Linking soil properties and pre-Columbian agricultural strategies in the Bolivian lowlands: The case of raised fields in Exaltación

Quaternary International xxx (2016) 1e13

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Linking soil properties and pre-Columbian agricultural strategies in
n the Bolivian lowlands: The case of raised fields in Exaltacio Leonor Rodrigues a, *, Umberto Lombardo b, Elisa Canal Beeby c, Heinz Veit a a

Institute of Geography, University of Berne, Hallerstrasse 12, CH-3012 Bern, Switzerland University of Pompeu Fabra, Ramon Trias Fargas 25-27, Merc e Rodoreda, ES-08005 Barcelona, Spain c Independent Researcher, Passeig de la Carena 33, Valldoreix, 08197, Barcelona, Spain b

a r t i c l e i n f o

a b s t r a c t

Article history: Available online xxx

This paper aims to further our understanding of pre-Columbian agricultural systems in the Llanos de Moxos,
n, in Bolivia. Three different types of raised fields co-existing in the same site near the community of Exaltacio north-western Beni, were studied. The morphology, texture and geochemistry of the soils of these fields and the surrounding area were analysed. Differences in field design have often been associated with the diversity of cultural practices. Our results suggest that in the study area differences in field shape, height and layout are primarily the result of an adaptation to the local edaphology. By using the technology of raised fields, preColumbian people were able to drain and cultivate soils with very different characteristics, making the land suitable for agriculture and possibly different crops. This study also shows that some fields in the Llanos de Moxos were built to prolong the presence of water, allowing an additional cultivation period in the dry season and/or in times of drought. Nevertheless, the nature of the highly weathered soils suggests that raised fields were not able to support large populations and their management required long fallow periods. © 2015 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Amazonia Llanos de Moxos Bolivia Raised fields Pre-Columbian Agricultural strategies

1. Introduction The Llanos de Moxos (LM), a seasonally inundated savannah located in the southernmost part of the Amazon basin, is an important area for the study of pre-Columbian human-environment interactions in the region. Recent archaeological studies have shown that pre-Columbian settlements date back more than 10,000 years here (Lombardo et al., 2013b). Increasing research shows that several cultures settled in the LM at different times, building a variety of earthworks (Denevan, 2001; Erickson, 2008; Lombardo et al., 2011a; Jaimes Betancourt, 2013; Prümers, Jaimes Betancourt, 2014). One of the most striking examples are the thousands of hectares of pre-Columbian raised fields found in the LM. Raised fields are elevated agricultural earth platforms. Why they were built and how they were managed in the past are matters of debate. Some authors suggest that raised fields were a form of highly productive agriculture able to sustain large populations (Walker, 2004; Erickson, 2008; Whitney et al., 2014), and others propose that fields were built as a mitigation strategy against severe floods during periods of frequent extreme events (Lombardo

* Corresponding author. E-mail address: [email protected] (L. Rodrigues).

et al., 2011b, 2013a; Baveye, 2013; Rodrigues et al., 2015). Unlike other earthworks, which tend to be concentrated in relatively small areas (e.g. monumental mounds, fish weirs and ring villages), raised fields can be found in areas with different edaphological conditions and are distributed all over the western part of the LM, where flooding is much more severe than in the eastern LM (Hanagarth, 1993; Lombardo et al., 2011a). Different types of raised fields, ranging from round mound fields to large rectangular platforms, have been identified in the LM; the different fields do not generally coexist in the same areas (Denevan, 2001; Lombardo et al., 2011a). It has been suggested that these regional differences in raised field types are the result of the diversity of cultural practices (Walker, 2011; Rostain, 2013, p. 136) and/or adaptation strategies to different local environments (soils, hydrology, topography) (Denevan, 2001, p. 220; McKey et al., 2010; Lombardo et al., 2011b; Rostain, 2013, p. 136). In the case of raised fields in French Guyana, the diversity of shapes has been associated to a combination of available technology and adaptation strategies to the local hydrology (Rostain, 2013, p. 136). Raised fields in the LM have been used as a proxy for population density, by estimating their carrying capacity and amount of labour needed for their construction (Walker, 2004; Erickson, 2006). For his study site in the region of Santa Ana (Fig. 1), Walker (2004, p. 48)

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Fig. 1. a.) MODIS image of the LM and map of South America. Main rivers are represented as blue lines. Red dots indicate major cities mentioned in the text. The red square
n. b.) Digital Elevation Model of the study area with raised fields (yellow polygons) and the three sites where the fieldwork was carried out: represents the study area of Exaltacio Ex1, EX2, Ex3. Dark blue lines are paleo rivers (note: slightly elevated parts in orange are the associated remains of paleo levees). Archaeological sites are indicated: El Cerro and San Juan. DEM data: CGIAR-CSI SRTM90m Database (http://srtm.csi.cgiar.org). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

estimated a population density ranging from a minimum of 3 people/km2, considering the amount of labour needed to build all the fields, to a maximum of 30 people/km2, taking into account their carrying capacity. However, there is no evidence that the fields were built at the same time. It has been suggested that the fields here could have been built over a period of hundreds of years, in which case a much smaller population, of 1 person/10 km2, could have been enough to construct all the fields (Lombardo, 2010). Understanding why people built raised fields and how these were managed can contribute to better estimate the amount of people living in the LM in pre-Columbian times. Although increasing research on the subject of raised fields in the LM (Michel, 1993; Erickson, 1995; Lombardo et al., 2011b; Denevan, 2001; Walker, 2004; Rodrigues et al., 2015), not much is known about their carrying capacity and ancient management. Until now, the combination of mapping and the few existing field studies of raised fields in the LM show that the fields were built mainly on fluvial levees and other naturally well drained areas, often on silty to sandy sediments, in order to further improve their drainage (Lombardo et al., 2011b; Walker, 2004; Rodrigues et al., 2015). However, raised fields constructed in permanent wetlands on impermeable sediments have also been reported (Denevan, 1966, 2001; Erickson, 1995; Michel, 1999). Unfortunately, no soil or hydrological studies have been carried out on these fields.
n, north of Santa Ana This study focuses on the area of Exaltacio de Yacuma (Fig. 1), where raised fields are found in high density. The site is of particular interest as it has three different types of raised fields coexisting in very different environmental settings. This case study can contribute to a better understanding of why and how pre-Columbian people used raised fields in the LM and whether different field shapes and dimensions are the result of natural or cultural factors. 2. Study area 2.1. Geography and environment The study area is located in the northwestern part of the Beni Department, in the Yacuma Province, in the vicinity of the indige
n de La Cruz (Fig. 1). It is situated on nous community of Exaltacio


, a few kilometres south of the western bank of the Río Mamore ~ ez joins the Mamore
. The climate in the LM is where the Río Iruyan controlled by the South American Summer Monsoon, leading to heavy convective rainfalls in austral summer and dry conditions in winter (Zhou, Lau, 1998; Garreaud et al., 2009). The mean annual precipitation in the region is 1500 mm (Hijmanns et al., 2005). The extent of flooding during the rainy season varies greatly, within a range of 30,000 km2 and >80,000 km2 (Hanagarth, 1993; Hamilton et al., 2004).
n is The geomorphology of the landscape in the area of Exaltacio complex and characterized by a mosaic of fluvial deposits of the
partially covering fluvial features of active white water Río Mamore ~ ez and Río Omi, which cross the paleo rivers (Fig. 1). The Río Iruyan area, are underfit rivers that flow in ancient courses of the Río Beni (Dumont and Fournier, 1994) and do not overflow during the rainy season (Walker, 2004). The presence of underfit rivers, combined with other evidence of neotectonic activity, suggests that the area is subject to uplift (Lombardo, 2014). Because of this uplift, floods in the Yacuma region are caused by local precipitation and not by river overflow (Hanagarth, 1993; Bourrel and Pouilly, 2004) (Fig. 1). The combination of these active and relict morphological features, such as natural levees, back swamps and ox-bow lakes, creates local relief and differences in sediment grain size within the study site. This is of great importance as slight differences in topography and soil properties are the main drivers determining vegetation type and distribution (Mayle et al., 2007). According to Beck (1983) and Langstroth (1996), the landscape can be broadly divided into three natural ecosystems in relation to their topography: the alturas, semi-alturas and the bajíos. The alturas are the river levees, generally covered with forest of evergreen species; the semi-alturas comprise levee back-slopes and river splays and are typically scattered with fire tolerant trees and bushes; the baíjos, which are seasonally to permanently flooded areas, comprise mostly grasses and wetland vegetation. The studied raised fields are located on the alturas (paleo levees) and the baíjos, whereas in the semi-alturas there are no raised fields (Figs. 1 and 2). The study site has sartenejas (Fig. 5b), small earth mounds built by worms, ants and termites (Hanagarth, 1993) that create micro relief (Fig. 5). Sartenejas are an indicator of badly drained soils, as worms construct their mounds above the water

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level for soil aeration (Hanagarth, 1993; McKey et al., 2010). Soils in this area have been described as very acid, hydromorphic, and nutrient poor (Lombardo et al., 2013a). The parent material of the soils comes from the Andes and has a high content of highly weathered clays, such as kaolinite (Guyot, 1992). The Santa Ana region is covered with alluvia, probably deposited by the Río Beni in the past, when it used to flow eastward (Hanagarth, 1993; Dumont, 1996). It is unclear when these sediments were deposited. However, several studies suggest that sediments in the northern part of the Beni are relatively old compared to the southern part (Hanagarth, 1993; Dumont, 1996; Lombardo et al., 2012). The fine and organic rich sediments found in the bajíos could have been deposited during periods in which the transport of suspended sediments decreased due to a reduction in river slope caused by a neotectonic uplift in the northern LM during the late Pleistoceneeearly Holocene (Lombardo, 2014).

1311e1446 for the El Cerro site (Walker, 2004). It is suggested that raised field agriculture was sustained throughout this period. However, it seems that raised fields in the San Juan site are more recent than the settlement there, as the fields are rich in artefacts (Prümers and Jaimes Betancourt, 2014), suggesting that they were built with sediments from the settlement once it had been abandoned. Raised fields do not normally contain artefacts and are therefore difficult to date directly (Erickson, 2008). Pre-Columbian land management practices at el Cerro site have been recently studied by Whitney et al. (2014). The study suggests that raised fields were continuously in use and cultivated with maize from AD 310 until at least the 14th century.

2.2. Archaeology in the study area

The location for the study of raised fields was first predefined with the help of Google Earth. An area of about 18 km2 was selected, where different types of raised fields coexist. Once in the field, three sites were chosen for excavation (Ex1, Ex 2 and Ex 3) (Fig. 2). To serve as reference, an additional profile was excavated away from the raised fields (REF 1). The local relief in the bajío (Ex1) and the altura (Ex2) was measured (Fig. 3) using a digital level Sokkia D50. In Ex1, a 630 m topographic profile was drawn based on measurements taken approximately every 1 m from the paleo levee across the bajío. In Ex2, 1233 elevation points were measured and a digital elevation model (DEM) of the fields was generated using the 3D analyst extension of ArcGis with natural neighbour interpolation (Fig. 4). In total, eight soil profiles were dug in the study site: in the bajío and the altura pits were dug both in the elevated bed of a field (Ex1F and

The study area has archaeological evidence of pre-Columbian occupational sites, forest islands and raised fields. Causeways, which are commonly associated to fields in the southern LM (Erickson and Walker, 2009; Lombardo et al., 2011a), are absent. The density of raised fields in this area is very high. Lombardo (2010) showed that 6.4% of this region is covered with raised fields. Unfortunately, archaeological excavations of settlements are scarce and there is only one detailed study of this area (Walker, 2004). Walker's study includes two major pre-Columbian settlements, the el Cerro Site and the San Juan site (Figs. 1 and 2). Both settlements are about 20 km away from the raised fields studied in the present work. The settlements have been dated using 14C, revealing an age of AD 446e613 for the San Juan site and AD

3. Material and methods 3.1. Soil sampling and laboratory analysis

Fig. 2. Detailed section of the study area. Each site Ex1, EX2, Ex3 is represented in detail below.

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Fig. 3. Study site Ex1: a.) Green polygons are raised fields, white dashed circles represent the ponds and the white dashed line the wide canal described in the text; the red line from A to B illustrates the location of the topographic. b.) Picture of studied raised fields. c.) Topographic profile from A to B. d.) Depth profile of excavated raised field. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Ex2F) and in the associated canal (Ex1C and Ex2C). A reference profile (REF) was excavated in the semi-altura, away from raised fields, between the border of the bajío and the altura (Fig. 2). The profile represents an unmanaged soil, against which we can compare the area with raised fields. In the area Ex3, three pits were excavated: two in adjoining fields (Ex3Fa and Ex3Fb) and one between the fields in the canal (Ex3C). Sampling was conducted every 10 cm. The description of the horizons/layers follows the guidelines of FAO (IUSS Working Group WRB, 2014). In order to carry out elemental- and C/N analyses and measure soil pH, 20 g of each sample were sieved through a 2 mm sieve and pulverized with a disk mill afterwards. For the pH, the samples were mixed with a 0.01 M CaCl2 solution and afterwards measured with a glass-

electrode. Concentrations of C and N were measured by dry combustion and gas chromatographic separation with a CNS analyser. For particle size distribution, organic matter was removed with 30% H2O2 and then measured with a Malvern Mastersizer Hydro 2000S. Cation Exchange Capacity (CEC) was measured after extraction of the exchangeable Cations Ca2þ, Mg 2þ, Kþ, Naþ,Mn2þ, and Al3þ, with 1 M ammonium nitrate solution (NH4NO3) and afterwards concentrations were measured using an atomic absorption spectrometer of the type analytikjena ZEEnit 700P. Effective Cation Exchange Capacity (CECeff) and Base Saturation (Bs) were calculated P as follows: CECeff ¼ Cations (exchangeable Cation ¼ Cation mg/ kg/molar mass mmol/l*valence) and BS ¼ ([Ca] þ [Mg] þ [K] þ [Na]/ CECeff)*100.

Fig. 4. Study site Ex2: a.) Orange polygons are raised fields, red points represent excavated pits. b.) Digital elevation model of the present-day morphology of the raised fields studied. The Topographic profile is indicated with a line from A to B. c.) Picture of the studied raised field. Trees like the chaparro tree (Curatella americana L.) and the tajibo tree (Tabebuia impetiginosa) grow on top of the fields. These trees are characteristic of the semi alturas and acid soils of the LM (Navarro and Maldonado, 2002). d.) Excavated profiles Ex2 from the field to the canal and REF profile. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. Study site Ex3: a.) Light blue polygons are raised fields, red points represent excavated pits. The straight lines cutting across the fields are modern fences. b.) Picture of the landscape at the site showing sartenejas (little earth mounds) built by invertebrates. c.) Excavated profiles of raised fields and canal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4. Results 4.1. Morphology of the fields
n, Following Denevan's definition (1970), the fields of Exaltacio in general, can be classified as platform fields. In all sites (Ex1, Ex2 and Ex3) the fields are more or less uniform in width (12e20 m). There is some variation in length, but most range from 100 m to 600 m. Platform fields are normally only slightly elevated (~20e50 cm), like in the case of the fields in Ex1 and Ex3. However, this does not apply in the case of Ex2, where the fields are up to 100 cm high. The arrangement of the fields, layout and shape, differs remarkably in each area. The fields in Ex1 are elongated, with an average length of 250 m and width of 8e10 m, and were built in a bajío between paleo levees (Fig. 3). Some of these fields are embanked. From what can be seen clearly today, there are at least two relatively wide canals of about 500 m long going from the northwest to the southeast and ending up, at both ends, in more or less circular ponds. The fields here are parallel to each other and perpendicular to the wide canals, which drain the water towards the ponds (Fig. 3). The topographic profile from the altura to the bajío reveals a maximum difference in altitude of 1.7 m and illustrates that the pond is located at the lowest point (Fig. 3). Between the fields and the canals in Ex1 there is an average difference in height of 23 cm. The fields in Ex2 follow a very different pattern (Fig. 4a). Fields come together in an angle of more or less 80 , in the shape of an arrowhead. The fields are on average 10 m wide and up to 500 m

long. The DEM shows that these fields are much higher than in the two other sites. Most fields are not embanked; however, some of them are closed off at one end, where they come together (Fig. 4a). The maximum difference in height between the canals and the fields is 94 cm (Fig. 4c). The fields in Ex3 are located between the altura and the semialtura (Figs. 2 and 5). The fields in Ex3 follow the more typical design of platform fields as described by Denevan (2001) and Walker (2004) (Fig. 5). Fields are 15e20 m wide and at least three times as long and about 20 cm high. Some of the fields are highly eroded and were therefore not digitalised, as their reconstruction is difficult. It seems that there are several generations of fields lying on top of each other (Fig. 5a). 4.2. Soil physical properties Field descriptions of each profile are summarized in Table 1 and results of laboratory measurements in Table 2 respectively. The fields in the bajío Ex1 consist of dark organic-rich clay, while the others, on the nearby altura (Ex2 and Ex3), are built on weathered loamy sediments. Ex2 and Ex3 have a thin organic poor Ah horizon and well-developed iron and manganese concretions in the lower part of the profiles (compare Figs. 3e5 and Table 1). According to their topographic position, the soil profiles show different textures. The particle size distribution can be seen in the soil texture triangle (Fig. 6). Each group of fields is associated to a different texture: clay to silty-clay in Ex1, silty-clay loam in Ex2 and REF and silty loam in Ex3.

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Table 1 Field descriptions of profiles. Depth cm:

Munsell Colour

Soil properties

Ex1 Field 8e10 24e30

Greyish brown 7.5 YR 5/2 Greyish brown 7.5 YR 5/3

Top soil covered with grass, disturbed by cattle steps. No clear boundaries exist, however colour of mottles change with depth. The structure is moderate with blocky subangular aggregates. Cracks up till 120 cm, very weak structure and increasing density towards 200 cm.

40e44 53e57 66e72 103e107 121e128 139e145 158e167 178e184 192e200 230e240 Ex1 Canal 7e12 28e32 38e44 48e53 58e64 68e73 78e84 88e93 97e102 108e114 121e130 138e142 Ex2 Field 5e10 20e30 30e40 45e50 58e63

Brownish Brownish Brownish Brownish Brownish Brownish Brownish Brownish Brownish Brownish

73e80

Light reddish grey matrix 2.5 YR 7/1, 20% orange 2.5 YR 6/8 mottles and red pisoliths 10 R 4/6 Light reddish grey matrix 2.5 50% orange 2.5 YR 6/8 mottles and red pisoliths 10 R 4/6

90e95 109e116 Ex2 Canal 15e24 30e40 45e50 60e68 73e83 90e98 107e115 Ex3a Field 0e10 20e30 30e40 40e50 50e60 60e70 70e80 80e90 90e100 Ex3b Field 0e10 10e20 20e30 30e40 40e50 50e60 60e70 70e80 80e90 90e100 Ex3 Canal 0e10 10e20 20e30

black 7.5 YR 3/3 black 7.5 YR 3/4 5% yellow orange 10 YR 7/8 mottles black 7.5 YR 3/4 5% yellow orange 10 YR 7/8 mottles black 7.5 YR 3/4 5% yellow orange 10 YR 7/8 mottles grey 7.5 YR 5/1 orange mottles 5% 5 YR 6/8 grey 7.5 YR 5/1 orange mottles 5% 5 YR 6/8 grey 7.5 YR 5/1 orange mottles 5% 5 YR 6/8 grey 7.5 YR 5/1 orange mottles 5% 5 YR 6/8 grey 7.5 YR 5/1 mottles 5% red 10 R 4/8 grey 7.5 YR 5/1 mottles 5% red 10 R 4/8

Greyish brown 7.5 YR 5/3 Brownish black 7.5 YR 3/3 Brownish black 7.5 YR 3/4 5% yellow orange 10 YR 7/8 mottles Brownish grey 7.5 YR 5/1 orange mottles 5% 5 YR 6/8

Top soil covered with grass, disturbed by cattle steps, the structure is moderate with blocky subangular aggregates Down the profile there are no clear boundaries, however colour of mottles change with depth, very weak structure.

Brownish grey 7.5 YR 5/1, red 10 YR 7/8 mottles

Light grey 10 YR 7/1 orange-red 10 YR 7/8 mottles Light grey 5 YR 7/1 Light grey 5 YR 8/1 matrix 2.5 YR 7/1, 5% orange 2.5 YR 6/8 mottles Light reddish grey matrix 2.5 YR 7/1, 10% orange 2.5 YR 6/8 mottles Light reddish grey matrix 2.5 YR 7/1, 15% orange 2.5 YR 6/8 mottles

Light grey 5 YR 7/1 Light grey 5 YR 8/1 matrix 2.5 YR 7/1, 7% orange 2.5 YR 6/8 mottles Light grey 5 YR 8/1 matrix 2.5 YR 7/1, 10% orange 2.5 YR 6/8 mottles Light reddish grey matrix 2.5 YR 7/1, 50% orange 2.5 YR 6/8 mottles and red pisoliths 10 R 4/6 Same as upper layer however decreasing amount of pisoliths.

Light grey 5 YR 7/1 Light grey 5 YR 7/1 3% orange mottles 5 YR 7/8 Light reddish grey matrix 2.5 YR 7/1, 15% orange 2.5 YR 6/8 mottles and soft pisoliths Light reddish grey matrix 2.5 YR 7/1, 20% orange 2.5 YR 6/8 mottles

Really thin top soil, low amount of organic, fine dusty structure. Beginning of hydromorphic affected depth, medium structure with subangular blocky aggregates, few manganese concretions and very few charcoal pieces diameter of 5 mm. Transition to pisolithic layer with some few pisoliths. Highly hydromorphic affected with hard pisoliths, very dense structure, and subangular aggregates. Really thin top soil, low amount of organic, fine granular structure. Beginning of hydromorphic affected depth, structure is medium with subangular blocky aggregates. Highly hydromorphic affected with hard pisoliths increasing with depth, very dense, structure with subangular aggregates.

Fine granular structure.

Moderate structure with subangular blocky aggregates.

Same with increasing amount of mottles 50%

Greyish brown 5 YR 6/2 Light grey 5 YR 7/1 Light grey 5 YR 7/1 3% orange mottles 7.5 YR 6/8 Light reddish grey matrix 2.5 YR 7/1, 10% orange mottles 7.5 YR 6/8 and orange 2.5 YR 6/8 soft pisoliths

Fine granular structure.

Medium granular structure. Moderate structure with subangular blocky aggregates. Moderate structure, subangular blocky aggregates.

Same with increasing amount of mottles 40%

Greyish brown 5 YR 6/2 Greyish brown 5 YR 6/2 Light grey 5 YR 7/1 3% orange mottles 7.5 YR 6/8

Massive structure with blocky aggregates.

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Table 1 (continued ) Depth cm:

Munsell Colour

Soil properties

30e40 40e50 50e60 60e70 70e80 REF 1 10e15 20e25 35e40 47e52

Light reddish grey matrix 2.5 YR 7/1, 10% orange mottles 7.5 YR 6/8 and orange 2.5 YR 6/8 soft pisoliths

Moderate structure with subangular blocky aggregates.

Light grey 5 YR 7/1 Light grey 5 YR 7/1 Light grey 5 YR 7/1 Light grey 5 YR 8/1 matrix, 5% red 10 R 4/8 to reddish brown 2.5 YR 4/8 mottles and concretions Reddish grey matrix 2.5 YR 5/1, increasing mottles up to 50% red 10 R 4/8 to reddish brown 2.5 YR 4/8 mottles and concretions

Fine dusty structure.

57e61 67e72 75e80 85e90 99e105

Dense granular structure. Hard dense structure with subangular blocky aggregates, highly affected by hydromorphisme, pisoliths start in about 65 cm.

Table 2 Physicochemical properties of profiles. Depth cm:

Ex1 Field 8e10 24e30 40e44 53e57 66e72 103e107 121e128 139e145 158e167 178e184 192e200 230e240 Ex1 Canal 7e12 28e32 38e44 48e53 58e64 68e73 78e84 88e93 97e102 108e114 121e130 138e142 Ex2 Field 5e10 20e30 30e40 45e50 58e63 73e80 90e95 109e116 Ex2 Canal 15e24 30_40 45e50 60e68 73e83 90e98 107e115 Ex3a Field 0e10 20e30 30e40 40e50 50e60 60e70 70e80 80e90

Sand%

Silt%

Clay%

63 mme2000 mm

4 mme63 mm

<4.00 mm

pH

Corg

3.2 1.3 1.4 0.6 1.1 9.5 10.2 0.8 17 6.9 6.5 5.3

37.0 33.6 32.5 30.9 31.1 31.6 32.0 26.8 30.2 52.0 47.1 56.3

59.8 65.1 66.1 68.5 67.8 58.9 57.8 68.4 52.9 41.1 46.4 38.3

3.93 3.75 3.80 3.80 3.85 3.87 3.85 3.99 3.99 3.90 4.10 3.87

2.09 1.78 1.70 1.18 1.39 1.16 1.12 0.94 0.82 0.74 0.34 0.32

0.22 0.20 0.20 0.15 0.17 0.16 0.15 0.11 0.11 0.10 0.05 0.08

17.8 3.9 1.5 1.4 1.1 0.7 0.9 1.2 8.6 3.0 4.3 9.6

44.1 39.2 36.7 35.5 35.3 32.4 35.4 35.6 33.0 37.1 43.5 50.3

38.1 56.9 61.8 63.1 63.6 66.9 63.7 63.2 58.4 59.9 52.3 40.1

4.24 4.04 3.99 3.99 3.98 3.83 3.88 3.89 3.98 4.01 3.99 3.97

4.22 1.68 1.61 1.50 1.43 1.31 1.04 1.04 0.76 0.61 0.43 0.27

4.7 6.5 7.0 8.9 9.5 6.9 21.7 17.1

66.7 62.7 62.7 58.0 54.4 55.2 45.7 54.0

28.5 30.8 30.3 33.1 36.1 37.8 32.5 28.9

3.85 3.93 3.85 3.85 3.89 3.83 3.91 3.85

16.9 10.6 8.4 6.7 10.3 9.6 8.3

59.5 57.3 58.8 59.4 56.2 55.2 63.6

23.6 32.1 32.8 33.9 33.5 35.2 28.1

19.1 24.1 15.3 13.0 18.7 27.7 24.8 20.6

68.6 63.6 65.0 63.9 63.5 55.4 56.3 60.1

12.3 12.3 19.7 23.1 17.7 17.0 18.9 19.4

N%

C/N

CECeff

Bs

mmolc/kg

%

9.65 9.03 8.35 7.71 8.09 7.45 7.34 8.33 7.73 7.77 6.51 4.05

59.9 128.5 217.6 190.7 198.4 187.6 191.7 192.6 202.0 155.6 168.8 59.9

79.3 35.2 22.5 24.8 32.7 34.4 31.9 32.2 29.6 33.9 34.4 79.3

0.44 0.20 0.19 0.18 0.17 0.16 0.14 0.13 0.10 0.09 0.07 0.07

9.60 8.20 8.42 8.41 8.47 8.06 7.50 8.24 7.80 6.67 6.18 3.83

94.9 218.7 230.3 216.9 221.1 209.8 192.7 229.1 203.4 227.0 203.2 165.9

62.8 26.6 18.3 20.9 19.9 19.0 18.5 17.9 17.9 16.1 17.3 16.7

0.94 x 0.60 0.49 0.51 0.40 0.31 0.21

0.10 x 0.08 0.08 0.09 0.08 0.09 0.06

9.56 x 7.73 6.49 5.92 4.95 3.60 3.49

54.1 58.7 x 62.7 84.3 84.7 98.3 x

27.9 4.4 x 4.4 14.8 3.5 3.9 x

3.87 3.87 3.84 3.77 3.89 3.85 3.96

0.96 0.59 0.55 0.53 0.39 0.32 0.23

0.11 0.09 0.09 0.09 0.09 0.09 0.07

8.53 6.81 6.22 6.06 4.48 3.76 3.39

x x x x x x x

x x x x x x x

3.84 3.79 3.84 3.87 3.92 3.92 3.90 3.92

1.27 0.68 0.38 0.37 0.30 0.28 0.24 0.23

0.12 0.07 0.05 0.06 0.05 0.06 0.06 0.05

10.42 9.21 6.95 6.46 5.70 5.13 4.24 4.59

18.4 23.6 42.1 54.7 54.2 47.6 51.2 56.8

20.8 5.6 3.2 2.2 2.0 4.1 2.5 5.6

%

(continued on next page)

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Table 2 (continued ) Depth cm:

90e100 Ex3b Field 0e10 10e20 20e30 30e40 40e50 50e60 60e70 70e80 80e90 90e100 Ex3 Canal 0e10 10e20 20e30 30e40 40e50 50e60 60e70 70e80 REF 1 10e15 20e25 35e40 47e52 57e61 67e72 75e80 85e90 99e105

Sand%

Silt%

Clay%

63 mme2000 mm

4 mme63 mm

<4.00 mm

pH

Corg

18.6

60.9

20.5

3.91

0.19

0.05

24.9 25.5 24.4 18.1 18.0 15.9 19.1 21.2 23.3 15.2

63.8 62.9 62.8 67.0 65.4 65.7 62.0 59.8 58.3 63.3

11.3 11.6 12.8 14.9 16.6 18.3 18.9 19.0 18.4 21.5

3.97 3.88 3.90 3.83 3.90 3.89 3.86 3.86 3.98 3.81

1.11 0.63 0.52 0.33 0.28 0.21 0.26 0.16 0.15 0.17

10.95 10.30 15.87 12.58 14.40 19.91 14.41 17.37

68.1 67.7 62.0 65.1 63.4 59.2 64.2 61.2

20.96 22.00 22.16 22.29 22.21 20.87 21.41 21.43

3.83 3.75 3.78 3.94 3.87 3.92 3.91 3.94

18.7 20.9 19.7 19.7 25.1 18.6 20.5 21.4 20.5

57.8 56.2 57.0 57.0 52.2 56.6 55.2 54.7 55.2

23.5 22.9 23.3 23.2 22.7 24.7 24.3 23.9 24.3

3.74 3.79 3.83 3.95 3.89 3.87 3.88 3.88 3.89

N%

C/N

CECeff

Bs

mmolc/kg

%

3.88

45.7

3.0

0.10 0.07 0.06 0.05 0.05 0.05 0.05 0.05 0.04 0.04

10.68 9.14 9.08 6.58 5.82 4.42 4.99 3.47 3.61 3.72

16.6 15.3 16.7 26.1 29.9 29.3 31.2 39.9 31.6 42.9

36.9 13.4 5.4 8.4 9.2 6.0 4.9 11.0 7.2 7.2

1.72 1.21 0.81 0.53 0.43 0.40 0.47 0.36

0.17 0.12 0.09 0.08 0.07 0.06 0.08 0.06

10.24 10.29 9.06 7.05 6.28 6.34 6.20 5.96

42.8 47.4 40.8 45.7 43.6 54.7 53.5 50.7

17.0 5.9 3.1 2.9 2.4 2.8 4.0 4.1

1.07 0.65 0.52 0.44 0.42 0.35 x x x

0.11 0.08 0.07 0.06 0.06 0.07 x x x

9.55 8.46 7.93 8.01 6.55 5.15 x x x

34.7 44.2 34.7 40.7 47.7 74.9 81.6 80.3 87.1

30.0 18.3 11.2 8.6 8.2 8.2 8.1 9.3 9.7

%

Fig. 6. Texture triangle indicating clustering of grain size classes for each site.

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4.2.1. Profiles in Ex1 In the bajío, a pit of 240 cm was dug in a raised field (Fig. 3). The texture of the profile is quite homogenous from the top to a depth of 190 cm and can be classified as clay, with an average content of 58% clay, 36% silt and 6% sand (Table 2). From 185 to 240 cm there is a decrease in clay content to 38% and also a clear change in colour, from dark brown to light grey (Table 1). Deep cracks are present up to a depth of 120 cm, due to the swelling and shrinking of the clay following seasonal fluctuations in soil moisture. The high percentage of clay in the bajío results in waterlogging. However, hydromorphic features present throughout the profile, such as soft mottles, and also the presence of desiccation cracks indicate that it completely dries out in the dry season. A similar granulometric profile can be found in the canal (see Tables 1 and 2). 4.2.2. Profiles in Ex2 In the altura, a pit of 120 cm was dug in a raised field (Fig. 4). The soil texture here is silty clay loam, slightly coarser than in Ex1. The dominant fraction is silt, with an average value of 58%, followed by clay with 31.8% and sand with 10.2% (Fig. 6 and Table 2). Given the soil texture, the permeability rate can be classified as slow to moderately slow, leading to temporary water stagnancy (Cobb et al., 2003; IUSS Working Group WRB, 2014). This is confirmed by the abundant hydromorphic features, which are characterized by red mottling in the upper part of the profile Ex2F until a depth of 75 cm and, from thereon, present as hard nodules of iron and

9

pisoliths, known as plinthite formation. The formation of a pisolithic horizon is a common consequence of fluctuating water conditions in pre-weathered and iron rich sediments (Schachtschabel et al., 2002). In the adjoining canal, Ex2c, the pisolithic horizon is present at a depth of 65 cm. 4.2.3. Profiles in Ex3 In the transition area between the altura and the semi-altura, 2 pits of 100 cm were dug in raised fields and 1 pit of 100 cm was dug in the canal between the raised fields (Fig. 5). The texture of all three profiles dug in the fields and the canal in Ex3 is silt loam (Fig. 6 and Table 2). Given the soil texture, the permeability can be classified as moderate (IUSS Working Group WRB, 2014). Hydromorphic features are present especially as orange iron mottles and a few manganese and iron pisoliths. In contrast to Ex2, the iron nodules here are in the pre-stage of pisolith formation and still soft. The pisolithic horizon starts where grain size decreases, in Ex3a at a depth of 35 cm and in Ex3b at a depth of 45 cm. This suggests that most of the hydromorphic processes taking place here are due to the stagnation of rainfall water (pseudogley conditions). Therefore, the amount of time in which the soil is waterlogged depends almost exclusively on the permeability of the sediments, as runoff is impended by the lack of slope. The soils in the altura can be characterized as Plinthosol (PT), comprising a pisoplinthic horizon (px), the formation of which is controlled by stagnic conditions.

Fig. 7. Results of CECeff, BS, Ca/Al ratios, Corg of all the profiles.

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4.2.4. Reference profile REF The REF profile was dug in the semi-altura (Figs. 2 and 4). It shares some characteristics with profiles in Ex2 and Ex3, but differs in one important aspect which is a result of the REF profile's topographic position. The profile is also affected by hydromorphic processes, starting at a depth of 65 cm, characterized by abundant iron mottling and iron pisoliths in the pre-stage formation, similar to Ex3. The grain size is silty loam, similar to the altura, however, it is slightly finer than in EX2 and slightly coarser than in EX3 (Fig. 6 and Table 2). Contrary to the Ex2 and Ex3 profiles, there is no stagnant layer in the REF profile and pisoliths are present from 65 cm downwards which indicated the range of groundwater fluctuations. 4.3. Soil chemical properties In all profiles Corg decreases with depth. In the altura, Corg ranges from 1.2 to 0.2%, whereas in the bajío values are much higher, going from 4% to 0.35% (Table 2). The highest value of 4% is from the canal in Ex1 where, due to prolonged waterlogging, the mineralization of Corg decreases (Schachtschabel et al., 2002). In the fields, N values lie between 0.2% in Ex1 and 0.1% in all the others. The highest value of 0.44% was measured in the canal in Ex1, where organic matter is highest too. In all profiles the C/N values in the top layers range between 8 and 10 and decrease to 3.4e5 in the lower layers, which are typical values for the tropics, where organic matter is mineralized fast (Schachtschabel et al., 2002). The pH in all the profiles is very acid, with values between 3.7 and 4.2. According to the criteria of (Hazelton and Murphy 2007), the CECeff is low to medium in Ex2, Ex3, Ex3b and REF. In Ex1 the CECeff is high, ranging from 60 to 198 mmol/kg (Fig. 7). The composition of CECeff is highly dependent on pH. At pH levels below 5.5, the availability of useful elements for plants declines sharply and aluminium approximates toxic levels (Jones, 2012). This is reflected in the results of Ex2, Ex3, Ex3b and REF, where the concentration of aluminium in the soil ranges from 60% to 90% and the base saturation (Bs) levels are very low. The ratio of Ca/Al is often used to express Aluminium toxicity in soils, as Ca acts as a buffer. According to the classification of Cronan and Grigal (1995) the potential risk of aluminium toxicity is very high (100%) to high (75%) in the topsoil of all the profiles, except for Ex1 (Fig. 7). In all the stratigraphic profiles CECeff increases with depth, together with an increase in clay, while Bs decreases with depth in all profiles, together with Corg. 5. Discussion The study focuses on the link between variability in field design and local edaphology. Three different field types were identified in the same area. Results show that the soil properties of each differ considerably. The difference in grain size distribution among the sites is the result of differences in the depositional environment. The sediments in this area have clear signs of advanced weathering. While the rate of soil weathering is dependent on the properties of the parent material and climate, the current soil development status is a function of time (Schachtschabel et al., 2002). The more soluble minerals, like carbonates, which are important nutrients for plants, are washed away relatively fast in wet tropical climates (Martini and Chesworth, 1992). The sediments in the Yacuma region were deposited most probably during the late Pleistoceneeearly Holocene. The data here presented reflects modern soil status; however, the soil might have changed slightly since the fields were constructed. Nevertheless, it can be safely assumed that conditions were similar in pre-Columbian times, as the estimated amount of time passed since the construction of the fields (between 500 and

1500 years) is relatively negligible, compared to the thousands of years of previous weathering since deposition.

5.1. Fields in the altura The fields in the altura were built on plinthosols, which are highly weathered soils. The profile shows a stagnic horizon; hydromorphic features, such as abundant orange-red mottling and the formation of pisoliths, are present. This clearly shows that waterlogging is a major problem for agriculture in this site, despite being elevated. The depth of the plinthic horizon marks the boundary where, due to the presence of finer sediments, permeability decreases (Table 2). A slight difference in grain size can significantly alter the depth of this horizon. This becomes apparent when comparing the profiles Ex3a and Ex3b. The onset of the plinthic horizon in each field is at different depths and is located at the boundary where grain size becomes slightly finer: at 35 cm in Ex3a and at 45 cm in Ex3b (compare Table 2 and Fig. 5). In Ex3, elevating the field 20e30 cm was enough to improve drainage, as has also been noted in similar fields in the vicinity of El Cerro site (Lombardo et al., 2011b). The REF profile is similarly affected by hydromorphic processes, however, results show that in this case it is a consequence of ground water fluctuation. This can be explained by the position of the REF profile, located between the altura and the bajío (Figs. 2 and 3). As can be seen in Fig. 2, the semi-altura was not used for the construction of raised fields. Even though the grain size in the REF profile is coarser than in Ex2, facilitating the drainage of the semi-altura, the area seems to be unfavourable for the construction of raised fields as it is affected by the rise of the groundwater table during the rainy season. Because of this, only the more elevated areas were used for the construction of raised fields (The fields in the bajío are an exception and will be discussed in the following section). The fields in Ex2 are not typical of this region. They are around 50% higher than the fields in Ex3 and other fields in the area (Fig. 4). Whereas both Ex2 and Ex3 sites have similar characteristics, in terms of the geochemistry of the soil and weathering processes e.g. plinthite formation, there is a significant difference in grain size, which seems to be an important factor in determining field height. The sediments in Ex2 are considerably finer, compared to Ex3, hence there is poorer drainage and fields need to be built higher. What is interesting in Ex2 is the untypical arrow-like design of the fields and the fact that some of them are embanked and some are not (Fig. 4). Embanked fields built on poorly drained soils would favour the storage of water into the dry season. Fields with similar shape have been described in the lowlands of Ecuador, referred to as L-shaped fields (Cadudal, 2007). The fields in Ecuador were built on soils with comparable texture to that in Ex2. Experiments on the L-shaped fields, planting different crops, including non-native crops like rice, on the elevated bed during the rainy season and in the canal during the dry season, permitted a year-round cultivation (Cadudal, 2007). This management system is however unlikely in the case of Ex2, as chemical soil properties are different and the soils much poorer. A DEM covering the whole area would help to better understand why, contrary to EX3, the raised fields in EX2 are partially embanked. Multiple crops, including maize (Whitney et al., 2014), peanuts, squash, sweet potatoes (Erickson, 2006) and cassava (Lombardo et al., 2011b; Denevan, 2001, pp. 248e249; Walker, 2004, pp. 47e49) have been proposed as possible cultivars grown on raised fields. Maize, and especially cassava, are considered as the most important crops in Amerindians diets today (Dufour, 1991). Cassava can be successfully grown on poor soils (Fageria et al., 2011, p. 458). Furthermore, provided the soil is well drained, cassava can

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be stored underground for long periods of time (Walker, 2004, p. 43). In contrast, maize is a nutrient demanding crop that requires fertilization in order to sustain satisfactory yields (Mengel, Kirkby, 1987; Fageria et al., 2011, p. 314). Typically attainable yields for traditional farming, excluding intensive fertilisation, in the tropics are around 1130e1280 kg/ha for maize and 8690e9490 kg/ha for cassava (Landon, 1991, p. 270). Nutritional values per 100 g of cassava and maize are 160 kcal and 86 kcal/g respectively (Montagnac et al., 2009). The advantage of maize is clearly the higher value of protein containing essential amino acids which are lacking in Cassava (Young and Pellett, 1994). Nevertheless Cassava is a very energy dense crop, producing around 38,000 kcal/ha/d, compared with the 3070 kcal/ha/d obtained from maize. Pollen studies of lake sediments in the LM have revealed that maize was widely used in pre-Columbian times (Dickau et al., 2012; Whitney et al., 2013, 2014; Carson et al., 2015). Maize phytoliths found in raised fields in the area of Santa Ana de Yacuma have provided evidence that maize was one of the main crops cultivated here too (Whitney et al., 2014). However, while phytoliths are an indicator of the presence/absence of a cultivar, they do not say much about the period of time during which it was cultivated. In fact, maize phytoliths could have originated from maize cultivated during the first years after the construction of the fields, which afterwards could have been cultivated with cassava (Lombardo et al., 2013a). When constructing the fields, the A horizon is doubled and organic matter and nutrients are accumulated on the top, improving soil fertility. However, such acid soils become exhausted after 2e3 years of cultivation, requiring either a long fallow or the cultivation of less demanding crops (Schachtschabel et al., 2002; Fageria et al., 2011). Some authors suggest that the importance of maize in pre-Columbian diets has been overestimated, due to preferential maize specific studies (Arroyo-Kalin, 2010). Unfortunately, cassava produces very little pollen and phytoliths and starch grains are not well preserved in soils (Piperno and Pearsall, 1998). Studies of phytoliths and starch grains on artefacts, combined with the analysis of macro remains, have provided good evidence that both cassava and maize were abundantly used at habitation sites in the south-eastern LM (Bruno, 2010; Dickau et al., 2012). Similar studies, including the analysis of stable isotopes from human bones at habitation sites close to raised fields, are needed in order to assess how important maize was in pre-Columbian diets. Taking into account the nature of the soils here, we suggest that fields were not cultivated on a continuous basis and were managed using very long fallow periods in order to replenish nutrients. Other important factors for which long fallow periods would have been needed are: weed invasion, insect pests and plant diseases (Pearsall, 2007). The overlapping of fields in the study site supports the idea that they were not all built at the same time, but rather abandoned and then rebuilt and/or reused. This suggests that raised fields could have been used in a similar way as shifting cultivation is practiced today in the region. Raised fields could have been cultivated for two or three years and then left to rest for 15e20 years in order to replenish the soil's fertility; during this time new fields would be built elsewhere. It has been argued that organic manuring, using plants grown in the canals, could have been practiced to fertilize the fields (Lee, 1997; Saavedra, 2006; Erickson, 2008; Whitney et al., 2014). However, none of the raised fields studied here or elsewhere in the LM have shown any evidence of manuring (Lombardo et al. 2011b, 2013a; Rodrigues et al., 2015) and experiments performed in modern raised fields have been inconclusive (Lombardo et al., 2011b). The two most limiting factors for maize cultivation are nitrogen and sufficient water (Fageria et al., 2011, pp. 323e329). Nitrogen could in fact be partly provided by cyanobacteria growing in

11

the canal. However, the moment when maize requires most nutrients is around 30 days after sowing and in the LM this coincides with the beginning of the rainy season (Fageria et al., 2011, pp. 323e329), when the canals are dry, even those that are embanked (Rodrigues et al., 2015). From the analysis of the soil characteristics in Ex2 and Ex3, we can conclude that raised fields in the altura were primarily built in order to improve the drainage. Until archaeological evidence proves otherwise, there is no data to suggest that these fields were able to produce maize on a continuous basis. On the contrary, our data suggests that the cultivar that was primarily produced here was cassava. It is known that multi- and intercropping was a common strategy already in pre-Columbian times (Whitmore, Turner II, 1992; Denevan, 1995) and it is possible that the raised fields here could have been multi- or intercropped with other cultivars as suggested by the study of Whitney et al. (2014) estimated that raised fields at el Cerro, close to the present study site, were in use during a period of at least 1000 years. The exact timing of the introduction of raised field agriculture is difficult to estimate as fields are difficult to date (see introduction). Given the unfertile soils in the area, the continuous use of the raised fields during such a long period of time would have required intensive fertilization and field maintenance. This would have caused noticeable changes in soil properties, such as the presence of abundant charcoal, accumulation of organic material and soil stable phosphorous, and considerable changes in soil structure and evidence of organic rich dusty clay coatings due to the repeated working of the soils (See review in Rodrigues et al. 2015). Such changes have never been found in fields studied in the LM (Lombardo et al., 2011b; Rodrigues et al., 2015) and could not be found in the present study either. The results rather point towards more extensive land use practices, where raised fields required long fallow periods, comparable to the fields studied in Bermeo, which have been used intermittently for at least 700 years (Rodrigues et al., 2015). Archaeological evidence similarly suggest a relatively “flexible use of the land” (Walker, 2004, p. 107), where forest islands were most probably seasonally occupied and larger sites, such as the San Juan and el Cerro sites, were occupied during a rather short period of 167 years and 100 years respectively (Walker, 2004, p. 113). Probably there are many more similar sites, which could be part of the 1000 year land use history reported by Whitney et al. (2014). To better understand the link between population, sites and raised fields, considerably more archaeological surveys and excavations are needed, including forest islands. Nevertheless, the existing archaeological evidence and the present research point towards a rather extensive use of the land, where raised fields were constructed over a long period of time and used intermittently by small groups of mobile people. This activity, practiced during hundreds of years, created the vast anthropogenic landscape we see today, with thousands of raised fields spread all over the region. 5.2. Fields in the bajío Fields built in the bajío differ in many aspects from those in the altura. Soil characteristics in Ex1 are completely different. Fields in Ex1 consist of silty clay and organic rich material. Here, the presence of water is prolonged into the dry season, whereas in the altura it is completely dry. This is given by the fact that the bajío lies 1.7 m below the altura and sediments are rich in clay, with high water holding capacity. Similar raised fields have been described in the Mexican lowlands (Beach et al., 2009). These Maya raised fields share some key characteristics with the fields in Ex1, such as the high content of clay, up to 80%, and organic matter, and the fact that they were constructed in a bajío (Beach et al., 2009). Because of high organic carbon content and better supply of nutrients and water during the dry season, agricultural conditions in Ex1 would

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be more favourable for maize and other cultigens, such as beans, than in Ex2 and Ex3. The disadvantage here, however, is the high amount of clay and the fact that fields could only be cultivated during the dry season or during unusually dry years (Espinoza Villar et al., 2009; Latrubesse et al., 2009). Nevertheless, if water and nutrients are sufficiently available roots will not have to penetrate deep into the soil in order to obtain the needed nutrients (Nicoullaud et al., 1994). The differences in soil characteristics and hydrology between the bajío and the altura suggests that by building raised fields in both sites, assuming they were used at the same time, pre-Columbian people were able to diversify their production and created different and complementary cropping regimes. This would have had important benefits, such as the cultivation of different crops and a prolonged or additional cultivation period in the dry season. Studies of modern-day small scale subsistence farmers have shown that this kind of diversification strategies, such as cultivating different crops on different land, are common agricultural practices (Altieri, 1999; Gliessman, 2001). Diversification improves food security (Frison et al., 2011) and also reduces vulnerability on the long term by improving resilience against natural risks like climate change, pests and plant diseases (Lin, 2011). Similarly, it has been proposed that pre-Columbian agriculture took advantage of “multiple habitat exploitation” (Denevan, 2001, p. 73). Alternatively, fields could have been built in the bajío to overcome periods of severe and frequent droughts. People in the LM are particularly vulnerable to natural risks due to great annual variability in flooding, with extreme floods and also severe drought events resulting in a potential threat to their livelihoods. Studies in the Amazon Basin, and particularly in south-western Amazonia, have shown that extreme floods and drought are part of the natural climate variability (Ronchail et al., 2005; Marengo et al., 2013), with increasing frequency in the late Holocene (Mayewski et al., 2004; Wanner et al., 2008), when most raised fields were probably built in the LM. It is possible that fields in the bajío were built and used during a period of more frequent severe droughts. However, as we did not find any material that could be dated, the question about the exact time of use of these fields remains uncertain. Differences in the design of pre-Columbian raised fields have often been associated with the diversity of cultural practices (Walker, 2011; Rostain, 2013, p. 136). Nevertheless, this case study has shown that differences in layout and height of platform fields co-existing in the same cultural area seem to respond to differences in the local edaphology. Even though the raised fields region in Santa Ana might seem homogeneous, with regards to geology, climate, vegetation and fauna, soil properties can vary significantly at a very small scale. The design of raised fields per se might therefore not be appropriate for the interpretation of cultural spatial patterns. 6. Conclusions This study has identified different types of raised fields co
n, in northexisting in the same area in the vicinity of Exaltacio western Beni. Results show that the shape/dimension and spatial distribution of the raised fields are the result of an adaptation to the local edaphology. Raised fields in the altura were built primarily to improve the drainage. Nevertheless, the study of the raised fields in the bajío shows that some fields in the LM were also built to prolong the presence of water, allowing an additional cultivation period in the dry season and/or in times of drought. By using the technology of raised fields on soils under different hydrological
n were conditions, pre-Columbian people in the area of Exaltacio probably able to diversify their production and reduce risks associated with seasonal and/or annual climate variability. However,

the study has not found evidence to support the idea that raised field agriculture in this region was able to support large populations. Acknowledgements The present study has been funded by the Swiss National Science Foundation (SNSF), grant no SNF 200020-141277/1, and performed under authorisation N_ 017/2012 issued by the Unidad de Arqueología y Museos (UDAM) del Estado Plurinacional de Bolivia.
pez from the Ministerio de Culturas We thank Dr. M.R. Michel Lo and our Bolivian counterpart Dr. J.M. Capriles for their support. A
n for their logistical special thanks to the community of Exaltacio support in the field and for allowing us free access to their land. Fieldwork assistance by B. Vogt and C. Welker is gratefully acknowledged. We thank Dr. D. Fischer for technical support in the laboratory. X-ray fluorescence spectroscopy (XRF) was measured at the Geological Institute of the University of Fribourg. Special thanks to the anonymous reviewers whose comments helped improve this manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quaint.2015.11.091. References Altieri, M.A., 1999. The ecological role of biodiversity in agroecosystems. Agriculture, Ecosystems & Environment 74 (1), 19e31. Arroyo-Kalin, M., 2010. The Amazonian formative. Crop domestication and anthropogenic soils. Diversity 2, 473e504. Baveye, P.C., 2013. Comment on “Ecological engineers ahead of their time: the functioning of pre-Columbian raised-field agriculture and its potential contributions to sustainability today” by Dephine Renard et al. Ecological Engineering 52, 224e227. Beach, T., Luzzadder-Beach, S., Dunning, N., Jones, J., Lohse, J., Guderjan, T., et al., 2009. A review of human and natural changes in Maya Lowland wetlands over the Holocene. Quaternary Science Reviews 28, 1710e1724. €kologische Grundlagen der Viehwirtschaft in den Beck, S.G., 1983. Vegetationso Überschwemmungs-Savannen des Río Yacuma (Departamento Beni, Bolivien). Dissertationes botanicae. Dissertationes botanicae Vaduz. Cramer, Vaduz.
. In: Bourrel, L., Pouilly, M., 2004. Hidrología y din
amica fluvial del Río Mamore ~ ez, C. (Eds.), Diversidad biolo
gica en la Pouilly, M., Beck, S., Moraes, M., Iban
n del Río Mamore
. Bolivia: Centro de Ecología Simo
n I. llanura de inundacio ~ o, Santa Cruz, pp. 95e116. Patin Bruno, M., 2010. Carbonized plant remains from Loma Salvatierra, Department of Beni, Bolivia. Zeitschrift für Arch€ aologie Außereurop€ aischer Kulturen 3, 151e206. mes Agricoles Pre
hispaniques des Basses Cadudal, F., 2007. Camellones et Syste
^te Nord de l'Equateur. Terres de la Co Available online at: www.arqueoecuatoriana.ec. updated on 2007, checked on 6/8/2015. Carson, J.F., Watling, J., Mayle, F.E., Whitney, B.S., Iriarte, J., Prümers, H., Soto, J.D., 2015. Pre-Columbian land use in the ring-ditch region of the Bolivian Amazon. The Holocene, 0959683615581204. Cobb, K.M., Charles, C.D., Cheng, H., Edwards, R.L., 2003. El Nino/Southern oscillation and tropical Pacific climate during the last millennium. Nature 424, 271e276. Cronan, C.S., Grigal, D.F., 1995. Use of calcium/aluminum ratios as indicators of stress in forest ecosystems. Journal of Environmental Quality 24 (2), 209e226. Denevan, W.M., 1966. The Aboriginal Cultural Geography of the Llanos De Mojos of Bolivia. DTIC Document. Denevan, W.M., 1995. Prehistoric agricultural methods as models for sustainability. Advanced Plant Pathology 11, 21e43. Denevan, W.M., 2001. Cultivated Landscapes of Native Amazonia and the Andes. Oxford University Press. Dickau, R., Bruno, M.C., Iriarte, J., Prümers, H., Jaimes Betancourt, C., Holst, I., Mayle, F.E., 2012. Diversity of cultivars and other plant resources used at habitation sites in the Llanos de Mojos, Beni, Bolivia: evidence from macrobotanical remains, starch grains, and phytoliths. Journal of Archaeological Science 39, 357e370. Dufour, D.L., 1991. Diet and nutritional status of Amerindians: a review of the literature. Cadernos de Saúde Pública 7 (4), 481e502. Dumont, J.F., Fournier, M., 1994. Geodynamic environment of Quaternary morphostructures of the subandean foreland basins of Peru and Bolivia: Characteristics and study methods. Quaternary International 21 (0), 129e142.

Please cite this article in press as: Rodrigues, L., et al., Linking soil properties and pre-Columbian agricultural strategies in the Bolivian lowlands:
n, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2015.11.091 The case of raised fields in Exaltacio

L. Rodrigues et al. / Quaternary International xxx (2016) 1e13 Dumont, J.F., 1996. Neotectonics of the Subandes-Brazilian craton boundary using ~ on and Beni basins. Tectonophysics 257,137e151. geomorphological data. The Maran Erickson, C.L., 1995. Archaeological methods for the study of ancient landscapes of the Llanos de Mojos in the Bolivian Amazon. Cambridge University Press. Erickson, C.L., 2006. The domesticated landscape of the Bolivian Amazon. In:
e, William, Erickson, Clark L. (Eds.), Time and Complexity in Historical Bale Ecology: Studies in the Neotropical Lowlands. Columbia University Press, New York, pp. 236e278. Erickson, C.L., 2008. Amazonia: the historical ecology of a domesticated landscape. In: The Handbook of South American Archaeology. Springer, pp. 157e183. Erickson, C.L., Walker, J.H., 2009. Precolumbian causeways and canals as Landesque capital. In: Snead, J.E., Erickson, Clark L., Darling, J.A. (Eds.), Landscapes of Movement. University of Pennsylvania Museum of Archaeology and Anthropology, Philadelphia. Espinoza Villar, J.C., Ronchail, J., Guyot, J.L., Cochonneau, G., Naziano, F., Lavado, W., et al., 2009. Spatio-temporal rainfall variability in the Amazon basin countries (Brazil, Peru, Bolivia, Colombia, and Ecuador). International Journal of Climatology 29, 1574e1594. Fageria, N.K., Baligar, V.C., Jones, C.A., 2011. Growth and Mineral Nutrition of Field Crops. CRC Press; Taylor and Francis Group. Frison, E.A., Cherfas, J., Hodgkin, T., 2011. Agricultural biodiversity is essential for a sustainable improvement in food and nutrition security. Sustainability 3, 238e253. Garreaud, R.D., Vuille, M., Compagnucci, R., Marengo, J., 2009. Present-day South American climate. Palaeogeography, Palaeoclimatology, Palaeoecology 281, 180e195. Gliessman, S.R., 2001. Agroecology. Ecological Processes in Sustainable Agriculture. Lewis Publ., Boca Raton.
ochemie de fleuves de Amazonie Bolivienne (Doctoral Guyot, J.L., 1992. Hydroge
de Bordeaux, Bordeaux, France. thesis). Universite Hamilton, S.K., Sippel, S.J., Melack, J.M., 2004. Seasonal inundation patterns in two large savanna floodplains of South America. The Llanos de Moxos (Bolivia) and the Llanos del Orinoco (Venezuela and Colombia). Hydrological Processes 18, 2103e2116. Hanagarth, W., 1993. Acerca de la geoecología de las sabanas del Beni en el noreste de Bolivia. Instituto de Ecología, La Paz. Hazelton, P.A., Murphy, B.W., 2007. Interpreting Soil Test Results: What Do All the Numbers Mean? CSIRO Publishing. Hijmanns, R.J., Cameron, S.E., Parra, J.L., Jones, P.G., Jarvis, A., 2005. Very high resolution interpolated climate surfaces for global land areas. International Journal of Climatology 25, 1965e1978. IUSS Working Group WRB, 2014. World Reference Base for Soil Resources. International Soil Classification System for Naming Soils and Creating Legends for Soil Maps. FAO (World Soil Resources Reports), Rome. Jaimes Betancourt, C., 2013. Diversidad cultural en los Llanos de Mojos. In: Valdez, F. (Ed.), Arqueología Amazoníca; las civilizaciones ocultas del bosque tropical. Diversidad cultural en los Llanos de Mojos, pp. 235e278. Jones, J.B., 2012. Plant Nutrition and Soil Fertility Manual, second ed. In: How to Make Soil Fertility; Plant Nutrition Principles Work. CRC Press, Boca Raton, Fla. Landon, J.R., 1991. Booker Tropical Soil Manual. A Handbook for Soil Survey and Agricultural Land Evaluation in the Tropics and Subtropics. Booker Agriculture International Limited. Pbk. ed. Harlow, Essex, England: Longman Scientific & Technical. Langstroth, R.P., 1996. Forest islands in an Amazonian Savanna of Northeastern Bolivia (unpublished Ph.D. dissertation). Department of Geography, Madison, University of Wisconsin. Latrubesse, E.M., Baker, P.A., Argollo, J., 2009. Geomorphology of natural hazards and human-induced disasters in Bolivia. In: Latrubesse Edgardo, M. (Ed.), Developments in Earth Surface Processes, vol. 13. Elsevier, pp. 181e194.
nicas de las llanuras de Lee, K., 1997. Apuntes sobre las obras hidrauclicas prehispa Moxos. Paititi 11 (1), 24e26. Lin, B.B., 2011. Resilience in agriculture through crop diversification: adaptive management for environmental change. BioScience 61 (3), 183e193. Lombardo, U., 2010. Raised Fields of Northwestern Bolivia. a GIS based analysis. €ologie Außereurop€ Zeitschrift für Archa aischer Kulturen 3, 127e149. Lombardo, U., 2014. Neotectonics, flooding patterns and landscape evolution in southern Amazonia. Earth Surface Dynamics Discussions 2 (2), 635e679. Lombardo, U., Canal-Beeby, E., Veit, H., 2011a. Eco-archaeological regions in the Bolivian Amazon. Geographica Helvetica 66, 173e182. Lombardo, U., Canal-Beeby, E., Fehr, S., Veit, H., 2011b. Raised fields in the Bolivian Amazonia: a prehistoric green revolution or a flood risk mitigation strategy? Journal of Archaeological Science 38, 502e512. Lombardo, U., May, J.-H., Veit, H., 2012. Mid-to late-Holocene fluvial activity behind pre-Columbian social complexity in the southwestern Amazon basin. The Holocene 22, 1035e1045. Lombardo, U., Denier, S., May, J.-H., Rodrigues, L., Veit, H., 2013a. Humaneenvironment interactions in pre-Columbian Amazonia: the case of the Llanos de Moxos, Bolivia. Quaternary International 312, 109e119. Lombardo, U., Szabo, K., Capriles, J.M., May, J.-H., Amelung, W., Hutterer, R., et al., 2013b. Early and middle Holocene hunter-gatherer occupations in Western Amazonia: the hidden shell middens. PloS One 8 (8), e72746.

13

Marengo, J.A., Borma, L.S., Rodriguez, D.A., Pinho, P., Soares, W.R., Alves, L.M., 2013. Recent Extremes of Drought and Flooding in Amazonia: Vulnerabilities and Human Adaptation. Martini, I.P., Chesworth, W., 1992. Weathering, Soils & Paleosols. In: Developments in Earth Surface Processes, vol. 2. Elsevier, Amsterdam, New York.
n, W., Maasch, K.A., Meeker, D.L., et al., Mayewski, P.A., Rohling, E.E., Stager, C.J., Karle 2004. Holocene climate variability. Quaternary Research 62, 243e255. Mayle, F.E., Langstroth, R.P., Fisher, R.A., Meir, P., 2007. Long-term forestsavannah dynamics in the Bolivian Amazon. Implications for conservation. Philosophical Transactions of the Royal Society B: Biological Sciences 362, 291e307. McKey, D., Rostain, S., Iriarte, J., Glaser, B., Birk, J.J., Holst, I., Renard, D., 2010. PreColumbian agricultural landscapes, ecosystem engineers, and self-organized patchiness in Amazonia. Proceedings of the National Academy of Sciences 107, 7823e7828. Mengel, K., Kirkby, E.A., 1987. Principals of Plant Nutrition, fourth ed. International Potash Institute, Basel.
n arqueolo
gica de San Ignacio de Moxos. Prov. Moxos, Michel, M., 1993. Prospeccio
s, La Paz. Facultad de Departamento de Beni. Universidad Mayor de San Andre Ciencias Sociales. Michel, M., 1999. Desarrollo temprano de la agricultura de campos elevados en los llanos de Moxos, depto. de Beni, Bolivia. In: Ledergerber-Crespo, P. (Ed.), For
n. Ponencias presentadas en el Simposio mativo sudamericano, una revaluacio
lez y internacional de arqueología sudamericana. Homenaje a Alberto Rex Gonza Betty J. Megger e Cuenca e Ecuador. Abya-Yala, Quito, pp. 271e281. Montagnac, J.A., Davis, C.R., Tanumihardjo, S.A., 2009. Nutritional value of cassava for use as a staple food and recent advances for improvement. Comprehensive Reviews in Food Science and Food Safety 8, 181e194.
gica de Bolivia. Vegetacio
n y Navarro, G., Maldonado, M., 2002. Geografía ecolo
ticos, Quinta. Santa Cruz de la Sierra, Bolivia. ambientes Acua Nicoullaud, B., King, D., Tardieu, F., 1994. Vertical distribution of maize roots in relation to permanent soil characteristics. Plant and Soil 159, 245e254. Pearsall, D.M., 2007. Modeling Prehistoric Agriculture through the Palaeoenvironmental Record: Theoretical and Methodological Issues. In: Rethinking Agriculture: Archaeological and Ethnoarchaeological Perspectives, pp. 210e230. Piperno, D.R., Pearsall, D.M., 1998. Origins of Agriculture in the Lowland Neotropics. Academic Press. ~ os de investigacio
n arqueolo
gica en Prümers, H., Jaimes Betancourt, C., 2014. 100 an
gicas 4, 11e53. los Llanos de Mojos. Arqueoantropolo Rodrigues, L., Lombardo, U., Fehr, S., Preusser, F., Veit, H., 2015. Pre-Columbian agriculture in the Bolivian Lowlands: construction history and management of raised fields in Bermeo. Catena 132, 126e138. Ronchail, J., Bourrel, L., Cochonneau, G., Vauchel, P., Phillips, L., Castro, A., et al.,
basin (south-western AmazondBolivia) and 2005. Inundations in the Mamore sea-surface temperature in the Pacific and Atlantic Oceans. Journal of Hydrology 302, 223e238. Rostain, S., 2013. Islands in the Rainforest. Landscape Management in PreColumbian Amazonia. Left Coast Press (New Frontiers in Historical Ecology), Walnut Creek.
nico de camellones en la Amazonía Saavedra, O., 2006. El sistema agrícola prehispa Boliviana. In: Valdez, F. (Ed.), Agricultura ancestral. camellones y albarradas. Contexto social, usos y retos del pasado y del presente. Abya Yala, Quito, pp. 295e311. Schachtschabel, P., Blume, H.P., Brümmer, G., Hartge, K.H., Schwertmann, U., 2002. Scheffer/Schachtschabel. Lehrbuch der Bodenkunde, 15th ed. Spektrum Akademischer Verlag, Heidelberg. Walker, J.H., 2004. Agricultural change in the Bolivian Amazon: center for comparative. Archaeology 13. Walker, J., 2011. Social implications from agricultural taskscapes in the Southwestern Amazon. Latin American Antiquity 22, 275e296. Wanner, H., Beer, J., Bütikofer, J., Crowley, T.J., Cubasch, U., Flückiger, J., et al., 2008. Mid- to late Holocene climate change: an overview. Quaternary Science Reviews 27, 1791e1828. Whitmore, T.M., Turner II, B.L., 1992. Landscapes of cultivation in mesoamerica on the eve of the conquest. Annals of the Association of American Geographers 82, 402e425. Whitney, B.S., Dickau, R., Mayle, F.E., Soto, J.D., Iriarte, J., 2013. Pre-Columbian landscape impact and agriculture in the Monumental Mound region of the Llanos de Moxos, lowland Bolivia. Quaternary Research 80, 207e217. Whitney, B.S., Dickau, R., Mayle, F.E., Walker, J.H., Soto, J.D., Iriarte, J., 2014. PreColumbian raised-field agriculture and land use in the Bolivian Amazon. The Holocene 24, 231e241. Young, V.R., Pellett, P.L., 1994. Plant proteins in relation to human protein and amino acid nutrition. American Journal of Clinical Nutrition 59, 1203Se1212S. Zhou, J., Lau, K.M., 1998. Does a monsoon climate exist over South America? Journal of Climate 11, 1020e1040.

Please cite this article in press as: Rodrigues, L., et al., Linking soil properties and pre-Columbian agricultural strategies in the Bolivian lowlands:
n, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2015.11.091 The case of raised fields in Exaltacio