1 Introduction

Marginal lands are economically characterized as low productivity lands under normal agricultural inputs (fertilization, irrigation, common tillage, weeding, and other agronomic measures). The net income generated from these lands may just compensate for the input or even lower than the incurred cost (Qaseem and Wu 2020). These lands usually have little or no agricultural value due to poor soil conditions including salinization, acidification, desertification, contamination, and low nutrients (Stephanie et al. 2013; Tilman et al. 2006). The tough climatic conditions like drought and cold and poor agricultural infrastructure also limit crop growth on these lands (Qaseem and Wu 2020).

The marginal lands in China constitute about 290 million ha, which is more than twice as much as the arable land (128 million ha). In China, arable land per capita is 0.09 ha, which is less than half of the global average. Constant rise in the utilization of arable land for developing infrastructure construction projects is challenging. According to the Third National Land Survey Main Data Bulletin, the average annual reduction in arable land in China was estimated to be about 0.75 million ha for the period 2009 to 2019 (National Land & Resources 2016). Therefore, it is a great challenge for Chinese government to maintain the minimum level of arable land at 120 million ha in the next few decades, a threshold set by the central government (Liu et al. 2015). Rational development and utilization of the marginal lands can be explored as one of the strategies to deal with the issue. Therefore, it is critically important for Chinese government to effectively manage the marginal lands, in order to deal with key strategic issues of both food security and ecological and environmental safety (Tang et al. 2010). This strategy can also be advantageous to expand the carbon pool capacity in marginal lands for their low carbon density (Wong et al. 2010), which may remarkably contribute to the ambitious goals of peaking carbon dioxide emissions and achieving carbon neutrality in 2030, which is the target year to achieve such goals. This study aims to explore the salt-affected land, a major type of marginal lands, which is readily grown with halophytes as biochar feedstock. The purpose of this study is to introduce a new approach to the extension of biochar soil amendment and effective management of marginal lands. Specifically, this study aims to clarify the importance and necessity of the ecological restoration and social benefits of the marginal lands with a particular focus on salt-affected land. We also put forward to an all-win means for effective management of the salt-affected marginal land with halophytes.

2 Soil salinization: a major constraint in marginal lands

Based on the natural conditions and economic value, marginal lands can be divided into different groups like shrub land, sparse forest land, grasslands with moderate/low coverage, bottomland, shoal, salt-affected land, and bare land (Table 1). Most of the marginal lands do not hold significant economic value and are not feasible to develop for agricultural production because of their small size, scattered distribution, poor soil fertility and environmental conditions (Qaseem and Wu 2020). Salt-affected lands are usually distributed in arid and semi-arid regions, accounting for about 950 million ha worldwide. According to the estimation, the proportion of these lands is increasing at an annual rate of 1–2% due to climatic changes, inappropriate irrigation, and farming measures (Rana et al. 2008; Wong et al. 2010).

Table 1 Main marginal lands in China
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China has 131 million ha of salt-affected land, which accounts for 4.3% of the marginal lands (Table 1). The salt-affected lands in China are usually divided into four types based on the soil salinity; Songnen Plain in northeast China is dominated by Na2CO3 and NaHCO3; eastern coastal region is distributed with NaCl and MgCl2; Huang-Huai-Hai Plain with NaCl, Na2CO3, and Na2SO4; and the regions of Heilongjiang, Jilin, Inner Mongolia, Xinjiang, Hebei, and Shandong with Na2SO4 (Du and Hu 2021).

Salt-affected lands are mostly prevalent in the coastal, arid, and semi-arid regions of northwestern China. The Yellow River Delta is a typical coastal salt-affected area in northern China. The saline-alkali lands are mainly distributed in the inland regions of northeast and south China (Cao et al. 2021). In some provinces such as Xinjiang, Gansu, and Ningxia regions, the salt-affected lands are continuously expanding at an annual rate of 1% due to inappropriate irrigation practices (Li and Wang 2018). The Songnen Plain in northeastern China, a typical example, had only 2.4 million ha of saline land in the 1950s, which reached up to 3.9 milloin ha in 2016.

Since earlier 1990s, developed countries like the United States have achieved outstanding progress in improving salt-affected lands for agriculture production (McKell et al. 1986; Shih and Myhre 1994; Tanji et al. 1972). In contrast, in the countries like China with very limited arable land resources, the initial consideration must be to transform the salt-affected land into arable land to produce grain, oil, and fiber (Qaseem and Wu 2020; Tang et al. 2010; Wicke et al. 2011). Since the mid-twentieth century, China has made significant progress in the large-scale development and utilization of the salt-affected land in the regions of Huang-Huai-Hai River Basin (Shi 2003), Hetao region (Dong et al. 2019), and the inland of northwest China (Huang et al. 2022). The salt-affected lands in these regions have now transformed into an important base for producing grain, oil, cotton, fruit, and vegetables (Zhang 2014).

The remaining salt-affected lands, accounting for about 52% of the total marginal land in China, fall into tough natural and geographic conditions such as high dryness coefficient, low-lying terrain, and poor drainage (Shrivastava and Kumar 2015). Therefore, it is difficult and costly to produce food and bioenergy crops on these types of primary saline-affected lands (Qaseem and Wu 2020). This is particularly due to the huge consumption of freshwater in saline lands to cultivate crops. For instance, in the saline lands of the Hetao region, more than 1 m3 of freshwater from the Yellow River is required to produce one-kilogram rice (Duan and Zhang 2000; Qiu et al. 2015). This is, therefore, impracticable in the countries like China with limited freshwater resources standing at 2300 cm3 per capita, less than one-fourth of the global average (Qin et al. 2019). Furthermore, the salt-affected land has readily suffered from serious secondary soil salinization, with high ecological and environmental risks. Furthermore, the salt-affected land also suffer from secondary soil salinization, with high ecological and environmental risks. For example, the desertification of Minqin Oasis (Gansu Province, China) is alarming as the landscape of these areas turned into a desert when it was cultivated with glycophytes of cereals, cotton, and maize (Shi 2000).

Human society is facing the greatest challenges of the time in terms of achieving food security and eco-environmental safety. The availability of land and water resources on marginal lands is intrinsically linked to these global issues, but their exploitation for food production may cause severe ecological disasters. Therefore, it has become urgent to take innovative measures for proper management of the marginal lands, of which a major part constitutes of salt-affected lands in many countries like China.

3 Halophyte cultivation: the most natural approach to manage salt-affected lands

Numerous measures have been recommended to improve the quality of the salt-affected lands (Huang et al. 2022; Liu and Wang 2021; Sun et al. 2022). Hydraulic facilities are the most common and effective measure to reduce salinity and irrigate crops if abundant freshwater resources are available (Amini et al. 2016; Wang and Zhou 2013). Calcium-containing materials such as desulfurized gypsum can effectively replace sodium ions (Na+) with calcium ions (Ca2+) and then organic amendments are incorporated to enhance soil fertility and stabilize the soil structure (Gupta and Abrol 1990; Gharaibeh et al. 2009, 2010, 2011). Another effective measure can be phyto-reclamation, in which halophytes are used to remediate soil salinity (Devi et al. 2019; Mirza et al. 2014; Rozema and Schat 2013).

Halophytes have demonstrated their high tolerance under extremely saline conditions even higher than 200 mmol L− 1 NaCl (Abbas et al. 1992; Glenn et al. 1999; Rozema and Schat 2013). More than 1560 halophyte species from 10 families have been documented around the world, including 500 species recorded in China (Glenn et al. 1999). They are often classified into three types of euhalophytes (true halophytes), recretohalophytes (salt excretors), and pseudohalophytes (salt avoiders) (Glenn et al. 1999) according to their physiological mechanisms of salt-tolerance. Generally, euhalophytes are a class with succulent leaves and stems that can store salts within vacuoles (Liu and Wang 2021). Recretohalophytes can actively expel salts from plant tissues through their unique structures like salt glands and salt sacs (Yuan et al. 2016). Pseudohalophytes can prevent salt from entering cells (Aslam et al. 2011). The common halophytes in China include Suaeda altissima, Suaeda altissima (L.) Pall., Kalidium foliatum (Pall.) Moq., Phragmites australis, Tamarix chinensis Lour., Phragmites australis (Cav.) Trin. ex Steud, etc. (Xiao et al. 2022).

There is abundant literature available citing the numerous functions of halophytes depending on their botanic nature. Some halophytes such as Atriplex patens, Kalidium foliatum (Pall.) Moq. and Salicornia europaea L. can serve as forage (Al-Amro et al. 2019; Alkhuzai et al. 2015; Esfahan et al., 2010; Ghazanfar, 1999; Shamsi et al., 2020). The young leaves of Lycium barbarum L. and Suaeda salsa (L.) Pall. are edible for human consumption due to rich dietary fiber, vitamins, protein, and minerals (Al-Amro et al. 2019; Alkhuzai et al. 2015; Ghazanfar 1999). The seeds of Salicornia europaea L. contain a high percentage of oil and have the potential to produce biodiesel. Apocynum venetumL. and Suaeda vermiculata are valuable as herbs for their richness in flavonoids and conjugated linoleic acids (Abbas and El-Oqlah 1992; Al-Amro et al. 2019; Alkhuzai et al. 2015; Esfahan et al. 2010; Shamsi et al. 2020). Both Apocynum lancifolium and reed can be used as papermaking materials (Hamidov et al. 2007; Liu and Wang 2021; Mirza et al. 2014), while both Tamarix chinensis and Iris lactea Pall are widely used as urban green plants in western China.

Most euhalophytes have evolved with succulent stems and leaves, which can store a large amount of salts in a compartmentalized manner. The dry matter of these euhalophytes often has an ash content as high as 34%. A study has reported that during a four-month experiment, a large amount of salts, 504 kg ha−1 for S. maritima, and 474 kg ha− 1 for Sesuvium portulacastrum can be removed annually from the extremely saline lands by harvesting the dry matter of euhalophytes (Ravindran et al. 2007). By cultivating the halophytes, soil salinity is gradually reduced and healthier crops can grow in less saline environments assuming the salt quantity added from irrigation water must be much lower than that removed by halophyte harvest. Tian et al. (2021) has conducted multi-year experiments on salt-affected soils in Kashgar (Xinjiang Autonomous Region, China) and reported that more than 400 kg of salts per hectare could be removed seasonally using euhalophytes. Soil salinity was reduced by 40%, 60%, and 85 to 90% in the first, second, and third years, respectively, and was finally suitable to cultivate glycophytes of most crop species.

Halophytes often have well-developed roots, supply a remarkable number of residues, and enhance the organic matter content in the salt-affected soil, which is extremely low under saline conditions (Wong et al. 2010; Xiao et al. 2022). The most popular halophytes are leguminous plants such as Melilotus miller and Vicia villosa Roth, which accelerate soil desalination and improve the soil organic matter content and minerals. Halophytes offer great potential to enhance organic carbon pools in salt-affected lands. Additionally, halophyte cultivation can greatly increase the vegetation coverage of land affected by saline conditions, which improves the ecological and environmental conditions for local inhabitants (Hasanuzzaman et al. 2014). The cultivation of halophytes could increase farmers income. Therefore, it is suggested that halophyte cultivation can be an economical, feasible, and sustainable approach to manage the under-utilized salt-affected lands in China (Behera and Ramachandran 2021; Jing et al. 2019; Liu and Wang 2021). The potential use of halophyte resources and extending the innovative technology for phytoreclamation is essential to remediate the salt-affected land.

4 Biochar derived from halophytes as soil amendment can achieve all-win outcomes

Biochar is a fine-grained, porous, and carbon-rich material obtained by pyrolyzing biomass under oxygen-limited conditions (Lehmann and Joseph 2015). It has been demonstrated through several pot and field trials, that biochar-amended soil in tropical and subtropical regions can significantly improve soil physical, chemical, and microbiological properties (Jeffery et al. 2017). The biochar-amended soil usually has lower bulk density, higher aggregates, porosity, water infiltration rate, water holding capacity, and available water content. However, these benefits may vary depending on the conditions of soil, the nature of biochar, and its rate of application (Chen et al. 2019; Fan et al. 2020; Joseph et al. 2021; Li et al. 2018; Schmidt et al. 2021; Shaaban et al. 2018; Xu et al. 2021).

Commonly, biochar amendment distinctly improves acidic soil by increasing pH value and reducing aluminum toxicity (Joseph et al. 2021; Wang et al. 2018; Wu et al. 2018; Zhu et al. 2017). However, biochar with low pH has also improved the quality of saline-alkali soil (Saifullah et al. 2018). Biochar amendment in addition to improving minerals content such as phosphorus, potassium, calcium, magnesium, etc. (Dai et al. 2020; Sheng and Zhu 2018; Joseph et al. 2021), also improves soil cation exchange capacity (CEC) (Joseph et al. 2021). Data from a few studies have proved that addition of biochar increases microbial biomass and activity in soils, and even changes the structure of its microbial community (Dai et al. 2018; Luo et al. 2013; Wu et al. 2019; Zhou et al. 2019a, b). The production of cereals, roots, and stems has all been reported to benefit from biochar amendment, with varying rates depending on crop, climate, soil, and the nature of biochar used (Hagemann et al. 2017; Kloss et al. 2014; Mao et al. 2012; Novak et al. 2009).

Existing studies show that biochar is useful in remediating the contaminated soils as it can reduce the bioavailability and mobility of heavy metals, and accelerate the degradation of pesticides, pigments, and petroleum (Ahmad et al. 2014; Fang et al. 2014; Qi et al. 2017; Safari et al. 2019). Additional benefits of biochar amendment are its ability to lock up carbon within soil pedon and reducing CH4 and N2O emissions (Duan et al. 2018; Liu et al. 2019; Luo et al. 2011; Nan et al. 2022). As a result, biochar soil amendment is considered a zero net carbon or even negative carbon strategy for sustainable agricultural development (Arneth et al. 2019; Buss et al. 2022; Lehmann et al. 2020, 2021).

However, despite these benefits, large scale adoption of biochar is challenging due to its high cost, as well as technical imperfections in biochar production. At present, one ton of biochar costs more than 300 US dollars in Chinese markets, far higher than the increased income of agricultural products. Utilizing, cheap feedstock may help to bring down these costs, since feedstock accounts for more than 11% of biochar cost (Fig. 1). Similarly, most of the organic wastes, like crop straw, sawdust, manure, and sludge can also be potentially used as feedstock for producing biochar through either slow or fast pyrolysis (Ippolito et al. 2020). Crop straw among these is highly suitable for biochar production since as feedstock, however, cannot be used for large-scale biochar production since as it is already directly incorporated into soil in China and other countries as well. It is therefore evident that abundant and cheap feedstock is an important prerequisite and significant limitation for large scale application of biochar in agriculture.

Fig. 1
figure 1

(modified from Homagain et al. 2016)

The cost assessment (%) of biochar in the life cycle. a Scenario 1 is the low availability of feedstock and land application. The distance of the feedstock and land application (50 t ha−1) is within 300 km. b Scenario 2 is the high availability of feedstock and land application. The distance of the feedstock and land application (50 t ha−1) is within 100 km.

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It may be feasible to have ample and cheap biochar feedstock from marginal lands such as salt-affected land (Fig. 2). For example, it is well known that halophytes can grow very well in most of the salt-affected lands (Xiao et al. 2022), and can be harvested each season from land containing more than 10% soluble salts without much additional investment (Al-Azzawi and Flowers 2022; Behera and Ramachandran 2021; Hasanuzzaman et al. 2014; Liu and Wang 2021). Our previous results showed that the typical halophytes could be used to derive 23–47% biochar, 18–37% biogas, and 27–53% bio-oil under a fixed-bed slow pyrolysis process for 2 h (Irfan et al. 2016a, b; Yue et al. 2016a, b). Both biochar and bio-oil have a certain calorific value, for example, biogas contains rich combustible gases, such as CO and H2 that can account for 20–80% of the total and therefore, both biogas and bio-oil produced during the halophyte-derived biochar production process can also be used as direct fuel and industrial raw materials (Table 2).

Fig. 2
figure 2

Schematic diagram of comprehensive benefits of biochar derived from halophyte for marginal lands amendment

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Table 2 The outputs of bio-oil, bio-gas, and biochar derived from typical halophytes under 300 °C, 500 °C, and 700 °C for 2 h in a fixed bed pyrolysis system
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Limited data so far have shown that halophyte-derived biochar, unlike glycophytes-derived biochar, has some special properties such as a higher ash content (up to 23.64%), more Na+ (33.93 g kg− 1), and lower point of zero net charge value (Xiao et al. 2022). Pot trials have shown that amending the halophyte biochar helps to improve salt-affected soil and plant growth (Irfan 2016c; Yue 2017). Moreover, halophyte biochar may have many advantages as a conditioner of highly-weathered oxisols for their high ash and rich minerals such as sodium, potassium and magnesium. The rich ash can have a distinct liming effect, while the base metals may enhance the base saturation of the highly weathered acidic soil (Xiao 2021). Particularly, the luxuriant sodium in the halophyte biochar may partially act as potassium and benefit sodium-loving crops such as turnip and spinach. There are potentially all-round benefits from the halophytes-derived biochar production processes, which can then be useful to amended local saline soil or the highly weathered acid soil.

During recent decades, glycophyte-derived biochars, such as crop straw, sawdust, wood, etc. have received greater attention, which has resulted in remarkable progress in improving the manufacturing process of biochar. This enhances our understanding of its nature and impacts on soils and crops. However, there are still important knowledge gaps in halophyte-derived biochar that could help us gain potentially multiple benefits from halophytes.

5 Further research

Biochar-based soil amendment has multiple benefits for both agricultural and environmental applications. However, readily accessible and cheap feedstocks can be one of the important constraint factors for the widespread application of biochar in agriculture. The vast marginal lands, such as saline-alkali land widely distributed worldwide, offer a sustainable solution for biochar production. Further research can be as follows:

  1. 1)

    Most of the marginal lands are located in fragile ecosystems with poor soil and natural conditions. It is a great challenge to establish a native vegetation system in marginal lands that meets the requirements to sustain an ecological environment. Contemporary technology involving the use of genetic engineering to reestablish the ecosystem by maintaining biological diversity, stability, and sustainability in above, and below the marginal lands.

  2. 2)

    The sustainable development of marginal land ecosystems largely relies on the effective management of above-ground biomass resources. It is necessary to develop innovative techniques to build a stable value chain and industry chain for biomass-derived products. Much concern should be focused on the development of biochar-based poly-generation technology with marginal land biomass such as halophytes.

  3. 3)

    Marginal lands are characterized by low carbon density with low agricultural and ecological potential. Much attention should be paid to enhance soil organic matter content and carbon sequestration, and reduce greenhouse gas emissions. Further research is required to understand the processes, mechanisms and functions of biochar-derived carbon cycling in different terrestrial ecosystems.

  4. 4)

    It is particularly important for developing countries like China to maintain sufficient arable land to maintain self-sufficiency and guarantee food security. It is suggested to adopt compensating the arable land occupation with marginal lands as a national strategy for social and economic development. It is also realistic to improve the productivity of arable lands using halophyte biochar-based techniques, coupled with the National projects dealing with arable land construction. Mutual benefits obtained from both marginal and arable lands should be evaluated when the biochar-based amendment is applied on a large-scale.