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Introduction

Soil salinity and sodicity are serious land degradation problems in arid and semiarid regions of the world. The excessive irrigation in agriculture has mainly contributed to the increasing problems of secondary salinization, alkalization, and waterlogging (Szabolcs 1994; Rengasamy 2006; Qadir et al. 2007). Waterlogging and salinity are major impediments to the sustainability of irrigated lands and livelihood to the farmers, especially the smallholders in the affected canal-irrigated as well as nonirrigated areas. Salt-prone soil degradation results in loss of ecosystem goods and services, which affects the livelihood of people dependent on soil and water resources of marginal lands (Qadir et al. 2007). Agroforestry applications in saline areas can help in mitigation of rising atmospheric carbon dioxide concentrations through carbon sequestration in the plant-soil system. The mixing of woody plants into crop, forage, and livestock operations provides greater resiliency to the interannual variability through crop diversification as well as through increased resource use efficiency (Olson et al. 2000). In a recent estimate, salt-induced land degradation in irrigated areas may cost 27.3 billion US dollars because of lost crop production (Qadir et al. 2014). To meet increasing demands of human society for food, fodder, biomass energy, and industrial products for the ever growing population, it is critical to formulate management strategies for ecological restoration of salt-affected lands.

Agroforestry systems may be an alternative land use option for utilizing salt-affected soils by integrating salt-tolerant tree species with crops or other useful plants (Singh et al. 1988; Bell 1999; Turner and Lambert 2000; Singh et al. 1994; Dagar 2009; Wicke et al. 2013; Toderich et al. 2013). Biodrainage has been found to be effective in controlling waterlogging and salinity in irrigated canal command areas in arid and semiarid regions (Ram et al. 2007; Shakya and Singh 2010; Toky et al. 2011). Agroforestry can add biodiversity on degraded saltlands by enhancing their capacity for supporting numerous ecological and production functions. Agroforestry on salt-affected soils can help in mitigation of rising atmospheric carbon dioxide concentrations through carbon sequestration in the plant-soil system. Estimates of biomass production and the rates of carbon sequestration (Lal 2004) and exploration of greenhouse gas balance (Wicke et al. 2013) have been found to be useful to understand the potential of tree-based systems in climate change mitigation. Management practices that favor carbon sequestration in agroforestry also tend to enhance resilience in the face of climate variability and facilitating long-term adaptation to changing climates (FAO 2010). Soil carbon sequestration is an important factor in the greenhouse gas emission balance and is strongly related to site conditions (e.g., soil structure, initial soil carbon content, climate), structure of agroforestry, and soil management (Montagnini and Nair 2004; Nair 2007; Nair et al. 2009).

This paper gives an overview of salt-induced land degradation and the role of agroforestry systems in restoration of salt-affected soils with emphasis on soil enrichment, bio-amelioration, and carbon sequestration.

Salt-Induced Land Degradation

Estimations the global area of salt-affected land range from 400 million hectares (Mha) to 960 Mha (Szabolcs 1989; FAO 2001, 2008), depending on the datasets and the classification systems used. Recent estimates show that approximately 1 billion hectares of land are salt-affected worldwide (Wicke et al. 2011), of which about 76 (Mha) are affected by human-induced salinization and sodification (Oldeman et al. 1991). In India, about 6.75 Mha lands are either sodic or saline (Mandal et al. 2010). In Pakistan, nearly 6.3 million hectares are affected by different levels and types of salinity, out of which nearly half are under irrigated agriculture, especially Indus basin (Qureshi et al. 2008).

Salt-affected lands in Central Asian region are the most characteristic features of natural continental terrestrial salinization, sodification, and alkalinization, the predominant salinity being sulfate-chloride type (Toderich et al. 2013). Low organic matter (<1.0 %), high salt contents, and poor water-holding capacity render these soils unproductive (Toderich et al. 2013). Currently, salt-affected soils have been reported to occur within the sovereign borders of at least 75 countries (Qadir et al. 2014). Salt-affected soils are known to occur in many other arid and semiarid regions of the world including the Aral Sea Basin in Central Asia, the Yellow River Basin in China, the Euphrates Basin in Syria and Iraq, the Murray-Darling Basin in Australia, and the San Joaquin Valley in the United States (Qadir et al. 2014).

Characteristics of Salt-Affected Soils

The characterization of salt-affected soils is useful to determine the restoration options under different climatic conditions, quality and quantity of groundwater, and plant adaptation to salt stress. Depending upon physical and chemical nature, salt-affected soils are generally categorized into saline, alkaline/sodic, and saline-alkali soils (see Table 1). The saline soils have white encrustations on the surface and have high concentrations of soluble chlorides and sulfates of sodium, calcium, and magnesium as dominant salts. These soils have pH below 8.2 and electrical conductivity greater than 2–4 dS m−1 at 25 °C and sodium absorption ratio of the soil solution <15. Saline soils usually remain flocculated due to the presence of excess salts and have high osmotic pressure of soil solutions which induces physiological drought, tissue injury due to direct toxic effects of individual ions, and complex interactions between sodium, calcium, and magnesium.

Table 1 Some properties of saline, sodic, and waterlogged soils relevant to plant survival and growth
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The sodic soils are characterized by high soil pH (saturation soil paste pH >8.5 and often approaching 11), exchangeable sodium percentage (ESP) >15, and varying electrical conductivity (ECe <2–4 dS m−1). The presence of high exchangeable sodium percentage in soils imparts poor physical conditions to soils, low infiltration of water, and dispersion of soil organic matter. The precipitation of calcium in alkali soils causes deposition of thick CaCO3 layer known as kankar (CaCO3) pan.

The saline-alkali soils are characterized by high levels of soluble salts as well as sodium ions.

Rengasamy and Sumner (1998) have discussed various mechanisms for soil degradation under sodic conditions. Soil degradation under sodic conditions occurs due to hydration of dry aggregates, slaking and swelling of wet aggregates, and dispersion of clay platelets from soil aggregates (Cass and Sumner 1982; Rengasamy and Sumner 1998). The formation of structural crusts on the surface of sodic soils affects the infiltration of water in soils of arid and semiarid regions (Moore and Singer 1990).

Recent evidence from different regions of the world has distinguished another type of salt-affected soil, i.e., soil that is affected by magnesium (Vyshpolsky et al. 2008). With high levels of magnesium, when plowed, these soils form large clods that impede water flow resulting in poor water distribution and plant growth; magnesium-affected soils share several common features with sodic soils.

Approaches for Restoring Salt-Affected Lands

Several approaches including chemical amendments, tillage operations, crop-assisted interventions, tree plantations, and phytoremediation have been used for the restoration of salt-affected soils (Qadir et al. 2002, 2007). The sodic soils have been reclaimed by growing salt-tolerant grasses (Malik et al. 1986; Rana and Parkash 1987), protecting natural vegetation cover (Gupta et al. 1990, 2015; Dagar 2014), and adopting reclamation forestry and agroforestry (Singh and Gill 1992; Singh 1995). Agroforestry systems have been developed on sodic soils (Dagar et al. 2001; Kaur et al. 2000, 2002a; Singh and Dagar 2005), on saline wastelands (Tomar et al. 1998, Tomar et al. 2003; Dagar et al. 2006, 2009), and for controlling waterlogging in semiarid regions (Ram et al. 2007, 2008, 2011; Kumar 2012; Dagar et al. 2015a, b). Some examples of agroforestry systems for salt-affected areas in India are summarized in Table 2. These systems are comprised of temporary agroforestry system, silvi-agricultural to agrohorticultural, halophytic trees to remediate soil, conventional agroecosystem, trees for biodrainage, energy plantations, and silvopastoral agroforestry.

Table 2 Some examples of agroforestry systems on salt-affected soils in India
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Agroforestry for Sodic Soils

On a highly sodic soil, mesquite (Prosopis juliflora) and Kallar grass (Leptochloa fusca) silvopastoral practices were found to be promising for firewood and forage production and also for soil amelioration (Singh et al. 1993). Leptochloa fusca grown with P. juliflora produced 55.6–80.9 Mg ha−1 green forage without application of any fertilizer or other amendment (Singh et al. 1993; Singh 1995). The integration of salt-tolerant trees with naturally growing grasses has been reported to be a viable land use option for improving the biological productivity and fertility of highly sodic soils at Bichhian, northwestern India (Kaur et al. 2002a, b). In silvopastoral agroforestry systems, comprising Acacia nilotica + Desmostachya bipinnata, Dalbergia sissoo + Desmostachya bipinnata, and Prosopis juliflora + Desmostachya bipinnata, the bole wood (that can be used as small timber) was 4.62–9.78 Mg ha−1, and branch wood biomass (that can be used as fuelwood) production ranged from 4.16 to 20.82 Mg ha−1 year−1 (Kaur et al. 2002a, Table 3). In these systems, increased input of plant residues from aboveground and belowground parts into the soil played a significant role to improve soil properties and fertility of highly sodic soils.

Table 3 Timber and fuelwood production in silvopastoral systems of Acacia nilotica, Dalbergia sissoo, and Prosopis juliflora along with Desmostachya bipinnata and Sporobolus marginatus after 6 years on a sodic soil at Bichhian, northwestern India
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In a well-conducted site-specific field study on sodic soils at Saraswati in semiarid Haryana, out of 30 tree species planted in highly alkali soil (pH of soil profile 10.1–10.6), only three tree species, i.e., Prosopis juliflora, Acacia nilotica, and Tamarix articulata, were found to be economically viable with biomass production of 51, 70, and 93 Mg ha−1 in 7 years, respectively (Dagar et al. 2001; Singh and Dagar 2005). At the same site, the different species of Prosopis such as P. juliflora, P. alba, P. articulata, P. levigata, and P. nigra produced high biomass. Aboveground air-dried biomass (Mg ha−1) of different trees grown in sodic soil ranged from 56.5 to 19.2 Mg ha−1, the values being high in the case of Prosopis juliflora, Acacia nilotica, and Casuarina equisetifolia (Singh et al. 2008, Fig. 1).

Fig. 1
figure 1

Aboveground air-dried biomass (Mg ha−1) of different trees grown in sodic soil Pj Prosopis juliflora, An Acacia nilotica, Ce Casuarina equisetifolia, Ta Terminalia arjuna, Pd Pithecellobium dulce, Et Eucalyptus tereticornis, Pa Prosopis alba, Pp Pongamia pinnata, Cs Cassia siamea, Ai Azadirachta indica (Source: Singh et al. 2008)

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Some aromatic grasses such as palmarosa (Cymbopogon martinii) and lemon grass (C. flexuosus) could successfully be grown on moderate alkali soils up to pH 9.2, while Vetiveria zizanioides could withstand both high pH and stagnation of water (Dagar et al. 2004, 2006). Plantago ovata produced 1.47–1.58 Mg ha−1 unhusked grain at pH 9.2 and 1.03–1.12 Mg ha−1 at pH 9.6 showing its potential for utilizing moderate alkali soil (Dagar et al. 2006). Matricaria chamomilla, Catharanthus roseus, and Chrysanthemum indicum were other interesting medicinal and flower-yielding plants which could be grown on moderate alkali soil (Dagar et al. 2009). All these crops can be integrated suitably as intercrops in agroforestry systems on moderate alkali soils.

Dagar et al. (2015b) reported that licorice (Glycyrrhiza glabra) could successfully be grown on alkali soils ranging in pH from 8.4 to 9.8. The forage yield was 2.4–6.1 Mg ha−1 per annum, and root biomass ranged from 6.0 to 7.9 Mg ha−1 after 3 years of growth. The sodic lands were reclaimed considerably in terms of reducing soil pH and exchangeable sodium percentage by growing licorice.

Agroforestry Systems for Saline Soils

A special feature of many dryland soils is salinity, either through natural occurrence or increasingly as a result of irrigation (Glenn et al. 1992). Many halophytic plants are especially adapted to these conditions, and there is a large potential for sequestering carbon in saline soils. The saline soils can be used for growing halophytes, which can be used for forage, feed, and oilseed (Glenn et al. 1992, 1993; Dagar 2014). Aboveground biomass of trees on saline soils after 9 years of planting on saline soils, Acacia nilotica, Prosopis juliflora, Casuarina glauca were best suited to saline conditions; the biomass ranged from 52 to 98 Mg ha−1 (Tomar et al. 1998). It may be stated that these species could be used successfully as energy plantations on both saline and sodic soils.

In waterlogged saline areas, several grasses such as Leptochloa fusca and species of Aeluropus, Eragrostis, Sporobolus, Chloris, Panicum, and Brachiaria can be successfully grown along with salt-tolerant trees for viable and sustainable silvopastoral systems to sustain livestock productivity (see Dagar 2014). Aeluropus lagopoides, Sporobolus helvolus, Cynodon dactylon, Brachiaria ramosa, Paspalum spp., Echinochloa colonum, E. crusgalli, Dichanthium annulatum, Vetiveria zizanioides, and Eragrostis sp. are important grasses which are tolerant to both salinity and stagnation of water and can successfully be grown in silvopastoral systems. Species of Ziziphus, Atriplex, Kochia, Suaeda, Salsola, Haloxylon, and Salvadora are prominent forage shrubs of saline regions and browsed by camel, sheep, and goat (see Dagar 2014). Salt-tolerant tree plantations and forage grasses could be raised using saline water irrigation (Minhas et al. 1997a, b; Tomar and Minhas 1998; Tomar et al. 2003).

Oil-yielding saltbush Salvadora persica can perform well both in dry and waterlogged situations in saline soils. S. persica-based silvopastoral system was developed with forage grasses (Leptochloa fusca, Eragrostis sp., and Dichanthium annulatum) on clay loam saline vertisol (clay 40 %, silt 31 %, sand 29 %; pH ranging from 7.2 to 8.9; ECe from 25 to 70 dS m−1) in Gujarat (Rao et al. 2003). Leptochloa fusca, Eragrostis sp., and Dichanthium annulatum, when planted on 45 cm high ridges, could produce 3.17, 1.85, and 1.09 Mg ha−1 forage, respectively. When planted in furrows, the forage yield was 3.75, 1.76, and 0.54 Mg ha−1 in the case of Leptochloa fusca, Eragrostis sp., and Dichanthium annulatum, respectively, showing their potential for these highly degraded lands.

In northwestern India, long-term field trial with tree species on a calcareous soil of semiarid region showed that aboveground biomass production after 20 years varied from 20.37 to 391.53 Mg ha−1. The differences in biomass among the tree species showed that preferred choice for carbon sequestration in aboveground plant biomass was Tamarix articulata, Acacia nilotica, A. tortilis, Prosopis juliflora, Eucalyptus tereticornis, Azadirachta indica, and Cassia siamea. The species with moderate biomass production are Acacia tortilis (hybrid), Ziziphus mauritiana, Pithecellobium dulce, Melia azedarach, Cassia fistula, C. javanica, Callistemon lanceolatus, and Acacia farnesiana, whereas tree species like Acacia auriculiformis, Albizia lebbeck, Bauhinia variegata, Cassia glauca, Syzygium cuminii, Crescentia alata, Samanea saman, and Terminalia arjuna showed lower biomass. For sustaining viable wood production under saline irrigation, tree species should be tolerant to salinity and drought as well as adapted to the local agro-climate (for details, see in chapter “Innovations in Utilization of Poor-Quality Waters for Sustainable Agricultural Production”).

In the Hungary Steppes in Uzbekistan, the highly saline abandoned soils were restored by growing licorice (Glycyrrhiza glabra), which is known to be a salt-tolerant perennial shrub species (Kushiev et al. 2005). Licorice was grown on 13 ha that had been abandoned due to high levels of salts and shallow groundwater, an adjacent field of 10 ha served as the control during the study period 1999–2003. The licorice fodder that was harvested in 2001 showed dry matter yield of 3.6 Mg ha−1, with a protein content of 12 %. By 2003, licorice fodder and root yields were 5.1 Mg ha−1 and 8.5 Mg ha−1, respectively. At the end of the 2003 growing season, the control and licorice grown fields were used for growing wheat-cotton crop in rotation, wheat yield being 2.42 Mg ha−1 after licorice cultivation and 0.87 Mg ha−1 from the control plots (Dagar et al. 2015b). The restoration of soil also caused an increase in cotton yield from 0.31 to 1.89 Mg ha−1. Comparing the average yields of wheat and cotton in the study area as 1.75 and 1.5 Mg ha−1, respectively, licorice showed the potential to increase productivity and farm-level income from abandoned saline fields because of lowering of the water table, enhancing the leaching of salts, as well as increase in the soil organic carbon content (Kushiev et al. 2005).

Lamers et al. (2008) studied the prospects of establishing agroforestry systems, comprised of three tree species (Elaeagnus angustifolia, Ulmus pumila, and Populus euphratica), on saline wastelands in the lower reaches of Amu Darya River in Uzbekistan. The biomass data were collected for 4 years as well as complemented this with data of mature trees (15–20 years) growing naturally on the marginal land. The potential for capital investment in small-scale woodlots was computed from annual fuelwood, fodder, and fruit production, plus the stumpage value after 20 years. The benefit to cost ratio (BCR) and net present value (NPV) were compared at 10 %, 16 %, and 24 % discount rates. At 16 % discount rate, the NPV for the three tree species was Elaeagnus angustifolia US$ 13,924 ha−1, Populus euphratica US$ 4096 ha−1, and Ulmus pumila US$ 1717 ha−1. The benefit to cost ratio (BCR) ranged from 7.8 to 1.8 for these species, respectively. Thus, tree plantations may provide positive returns to investment and significant economic and social benefits to land users and providing an opportunity for capital investment in afforesting abandoned salt-affected lands with multipurpose tree species. Although the financial assessment of afforestation is an important criterion, many additional factors such as risk assessment, planting techniques, and availability of resources need to be taken into consideration (Lamers et al. 2008).

In the Yangiobod farm, northern Tajikistan, an agrisilvicultural trial of trees intercropped with deep-rooted, early maturing, and frost-tolerant legume was established for evaluating suitable tree species and silvicultural techniques for utilizing degraded, saline lands (Toderich et al. 2013). Soil salinity at root zone was about 45 dS m−1, whereas the groundwater salinity ranged from 8.0 to 16.5 dS m−1. The agroforestry model was characterized of native tree/shrubs plantation intercropped between rows with annual halophytes and forage crops on saline marginal lands in the region. The results of this experiment revealed that the leading tree species with regard to survival rate, growth characteristics, and adaptability to high-saline natural environment included H. apphyllum, Salsola paletzkiana, and S. richteri at saline sandy sites, followed by E. angustifolia, Populus euphratica, P. nigra var. pyramidalis, Robinia pseudoacacia, Morus alba, and M. nigra. During the first 3 years, the growth performance of trees on marginal land was found to be comparable to those reported for trees on irrigated agricultural land (Khamzina et al. 2008). The species of genus Tamarix and Elaeagnus angustifolia showed the highest potential for growing on both loamy and sandy soils, the dominant soil textures in the region. Thus, incorporation of fodder halophytes into the agro-silvo-pastoral system represents low-cost strategies for rehabilitation of desert degraded rangelands and abandoned farmer lands affected both by soil and water salinity (Toderich et al. 2013).

Agroforestry Systems for Waterlogged Areas (Biodrainage)

Waterlogging may be defined as stagnation of water on the land surface or where the water table rises to an extent that soil pores in the crop root zone become saturated, resulting restriction in normal circulation of air leading to decline in the level of oxygen and increase in the level of carbon dioxide (Heuperman et al. 2002; Setter et al. 2009). Much of the world’s saline land is also subject to waterlogging (saturation of the soil) because of the presence of shallow water tables or decreased infiltration of surface water due to sodicity (Ghassemi et al. 1995; Qureshi and Barrett-Lennard 1998). Biodrainage may be defined as “pumping of excess soil water by deep-rooted plants using their bio-energy” (Ram et al. 2008, 2011). The Eucalyptus-based agroforestry on waterlogged soils have been developed in semiarid regions of Haryana (Ram et al. 2007, 2008, 2011; Kumar 2012; Dagar et al. 2015a).

Biomass accumulation has been studied in farmer’s plantation model of biodrainage in northwestern India (Toky et al. 2011). An abandoned waterlogged area (water table up to 2 m) on a farm adjacent to Balsamand canal at HAU Hisar was planted with ten tree species. After 6 years of establishment of the plantations, the cone of depression of the water table beneath the plantation strips was observed, and the decline in water table was found to be 20 cm over the entire area (Toky et al. 2011). The aboveground and belowground biomass accumulation after 6 years was greater in different clones of Eucalyptus tereticornis (102–186 Mg ha−1) as compared to other tree species (12–95 Mg ha−1).

Dagar et al. (2015a) reported timber and fuelwood biomass in clonal Eucalyptus tereticornis plantation in different spacing in shallow water table areas at Puthi, Hisar. After 6 years, total dry biomass production of 49.5 Mg ha−1 was obtained from row plantation (1 × 1 m space) and 193 Mg ha−1 in block plantation (2 × 4 m space) showing potential of clonal Eucalyptus on waterlogged farmlands (Table 4).

Table 4 Growth performance of 6-year-old cloned Eucalyptus plantation grown as strip and block plantation
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The naturally growing halophyte such as Atriplex, Suaeda, Haloxylon, Kochia, and Salsola and grasses such as L. fusca, B. mutica, V. zizanioides, and Paspalum spp can provide vegetation cover over the barren areas so as to check the upward flux of salts from the soil surface (Dagar 2014). When trees are raised in wider rows, the grasses in combination with trees are more efficient than the trees alone in waterlogged and saline areas (Dagar 2014). Khamzina et al. (2006) evaluated the potential of nine multipurpose tree species to reduce saline groundwater tables in the lower Amu Darya River region of Uzbekistan. On the basis of water use characteristics, salinity tolerance, growth rate, and the ability to produce fodder and fuelwood, Elaeagnus angustifolia performed best for biodrainage; Populus spp. and Ulmus pumila also showed good potential for biodrainage (Khamzina et al 2006). The fruit species in the region such as P. armeniaca and Morus alba showed low biodrainage potential.

Lamers et al. (2008) and Lamers and Khamzina (2008) evaluated the prospects of establishing agroforestry systems on saline wastelands in the lower reaches of the Amu Darya River in Uzbekistan by using three tree species including Russian olive (Elaeagnus angustifolia), Siberian elm (Ulmus pumila), and Euphrates poplar (Populus euphratica). The data collected for 5 years were compared with data of mature trees (15–20 years) growing naturally on the marginal land. The 1 ha plantation of Euphrates poplar produced fuelwood to meet the average annual per capita energy needs of 89 people, followed by Russian olive (72 people) and Siberian elm (55 people) (Table 5; Lamers and Khamzina 2008).

Table 5 Fuelwood production and estimated energy value of 5-year-old tree plantations based on conversion into oil and coal equivalents
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The potential for capital investment in small-scale woodlots was assessed by considering annual fuelwood, fodder, and fruit production, plus the stumpage value after 20 years. The benefit to cost ratio (BCR) and Net Present Value (NPV) were compared at 10, 16, and 24 % discount rates. At 16 % discount rate, the Net Present Value (NPV) for Russian olive was the highest (US$ 13,924 ha−1), followed by Euphrates poplar (US$ 4096 ha−1) and Siberian elm (US$1717 ha−1) showing a BCR of 7.8, 2.2, and 1.8 for these tree species, respectively (Lamers et al. 2008). This study demonstrated that tree plantations may provide positive returns to investment and significant economic and social benefits to land users, facilitating the use of abandoned salt-affected lands with multipurpose tree species. For successful agroforestry on such soils, many additional factors such as risk assessment, planting techniques, and availability of resources need to be taken into consideration (Lamers et al. 2008).

Improvement of Soil Biological Properties under Agroforestry

The soil microbial activity includes measures of the respiratory activity of soil organisms (Singh and Gupta 1977), soil microbial biomass (Jenkinson and Ladd 1981; Vance et al. 1987; Wong et al. 2010; Balota et al. 2014; Campos et al. 2014), and microbial respiration (Ingram et al. 2005), decomposition of soil organic matter (Rietz and Haynes 2003), and carbon dioxide (CO2) emission from soil (Singh and Gupta 1977). These activities are regulated by both biotic and abiotic factors (Yuan et al. 2007). In general, these activities are likely to be optimum in moist neutral (pH around 7) soils at high temperature with adequate organic matter. The increased salt content (saline soils) and decreased structural stability (sodic soils) along with other chemical alterations affect soil quality in many different ways (Singh 2015). The available information concerning microbial and enzymatic activities in use and management of salt-affected soils has been recently reviewed (Singh 2015).

Bacteria and archaea play essential roles in biogeochemical processes in saline soils. According to a recent meta-analysis, the global microbial community composition is influenced more by salinity than by extremes of temperature, pH, or other physical and chemical factors (Lozupone and Knight 2007; Ma and Gong 2013). The study of microbial diversity in saline soils is significant for understanding the ecological functions, saline adaption mechanisms, and biotechnical potentials of microorganisms. In a recent study, the collective bacterial and archaeal diversity in saline soils has been analyzed using a meta-analysis approach and representing a global overview of the microbial diversity of saline soils (Ma and Gong 2013). Bacteria and archaea play essential roles in biogeochemical processes in saline soils.

Microbial aspects of saline environments have been studied by only a few workers (Zahran 1997; Kaur et al. 2000, 2002b; Rietz and Haynes 2003). Soil microbial communities and their activity are greatly influenced by salinity (Rietz and Haynes 2003), microbial biomass, and soil respiration, and fluoresce in diacetate-hydrolyzing activity can be measured as a sensitive indicator of ecosystem disturbance. The application of gypsum followed by cropping increased the urease and dehydrogenase activity (Rao and Ghai 1985) in alkali soils. The biological activity of alkali soils, in terms of increased dehydrogenase activity, was found to improve under crop, forage grasses, and tree cover and resulted in decrease in CaCO3 content by 1, 1.5, and nearly 2 % (Rao and Ghai 1985). Rao and Pathak (1996) have reported increase in urease and dehydrogenase activities due to green manuring with Sesbania. Using various combinations of gypsum, Kallar grass (Leptochloa fusca), and cropping systems on sodic soils, Batra et al. (1997) have found an increase in dehydrogenase activity and microbial biomass carbon in alkali soils. Tripathi et al. (2006) have studied the effect of salinity on the microbial and biochemical parameters of salt-affected soils of coastal regions of the Bay of Bengal indicating that microbial biomass and microbial activities decrease with an increase in salinity (Tripathi et al. 2006).

After 10 years plantation on sodic soils, the ameliorative effects of various tree species showed that Prosopis juliflora, Acacia nilotica, Pongamia pinnata, and Casuarina equisetifolia resulted in greater improvement of soil carbon and lowering of soil pH as compared to other tree species (Singh et al. 1993). The tree plantations and silvopastoral agroforestry systems raised on sodic soils have been found to improve soil carbon and microbial activity through input of organic matter from aboveground and belowground parts of the plants (Kaur et al. 2000, 2002b). Plant cover through its effects on quantity and quality of organic matter influenced the levels of soil microbial biomass. Kaur et al. (2002b) also showed a significant relationship between microbial biomass carbon and plant biomass carbon (r = 0.92) as well as the flux of carbon in net primary productivity (r = 0.92). Nitrogen mineralization rates were found greater in silvopastoral systems compared to sole grass system. Soil organic matter was linearly related to microbial biomass carbon, soil N, and nitrogen mineralization rates (r = 0.95–0.98, p < 0.01) (Kaur et al. 2002b).

At another sodic soil site at Banthra, Lucknow, with the increase in age of Prosopis juliflora and Dalbergia sissoo plantations from 3 to 9 years on sodic soils, there was increase in soil porosity, decrease in bulk density of soil, improved soil aggregation, and increase in mean permeability of soil due to increase in the levels of soil organic matter (Mishra and Sharma 2003; Mishra et al. 2003). Revegetation of salt wastelands has been found to ameliorate soil conditions and improve soil biological activity (Tripathi and Singh 2005). Creation of new forests on barren land has contributed significant soil amelioration in the degraded sodic soil of the Indo-Gangetic plains (Tripathi and Singh 2005).

Jatropha curcas, a biodiesel plant, has been found to improve soil fertility and decrease soil sodicity after 6 years of its growth at Banthra Research Station, Lucknow (Singh et al. 2013). Soil amelioration potential of Jatropha curcas on soil properties was significant when compared to initial soil properties at 0–15 cm soil depth (Table 6). Soil bulk density, pH, electrical conductivity (EC), and exchangeable sodium percentage (ESP) decreased. There was significant increase in soil organic carbon (SOC), nitrogen (N), phosphorus (P), and microbial biomass (MB-C, MB-N, and MB-P), beneath the canopy of Jatropha curcas than outside canopy (Singh et al. 2013, Table 6).

Table 6 Change in soil organic carbon (SOC), nitrogen and microbial biomass in soils beneath and outside canopy of Jatropha curcas after 6 years growth on sodic soils at Banthra Research Station, National Botanical Research Institute, Lucknow
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Nitrogen cycling in silvopastoral systems in a highly sodic soil showed that nitrogen pool in vegetation was 32.47 % and 29.52 % of the soil pool in Prosopis juliflora+ Desmostachya bipinnata and Prosopis juliflora+ Sporobolus marginatus silvopastoral system, respectively, on a sodic soil at Bichhian, northwestern India (Fig. 2). The return of nitrogen in litterfall varied from 0.0 75 to 0.14 Mg N ha−1year−1. The turnover of fine root biomass returned 0.019–0.037 Mg N ha−1year−1. The return of total nitrogen to soil was 0.11–0.177 Mg N ha−1year−1. Total nitrogen uptake was 0.156–0.277 Mg N ha−1year−1. Thus, nitrogen sequestration in the system was 0.046–0.10 Mg N ha−1year−1, which was 28.84–36.10 % of total uptake of the nitrogen in the agroforestry system (Kaur et al. 2002a).

Fig. 2
figure 2

Nitrogen cycling in Prosopis juliflora + Desmostachya bipinnata and P. juliflora + Sporobolus marginatus silvopastoral system on a sodic soil in Bichhian in Haryana, northwestern India. The values in compartments represent the nitrogen stock (Mg N ha−1). The values on arrows represent the flows (Mg N ha−1year−1) (Source: Based on Kaur et al. 2002a)

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Impacts of agroforestry systems on nitrous oxide (N2O) and methane (CH4) emissions need to be considered. Integrating agroforestry into agricultural operations reduces N2O emissions by reducing nitrogen (N) application. Additionally, emissions may be further reduced through tree uptake of excess nitrogen (Bergeron et al. 2011; Schoeneberger et al. 2012).

The silvopastoral agroforestry systems on calcareous soils irrigated with saline water at Hisar, northwestern India, were studied for their ameliorative effects on soil properties (Kumari 2008). These systems were comprised of Acacia nilotica and Salvadora persica along with native grasses of Cenchrus ciliaris and Panicum miliare. The litter accumulation on the soil surface was found to be greater in Salvadora persica system (3682–2712 kg ha−1) as compared to Acacia nilotica system (2216–2442 kg ha−1). The total organic carbon (0–30 cm soil depth,) was 6.839, 21.195, and 20.181 Mg C ha−1 for the native grassland, the Acacia nilotica+ Cenchrus ciliaris silvopastoral systems, and Salvadora persica system, respectively. The soil organic carbon was greater (20.181–21.195 Mg C ha−1) in the case of silvopastoral systems as compared to the native grassland (6.839 Mg C ha−1). Thus, integration of trees with forage grasses improved soil organic carbon significantly on calcareous soils irrigated with saline water (Kumari 2008).

In northwestern India, long-term field trial with 31 tree species was conducted from 1991–1992 to 2010–2011 on a calcareous soil of semiarid region with average annual rainfall 498 mm and open pan evaporation 1888 mm (Dagar et al. unpublished). In this study, the tree species were irrigated with saline water (9.3 ± 0.7 dS m−1) during the first 3 years. After a growth period of 20 years, litterfall from the most of tree species resulted in an improvement in organic carbon content of the underlying soil. The tree species including Acacia nilotica, A. tortilis, Azadirachta indica, Eucalyptus tereticornis, Feronia limonia, Tamarix articulata, and Guazuma ulmifolia increased organic carbon content (>0.5 %) considerably as compared to other species (Fig. 3). The increase in soil organic carbon was more pronounced in the upper 0–0.3 m layer as compared to the lower soil layers. Thus, the tree-based saline water management strategies can lead to productive use of abandoned lands as well as providing ecological security (Dagar et al. unpublished).

Fig. 3
figure 3

Soil organic carbon after 20 years of growth under different tree plantations. Depictions: Af Acacia farnesiana, An Acacia nilotica, At A. tortilis, Al Albizia lebbeck, Ai Azadirachta indica, Cs Cassia siamea, Cj C. javanica, Cf C. fistula, Cl Callistemon lanceolatus, Et Eucalyptus tereticornis, Fl Feronia limonia, Gu Guazuma ulmifolia, Ma Melia azedarach, Mo Moringa oleifera, Pc Prosopis cineraria, Pd Pithecellobium dulce, Pj Prosopis juliflora, Ta Tamarix articulata, Zm Ziziphus mauritiana

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Diversity of Arbuscular Mycorrhizal Diversity under Salt Stress

At Bichhian, Desmostachya bipinnata and Sporobolus marginatus have been integrated with trees in the silvopastoral systems. These salt-adapted grasses showed moderately high diversity of arbuscular mycorrhizal (AM) fungal fungi in their rhizosphere growing on sodic soils (Jangra et al. 2011; Gupta et al. 2015). A total of 27 species of AM fungal spore belonging to six genera, i.e., Acaulospora, Entrophospora, Gigaspora, Glomus, Sclerocystis, and Scutellospora, were identified in sodic soils. In Desmostachya bipinnata grassland system, a total of 18 AM fungi belonged to four genera, i.e., Acaulospora, Entrophospora, Gigaspora, and Glomus. In the Sporobolus marginatus grassland, ten species of Acaulospora, three species of Entrophospora, three species of Gigaspora, 19 Glomus spp., and one species each of Scutellospora and Sclerocystis were recorded. The arbuscular mycorrhizal species of Glomus and Acaulospora dominated the AM fungal communities. The density of arbuscular mycorrhizal (AM) fungal spores in soil of the sodic grassland systems was 0–15 cm soil depth, 22.8–60.8 g−1 soil, and 15–30 cm soil depth, 9.6–18.4 g−1soil.

The root colonization of Desmostachya bipinnata showed the presence of abundant fungal mycelium, the AM fungal infection consisted of both fine and coarse hyphae with distinct vesicles and arbuscules. In the case of Sporobolus marginatus, the mycorrhizal fungal colonization of the roots varied from 68 to 80 % in different seasons with abundant mycelia, arbuscules, and vesicles in the cortical cells (Fig. 4). The capability of becoming densely colonized by AM fungi is an important trait of sodicity-tolerant plants. The mycorrhizal root colonization was observed in different forms such as mycelium (H, Y types) and formation of arbuscules and vesicles (elliptical, globose, and round types). Garcia and Mendoza (2007) studied AM fungal colonization of plant roots in a saline-sodic soil. These workers have shown AM fungal colonization of 90–73 % at high soil pH of 9.2 and exchangeable sodium percentage of 65 %. Garcia and Mendoza (2007) reported that AM fungi can survive and colonize roots of Lotus glabra and Stenotaphrum secundatum under extreme saline-sodic soil conditions.

Fig. 4
figure 4

(a, b) Arbuscular mycorrhizal fungal infection in roots of Sporobolus marginatus with presence of vesicles and hyphae in cortical cells (From Jangra 2010)

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In Acacia nilotica and Salvadora persica silvopastoral system on saline-sodic soils at Hisar, the AM root colonization in various grass species varied from 47.8 to 71.2 % (Kumari 2008). In the agrohorticultural system of Carissa carandas along with Hordeum vulgare, some 23 species of mycorrhizal fungi belonging to Glomus, Acaulospora, and Gigaspora were identified. The various species of Glomus were Glomus macrocarpum, Glomus caledonium, Glomus constrictum, Glomus pallidum, Glomus mosseae, Glomus intraradices, Glomus reticulatum, and six unidentified species. In silvopastoral system and agrohorticultural system, the spore density in the rhizosphere of predominant plant species varied from 57.6 to 203.2 spores/10 g soil, the value being maximum in the case of Hordeum vulgare. Arbuscular mycorrhizal fungal infection in roots of Hordeum vulgare showed presence of vesicles and hyphae. The species of Glomus and Acaulospora dominated the AM fungi in the agrohorticultural and silvopastoral systems (Kumari 2008).

A large amount of carbon found in tissues of mycorrhizal fungi could be long-lived in the soil (Treseder and Allen 2000). For example, chitin, which is not readily decomposed (Gooday 1994), can constitute up to 60 % of fungal cell walls (Muzzarelli 1977). Arbuscular mycorrhizal fungi are also the sole producers of glomalin, a potentially recalcitrant glycoprotein (Wright et al. 1996; Wright and Upadhyaya 1999). Mycorrhizal fungi could sequester increased amounts of C in living, dead, and residual hyphal biomass in the soil (see Treseder and Allen 2000) and may play key role in soil carbon sequestration.

Carbon Sequestration in Plant Biomass during Restoration

Carbon sequestration involves the removal and storage of carbon from the atmosphere in vegetation and soils through physical or biological processes. Soils represent the largest terrestrial carbon pool on earth’s surface, which contains three times more carbon than the amount in all living matter (IPCC 2001). Plant biomass and soil organic matter constitute the major pool of carbon in terrestrial ecosystems. The biotic pool in vegetation stores about 610 Pg C at any given time. The total amount of carbon in the world’s soil organic matter is estimated to be 1500–1580 Pg C (Amundson 2001; Lal 2004). The global soil carbon pool of 2500 gigatons (Gt) C includes about 1550 Gt of soil organic carbon and 950 Gt of soil inorganic carbon (Lal 2004).

The trees on salt-affected soils have the potential for carbon sequestration by increasing soil carbon and plant biomass production (Bhojvaid and Timmer 1998; Garg 1998; Kaur et al. 2002a, b). Prosopis juliflora has been grown on salt-affected soils in northwestern India and increased the soil organic carbon pool from 10 to 45 Mg ha−1 in an 8-year period (Garg 1998). In an age sequence of Prosopis plantations, trees have been found to ameliorate highly sodic conditions by alleviating sodium toxicity and improving the buildup of soil fertility (Bhojvaid and Timmer 1998). These workers showed the annual rate of increase of 1.4 Mg C ha−1year−1 over a 30-year period of plantation. Glenn et al. (1992) estimated that 0.6–1.2 Gt of C per year could be assimilated annually by halophytes on saline soils, evidence from decomposition experiments suggesting that 30–50 % of this carbon might enter long-term storage in soil. Thus halophytes adapted to saline soils could play an important role in soil carbon sequestration.

In southwestern Australia, the rates of C sequestration in biomass of E. globules over a 10-year period ranged from 3.3 to 11.5 Mg C ha−1year−1 on a large-scale watershed, the rates of C sequestration being higher as compared to other systems (Harper et al. 2005, 2012).

In silvopastoral agroforestry systems on sodic soils at Bichhian, northwestern India, the total carbon storage was 1.18–18.55 Mg C ha−1, and carbon input in net primary production varied between 0.98 and 6.50 Mg C ha−1year−1 (Kaur et al. 2002a). Carbon storage potential in the plant biomass (Mg C ha−1) and annual carbon flux (Mg C ha−1year−1) in the Prosopis juliflora + Desmostachya bipinnata and Prosopis juliflora + Sporobolus marginatus agrosilvopastoral systems on sodic soils at Bichhian, India, are shown in Fig. 4. The aboveground woody biomass carbon in Prosopis juliflora + Desmostachya bipinnata silvopastoral systems, bole, and branches comprised 82 % of the total biomass carbon in 6-year-old systems (Kaur et al. 2002a). Total carbon storage was 18.54–12.17 Mg C ha−1, and carbon input in net primary production varied between 6.50 and 3.24 Mg C ha−1year−1.

Biomass and carbon sequestered by 5-year and 4-month-old clonal E. tereticornis on waterlogged soils at Puthi, Hisar, northwestern India, was estimated to be 15.5 Mg ha−1 (Ram et al. 2011) and by 7 years old plantations 145.1 Mg ha−1 in a space of 1 × 1 m in two parallel lines on farm bunds and 265 Mg ha−1 in block plantations (2 × 2 m space) (JC Dagar, personal communication). The Eucalyptus-based agroforestry on waterlogged soils showed soil carbon storage of 15.823 Mg C ha−1. Compared to baseline of the cropland, the net carbon sequestration amounted to 4.452 Mg C ha−1 over a period of 4 years.

Carbon storage in plant biomass (Mg C ha−1) and carbon flux in net primary productivity (Mg C ha−1year−1) of clonal Eucalyptus tereticornis agroforestry and tree plantation at different spacing in shallow water table areas in Puthi, northwestern India (Kumar 2012), showed that at 4 years of age, total carbon storage in plant ranged from 5.85 to 16.46 Mg C ha−1. Total carbon sequestration by the same plantations after 6 years was 22.8 Mg C ha−1 in 1 × 1 m space and 90.6 Mg C ha−1 in block plantations (Dagar et al. 2015a). Carbon flux in net primary productivity was 2.01–4.7 Mg C ha−1year−1. Soil organic carbon storage varied from 29.02 to 32.63 Mg C ha−1 (Kumar 2012, Fig. 5).

Fig. 5
figure 5

Carbon pools and fluxes in Eucalyptus clonal agroforestry on waterlogged soils in Puthi at 4 years growth stage. The values in compartments represent the carbon stock (Mg C ha−1). The values on the arrows represent the flows (Mg C ha−1year−1) (Based on Kumar 2012)

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Carbon sequestration was estimated both in plant biomass and soil in two pasture systems (Cenchrus ciliaris and Cenchrus setigerus), two tree systems (Acacia tortilis and Azadirachta indica), and four silvopastoral system (combination of one tree and on grass) on moderately alkaline soils (pH 8.36–8.41) at Kachchh, Gujarat, northwestern India (Mangalassery et al. 2014). The systems were characterized with sole tree plantation of Acacia tortilis and Azadirachta indica, the grass-only systems of Cenchrus ciliaris and Cenchrus setigerus, and four silvopastoral systems with combinations of one tree and one grass. This study showed that maximum carbon was sequestered by silvopastoral system of Acacia + C. ciliaris (6.82 Mg C ha−1) followed by Acacia + C. setigerus (6.15 Mg C ha−1) compared to 6.02 Mg C ha−1 sequestered by sole plantation of Acacia tortilis. The silvopastoral system of Azadirachta indica + C. ciliaris and Azadirachta indica + C. setigerus registered a total carbon stock of 4.91 and 4.87 Mg C ha−1, respectively, against sole plantation of neem (3.64 Mg C ha−1) (Mangalassery et al. 2014).

Soil Carbon Sequestration Benefits

Soil carbon sequestration in agroforestry systems on salt-affected soils has been studied by several workers as summarized in Table 7. In the Prosopis juliflora + Desmostachya bipinnata and Prosopis juliflora + Sporobolus marginatus agrosilvopastoral systems on sodic soils at Bichhian, northwestern India, the soil carbon pool was 13.431 Mg C ha−1, Prosopis juliflora + Desmostachya bipinnata, and 9.621 Mg C ha−1, Prosopis juliflora + Sporobolus marginatus (Kaur et al. 2002a). The soil carbon sequestration in Acacia nilotica and Salvadora persica silvopastoral systems on saline-sodic calcareous soils at Hisar, India, varied from 20.393 to 19.930 Mg C ha−1 (Kumari 2008). The soils at different sites were found to store 25.86–99.33 Mg CO2 ha−1, which accounted for 25.86–99.33 73.927 carbon credits ha−1 for soil carbon sequestration. Assuming $10 price for one Carbon Credit, the monetary value of carbon storage comes out to be ranging from 259 to 993 US $ha−1.

Table 7 Soil carbon sequestration in some agroforestry systems in 0–30 cm soil on salt-affected soils at different locations in India (compiled from various sources)
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Soil inorganic carbon and its dynamics in arid and semiarid regions are important (Lal and Kimble 2000; Lal 2008). The soluble salts that occur in soils consist mostly of various proportions of anions such as sulfate, chloride, and bicarbonate and the cations such as calcium, sodium, and magnesium. Schlesinger (1985) calculated that the input rate of SIC in Aridisols was 0.24 g C m−2years−1 in the Mojave Desert and the accumulation rate of secondary carbonates ranged between 0.12 and 0.42 g C m−2years. The soil inorganic carbon represents a large proportion of total soil carbon in Indian soils with long turnover time (Bhattacharyya et al. 2008).

In Prosopis juliflora and Eucalyptus tereticornis plantations on reclaimed sodic soils at Saraswati Reserved Forest, northwestern India, the soil inorganic carbon (Mg IC ha−1) stocks in the tree plantations were 1.561–2.458 (30–45 cm soil depth), 4.242–5.252 (45–60 cm soil depth), and 18.596–16.901 (60–100 cm soil depth) (Jangra 2010). This study indicated that soil inorganic carbon (SIC) at increasing soil depth provided greater potential for carbon sequestration. The soil inorganic carbon pool has been considered to improve soil physical properties, as well as improve total carbon sequestration in the soils (Pal et al. 2000; Bhattacharyya et al. 2004). Recently, soil dissolved inorganic carbon (SDIC) and soil inorganic carbon (SIC) in saline and alkaline soil profiles up to 9 m soil depth from six profiles in the southern Gurbantunggut Desert, China, have been analyzed by Wang et al. (2013). This study showed that deep layer soil contained considerable inorganic carbon, with more than 80 % of the soil carbon stored below 1 m, in the form of SDIC or SIC. Thus, it is important to understand the role of inorganic carbon in soil carbon sequestration so as to optimize management strategies for carbon sequestration.

Soil carbon sequestration was studied in four silvopastoral system (combination of one tree and on grass) on moderately alkaline soils (pH 8.36–8.41) at Kachchh, Gujarat, northwestern India (Mangalassery et al. 2014). The silvopastoral system sequestered 36.3–60.0 % more total soil organic carbon stock compared to the tree system and 27.1–70.8 % greater in silvopastoral system as compared to the grass-only system (Mangalassery et al. 2014).

Climate Change Mitigation and Adaptation

In recent years, the increase in carbon dioxide in the atmosphere has gained a lot of attention as a greenhouse gas, as it has potential to influence the climate pattern of the world. The fastest growth in CO2 emission until 2025 is projected to occur in developing countries, whose collective emissions are projected to rise 84 % (compared to 35 % growth for industrialized countries). Agroforestry functions that support climate change mitigation and adaptation are summarized in Fig. 6.

Fig. 6
figure 6

Functions that address climate change mitigation and adaptation for biosaline agroforestry

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Scientific literature has found a large variation in sequestration rates, which is primarily explained by differences in the soil type, initial soil conditions, climate, and the tree species (as a result of different litter production rates) (Wicke et al. 2013). But even at the lower level of the range found in literature, soil carbon sequestration is an important benefit of biosaline (agro)forestry. Biosaline (agro)forestry systems may potentially have the positive effect of improving water infiltration and soil moisture retention and may provide an opportunity for improving yields of agricultural crops intolerant to waterlogging. Thus including trees in the agricultural production system can help remove excess water and thereby reduce waterlogging. Various studies have confirmed that, when properly implemented, these so-called biodrainage systems can lower groundwater tables (see Wicke et al. 2013).

Agroforestry can add a high level of diversity on degraded lands with an accompanied increased capacity for supporting numerous ecological and production services that impart resiliency to climate change impacts (Verchot et al. 2007; Turner et al. 2009; Schoeneberger et al. 2012). The mixing of woody plants into crop, forage, and livestock operations provides greater resiliency to the interannual variability through crop diversification as well as through increased resource use efficiency (Olson et al. 2000).

Conclusions

Agroforestry has the potential to affect numerous production and ecosystem services such as aesthetics, recreation, microclimate, carbon sequestration, natural pest control, pollination, water quality, soil erosion, and protection and that will be impacted by climate change. Carbon sequestration also provides associated ecosystem co-benefits such as increased soil water-holding capacity, better soil structure, improved soil quality and nutrient cycling, and reduced soil erosion. Under waterlogged conditions in saline environments, tree-based systems have been found effective for pumping out excess water more rapidly than only cropland systems. Agroforestry systems can provide economic stability and reduce risk under climate change by creating more diversified systems to resource poor farmers. Thus, climate change-integrated tools along with ecosystem functioning and services need to be developed to ensure sustainable agroforestry in saline environments. There is a need to generate information on soil inorganic carbon in soil profile for assessing from the point of view of optimizing strategies to reduce the accumulation of CO2 in the atmosphere. The roots of trees have the potential to take up excess N spatially and temporally that would otherwise be available for N2O emissions on- or off-site. Silvopastoral systems may offer several options for reducing CH4 and N2O emissions from the soil, particularly through increased nutrient use efficiency. The AM fungi associated with salt-adapted crops, grasses, and trees can play an important role in bio-amelioration and soil carbon storage. Role of microbiology in reclamation and bio-remediation of salt-affected soils needs top attention of researchers and policy makers.