997). ing winter and spring which correspond to the dry season. Since the profiles were difficult for Xochimilco is surrounded by a chain of moun division into genetic horizons, the morphological tains, the most important being Sierra de Gua description and soil sampling were done every dalupe, Sierra Chichinautzin, Cuautzin, and 10 cm down to the groundwater level. Figure 1. Schematic map of the study area in the chinampas area of Mexico City. Black boxes !"##$%&"'()*")%"+,)&#"-,$%. SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 72 73 K)!CA<><1).?@<1;)1 Table 1. 01$),"!2*+"')"3)%"+,)&#"-,$%)+')chinampas zone of Mexico City /".)&#"-,$ Coordinates Locality 1 19º16’9.7” N 99º05’16.1” W Xochimilco 2 19º16’46.5” N 99º05’17.3” W Xochimilco 3 19º1545.7” N 99º02’4.4” W San Luís Tlaxialtemalco 4 19º15’33.2” N 99º02’5.2” W San Luís Tlaxialtemalco 5 19º15’46.5” N 99º01’55.3” W San Luís Tlaxialtemalco 6 19º15’58.4” N 99º00’23.1” W Tláhuac 7 19º16’3.8” N 99º00’33.9” W Tláhuac 8 19º16’5.6” N 99º00’20” W Tláhuac 9 19º13’13.8” N 98º57’29.3” W Míxquic 10 19º13’17.8” N 98º57’28.3” W Míxquic UI)./#4+8, Morphological description was done according tion. The bulk density was determined using the to Schoeneberger et al. (2002). Colour was iden cylinder method, and the texture using the hy tified using Munsell Soil Color Charts (2000). The drometer method (Tan 1996). The electric con description was made by soil horizons, though ductivity and ionic composition were evaluated only a provisional division into genetic horizons in saturated paste (Jackson 1969), the ionic con could be done. The chinampas soils generally centrations of Ca2+ and Mg2+ were measured by have a homogeneous darkcoloured profile, with atomic adsorption spectrometer, K+ and Na+ by minor variation in organic matter content and flame photometer, chlorites by precipitation with texture which are difficult to distinguish in the silver nitrate and potassium dichromate indica field. Since soil horizons were difficult to discern, tor, sulphates – gravimetrically, carbonates and we collected samples for chemical analysis ev bicarbonates – volumetrically. Soils were clas ery 10 cm down to the groundwater level. The sified according to the World Reference Base samples were analyzed according to the routine (IUSS Working Group WRB 2006). methods of soil chemical and physical analysis (van Reeuwijk 2002; Burt 2004). In each sample we determined: pH with a glass electrode–calo M mel electrode pH meter method (H2O and 1 KCl extraction with a soil: solution ratio 1:2.5), organ ic C by wet combustion, exchangeable bases by NH4Ac method, and cation exchange capacity (CEC) by calcium chloride saturation. Soluble salts were washed out with alcohol before ex changeable bases extraction and CEC evalua SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 74 75 K)C!.;*)L?==;M)CIM)N!C1O!1!=E?CPDM)DI?IM);CA?N!)?*1;L!CM):I.I)Q)RC!*<=D VI)C/,J(#, Soil morphology and properties undisturbed lacustrine sediments. During the dry season the surficial layers down to 3050 cm The morphological characteristics of all the were dry or slightly moist, and the deeper soil studied profiles were similar. All the soils had a horizons were moist and wet, with moisture con relatively uniform profile down to 120160 cm. tents increasing with depth. Though soils were The division into horizons might be done mainly saturated with water, dark organic matter did by slight differences in organic matter con not permit observing gleycolours. Iron depletion tent and texture. Since distinguishing horizons was not observed in these soils, because Fe did was difficult and it was not clear whether this not reduce in alkaline environments even when discrepancy was a result of pedogenetic pro RedOx potential was low. Manganese deple cesses or lithological discontinuity, we preferred tion was possible, but it was difficult to detect describing the soil by 10cm layers. In general in black matrix. Some profiles of the sites San terms, the morphology of the chinampas soils Luís Tlaxialtemalco (profile 3), Tláhuac (profile 8) resembled that of deep organic soils (Histosols), and the profiles of Míxquichad efflorescences of but the organic matter content was lower. The soluble salts in overdried surficial horizons. colour of the profiles was black when moist (10YR 2/1), and varied from grey (10YR 5/1) to The bulk density was very low in all the profiles dark grey (10YR 4/1) when dry. The texture var (Figure 2), with maximum values in the topsoil ied from silty loam to clay, and in deeper layers ,$%%)*12')4.5)67861. At the sites Xochimilco, San of most of the profiles the texture seemed to be Luís Tlaxialtemalco and Tláhuac the bulk den heavier than in the topsoil layers. All the layers sity decreased with depth irregularly; at the site were soft and friable. The structure was weak Míxquic the decrease was strong and continu subangular blocky. At more than 1 m depth some ous. At the latter site the soils showed the low profiles had a thin lightgrey layer that has been est bulk density values, which in deeper layers previously reported to be a diatomic shells accu corresponded to the values reported for organic mulation (ReyesOrtigoza and GarcíaCalderón %"+,%)94.:;4.<)67861). 2004). The presence of such layers identified Figure 2.)=>,8)($'%+*?)"3)*1$)@!1+'2A&2%@)%"+,%B)'>AC$#%)"3)*1$)!>#D$%)!"##$%&"'()*")*1$)&#"-,$)'>AC$#%. SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 74 75 K)!CA<><1).?@<1;)1 The organic C content strongly varied between bimodal distribution with maximum values in the the sites and between the layers in the same topsoil and at more than 1 m depth. profile (Figure 3). In the surficial layer (more or less the upper 50 cm) the organic C concentra Clay content differed between the profiles in a tion was around 10% at the sites Xochimilco wide range, from 5% to almost 80% among the and Míxquic, and about 5% at the sites San profiles, even in those located at the same site Luís Tlaxialtemalco and Tláhuac. The vertical (Figure 4). The vertical distribution of clay was distribution of organic C showed two principal strongly irregular in almost all the profiles. Only patterns: in the profiles of the sites San Luís the profiles of the site Míxquic showed anuni Tlaxialtemalco and Tláhuac its content was rela modal distribution of clay with maximum values tively uniform or increased with depth, and in the at depths between 20 and 100 cm. profiles of Xochimilco and Míxquic it showed a Figure 3.)E#62'+!)!2#C"')!"'!$'*#2*+"')+')*1$)@!1+'2A&2%@)%"+,%B)'>AC$#%)"3)*1$)!>#D$%)!"##$%&"'()*")*1$)&#"-,$)'>AC$#%. Figure 4.)F,2?)!"'*$'*)+')*1$)@!1+'2A&2%@)%"+,%B)'>AC$#%)"3)*1$)!>#D$%)!"##$%&"'()*")*1$)&#"-,$)'>AC$#%. SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 76 77 K)C!.;*)L?==;M)CIM)N!C1O!1!=E?CPDM)DI?IM);CA?N!)?*1;L!CM):I.I)Q)RC!*<=D The reaction of the surficial layers of all the soils are considered to be more toxic than other salts. profiles was alkaline, with pH values around 8.5 According to this criterion, all the soils, except (Figure 5). In all the profiles the pH values de for those of the Xochimilco site, should be con creased with depth. In two profiles, No. 2 (Xochi sidered to be severely saline. It was remarkable milco) and 10 (Míxquic) the pH values dropped that within the same site the difference in soluble down at the depth of 100120 cm to values salts concentration in the surficial horizon was lower than 6 that meant slightly acid reaction. great. The vertical distribution of salts also var In some profiles the pH values increased slightly ied between the profiles. Most profiles showed with depth down to 50100 cm; in the profiles 1 an accumulative pattern with an intensive maxi (Xochimilco) and 8 (Tláhuac) the pH exceeded mum at the surface and a sharp decrease with 9. Even lower the pH values also decreased in depth, like the profiles mentioned above for high these profiles. salinity. In the profiles 4, 5 (San Luís Tlaxialte malco), 6, and 7 (Tláhuac) the salinity decreased Soil salinity and sodicity gradually with depth. Profiles 1 and 2 (Xochi milco) showed an irregular vertical distribution of Soil salinity data, expressed as electric con soluble salts. ductivity (EC), showed that all the profiles were characterized by elevated salt contents (Figure Soil sodicity could be expected already from 6). However, only several profiles had a soluble high pH values (Figure 5). However, alkaline re salt concentration that is recognized as severe action could be caused either by the presence salinity according to US Salinity Laboratory Staff of sodium carbonates or exchangeable sodium; 9GHI Figure 5.)01$)&P)D2,>$%)"3)*1$)@!1+'2A&2%@)%"+,%B)'>AC$#%)"3)*1$)!>#D$%)!"##$%&"'()*")*1$)&#"-,$)'>AC$#%. SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 76 77 K)!CA<><1).?@<1;)1 Figure 6.)K,$!*#+!)!"'(>!*+D+*?)"3)*1$)@!1+'2A&2%@)%"+,%B)'>AC$#%)"3)*1$)!>#D$%)!"##$%&"'()*")*1$)&#"-,$)'>AC$#%. Figure 7.)Q$#!$'*26$)"3)%"(+>A)%2*>#2*+"')"3)*1$)@!1+'2A&2%@)%"+,%B)'>AC$#%)"3)*1$)!>#D$%)!"##$%&"'()*")*1$)&#"-,$)'>AC$#%. The composition of soluble salts is shown in out the profile. In the anionic part, the topsoil Figure 8; only one profile per site was pictured, had the following sequence of concentrations: 2 2 because the distributions were similar among SO4 >Cl > HCO3 > CO3 . With depth, the con the profiles of the same site. At the Xochimilco centration of sulphates strongly increased, and site, the total concentration of ions of the satu that of bicarbonates decreased; the carbonates rated paste in the profile had a bimodal distribu were not present in the subsoil at all, and chlo tion, with maximum values in the topsoil and at rides concentration remained the same. At the more than 1 m depth. The topsoil had a cation site San Luís Tlaxialtemalco the soils had the concentration sequence Na+ > Mg2+> Ca2+, and highest ionic concentration in the surficial layer; Ca2+ concentration increased with depth, while the concentration sharply decreased with depth. Na+ and Mg2+ concentrations slightly decreased. The abundance of cations had the same se The concentration of K+ was negligible through quence as for Xochimilco profiles, but the con SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 78 79 K)C!.;*)L?==;M)CIM)N!C1O!1!=E?CPDM)DI?IM);CA?N!)?*1;L!CM):I.I)Q)RC!*<=D centrations of Na+ and Mg2+ were much higher than that of Ca2+ at all depths. In the anionic 2 part, also HCO3 and CO3 concentrations were 2 negligible compared with that of SO4 and Cl . At the Tláhuac site, the profiles had a distribu tion of total ionic concentration with two humps: in the surficial layer and at the depth of 4050 cm. The most abundant cations were Na+ and Mg2+; the ratio between these two cations varied with depth, while Ca2+ concentration was almost uniform. Among the anions, the most abundant was sulphate, its concentration varying with depth, while chloride and bicarbonate concentra tions were relatively uniform. The carbonate and potassium ionic concentrations were low. The profiles of the Míxquic site had a pattern of dis tribution of the ions in the saturated paste similar to that in the soils of the site San Luís Tlaxialte malco. The difference was that at the Míxquic site the concentration of Mg2+ was slightly higher than that of Na+, chlorides were more abundant than sulphates, and K+ concentration was higher than in the soils of other sites. Figure 8. Ionic composition of the saturated paste of selected *chinampas*soils: @2@)R)&#"-,$):S)T"!1+A+,!")%+*$S)@C@)R)&#"-,$) SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 78 79 K)!CA<><1).?@<1;)1 WI)E$,&J,,$+7 01$)"#+6+')2'()!,2%%+-!2*+"')"3)%"+,% the ratio of mineral particles in lacustrine sedi ments. Consequently, the recent upper layers of The parent material of all the studied soils con lake deposits have higher proportion of mineral sists of lacustrine sediments, disturbed by hu particles and lower percentage of organic car man activities. All the soils are characterized by bon. However, at the other two sites (Xochimilco nonuniform vertical distribution of the texture and Míxquic) the soils have two maximums of and some other properties, such as organic mat organic carbon concentration in the subsoil and ter content, bulk density, etc. The main question in the surficial layer. If we accept the hypothesis to answer is: the heterogeneity of texture and re of recent increase in mineral material sedimenta lated soil properties is a result of lithological dis tion due to humaninduced erosion, the increase continuity, human disturbance, or pedogenetic in soil organic matter in surficial layer should be processes? We believe that pedogenetic hy specially explained. The increase in organic mat pothesis should be disregarded for several rea ter might be ascribed to accumulation of the or sons. First, such a strong clay translocation re ganicrich materials, raised in the course of ex quires long time and stable environments (Birke cavation of channels. The other possibility is that land 1999; Targulian and Krasilnikov 2007), while strong accumulation of organic matter was due chinampas are known to be exposed to the sur to manuring and warping (GonzálezSalgado et face not very long ago, and the landscape has al. 2010). Finally, natural accumulation of organic undergone many changes in hydrological regime matter in the topsoil may be ascribed to a natu and microclimatic conditions since the expose ral humification process under grass vegetation (De Cserna et al. 1988). Secondly, the mineral (García et al. 1994; ReyesOrtigoza and García part of soil clays in chinampas consists mainly Calderón 2004). The latter hypothesis is not very of poorly ordered products of initial weathering convincing because the vegetation is practically of volcanic ashes (Huez et al. 1989); these com the same at all the studied sites, and it is dif ponents are known to be hardly mobile in soils ficult to explain why the vertical distribution is (Sedov et al. 2003). Finally, the vertical distribu completely different under the same natural con tion of clay content is different in soils at different ditions. The contribution of excavated material sites: it is difficult to expect that the pedogenetic versus manure addition is still under question; processes would act so differently at short dis the similarities in organic matter composition tances under almost the same environments. among the upper and deep soil layers reported Nonuniformity of lacustrine sedimentation was by ReyesOrtigoza and GarcíaCalderón (2004) reported for the lakes of the Valley of Mexico (e.g. allow giving a priority to the suggestion that the Bradbury 1971), thus the irregular vertical distri organic matter in the topsoil comes from deeper bution of clay content in chinampas soils might lacustrine sediments. However, further studies be also ascribed to the historical fluctuations in are needed to confirm this hypothesis. sedimentation rates. For better understanding of the origin of the soils of chinampas we also inter The classification of the soils under study was preted the vertical distribution of organic matter unclear until recently. The first round of soil sur content in the soils. The organic matter concen vey made by INEGI (1997) showed the chinam tration increases with depth in soils at the sites pas soils of metropolitan areas as Phaeozems, San Luís Tlaxialtemalco and Tláhuac. This type i.e. mineral soils with deep humusrich surficial of distribution corresponds well with the sedi horizon, with high base saturation throughout mentation history of the region. According to the profile and no secondary carbonates (IUSS the paleogeographical reconstructions, the Val Working Group WRB 2006). The name was giv ley of Mexico was covered with forest vegeta en using the earlier version of the international tion, but in the historical time the aridization of soil classification (CETENAL 1970), where an climate and human invasion resulted in almost thropogenically transformed soils were absent. complete deforestation of the area (Heine 1987). The disadvantage of this classification was that The loss of vegetation and intensive cultivation the anthropogenic transformation of the soil, the has led to intensive soil erosion, thus increasing layering of soilforming material, the presence of SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 80 81 K)C!.;*)L?==;M)CIM)N!C1O!1!=E?CPDM)DI?IM);CA?N!)?*1;L!CM):I.I)Q)RC!*<=D organic layers and salt accumulations were not except for one soil at the Xochimilco site also reflected. Recently, soil research and mapping in have the suffix modifier Sodic that means that Mexico accepted the latest version of the World the PSS is higher than 15% in the upper 50 cm Reference Base for Soil Resources (IUSS Work of the soils; in some cases the suffix Endosodic ing Group WRB 2006). The experts who devel is used to show that the PSS > 15% is found at oped this classification visited Mexico, includ the depth 50100 cm, or Hypododic if the PSS > ing the chinampas area, and provided a correct 15% is found only in a thin surficial layer, but the classification name for these soils: Terric An profile has PSS > 6% throughout. About half of throsols (ReyesOrtigoza and GarcíaCalderón the soils also received a suffix modifier Alcalic, 2004). The name means that the soils formed which means that the soil has pH > 8.5 in the under continuous intensive agricultural use with upper 50 cm of the profile. Three soils have or an addition of mineral and organic materials on ganic materials in the profile, and receive suffix the surface. For additional characteristics of modifiers Thaptohistic, if the organic material is these soils, other prefix and suffix modifiers may found between 50 and 100 cm, or Bathihistic, if be added (Table 2). All the studied profiles re the organic material is located at more than 100 ceive also a prefix qualifier Fluvic, because they cm depth. Finally, some soils having layers of have fluvic materials, evidenced by irregular clayey texture received a suffix Clayic; if the lay vertical distribution of clay and organic carbon er is found between 50 and 100 cm, the modifier content (Figures 3 and 4). The soils of the sites is used in the form Endoclayic. Summarizing, the San Luís Tlaxialtemalco and Míxquic also have a use of the latest version of the international clas prefix modifier Episalic that means that there are sification (IUSS Working Group WRB 2006) ade salic horizons in the upper 50 cm of these pro quately reflects the origin of chinampas soils and files, i.e. the electric conductivity is more than allows including practically all the details of par GI) (N7A1) 9L5) (N7A1 in the case of alkaline ticular soil properties. Also it reflects the internal soils) in the surficial layer. Almost all the profiles variation in soil properties in the chinampas zone. Table 2. 01$)!,2%%+-!2*+"')"3)%"+,)&#"-,$%)+')chinampas zone of Mexico City (IUSS Working Group WRB 2006) /".)&#"-,$ Locality F,2%%+-!2*+"' 1 Xochimilco Fluvic Terric Anthrosol (Endosodic, Alcalic) 2 Xochimilco Fluvic Terric Anthrosol (Alcalic) 3 San Luís Tlaxialtemalco Episalic Fluvic Terric Anthrosol (Hyposidic, Alcalic, Thaptohistic) 4 San Luís Tlaxialtemalco Episalic Fluvic Terric Anthrosol (Hyposodic) 5 San Luís Tlaxialtemalco Episalic Fluvic Terric Anthrosol (Hyposodic, Clayic) 6 Tláhuac Fluvic Terric Anthrosol (Hyposodic, Thaptohistic) 7 Tláhuac Fluvic Terric Anthrosol (Sodic, Alcalic, Clayic) 8 Tláhuac Fluvic Terric Anthrosol (Sodic, Alcalic, Endoclayic) 9 Míxquic Episalic Fluvic Terric Anthrosol (Hyposodic, Alcalic, Endoclayic) 10 Míxquic Episalic Fluvic Terric Anthrosol (Sodic, Endoclayic, Bathihistic) SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 80 81 K)!CA<><1).?@<1;)1 Salinization and alkalinization hazards presence of sodium carbonates (US Salinity Laboratory Staff 1954; Sánchez et al. 2003). The The soils of chinampas are both a cultural heri data obtained for metropolitan soils differ from tage and still a source of income for a significant that reported for exTexcoco lake, where sodium part of population of Mexico City (RojasRebela carbonate is a common component (Sánchez 1983). The traditional management practices et al. 2003). The ionic composition of salts is showed their effectiveness for ages, and are be closely connected with the composition of vol lieved to be the best option for actual agricul canic ash that constitutes the major part of the ture in the region (RojasRebela 1990). However, lacustrine sediments (Bradbury 1971). The ash chinampas recently faced new challenges due to is initially rich in sodium and magnesium; also soil degradation issues, especially soil saliniza the abundance of sulphates is due to the high tion, that require new approaches to soil man percentage of sulphur compounds in pyroclastic agement and conservation. sediments. The soils studied here show a significant varia The vertical distribution of soluble salts in most tion in salinity and sodicity. The reasons for the of the studied soils shows distinct accumulation difference in soluble salts content is not well at the surface (Figure 6); in some places one can understood yet (Ramos et al. 2001; Sánchez et see even morphologically evident salt accumula al. 2003). There is no dependence of salinity on tions in the topsoil. This type of vertical distribu clay or organic matter content (Figures 3, 4 and tion is usually observed in the dry season, whilst 6). Neither the accumulation of soluble salts in in the rainy season the maximum of salt concen topsoil layers depends on the depth to ground tration moves to the deeper horizons (Huez et water level that corresponds to the total depth of al. 1989). However, even periodical increase in profiles (the pits were excavated exactly down soluble salts concentration in the topsoil may to the groundwater table). The most probable have a negative effect on soil productivity. Gen explanation for the irregular spatial pattern of erally, increasing salinization has been reported salts accumulation is the proximity of uplifted repeatedly for chinampas soils for more than a areas, for example, of the volcano Xico that is half of a century (West and Armillas 1950; Coe situated close to Míxquic, the most saltaffected 1964; Escurra 1990; RojasRabiela 1983; Vallejo site. It explains why the Xochimilco site, which and Aguilera 1994; Jiménez et al. 1995; Ramos is strongly contaminated with urban waste, has et al. 2001). The authors agree that the main rea the lowest soil salinity level. The uplifted volca sons for salts accumulation are: water evapora nic bodies give an additional source of released tion in channels that leads to gradual lowering of cations to soils, while Xochimilco receives little the water level, increasing urban waste disposal, input due to its remoteness from uplifted areas. illegal settlement and conversion of agricultural lands into urban areas. Complex driving forces Most of the studied soils are characterized both of salinization require acomplex set of measures by high salts content in surficial layer and alka for soil protection and conservation. Simple line reaction. A combination of salinity and alka measures like washing the salts would not work linity often means that the soil is affected by so because highly mineralized water works as a dium carbonates that are regarded as very toxic source of salts (Huez et al. 1989). Also the salin salts. However, it is not the case for chinampas ity hazards should be treated together with other soils. The data on the ionic composition of soil degradation phenomena, such as contami soluble salts show that the most abundant salts nation with heavy metals and organic pollutants in the studied soils are sodium and magnesium (Vallejo and Aguilera 1994; Gutiérrez et al. 1995; sulphates and chlorides. The alkaline reaction of Ramos et al. 2001). For successful restoration of the soils results from the effect of exchangeable chinampas a combined approach should be ap sodium. Alkaline reaction caused by exchange plied, including soil remediation, water treatment able sodium also has a negative effect on plant and ecosystems protection. growth, but it is much less dangerous than the SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 82 83 K)C!.;*)L?==;M)CIM)N!C1O!1!=E?CPDM)DI?IM);CA?N!)?*1;L!CM):I.I)Q)RC!*<=D ]I)1+7&(J,$+7, XI)!&Y7+Z(/8[/\/7#, The soils of chinampas are unique products of The research was supported by the National a combination of lacustrine sedimentation under Counsel of Science and Technology of Mexico strong influence of pyroclastic materials, artifi (CONACyT) grant No. 30712. The authors wish cial soil construction, and longterm intensive to acknowledge our colleagues in the Laborato agricultural management. These soils are char ry of Edaphology, Faculty of Sciences, National acterized by layered structure, relatively uniform Autonomous University of Mexico (UNAM), who black and dark grey colour, irregular distribution contributed to this research or supported the au of organic carbon and clay with depth, gener thors at various stages of the work. ally high percentage of carbon, including organic materials in some layers. Some soils show an increase in organic matter with depth, and other profiles have maximum organic matter content in the surficial layers and in the subsoil. We con cluded that generally the dynamics of sedimen tation resulted in the decrease in organic mat ter in the upper layers of lacustrine sediments, because of recent increase in erosion rates and consequent increase in the proportion of mineral particles in the sediments. Most probably high organic matter content in surficial layers of the second group of soils studied is due to exca vation and accumulation of organicrich subsoil material in the course of digging the channels. The vertical structure of the profiles is a result of management practices and sedimentation dy namics rather than pedogenetic differentiation of soil into horizons. These soils are classified as Fluvic Terric Anthrosols and vary in the presence of organic layers, salinity, and alkalinity. The chinampas soils are characterized by al kaline reaction and high exchangeable sodium content. The alkalinity of soils depends more on exchangeable sodium than on free sodium bi carbonate; the dominant salts are sodium and magnesium sulphates and chlorides. Soluble salts content varies in a wide range and depends on the proximity to uplifted areas that serve as sources of freshly released ions. During the dry season we observed strong concentration of salts in the surficial layers of the majority of soils. Soil remediation in chinampas is a big challenge because a combined treatment of water, soil and vegetation is needed. SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 SJSS. SPANISH JOURNAL OF SOIL SCIENCE YEAR 2011 VOLUME 1 ISSUE 1 82 83 K)!CA<><1).?@<1;)1 REFERENCES 7) Heine K. 1987. Anthropogenic sedimentological changes during the Holocene in Mexico and Central America. In: Starkel L, editor. Anthropogenic sedi 7) Birkeland PW. 1999. Soils and Geomorphology. mentological changes during the Holocene. Striae New York, Oxford University Press, 3rd edition. 26. p. 5163. 7)Bradbury JP. 1971. Paleolimnology of lake Texco 7) Huez MA, Ortega M, Ramírez C, Tone JL. 1989. co, Mexico. Evidence from diatoms. Limnology and F2#2!*$#+[2!+\')($)2,6>'2%)&#"&+$(2($%)-%+!"Z>VA+ Oceanography 16:180200. cas de los suelos y espesores subyacentes del predio Montecillo. 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