Introduction

Salinity of soil or water is a major cause of agricultural crop stress and can seriously affect production, especially in arid and semiarid regions (Shannon 1998). Desertification processes affect almost 75 % of arid and semiarid regions in the world (Johnston et al. 1996). It is quite difficult to reverse desertification due to the high costs involved and the slow recovery process; therefore, it is necessary to develop integrated vegetation management techniques in these areas (Karlin 1998).

Saline soils exhibit electrical conductivity exceeding 4 dS/m in saturated extracts, or have an excess of Na+ in soil solution or cation exchange sites (called “sodic soils”). Salinity in soil may be due to natural or anthropogenic causes (derived from activities that produce an elevation of groundwater and / or increase the conductivity of the soil solution, associated with poor drainage, irrigation with poor quality water or excessive fertilization) (Lavado 2008).

Drylands salinity problems in Argentina

According to FAO-UNESCO, Argentina ranks third in the world (after Russia and Australia) in halomorphism processes (salinization, alkalinization and sodicity), which are increasing because of mishandling of irrigated soils (Taboada and Lavado 2009).

In Argentina, halomorphism processes affect arid, semiarid and even humid sectors, with and without irrigation, many of them coinciding with the country’s productive areas. An example is the Calchaquíes Valleys’ area, in the province of Salta, due mainly to its prevailing climatic and geological conditions. In the semi-arid Chaco—a sub-region of the Chaco Phytogeographic Region—the main limitation to growth is water availability, though soil salinity is also one of the primary constraints of land use (Angueira 1986). Water table dynamic processes in poorly drained soils are also present in the submeridional lowlands of Santiago del Estero and in large parts of the Southern and Eastern Córdoba, with large salinized areas. Other causes of salinization, such as clearings in Chaco and Formosa or drip irrigation in areas of Cuyo (Mendoza, San Juan and San Luis), leading to these phenomena and degradation should also be noted.

The replacement of mountain vegetation under semiarid to arid climates by irrigated crops has contributed to anthropogenic salinity. With supplementary irrigation, the water table rises, together with deep salts. In these areas, evapotranspiration exceeds precipitation, mobilizing water with higher saline concentration that—if evaporated—contributes to soil salinization. Mendoza, located at the Monte Phytogeographical Province, also has a high proportion of lands affected by salinity. Other affected provinces are Tucumán, Catamarca, La Rioja, Rio Negro and Chubut.

It is estimated that, in addition to naturally saline areas, anthropogenic salinity now affects on average 30 % of irrigated land in Argentina (Lavado 2008).

General characteristics of Atriplex spp

Atriplex is the largest and most diversified genus of the Chenopodiaceae family. Chenopodiaceae plants are distributed widely in temperate and subtropical saline habitats, particularly in coastal areas of the Mediterranean Sea, Caspian Sea and Red Sea, in the arid steppes of Central and East Asia, in the margins of the Sahara desert, in the alkaline prairies of the United States, in the Karoo of Southern Africa, in Australia, and in the pampas of Argentina. They also grow as weeds in the saline soils of urban areas, especially where water pollution and soil alteration have caused salinity problems (Mulas and Mulas 2004).

Species of the genus Atriplex are characterized by high biomass productivity, high tolerance to drought and salinity, and high efficiency in solar radiation use (Múlgura de Romero 1981). Moreover, they can provide a high quantity of leaf biomass during unfavorable periods of the year, being used as forage rich in protein and carotene and as means of combating desertification. Plants of this genus are able to fix CO2 following the C4 biosynthetic pathway. They also require good amounts of sodium—an essential element of metabolism. During C4 photosynthesis in these plants, the conversion of atmospheric carbon gas into plant material uses less oxygen, less water, fewer nutrients, and with minimum destruction of its own living tissue during the process.

Atriplex halimus (common names: saltbush, marisma, orgaza, sea purslane, shrubby orache) is a perennial shrub 2–3 m high. It is highly resistant to drought to the extent that it can survive for some time without rain. The growing season ranges from 60 to 120 days (FAO-Ecocrop 1993–2007). This shrub has a constant high salt content in its leaves, accumulating the salt excess in the plants hairs (Mozafar and Goodin 1970). These plants do not burn well because of their salt content, so their introduction to high risk fire areas could be of interest.

Some populations able to survive to salinity levels higher than those of sea water (up to EC: 60 mS/cm) have been the subject of several studies (Zid 1970; Franclet and Le Houérou 1971; Malcom and Pol 1986; Le Houérou 1986, 1993). When excreting salt through its leaves, Atriplex becomes a potential desalination plant, provided the material is removed periodically to prevent the salt from returning to the ground, which makes it ideal as a biomass producer. A. halimus is also able to grow and produce normally in non-saline soils (Mulas and Mulas 2004).

In Australia, many studies have focused on soil salinity combat using native species (Malcom 2000). Atriplex nummularia (common name: old man saltbush; http://www.florabank.org.au/lucid/key/species%20navigator/media/html/Atriplex_nummularia.htm) is an evergreen shrub, dioecious, erect, branched, evergreen, ashen colored, which reaches 1–3 m tall, is columnar in appearance and is originally from Australia (Enríquez-Carrillo et al. 2011). Its root system may develop over 3 m in depth and up to 10 m in width (Jones 1970). Thornburg (1982) reported that Atriplex nummularia grows where rainfall is at least 180 mm/year. Positive characteristics of this species are: good forage yields, strong resistance to extreme drought, resistance to grazing with fast resprouting, resistance to disease, and easy propagation. Moreover, its fuel wood has good energy content.

In the arid IV Region of Chile, A. nummularia is currently the most important species in reforestation and desertification control projects (Mulas and Mulas 2004). In these environmental conditions, forage shrub plantations can provide a supplement to cattle feed, which is based mainly on pasture, when forage availability from herbaceous cover is low due to drought.

Belkheiri and Mulas (2013a) compared both species after having subjected them to water stress. A. nummularia showed a greater osmotic adjustment and a positive net solute accumulation than A. halimus, suggesting that water stress resistance in A. halimus is linked to higher water use efficiency rather than a greater osmotic adjustment.

Uses and productivity

The benefits of Atriplex species are: long-term survival and biomass production on moderate-to-strong saline and waterlogged soils; drought tolerant; they can be used as plant-based “water pumps” to maintain saline water tables below critical depths; best used in conjunction with other feeds because they have: high protein concentration, adequate metabolizable energy, high salt concentrations, limited edible dry matter and moderate oxalate concentrations. Species of this genus also have a high ability to absorb nitrogen from the soil and can benefit from the action of nitrogen-fixing microorganisms (Ismaili et al. 2000).

A. halimus presents high bromatological values. The crude protein content is equivalent or superior to that of legumes and its digestibility is very high (Barroso et al. 2005). The literature suggests values of crude protein of 15–20 % and of 60 % digestibility (Correal 1993). It is tolerant to both intense and brief grazing, with full recovery the year after being grazed (Valderrábano et al. 1996). Salt content of edible parts of Atriplex plants, such as leaves and young shoots, is high. For this reason, sheep fed with Atriplex need to drink frequently and, in this case, the amount of ingested water may reach 11 L head−1 day−1 (Le Houérou 1991; Mirreh et al. 2000).

In the South of Spain, A. halimus produces edible biomass yields of 450–500 g/plant per year, in spite of its low nutritive value, due to its high content of non-proteic nitrogen (Correal et al. 1990).

At a research trial, Belkheiri and Mulas (2013b) observed that the leaves and roots of both species responded positively to increasing NaCl concentrations up to 600 mM NaCl for A. halimus, whereas the optimal growth of A. nummularia was recorded at 300 mM NaCl. A. halimus is a better ion accumulator and it may be used for phytoremediation.

The best yields, of about 15–20 ton ha−1 year−1, can be obtained with salt concentrations below 300 mM/L of NaCl equivalents (Le Houérou 1986). Trials performed at drylands with saltbush plantations are much modest than those with irrigated trials.

In some areas of Southern Coquimbo (Chile), with rainfall ranging from 100 to 220 mm/ year, yields of A. nummularia plantations have varied from 50 to 900 kg DM ha−1 year−1, depending on age, field management and plant density. In areas with 143 mm/year rainfall, average yields of 1,806 g/plant have been observed (Soto 1996). Planting density directly affects yield. So, as A. nummularia planting densities increase from 625 to 10,000 plants/ha, yields also experience an increase. However, leaf production decreases at planting densities over 2,500 plants/ha (Soto 1996).

A. nummularia has become commonly cultivated as a source of stock fodder and its potential to also address dryland salinity in southern Australia is under investigation (Hobbs et al. 2006). It has good production of biomass when irrigated with saline water (EC: 15–20 mS/cm) and yields may exceed 30 tons DM ha−1 year−1 (Le Houérou 1992); it also withstands long periods under soil flooded conditions: in North Africa, plants survived 3 months of flooding; they present fast and abundant regrowth after grazing and overgrazing resistance, which is the main limitation to the use of forage species. After elimination of the leaves, the plant needs a rest period of about 8–10 months to recover. On the other hand, if the plant is not grazed, its life is no longer than 12–15 years. Every 5 years, renewal pruning is recommended at 20–40 cm above the ground (El Mzouri et al. 2000).

Pruned woody branches are an important energy resource. For instance, the energy content of A. nummularia wood is about 4,538 kcal /kg (Garcia 1993). Yields of fuel wood obtained from Atriplex plants can also be considerable (Rivera 1996).

Atriplex can be utilized for biotechnical conversion in the production of gaseous or liquid fuels. Growing on saline land in Pacca Ana (Pakistan), the calorific value is 4,331 kcal /kg dried matter (El Basam 2010).

Carbon sequestration is a function of the biogeochemical exchange between plants and the atmosphere, and depends on land use and climate. According to Sochacki et al. (2012), A. nummularia produces a mean total biomass of 3.8 tons ha−1 year−1, that is to say 6.9 tons CO2-e ha−1 year−1 averaged over 4 years in Australia.

Ecological requirements

Selection of the best cultivation species is fundamental in semi-arid and arid climates and is determined by many factors, such as temperature and rainfall, soil type, water availability for irrigation and crop purposes.

A. halimus fits better in Mediterranean climates; however, it is a rustic species that tolerates harsh environmental conditions. It adapts to wet tropical and dry tropical climate (Aw), desert or arid climate (BW), or semi-arid steppe climate (BS) and subtropical dry summer (Cs) (FAO-Ecocrop 1993–2007).

Atriplex is a short-day plant (less than 12 h) (FAO-Ecocrop 1993–2007). It grows up to 1,500 m above sea level (Iglesias and Taha 2010).

Usually, the genus Atriplex can grow and reproduce under rainfall conditions ranging between 100 and 400 mm/year, with yields varying from 1 to 3 tons DM ha−1 year−1 (Sankary 1986).

The plant survives with precipitation ranging from 100 to 250 mm; however, better production is obtained with mean annual rainfall from 250 to 650 mm (Nefzaoui and El Mourid 2008). No problems are observed with precipitation of 50 mm per year and—in some cases—it survived one or more years without rainfall.

Atriplex has high water-use efficiency (Belkheiri and Mulas 2013a). In fact, A. nummularia, A. halimus, and A. canescens produce 10–20 kg DM ha−1 year−1, per millimeter of rain (Forti 1986; Correal et al. 1990; Le Houérou 1992). In spite of the high water-use efficiency of Atriplex plants (Silva and Lailhacar 2000) when planted in regions with an average rainfall of 200–300 mm/year, it is convenient to irrigate the crop with at least 200–250 mm water/year (Le Houérou 1992).

At irrigation levels ranging from 100 to 400 mm of water ha−1 year−1, some Atriplex species—A. halimus included—showed water-use efficiency values of 5–10 kg DM ha−1 year−1, per millimeter of water and yields of 2 to 4 tons DM ha−1 year−1. Therefore, these forage shrubs showed water-use efficiency high enough to produce a DM quantity twice higher than that of wheat and barley, and 4–5 times higher than that of Lucerne (Le Houérou 1992). In Western Australia, plantations of Atriplex irrigated with 500 mm/year yielded more than 5 tons DM ha−1 year−1 (Malcom and Pol 1986).

A. nummularia lives naturally in areas with rainfall between 150 and 400 mm (Enríquez-Carrillo et al. 2011). Winter temperatures decrease growth, e.g., saltbush presents very little or no growth at all when the average ambient temperature is lower than 11 °C. According to Florabank of Australia (http://www.florabank.org.au/…/Atriplex_nummularia), it inhabits sites where the mean annual temperature fluctuates from 13 to 24 °C, with the mean maximum temperature of the warmest month ranging from 32 to 37 °C and the mean minimum temperature of the coldest month ranging from 3 to 7 °C. Seeds usually germinate in 1–3 weeks at 13 °C (Rice 1988). Optimum temperatures fluctuate between 16 and 28 °C (FAO-Ecocrop 1993–2007) and they need an average solar radiation of 766 cal cm−2 day−1 (Iglesias and Taha 2010).

Regarding minimum temperatures that these species can withstand, we may say that they can resist frost intensity between −8 °C (A. nummularia) and −12 °C (A. halimus) (Huxley 1992). Plants can be damaged by severe frosts but they soon recover (Bean 1981).

In terms of soil requirements, they survive quite well in heavy textured thin soils, poor and sandy soils, but their best development and productivity can be found in deep medium textured soils with good drainage. They tolerate well a pH range of 6.8 to 8.2 (Iglesias and Taha 2010), with an optimum of 7.2 to 7.8 (FAO, Ecocrop 1993–2007).

This aim of this work consisted in defining the agro-ecological suitability of Argentine drylands saline soils to produce lignocellulosic biomass from A. halimus and A. nummularia.

Materials and methods

We used mean annual rainfall and temperature data available for the period 1981–2010, from meteorological and agrometeorological stations located within the geographical area under analysis.

In a first step and in order to analyze the bioclimatic requirements of Atriplex spp, we focused on the moisture factor, analyzing the average annual isohyets of 250, 400 and 650 mm. Those areas receiving from 100 to 250 mm annually (Nefzaoui and El Mourid 2008), were described as marginal areas; although the situation could be improved by implementing complementary irrigation. In the range of 250–400 mm (Sankary 1986) the area qualifies as suitable, and from 400 to 650 mm: optimal. However, rainfall higher than 650 mm to 1,200 mm also determined suitable areas (FAO-Ecocrop 1993–2007).

In order to consider the thermal factor, we took into account the isotherm corresponding to the average annual temperature of 13 °C.

The country’s geographic areas that do not meet this condition qualify as non-suitable areas. In addition, we considered that the coldest month’s average minimum temperature should exceed 3 °C and the warmest month’s average maximum temperature should not exceed 34 °C (average 32–37 °C).

Once satisfied that the mean minimum temperature of the coldest month was over 3 °C and the average maximum temperature of the warmest month was below 34 °C, we outlined the optimal area from the thermal point of view. When the average maximum temperature of the warmest month exceeds 34 °C, the area qualifies as a marginal area. Similarly, when the average temperature is over 13 °C and the average minimum temperature of the coldest month is higher than 3 °C, the area qualifies as suitable.

Furthermore, winter frosts intensity should be below −8 °C; otherwise, it would have a lethal effect on A. nummularia and −12 °C on A. halimus (Huxley 1992).

An agro-climatic zone map was then obtained by overlaying the previous maps. With the available database, we used the geographical limits for the different variables as five aptitude classes: optimal zone, very suitable, suitable with irrigation, suitable with constraints and non suitable were defined and mapped.

To obtain the maps, we used a series of previously interpolated bioclimatic variables, which were then processed with the Geographic Information System (GIS) tool of the Arc-GIS 9.3 program. These five bioclimatic variables, obtained from interpolation from data from 125 meteorological stations of the National Meteorology Service, and which cover all the Argentine Republic, were: (1) average annual temperature >13 °C, (2) mean minimum temperature of the coldest month (July) >3 °C, (3) average maximum temperature of the warmest month (January) <34 °C, (4) absolute minimum temperature −8 °C (A. nummularia) and −12 °C (A. halimus), and (5) average annual rainfall (240, 400 and 650 mm).

Climatic interpolations were made using the “Interpolate to Raster” tool, within the “3D Analyst” extension of the Geographic Information System (GIS) of the Arc-GIS 9.3. Program, following the Ordinary Kriging interpolation method.

Agroclimatic suitability and agro-ecological suitability mapped variables were obtained from multivariable integration geoprocessing, using the “Raster Calculator” tool of the “Spatial Analyst” extension of the same program.

Based on FAO (2008) salinity classification, drylands saline soils were plotted in Argentina, considering “moderately saline phase” when soil electrical conductivity ranges from 8–16 mmhos/cm and “strongly saline” when it exceeds that value, as possible sites for Atriplex spp. implantation, since both species tolerate values of 12.4 to 22.1 dS/m of high to extremely high salinity.

The overlap of the agroclimatic suitability map with the drylands saline soils map shows the agro-ecological zoning and defines the potential growing areas for Atriplex nummularia and A. halimus in Argentina for bioenergy on marginal soils.

According to Sochacki et al. (2012), A. nummularia produces a mean total biomass of 3.8 tons ha−1 year−1, that is to say, 6.9 tons CO2-e ha−1 year−1 averaged over 4 years in Australia. On the basis of this data, we calculated the potential carbon sequestration for each of the agro-ecological zoning aptitude classes. The obtained results provide an approximate idea of the ton CO2e that could be “sequestrated” in Argentina, by the implementation of Atriplex nummularia at each delimited zones.

Results and discussion

Figure 1 shows the delimitation of moisture regions. For this purpose, mean annual isohyets of 250, 400 and 650 mm were included. If an area receives from 100 to 250 mm rain annually (Nefzaoui and El Mourid 2008), it determines a marginal area and requires the application of complementary irrigation to obtain good yields of biomass; the range 250 to 400 mm (Sankary 1986) results in a suitable area; while 400–650 mm describes an optimal area. Annual rainfall of over 650 mm to 1,200 mm determines suitable areas (FAO-Ecocrop 1993–2007). Optimal areas comprise two sub-regions: one covers a strip running down the center of the country and extending to the province of Jujuy in Northern Argentina; and the other is located in the Patagonian mountain skirts, covering the provinces of Neuquén, Rio Negro and Chubut. Suitable areas extend on both sides of the optimal areas.

Fig. 1
figure 1

Moisture regimes in Argentina: annual rainfall

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Figure 2 shows thermal regions. Thus, the isotherm corresponding to the average annual temperature of 13 °C determines non suitable areas to the south and to the west. Suitable areas are circumscribed to conditions of 13 °C isotherm and 3 °C, corresponding to the average temperature of the coldest month. Optimal areas are determined by conditions of 3 °C isotherm and 34 °C isotherm, corresponding to the average temperature of the warmest month. When such a temperature range is exceeded, the area qualifies as marginal. The optimum area covers most of northern and central Argentina. This area is interrupted by two marginal sectors: one located in the province of La Rioja, which covers part of southern Catamarca, and the other located in the province of Formosa. Suitable areas include part of the Patagonian sector, most of the western part of the country and the rest of the pampas.

Fig. 2
figure 2

Thermal regime: 13 °C is the mean annual temperature, 3 °C represents the mean minimum temperature of the coldest month and 34 °C represents the mean maximum temperature of the warmest month

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The overlay of both maps (Figs. 1 and 2) allows an Agroclimatic suitability map to be generated (Fig. 3), which determined five suitability classes. The optimal area comprises Southwestern Santiago del Estero, Northwestern Cordoba, Northern San Luis, Western La Rioja and Catamarca. The area comprises two very suitable sub-regions: the Northwestern Region (Salta, Jujuy, Northern La Rioja and Western Catamarca) and another center sub-region covering Central San Luis, Western Mendoza, Central La Pampa, Southern Buenos Aires and a small sector of Rio Negro. To the South, it was possible to define a zone classified as suitable area with irrigation, which covers a small area of Neuquén, a small area of Rio Negro and Chubut, which could be irrigated with seawater at coastal areas. There were two areas suitable with constraints: one covers part of Chaco, Santiago del Estero, Formosa and Salta, and the other part of La Rioja and Catamarca.

Fig. 3
figure 3

Agroclimatic suitability for the cultivation of Atriplex halimus and Atriplex nummularia

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Figure 4 shows the lowest temperature that represents the minimum lethal temperature for each species, which is −8 °C for A. nummularia and −12 °C for A. halimus. While these low temperatures have a recurrence of one occurrence every 30 years, this must be weighed against a possible loss of biomass, because both species recover easily after frost.

Fig. 4
figure 4

Absolute minimum temperature: −8 °C (A. nummularia) and −12 °C (A. halimus)

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Figure 5 shows the Drylands saline soils in Argentina (FAO 2008).

Fig. 5
figure 5

Drylands salinity map (reference: FAO 2008)

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Finally, Fig. 6 shows the agroecological suitability map resulting from the overlap of the agroclimatic suitability map with the drylands salinity map (Fig. 5). The same six fitness classes delimited at the agroclimatic suitability map were observed. Lands classified with different degrees of fitness should be allocated to the cultivation of A. nummularia and A. halimus for energy purposes, knowing that such lands are not intended to compete with those designated for food.

Fig. 6
figure 6

Agro-ecological suitability map for the cultivation of Atriplex halimus and A. nummularia

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Furthermore, Atriplex species planted in soil with moderate to severe salinity problems can produce additional fodder for livestock, while stabilizing severely disturbed lands, meaning that these shrubs may be used for the rehabilitation of saltland.

Besides, A. nummularia is one of the plants that have been identified by the United Nations’ Carbon Emission Trading Scheme to assist with the battle against global greenhouse warming and the sequestration of atmospheric carbon back into the soil. Potential carbon sequestration was calculated according to Sochacki et al. (2012) considering that A. nummularia produced 6.9 tons CO2-e ha−1 year−1 averaged over 4 years. Table 1 presents potential carbon sequestration in Argentina for the different classes defined at the agro-ecological zonation.

Table 1 Potential carbon sequestration in the potential growing areas for A. nummularia and A. halimus
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The implementation of A. nummularia and A. halimus culture will provide feedstock for bioenergy or second generation biofuels and produce salt-tolerant forage for livestock. These crops do not compete for lands with those destined for food production.

This alternative approach to cultivating abandoned farmland affected by anthropogenic or natural salinization also serves the purpose of mitigating carbon emissions.

Native species of the genus Atriplex in Argentina should be investigated in production systems to exploit the country’s natural resources and preserve the biodiversity of the ecosystem.

Conclusions

Based on a search of the international literature, the authors outline an agro-climatic zoning model to determine potential production areas in Argentina for Atriplex halimus and A. numularia. This model may be applied to any part of the world, using the agroclimatic limits presented in this work.

Lands classified with different degrees of aptitude on the agroecological suitability map could be used for cultivation of A. nummularia and A. halimus for energy purposes, knowing that such lands are not intended to compete with those designated for food. These crops provide lignocellulosic biomass for solid fuel or for second-generation biofuels, at the same time providing food for livestock in times of scarcity and contributing to CO2 sequestration and thus mitigating climatic change. Besides, the use of saline lands—as long as they do not compete with those destined for food production—becomes of great importance for our growing global population.