1 Introduction

Salinity is a worldwide problem and approximately 20% of agriculture soils in the world are affected by salinity (Daliakopoulos et al. 2016; Minhas and Dagar 2016). With the steady increase in soil salinization, especially in arid conditions, the salinity tolerance of crops determines their suitability and success in salt-affected soils (Howell et al. 1995). Salt-affected soils are not suitable for cultivation of most crops; for example, the yield of durum wheat plants is reduced by 60–80% when cultivated in a soil has an ECe value of 3–4 dS m−1 (Houshmand et al. 2005). Pearl millet (Pennistum glaucum L.), which is a moderately salt-tolerant plant, could be an alternative crop for saline soils because it can tolerate salinity levels up to 15 dS m−1 (Ashraf and McNeilly 1987; Albassam 2001; Muscolo et al. 2003; Rai et al. 2008; Krishnamurthy et al. 2007). Given the importance of pearl millet plant as a fodder crop, its role in carbon emissions should be studied, especially under saline soil conditions in dry areas.

Biochar is considered one of the most important organic soil conditioners due to its superior chemical and physical properties over other soil amendments (Xiao et al. 2018; Eissa 2019). Biochar is made by the pyrolysis of organic material at temperatures below 700 °C (Lehmann and Joseph 2009; Xiao et al. 2018; Eissa 2019). Biochar amendments are carbon-rich products and are promising tools in the enhancement of soil fertility, remediation of saline soils and mitigation of gas emissions (Xiao et al. 2018; Eissa 2019). The results obtained from previous studies have revealed that application of biochar to saline soils increase the plant growth and improve the soil quality (Ali et al. 2017; Huang et al. 2019; Rekaby et al. 2020). Biochar has good properties to ameliorate saline soils (Rekaby et al. 2020) but little is known about its effect on pearl milt plants under saline conditions.

Emissions of CO2 from saline sodic soils are a serious problem and needs careful attention to mitigate greenhouse gas (Yang et al. 2019). Biochar is a promising tool to reduce gas emissions from agriculture land (Fidel et al. 2019). The results obtained by previous studies about the effects of biochar on CO2 emissions show wide variations (He et al. 2016; Liu et al. 2016; Fidel et al. 2019). Some of these studies reported that adding biochar reduces the gas emission through changing microbial populations and soil properties that could reduce the gas fluxes (Lehmann et al. 2011; Eissa and Ahmed 2016; Fidel et al. 2019; Majumder et al. 2019; Ding et al. 2020; Abou-Zaid and Eissa 2019; Wei et al. 2021). The effect of biochar on CO2 flux from soils is due to the sorption of dissolved organic carbon by the biochar surfaces (Zimmerman et al. 2011). Sequestration of soil carbon by agricultural management is a cost-effective and environmentally friendly method for mitigating of gas emission (Maucieri et al. 2017; Wei et al. 2021). Biochar decomposition is 10–100 times slower than fresh residues (Paustian et al. 2016) due to its chemical composition and relative resistance to microbial decomposition (Wei et al. 2021). Biochar amendments have the potential to reduce global warming by reducing CO2 emissions and increasing carbon sequestration in the soil (He et al. 2016). However, the exact mechanism of biochar in decreasing CO2 emissions remains unknown and the results obtained by previous studies are variable (Liu et al. 2016; He et al. 2016; Fidel et al. 2019). He et al. (2016) produced a meta-analysis of 91 research papers on the effects of biochar on CO2 emissions and they reported a 22.14% increase in CO2 emission as a result of adding biochar.

Biochar has been used to increases the productivity of saline sodic soils in several previous studies without enough information about its effects on CO2 emissions and carbon sequestration. We hypothesized that addition of biochar to saline sodic soils may change soil characteristics that could increase the soil C sequestration and mitigate the CO2 flux. Pearl millet plant is a promising fodder crop in arid regions, but its role in carbon emissions should be studied under these arid conditions.

2 Material and Methods

2.1 Biochar Preparation and Analysis

Corn cob pieces (less than 10 cm in length) were used to produce the biochar for this trial by slow pyrolysis of the corn cobs at 350 °C with a residence time of 2.5 h. The loss-on-ignition method was used to measure the organic carbon content of the biochar (Ball 1964). Biochar samples (0.5 g) were digested with H2O2 and H2SO4 (Burt 2004) and the total N, P, and K concentrations were then measured. The biochar pH was evaluated in a 1:5 H2O solution with a pH meter, and the electrical conductivity (EC) of the 1:5 extract was determined with an EC meter (Burt 2004). The main characteristics of the corn cob biochar were as follows: pH (10.5), EC (4.2 dS m−1), organic-C (522 g kg−1) and total N, P, and K of 22, 3.5, and 30 g kg−1, respectively.

2.2 Field Study

The field study was carried out at a private farm in Sohag Governorate, Egypt, in the summer seasons of 2018 and 2019. The soil in this site was clay loam and considered a saline sodic soil based on Chhabra (2004). Table 1 shows some physical and chemical properties of the experimental site. Table 2 shows the climatic conditions of the experimental site. Seeds of a local variety of pearl millet (Pennisetum glaucum L.), Shandweel 1, were sown by hand in hills at a rate of 133,000 plant ha−1. The plants were cultivated on one side of ridges (50 × 15 cm) in 1st first of May in 2018 and 2019. The experiment was repeated in different experimental plots each year. Biochar doses of 0, 5, 10, 15, and 20 ton ha−1 were applied to the field and mixed with the top layer (20 cm) before seed cultivation each year. The biochar treatments were arranged in a randomized complete block design (RCBD) and replicated five times with experimental unite (plot) of 5 × 10 m2 which containing 10 rows. Recommended doses of N: P2O5: K2O of 150:50:60 kg ha−1 were added each year. Superphosphate (15% P2O5) and potassium sulfate (50% K2O) fertilizers were added as a basal dose before cultivation. Urea (46%N) fertilizer was added in two equal doses; i.e., with the first irrigation and 3 weeks later. The plants were irrigated with ground water (EC = 2.0 dS m−1). The changes in soil moisture content in the 1 m soil surface layer were measured by using the profile probe (type PR1) then the amount of irrigation water was calculated and added to keep the soil moisture at the studied levels. The experimental site was irrigated at 70% of available soil water by adjusted surface irrigation system. The plants were cut manually (15 cm aboveground) three times per season; the first was done after 2 months of cultivation and subsequently the plants were harvested every month. The growth parameters were recorded and the total fresh forage yield for each cut was recorded at harvest. The forage yield was considered the sum of the three cuts.

Table 1 Some physical and chemical characteristics of the studied soils
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Table 2 Average monthly maximum (Tmax) and minimum (Tmin) temperature, relative humidity (RH), wind speed (WS), and solar radiation during 2018 and 2019 growing seasons
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2.3 Soil Analysis

A composite soil sample (0–30 cm) was collected from the experimental site before cultivation of the pearl millet plants. Composite soil samples were collected from each plot to investigate the effects of biochar on the characteristics of the studied soil after plant harvest in the last cut. The collected soil samples were air dried and crushed to pass through a 2-mm sieve. The percentages of sand, clay, and silt were measured by the pipette method (Burt 2004). The exchangeable Na was divided by the cation exchange capacity to give the exchangeable sodium percentage (ESP) (Burt 2004). Salinity of soil samples in a saturated soil extract was measured by an electrical conductivity meter (Burt 2004). Available nitrogen was extracted by 2M potassium chloride, and then determined using the micro-Kjeldahl method (Burt 2004). Available phosphorus was extracted by 0.5 M Na HCO3 at a pH of 8.5 and the extracted P was measured calorimetrically using the stannous chloride phosphomolybdic sulfuric acid system describe by Burt (Burt 2004). Available potassium was extracted by the ammonium acetate method and measured by flame photometry (Burt 2004). The soil organic carbon (SOC) was determined using the Walkley and Black method (Burt 2004). Walkley and Black method is appropriate for the estimation of soil organic matter up to 8%. The tested soil in the current study contained organic carbon less than 1%. So the method of Walkley and Black is suitable for organic-C determination. Briefly, 10 ml of 1 N K2Cr2O7 solution and 20 ml of concentrated H2SO4 were added to 2 g of soil sample. After the oxidation of soil organic matter, the remaining dichromate was titrated with 0.5 N FeSO4. Soil organic carbon was calculated based on the method described in Burt (2004). CO2 emissions were measured by the method described by Bono et al. (2008) and Schiedung et al. (2016). The method uses 2 M KOH to trap the CO2, which was then titrated with 0.01 N HCl to calculate the daily CO2 emissions.

2.4 Determination of Plant Nutrients and Biochemical Compounds

The plant samples, which were used to measure the plant nutrients and biochemical compounds, were collected before the first harvest each year. All the plant nutrients and biochemical analyses were measured in triplicates. The harvested plants were washed with distilled water and oven-dried at 70 °C for determination of the total N, P, K, Cl, and Na. The plants samples were digested with a mixture of 350 mL H2O2, + 0.42 g selenium powder + 14 g Li2SO4.H2O + 420 mL of concentrated H2SO4 (Parkinson and Allen 1975). N, P, K, and Na elements were analyzed according to the standard methods described by Burt (2004). The chloride (Cl) in the plant sample extract was measured by titration with a silver nitrate AgNO3 solution (0.05 N) after the pH was adjusted to 8.5 (Burt 2004). Crude protein (CP) and ash content were measured according to the animal nutrition and product quality laboratory manual (Zaklouta et al. 2011). The total phenolic compounds were extracted from fresh leaves from pearl millet plants using methyl alcohol (85%) and then measured by the Folin–Ciocalteu method (Kang et al. 2010). The concentrations of total phenolic content were measured by a spectrophotometer at a wavelength of 765 nm. Proline was extracted from the fresh leaves of pearl millet plants by sulphosalicylic acid method (Bastes et al. 1973). The extracted material was mixed with ninhydrin and glacial acetic acid reagents and measured spectrophotometrically at 520 nm. Three fully expanded fresh leaves of pear millet plants were collected randomly before the first harvest to measurer the photosynthetic pigments (chlorophyll and carotenoids). Chlorophyll and carotenoids were extracted from 100 mg of fully expanded fresh leaf using ethyl alcohol (95%) and measured by a spectrophotometer (Lichtenthaler 1987). Root samples were collected from each experimental unit to measure the dry matter from the roots. The root samples were collected by sieving soil from the top surface soil layer (0–20 cm) through a 0.5 mesh after the final harvest. The root biomass in the surface (0 cm to 100 cm) of soil depth was calculated based on the fact that roots in the 0 cm to 20 cm layer accounted for 60% of the root biomass (Jackson et al. 1996). The collected roots were dried at 60 °C and the organic carbon was measured (Jackson et al. 1996; Bono et al. 2008). The organic-C in the dry matter of shoot of each cut and of the roots for the final harvest was measured by the loss-on-ignition method as described in biochar analysis. The sum of organic-C in the shoots and roots was considered the total C sequestrated by pearl millet.

2.5 Statistical Analysis

The treatments were arranged in a randomized complete block design (RCBD) and replicated five times. The data were checked for normality before statistical analysis. Analysis of variance (ANOVA) and Duncan multiple range tests at 5% were conducted using SPSS 17.0 software package (SPSS, Chicago, IL, USA) to test whether the differences were significant. The different between the years was insignificant in most of the recoded data, so the data are presented as means of 2 years in some figures and tables.

3 Results

3.1 Effects of Applying Biochar on Soil pH, ECe, and Organic Carbon

The tested biochar doses that were applied to the saline sodic soils had clearly increased the soil organic carbon (SOC) and salinity level (Table 3). Biochar caused significant improvement in the soil organic carbon (SOC) and ECe (p < 0.05). The application of biochar caused a significant increase in the pH value of the soil compared with the control. The SOC increased from 8.07 g kg−1 in the control to 12.65 g kg−1 as a result of BC20 application. In the same trend, the application of BC20 increased the soil pH from 8.50 to 8.89, while the ECe increased from 9.00 to 12.80 dS m−1. Overall, increasing the biochar rates caused significant increases in the organic carbon content and soil salinity, while the tested rates caused a slight elevation in the soil pH.

Table 3 Effect of biochar application on soil pH, ECe and organic carbon (SOC) (after plant harvest)
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3.2 Effects of Applying Biochar on the Availability and Uptake of Nutrients by Pearl Millet

Amending the saline sodic soil with 5, 10, 15, and 20 ton ha−1 of corn cob biochar significantly (p < 0.05) enhanced the availability of nitrogen (N) phosphorus (P), and potassium (K), and therefore increased the uptake of these nutrients by pearl millet (Table 4). The addition of 5 ton ha−1 of corn cob biochar increased the availability of N, P, and K by 25, 27, and 18%, respectively, in comparison with the control soil, while the uptake of these nutrients was increased by 20, 14, and 50%, respectively. BC10 caused increases in the availability of N, P, and K by 41, 52, and 31%, respectively, compared to the control soil, while the uptake of these nutrients was increased by 40, 60, and 56%, respectively. BC15 increased the availability of N, P, and K by 25, 27, and 18%, respectively, in comparison with the control, while the uptake of these nutrients was increased by 24, 36, and 39%, respectively. Application of the highest rate of biochar (20 ton ha−1) increased the availability of N, P, and K by 43, 59, and 41%, respectively, and thus increased the uptake of these nutrients by 28, 43, and 39%, respectively, compared with the control soil. Overall, the applied rates of corn cob biochar increased the availability and uptake of plant nutrients.

Table 4 Effect of biochar application on availability and uptake of some nutrients
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3.3 Effect of Applying Biochar on the Growth and Yield of Pearl Millet Plants

Biochar significantly affected (p < 0.05) the tested growth parameters of pearl millet plants as shown in Table 5. The application of 5 ton of corn cob biochar (BC5) raised the height of plants, leaf area index, and leaf/stem ratio by 36, 21, and 8%, respectively, in comparison with the plants grown in the control soil. BC10 increased the plants’ height, leaf area index, and leaf/stem ratio by 40, 38, and 14%, respectively, compared with the plants grown in the control soil. BC15 increased the plants’ height, leaf area index, and leaf/stem ratio by 38, 29, and 12% respectively compared with the non-amended soil. The highest rate of corn cob biochar (BC20) increased the plants’ height, leaf area index, and leaf/stem ratio by 40, 30, and 8%, respectively, compared with the plants grown in the control soil. The forage yield and quality of pearl millet plants responded significantly (p < 0.05) to the application of biochar as shown in Fig. 1 A, B, C, and D. The addition of BC5, BC10, BC15, and BC20 increased the fresh forage yield by 36, 45, 27, and 18% in the first season and by 45, 65, 35, and 25% in the second season. BC5, BC10, BC15, and BC20 increased the protein content by 38, 25, 25, and 33%, respectively, compared with the non-amended soil. BC5, BC10, BC15, and BC20 increased the ash content by 25, 38, 38, and 50%, respectively, compared with the control soil. Overall, the addition of corn cob biochar to pearl millet grown in a saline sodic soil enhanced the plant growth and caused a clear increase in the total forage yield; moreover, it led to a clear improvement in the feed quality measured by the ash and crude protein content.

Table 5 Effect of biochar application on some growth parameters of pearl millet plants
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Fig. 1
figure 1

Effect of biochar application on yield and quality of pearl millet plants. A and B: forage yield in 2018 and 2019 growing season, C: crude protein, and D: ash. Data are means of two seasons and three cuts for ash and crude protein. Means (± SD) denoted by the same letter indicate no significant difference according to Duncan’s test at p < 0.05. C = without biochar application, BC5 = 5 ton ha−1, BC10 = 10 ton ha−1, BC15 = 15 ton ha−1, and BC20 = 20 ton ha−1

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3.4 Effect of Biochar on Biochemical Changes in the Leaf Tissue of Pearl Millet Plants

Applying of biochar to pearl millet plants grown in saline sodic soils caused significant changes in the biochemical compounds of the leaf’s tissues (Fig. 2A, B, C, D, and F). Biochar had a significant effect (p < 0.05) on the concentrations of proline, phenolic compounds, chlorophyll, and carotenoids as well as on sodium (Na+) and chloride (Cl−) in the leaves of pearl millet. The addition of BC5, BC10, BC15, and BC20 reduced the proline concentration by 33, 41, 33, and 42% in comparison with the non-amended soil. The concentrations of phenolic compounds decreased by 15, 33, 27, and 40% compared with the control soil as a result of applying BC5, BC10, BC15, and BC20. Chlorophyll and carotenoids in the leaves of pearl millet plants responded to the application of biochar, which resulted significant increases in the synthesis of chlorophyll and carotenoids compared with the non-amended soil. The addition of BC5, BC10, BC15, and BC20 increased the chlorophyll concentrations by 44, 60, 56, and 56% in comparison with the plants grown in the control soil. Carotenoid levels in the pearl millet plants increased by 47, 66, 47, and 60% compared with the control soil as a result of applying BC5, BC10, BC15, and BC20. Applying of biochar significantly affected (p < 0.05) the uptake of sodium (Na+) and chloride (Cl−) by pearl millet plants as shown in Fig. 2E and F. The addition of BC5 decreased the uptake of Na+ and Cl− by 20 and 30%, respectively, compared with the plants grown in the control soil. BC10 decreased the uptake of Na+ and Cl− by 18 and 25%, respectively, compared with the non-amended soil. BC10 decreased the uptake of Na+ and Cl− by 20 and 30%, respectively, compared with the control soil. BC20 decreased the uptake of Na+ and Cl− by 28 and 28%, respectively, compared with the non-amended soil.

Fig. 2
figure 2

Effect of biochar application on some biochemical compounds in the leaves of pearl millet plants. A: chlorophyll, B: carotenoids, C: proline, D: phenolic compounds, E: sodium, and F: chloride. C = without biochar application, BC5 = 5 ton ha−1, BC10 = 10 ton ha−1, BC15 = 15 ton ha−1, and BC20 = 20 ton ha−1. Data are means of two seasons and measured in the first cut. Means (± SD, n = 10) denoted by the same letter indicate no significant difference according to Duncan’s test at p < 0.05

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3.5 CO2 Sequestration by Pearl Millet and Effect of Biochar on CO2 Emissions

The organic carbon which was sequestrated by pearl millet was calculated as the sum of organic-C in the shoots of the three cuts plus that in the roots of the final harvest (Fig. 3A). Pearl millet plants cultivated in the saline sodic soils produced 6.00–9.45 ton of organic-C ha−1 and released 1.62–1.82 ton of organic-C ha−1 (Fig. 3B). The application of BC5, BC10, BC15, and BC20 increased the C storage in pearl millet by 50, 57, 43, and 28% compared with the control soil, while the total emissions of C were increased by 4.8, 5.3, 6.8, and 13.0%. The application of 10 ton ha−1 of biochar increased C sequestrated by pearl millet to 9.45 ton of organic-C ha−1 compared to 6.00 ton of organic-C ha−1 resulted from the non-amended soil.

Fig. 3
figure 3

Effect of biochar on carbon balance of pearl millet. A: total carbon-C in pearl millet plants and B: total carbon-C emission. C = without biochar application, BC5 = 5 ton ha−1, BC10 = 10 ton ha−1, BC15 = 15 ton ha−1, and BC20 = 20 ton ha−1. Data are means of two seasons. Means (± SD) denoted by the same letter indicate no significant difference according to Duncan’s test at p < 0.05

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Biochar significantly affected (p < 0.05) the emissions of CO2 during the different growth stages of pearl millet (Fig. 4A, B, and C). The daily emission of CO2 varied between 7 and 22, 10 and 26, and 9 and 26 kg C ha−1 in the first, second, and third cuts, respectively. The lowest CO2 emission values were recorded in the control soil, while the highest values were recorded in the soil amended with BC20. Emissions of CO2 from the soil increased in the order C > BC5 > BC10 > BC15 > BC20. Applying of BC5, BC10, BC15, and BC20 increased the emission of CO2 by 8.5, 9.3, 11.3, and 14.5% compared with the control soil in the first cut. The results of CO2 emissions in the second and third cuts of pearl millet plants were similar to those of the first one. In the second and third cuts, the CO2 emissions were higher in the soil amended with biochar than in the control soil. In the second cut, applying of BC5, BC10, BC15, and BC20 increased CO2 emissions by 5.3, 5.3, 8.2, and 12.9% compared with the control soil, while in the third cut, the increases were 2.3, 1.7, 2.2, and 10%. In general, the emissions of CO2 were increased with increasing the plant growth. The maximum CO2 emissions occurred when the plants reached the maximum vegetative growth.

Fig. 4
figure 4

Effect of biochar on daily CO2-C emission during the (A) first, (B) second, and (C) third cut of pearl millet. C = without biochar application, BC5 = 5 ton ha−1, BC10 = 10 ton ha−1, BC15 = 15 ton ha−1, and BC20 = 20 ton ha−1. Data are means of two seasons. Means (± SD, n = 10) denoted by the same letter indicate no significant difference according to Duncan’s test at p < 0.05. CO2 emissions were recoded from the cultivation date to the first harvest which was after 60 days of cultivation. The period of plant growth in second and third cuts was only 36 days

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4 Discussion

The obtained results of the current study indicated that the biochar addition increased the CO2 emissions from saline sodic soils. Increasing CO2 emissions from soils is a serious problem and needs careful attention to reduce greenhouse gas emissions (Yang et al. 2019; Fidel et al. 2019). The results which were obtained by previous studies about the effects of biochar on CO2 emissions show wide variations (He et al. 2016; Liu et al. 2016; Fidel et al. 2019). Most previous studies reported either an increase in CO2 emissions or no significant effect. The result of a meta-analysis of 91 research papers about the effects of biochar on CO2 emissions by He et al. (2017) showed that biochar increased the CO2 emission from soil by 22.14%. Emissions of CO2 in the current study increased as a result of applying biochar. Our results are consistent with He et al. (2016) and Liu et al. (2016). The current studies were conducted in an arid region with a high temperature in summer, moreover, the soil of the field trials were very poor in organic matter (less than 1% of organic carbon). The above mentioned conditions encouraged the emission of CO2. The high organic carbon content of the biochar may increase CO2 emissions from the soil (Yang et al. 2019). Reducing CO2 flux from soils by biochar needs high organic matter content in soil to increase the sorption of dissolved organic carbon (Zimmerman et al. 2011). In the current study, the high rate of biochar raised the soil organic carbon from 6 to 10 g kg−1 which was still very low to reduce the CO2 emissions, consequently, biochar under high temperature conditions failed to reduce CO2 flux. The emissions of CO2 varied during the growth stage of pearl millet and the maximum values were obtained before plant harvesting. Emissions of CO2 increase with increasing plant growth (Bono et al. 2008; Fidel et al. 2019).

In the current study, organic-C sequestrated by pearl millet was calculated as the sum of organic-C in the shoots and roots. Although pearl millet caused the emissions of 1.62–1.82 ton of organic-C ha−1 per season, it was able to sequestrate 6.00–9.45 ton of organic-C ha−1. The application of biochar increased amount of the organic-C that was sequestrated by pearl millet. The main reason of the increase of the sequestrated organic-C was due to the increase of pearl millet growth. The growth of plants in saline sodic soils was reduced due to the toxicity of the salt, the shortage of absorbed water, and nutrient imbalances (Shrivastava and Kumar 2015; Eissa and Roshdy 2018; El-Mahdy et al. 2018). Based on the findings of the current study, saline sodic soils amended with biochar increased the growth and productivity of pearl millet plants. The mechanism that was behind the positive effects of biochar may be related to the ability of biochar in increasing the water and nutrient availability in that soil type (Rekaby et al. 2020). Adequate amounts of essential plant nutrients in the plant rhizosphere provide optimum conditions for enhancing the aboveground and underground biomass (Pampuro et al. 2020). Additionally, a reduction in nutrient uptake has been observed in several plant species under saline conditions (Acosta-Motos et al. 2017). Biochar has two important effects on the remediation of saline and alkaline soils: the first is through improving the physical properties of soil e.g., permeability and structure, thus allowing the salt to leach (Raychev et al. 2001). The second action of biochar on alkaline soils is the increasing of nutrients release through the decomposition of organic materials and increasing the activity of soil microorganisms (Mahmoud et al. 2020; Rekaby et al. 2020). The high cation exchange capacity (CEC) of biochar is another advantage in saline sodic soils (Müller and Höper 2004; Eissa 2019). High CEC materials can adsorb soluble salts, especially Na and Cl, and encourage soil aggregation (Ali et al. 2017; Huang et al. 2019). Soil microbes have many important functions in reducing the effect of salinity; however, their activity decreases under saline conditions (Giri and Varma 2019). Adding biochar to saline soils increases soil microbial activity and thus improves plant resistance to salt stress (Giri and Varma 2019; Gorovtsov et al. 2020).

The results of the current study indicated that adding biochar to pearl millet plants grown in saline sodic soils produced significant changes in some biochemical compounds in the leaf tissue as well as in the concentrations of Na+ and Cl−. Pearl millet plants grown in the control saline sodic soil (without biochar) contained higher levels of proline and phenols. Applying biochar minimized the levels of these compounds in the leaves of pearl millet and this may be the reason for the increase in the nutrient uptake (Ali et al. 2017; Doganlar et al. 2010). Humic materials in the biochar have a vital role in increasing the nutrient uptake and act to reduce the stress by reducing the concentrations of proline in plant tissues (Rady et al. 2016). Under stress conditions, plants tend to reduce their leaf area and synthesize of chlorophyll (Akça and Samsunlu 2012; Qiu et al. 2014); however, plants in these conditions increase their concentrations of proline, oxalic acid, and some phenolic compounds (Akça and Samsunlu 2012; Eissa and Abeed 2019; Almaroai and Eissa 2020). Reducing salt stress will encourage plant growth and increase the synthesis of photosynthetic pigments (Rady et al. 2016; Eissa and Abeed 2019; Al-Sayed et al. 2020).

5 Conclusions

Agricultural production in saline soils is not economically viable in the case of using crops that do not tolerate high concentrations of salts. The use of biochar to increase the quality of saline sodic soils, cultivated with pearl millet, was investigated in two-year field studies. Cultivation of fodder plants, such as pearl millet, in saline soils leads to decrease in the quality of forage material due to the negative impact of salts. Adding biochar to pearl millet plants in saline sodic soils compensates for the yield loss and improves the quality. The emissions of CO2 from saline sodic soils cultivated with pearl millet plants ranged between 1.62–1.82 ton of organic-C ha−1, while the sequestrated organic-C ranged between 6.00–9.45 ton of organic-C ha−1. Although the application of biochar increases the CO2 flux, it increases the plant growth and carbon sequestration in saline sodic soils. Methods must be sought to improve the quality of these soils and to select suitable crops to exploit the saline soil resources, thus more field trials are required to determine the optimal biochar for maximum forage yield without significant increases in CO2 emissions.