Abstract
Soil salinization is a major ecological threat to crop growth and production. Biochar addition can alleviate the negative impacts of saline-sodic stress in crops. Here, a two-year field experiment was conducted in a highly saline-sodic paddy field to evaluate the response of soil physico-chemical properties, ionic concentration, and rice yield to biochar applications. The soil was amended with peanut shell biochar as follows: zero biochar (B0), 33.75 t ha−1 (B1), 67.5 t ha−1 (B2), and 101.25 t ha−1 (B3). Biochar significantly reduced soil bulk density (BD), while it markedly increased total porosity (TP) and saturated hydraulic conductivity (Ks). Furthermore, biochar markedly decreased the Na+ concentration, Na+/K+ ratio, Na+/Ca2+ ratio, HCO3-, and CO32- while it increased the concentrations of K+, Ca2+, and Mg2+. Biochar significantly decreased the electrical conductivity of soil saturation extract (ECe). The exchangeable sodium percentage (ESP) of B1, B2, and B3 were 53.6%, 62.3%, and 71.0% lower, respectively, than that of B0, and the corresponding decrease in sodium adsorption ratio (SARe) was 51.2%, 58.1%, and 60.5%. Biochar had no effect on the soil pH but significantly increased the soil cation exchange capacity (CEC). The rice biomass yield, grain yield, and harvest index significantly increased after biochar application.
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Peanut Shell Biochar’s Effect on Soil Physicochemical Properties and Salt Concentration in Highly Saline-Sodic Paddy Fields in Northeast China
Xuebin Li, Weikang Che, Junlong Piao, Xiang Li, Feng Jin,* Tianxu Yao, Pingyue Li, Wei Wang, Tan Tan, and Xiwen Shao *
Soil salinization is a major ecological threat to crop growth and production. Biochar addition can alleviate the negative impacts of saline-sodic stress in crops. Here, a two-year field experiment was conducted in a highly saline-sodic paddy field to evaluate the response of soil physico-chemical properties, ionic concentration, and rice yield to biochar applications. The soil was amended with peanut shell biochar as follows: zero biochar (B0), 33.75 t ha−1 (B1), 67.5 t ha−1 (B2), and 101.25 t ha−1 (B3). Biochar significantly reduced soil bulk density (BD), while it markedly increased total porosity (TP) and saturated hydraulic conductivity (Ks). Furthermore, biochar markedly decreased the Na+ concentration, Na+/K+ ratio, Na+/Ca2+ ratio, HCO3–, and CO32- while it increased the concentrations of K+, Ca2+, and Mg2+. Biochar significantly decreased the electrical conductivity of soil saturation extract (ECe). The exchangeable sodium percentage (ESP) of B1, B2, and B3 were 53.6%, 62.3%, and 71.0% lower, respectively, than that of B0, and the corresponding decrease in sodium adsorption ratio (SARe) was 51.2%, 58.1%, and 60.5%. Biochar had no effect on the soil pH but significantly increased the soil cation exchange capacity (CEC). The rice biomass yield, grain yield, and harvest index significantly increased after biochar application.
DOI: 10.15376/biores.17.4.5936-5957
Keywords: Saline-sodic soil; Biochar; Ionic concentration; Physicochemical properties; Rice paddy field; Peanut shell; Rice yield
Contact information: Agronomy College, Jilin Agricultural University, Changchun 130118, China; Xuebin Li, Weikang Che, and Junlong Piao contributed equally to this work;
* Corresponding authors: jinfeng@jlau.edu.cn, 616530670@qq.com
INTRODUCTION
The Songnen Plain, located between 42°30′ to 51°20’N and 121°40′ to 128°30’E, is the largest plain in Northeast China. The western area of this plain occupies a total of 3.42 million hectares of saline-sodic soil, which severely restricts its agricultural development and utilization (Wang et al. 2003). Over the years, overgrazing by livestock, population growth, and improper soil management have been the main anthropogenic factors responsible for increased soil salinization in the Songnen Plain (Huang et al. 2022). Currently, the Songnen Plain is more explored for agriculture than for livestock herding, but low-quality water and improper irrigation-resource management continue to be main factors in increasing soil salinity, sodicity, and alkalinization in recent years. Sodium carbonate and sodium bicarbonate are the main sodium salts in saline-sodic soil in this region, which have strong alkalinity (Chi and Wang 2010). Osmotic stress, ion toxicity, and high pH stress in saline-sodic soil are the main factors that inhibit crop growth and soil organism activity (Al-Karaki 1997; Chi et al. 2012). These negative effects can cause nutritional disorders in plants and a decrease in the soil water potential and can limit the uptake of essential plant nutrients (K, Ca, Mg, and P) and water, with decreases in soil infiltration and hydraulic conductivity as well as root respiration, thus reducing the yield (Wong et al. 2010; Chaganti and Crohn 2015). Saline-sodic soils can be reclaimed successfully for plant growth following steps aimed at the removal of excessive amounts of exchangeable Na and soluble salts from the cation exchange sites via other cations, such as Ca2+, and then leaching the replaced Na+ from the soil profile with good-quality water (Chi et al. 2012; Huang et al. 2022). The application of organic conditioners is another important way to ameliorate the impact of saline-alkali stress on plants, which can both improve the physicochemical properties and enhance soil fertility (Yaduvanshi and Swarup 2005; Vijayasatya et al. 2015; Srivastava et al. 2016). It is a fact that washing or leaching saline-sodic soils with good-quality (low-salinity) water is the most accepted method for mitigating the problem (Huang et al. 2022). However, freshwater is increasingly becoming scarce in arid/semiarid areas afflicted by saline-sodic soils. These soils have high concentrations of montmorillonite clay, which is high in negative charges and adsorb Na+ very efficiently, leading to clay-particle dispersion into pores, decreasing soil permeability and drainage (Chi et al. 2012). Therefore, it is necessary to use tested and proven methods to mitigate soil salinity and sodicity through the lixiviation of salts away from the crop root zone.
Similar to organic matter, a biochar application is effective in reducing salinity stress by limiting Na+ uptake by plants (Lashari et al. 2013; Thomas et al. 2013). Recent research shows that the benefit of biochar added to salt-affected soil is related to the stabilization of the soil structure, an improvement in the soil physical properties, an increase in the content of soil organic carbon and nutrients, and an enhancement of the soil cation exchange capacity (CEC) and soil surface area (Esfandbod et al. 2017; Liu et al. 2020; Yao et al. 2021). In addition, a biochar addition can enhance the nutrient levels of salted-affected soils by providing habitats for soil microorganisms and improving their vitality (Saifullah et al. 2018). The authors’ previous research showed that a biochar addition clearly reduced the Na+ concentrations in rice plants and decreased the Na+/K+ ratio of the rhizosphere soil in a saline-sodic paddy field mainly because of its high Na+ adsorption potential and K+ supply capacity (Jin et al. 2018; Ran et al. 2020; Zhao et al. 2020; Li et al. 2022). Moreover, biochar applied to saline-sodic soil can promote rice growth through an improvement in the soil nutrient status and an increase in the soil enzyme activity (Yao et al. 2021). A laboratory column-leaching experiment and a greenhouse study demonstrated that biochar effectively removed salts from saline-sodic soil, promoted a balanced ratio of Na+/K+ in the soil solution, and significantly reduced the electrical conductivity of soil saturation extract (ECe), exchangeable sodium percentage (ESP), and sodium adsorption ratio (SAR) because of its influence on pore size distribution and Na+ displacement (Santos et al. 2021). Similar results were observed in soybean amended with modified biochar (Mehmood et al. 2020). Generally, biochar is considered to be an effective organic ameliorant for saline soils; however, most research has been primarily conducted using small buckets in laboratories or greenhouses, and mainly focused on dry crops or on the aboveground parts of crops (Lashari et al. 2013; Drake et al. 2016; Mehmood et al. 2020; Santos et al. 2021). It is not clear how biochar is involved in improving the soil physicochemical characteristics, increasing the ion concentration, and regulating the yield formation of rice in highly saline-sodic paddy fields. Therefore, long-term field experiments on the effect of a biochar application in paddy fields with saline-sodic soil are necessary. The authors hypothesize that peanut shell biochar can improve the soil physicochemical properties by reducing the soil Na+ concentration, Na+/K+ ratio, and Na+/Ca2+ ratio, and offsetting saline-alkali stress, which in turn promotes the yield formation of rice in saline-sodic paddy fields.
In this study, the response of the soil physicochemical properties, ion concentration, and yield formation of rice was evaluated in response to peanut shell biochar additions under highly saline-sodic paddy field conditions. The mechanisms through which peanut shell biochar promoted the yield formation of rice were explored by measuring the soil bulk density (BD), total porosity (TP), saturated hydraulic conductivity (Ks), ionic concentrations, and soil saline-alkali parameters in two planting years. The research findings provide new insight into the amelioration of saline-sodic stress in rice and the improvement of health parameters of saline-sodic paddy soil by biochar applications.
EXPERIMENTAL
Setup of Experiment
Experimental site and soil sampling analysis
A 2-year field experiment was conducted in Sheli, Da’an Country, Jilin Province, Northeast China (45°35′N, 123°50′E). This area has a typical dry-cold monsoon climate, with an average annual air temperature of 4.7 °C, average precipitation of approximately 413.7 mm, and average evaporation of approximately 1696.9 mm. The imbalance between precipitation and evaporation is attributed to a high ground water level in these low-lying areas, in combination with an arid or semi-arid climate. The basic physicochemical characteristics of the soil in this experiment were measured before experiment, and the relevant indexes are shown in Table 1. The soil type at this experimental site is Solonchak (IUSS Working Group 2014).
Table 1. Physicochemical Properties of the Soil
Experimental design
The field trial was performed from April 2017 to October 2018. The experiment was designed as a randomized complete block with three replications, with a total of 12 plots (each 5 m × 6 m). The peanut shell biochar was applied to the saline-sodic paddy field at the following rates: 0 biochar (B0), 33.75 tons per hectare (B1), 67.50 tons per hectare (B2), and 101.25 tons per hectare, based on 0 g, 15 g, 30 g, and 45 g per kilogram of soil in the 0 to 20 cm plow layer. Biochar was only applied in the spring of 2017. The biochar was uniformly spread on the surface of the saline-sodic paddy soil before rice planting and then thoroughly ploughed into the topsoil (0 to 20 cm) using a wooden rake. Each experimental plot was separated by a 60-cm-wide soil ridge. Individual plots were equipped with an independent inlet and drainage valve.
Field management
The rice variety planted in this field study was japonica rice Changbai 9, one of the elite cultivars used in saline-sodic paddy soil in Northeast China. Rice seeds were sown in a greenhouse on 10 April 2017 and 9 April 2018. On May 20, 2017 and May 19, 2018, the rice seedlings were transplanted to the field plots. The transplanting density (per hill) was 30 cm × 16.5 cm, and each hill contained three seedlings. The rice was harvested on September 30, 2017, and October 1, 2018. In the four biochar treatments, the application rates of chemical (NPK) fertilizer were as follows: 250 kg N per hectare, 75 kg P per hectare, and 100 kg K per hectare. Before transplanting, 300 kg (NH4)2SO4 per hectare, 150 kg diammonium phosphate per hectare, and 50 kg K2SO4 per hectare were incorporated into the 0 to 20 cm topsoil layer. Ten days after transplanting, urea was added at 150 kg ha-1 on the water-soil surface to support tillering. At the rice panicle stage (52 days after transplanting), urea (60 kg per hectare) and K2SO4 (50 kg per hectare) were applied. Field management was the same as that used in local production fields to minimize yield loss.
Methods
Biochar characterization
The biochar was produced from peanut shells using a vertical kiln, manufactured by Jinhefu Agricultural Development Company, Liaoning Province, China, and the pyrolysis temperature was 350 to 550 °C for 4 h. The peanut shells were obtained from Jinhefu Agricultural Development Company, AnShan city, Liaoning Province, China. The physiochemical properties of the biochar and peanut shells were measured before experiment, and the relevant indexes are presented in Table 2.
Measurements of soil properties
In each plot, three undisturbed soil cores (100 cm3) at a depth of 20 cm from the plough layer were randomly collected after the rice harvest to measure the soil BD, TP, and Ks. The bulk density was calculated as the ratio of the oven-dry weight (105 °C) and the core volume. The soil specific gravity was measured by the drainage weighing method, and then the soil TP was calculated. A soil saturated hydraulic instrument (TST-55A, Nanjing Soil Instrument Co., Ltd., Nanjing, China) was used to collect undisturbed soil samples. The Ks values of soil samples from all treatments were determined using the constant water head method (Wang et al. 2008), and Ks was calculated by measuring the volumes drained (Q, unit) at the same time intervals (t) using Darcy’ law,
(1)
where S (cm2) is the cross-section of the penetration soil column, L (cm) is the thickness of the soil sample, and H (cm) is the height of the constant water head.
Table 2. Basic Properties of Raw Peanut Shell and Biochar
After the rice harvest, five soil samples (0 to 20 cm depth) from randomly selected sites in each plot were collected using an auger. All samples were air dried and sieved through a 2-mm mesh. The concentrations of Na+, Ca2+, and Mg2+ were measured using 1:5 soil to water extracts. These extracts were prepared by adding 20 mL of distilled water to 4 g soil in a 100-mL bottle. The bottle was sealed with a stopper, agitated for 15 min on a mechanical shaker (100 rpm), allowed to stand for 1 h, and then agitated again for 5 min. A sample was then obtained by filtration. The concentrations of sodium, magnesium, and calcium were detected by inductively couple-plasma spectroscopy (GBC-906AAS, GBC Scientific Equipment Pty Ltd., Melbourne, Australia). The K+ concentration was quantified using a flame photometer (M410, Sherwood Scientific Ltd., Cambridge, England). The Na+/K+ ratio and Na+/Ca2+ ratio were calculated after the determination of Na+, K+, and Ca2+.
The pH was measured in a 1:5 suspension of soil to water using a pH meter (Mettler Toledo International Trade Co., Ltd., Shanghai, China). The electrical conductivity (EC) was measured in a 1:5 extract (EC1:5) of soil to water using a conductivity meter (DDS-307, Shanghai Precision Scientific Instrument Co., Ltd., Shanghai, China), and the EC of a saturated paste extract (ECe) was estimated according to Chi and Wang (2010):
ECe = 10.88 EC1:5 (2)
The sodium adsorption ratio (SAR) was determined after measuring the Na+, Ca2+, and Mg2+ contents in a 1:5 soil to water extract (USDA 1954). According to the method of Chi and Wang (2010), the SAR of a saturated paste extract (SARe) was measured:
(3)
SARe = 13.19 SAR1:5 (4)
The soil CEC was determined according to Bower saturation (Richards 1954). Soil extractable cations were determined by rinsing soils for 10 min with 1 M ammonium acetate solution buffered at pH 8.5. Exchangeable cations were determined by the difference between the extractable and soluble cations.
The exchangeable sodium percentage (ESP) was determined according to Eq. 5:
(5)
The HCO3– and CO32- concentrations were determined via sulfuric acid titration following Bao (2000) and Richards (1954). The content of chloride ions (Cl–) in the soil was determined by silver nitrate (AgNO3) titration, and the content of sulfate ions (SO42-) was measured via the EDTA complexometric method (Richards 1954; Bao 2000).
Measurements of rice yield properties
At the mature stage (132 days after transplantation), 15 rice plants were randomly harvested in each plot. These plants were oven-dried at 105 °C for 30 min and then at 60 °C to a constant weight. The biomass was recorded. The rice plants were selected from 5 m2 in each experimental plot, and then the rice grain yield was calculated. The harvest index was calculated as the ratio of the rice grain yield to the biomass yield.
Statistical analysis
The data were analyzed using SPSS 18.0 software (IBM Corp., Armonk, NY, USA) based on the trial design. A two-way analysis of variance (ANOVA) and Tukey tests were applied to evaluate the interactive effects between the biochar treatment and year. One-way ANOVA and Tukey tests were employed to analyze the effect of biochar on the relevant test indicators. The mean value was determined with the least significant difference at the p < 0.05 level.
RESULTS AND DISCUSSION
Effect of Biochar on Soil Physical Properties
Year and biochar treatment significantly affected the soil physical properties in the saline-sodic paddy soil (Fig. 1).
The average bulk density (BD) was 4.05% lower in 2018 compared with 2017, while the TP and Ks values increased by 4.42% and 6.33%, respectively. Compared with B0, the BD was reduced by 22.29% in B3, by 15.66% in B2, and by 12.65% in B1 (Fig. 1a). The TP and Ks values were significantly increased by the biochar application (Fig. 1b, c). The TP value of B0 was 17.88, 22.41, and 33.40% lower than that of B1, B2, and B3, respectively. The Ks value was ranked as B3 > B2 > B1 > B0, and obvious differences were detected among all treatments.
Fig. 1. Main effects of year and biochar treatment on bulk density (BD), total porosity (TP), and saturated hydraulic conductivity (Ks) in saline-sodic paddy soil; Different letters indicate significantly different between biochar application rates (P < 0.05); NS, *, and **, Not significant, Significant at P < 0.05, and P < 0.01 level, respectively
Effects of Biochar on Na+, K+, Ca2+, Mg2+, Na+/K+ Ratio, and Na+/Ca2+ Ratio
Table 3 shows that the concentrations of Na+ and K+ were influenced by year and treatment. The average concentrations of Na+ and K+ were 7.51% and 9.07% lower, respectively, in 2018 than in 2017. Compared with B0, there were significant reductions of Na+ concentrations in B3, B2, and B1, by 30.21%, 25.19%, and 17.73%, respectively, while the K+ concentrations were 467.58%, 367.19%, and 267.02% higher, respectively. The differences among all treatments reached a significant level. Furthermore, year and biochar treatment exhibited an interactive effect on the Na+ concentration. The concentrations of Ca2+ and Mg2+ increased markedly with increasing biochar application rate (Table 3). On average, compared to B0, the treatments of B1, B2, and B3 increased the Ca2+ concentration by 43.8% to 84.7% and the Mg2+ concentration 17.9% to 42.5%. However, no significant difference was observed between different planting years.
Year and biochar treatment influenced the Na+/K+ ratio and Na+/Ca2+ ratio (Table 3). The Na+/K+ ratio and Na+/Ca2+ ratio in 2018 was 1.66% and 3.99% lower, respectively, than that in 2017. Compared to B0, the Na+/K+ ratio and Na+/Ca2+ ratio decreased by 69.2% and 42.9% under B1, 79.6% and 53.5% under B2, and 85.1% and 62.3% under B3, respectively. In addition, the differences among all biochar treatments reached a significant level, but there was no obvious interactive effect of year and biochar on the Na+/K+ ratio and Na+/Ca2+ ratio.
Table 3. Main Effects of Year and Biochar Treatment on Na+, K+, Ca2+, Mg2+, Na+/K+, and Na+/Ca2+ in Saline-Sodic Paddy Soil
Effects of Biochar on HCO3–, CO32-, Cl–, and SO42-
The concentrations of HCO3–, CO32-, and SO42- were significantly affected by year and biochar treatment, but there was no significant effect on the concentration of Cl– (Table 4). The concentrations of HCO3– and CO32- were generally higher in 2017, while the concentration of SO42- was significantly lower in 2017. The HCO3–, CO32-, and SO42- concentrations decreased markedly with the addition of biochar. The concentration of HCO3– in B1, B2, and B3 was 7.9%, 20.6%, and 31.0% lower, respectively, than that in B0. The CO32- concentration of B1 was 30% lower than that of B0, and the corresponding decreases in B2 and B3 were 47.8% and 61.1%, respectively. The corresponding decreases in the SO42- concentration were 17.4%, 29.2%, and 32.3%, respectively.
Table 4. Main Effects of Year and Biochar Treatment on HCO3–, CO32-, Cl–, and SO42- in Saline-sodic Paddy Soil
Effects of Biochar on Soil Chemical Properties
Year and biochar treatment markedly influenced the soil ECe, CEC, ESP, and SARe, while there was no significant effect on pH (Table 5). The ECe, CEC, ESP, and SARe values were 20.8%, 0.37%, 7.38%, and 20.2% lower, respectively, in 2018 than in 2017. Compared with B0, the soil ECe values of B1, B2, and B3 were 54.2%, 65.2%, and 76.1% lower, respectively.
The biochar application also markedly decreased the soil ESP and SARe values. The soil ESP values of B1, B2, and B3 were 53.6%, 62.3%, and 71.0% lower, respectively, than that of B0. The corresponding decrease in SARe was 51.2%, 58.1%, and 60.5%, respectively. However, the CEC values increased significantly (P < 0.05) with increasing biochar application rate. Year and biochar treatment exhibited an obvious interactive effect on the soil ECe value and soil SARe value.
Table 5. Main Effects of Year and Biochar Treatment on pH, ECe, CEC, ESP and SARe in Saline-Sodic Paddy Soil
Effects of Biochar on Rice Yield and Harvest Index
Year and biochar treatment significantly affected the rice yield (Fig. 2). The average biomass and grain yield were 8.7% and 9.7% higher, respectively, in 2018 than in 2017, while there was no significant effect on HI. Compared with B0, the biomass yield (Fig. 2a), grain yield (Fig. 2b), and harvest index (Fig. 2c) increased considerably after the biochar addition in both years, and an obvious difference was observed between B3, B2, and B1 compared with B0, while no marked difference was detected among the biochar treatments. The biomass was ranked as B3 > B2 > B1 > B0; the order of rice grain yield was as follows: B2 > B3 > B1 > B0; and the harvest index was ranked as B2 > B1 > B3 > B0. However, there was no significant interaction between year and treatment on the rice yield.
Fig. 2. Main effects of year and biochar treatment on biomass yield, grain yield, and harvest index of rice under saline-sodic paddy field; Different letters indicate significant differences between biochar application rates (P < 0.05); NS, *, and **, Not significant, Significant at P < 0.05, and P < 0.01 level, respectively.
Correlations Among the Rain Yield, Soil Physiochemical Parameters, Na+/K+ Ratio, and Na+/Ca2+ Ratio
Correlations among the grain yield, soil physiochemical parameters, Na+/K+ ratio, and Na+/Ca2+ ratio are presented in Table 6. The rice grain yield was positively correlated with the total porosity (TP), saturated hydraulic conductivity (Ks), and CEC (P < 0.01), but negatively correlated with the BD, ECe, ESP, SARe, Na+/K+ ratio, and Na+/Ca2+ ratio (P < 0.01). Moreover, BD was negatively correlated with pH (p < 0.05) and CEC (p < 0.01), but positively correlated with the ECe, ESP, SARe, Na+/K+ ratio, and Na+/Ca2+ ratio (P < 0.01). Both TP and Ks were negatively correlated with the ECe, ESP, SARe, Na+/K+ ratio, and Na+/Ca2+ ratio (P < 0.01), but positively correlated with CEC (P < 0.01).
Table 6. Correlation Among Grain Yield, Soil Physiochemical Parameters, Na+/K+, and Na+/Ca2+
Role of Biochar in Improving BD, TP, and Ks of Highly Saline-sodic Paddy Soil
Excessive amounts of Na+ lead to slaking, swelling, and dispersion in saline-sodic soil and can cause surface crusting and hard setting of soil; thus, saline-sodic soils exhibit poor physical conditions and infiltration problems (Oster et al. 1999). A reduction in plant growth in saline-sodic soil decreases the input of soil organic carbon, leading to a poor soil structure (Wong et al. 2010). Qadir et al. (2007) reported that the elimination of excessive amounts of sodium ions from the soil profile together with a suitable increase in the soil EC value are crucial for ameliorating the physical characteristics of saline-sodic soil.
Biochar has a high specific surface area and low bulk density due to its abundant pore structure (Brown et al. 2006). Many prior studies have indicated that the soil bulk density, total porosity, and water-holding capacity were significantly ameliorated in saline soil (Burrell et al. 2016; Obia et al. 2016; Liu et al. 2020), and the amount of biochar applied to the soil affected the improvement (Vaccari et al. 2011; Zhang et al. 2012). Similarly, biochar addition to the highly saline-sodic paddy field in this study clearly decreased the bulk density over two years, and the significant reduction in the soil bulk density was directly associated with the quantity of biochar added (Fig. 1). Moreover, the biochar application significantly increased the TP and Ks in both years (Fig. 1). The results of this study suggest that a biochar application, especially at a high application rate, can improve the physical properties of soda saline-alkali soil. Similar results were also obtained by Huang et al. (2019), who observed that 20 t/hm2 biochar addition increased the soil TP, Ks, and winter wheat yield under saline water irrigation. The results can be explained from three perspectives. First, biochar, as a high-surface-area, porous, and surface-charged material, can improve the BD, TP, and Ks of salt-affected soil through a dilution effect and the complex interactions that it establishes with soil particles (Obia et al. 2016; Blanco-Canqui 2017; Huang et al. 2019). In addition, it is well known that Ca2+ is conducive to the leaching of sodium ions from the soil profile, promoting the formation of soil aggregates, reducing the ESP and Na+ content, and thus significantly increasing the calcium content (Table 3); therefore, biochar application can ameliorate the physical characteristics of salt-affected soils (Clark et al. 2007). Third, biochar has a positive effect on the structure of salt-affected soils through soil structure-building processes, such as aggregation, thus promoting the activity of microorganisms in the root zone (Fletcher et al. 2014; Kolton et al. 2016; Jin et al. 2018; Li et al. 2022).
Role of Biochar in Balancing in the Ion Content and Improving the Chemical Characteristics of Highly Saline-sodic Paddy Soil
For the reclamation of saline-sodic soils, excess Na+ must be removed from the colloid’s cation exchange sites and then filtered out from the root zone (Rengasamy and Olsson 1991). Previous studies have shown that the application of divalent cations, such as Ca2+ and Mg2+, are vital to the reclamation of saline-alkali soil to reduce excess exchangeable sodium, and biochar plays a positive role in this aspect (Chaganti and Crohn 2015; Melas et al. 2017; Torabian et al. 2018). Laird et al. (2010) reported that biochar significantly increased Ca2+ levels in Clarion soil. Likewise, in salt-affected upland soil, Phuong et al. (2020) and Zheng et al. (2018) showed that the availability of Ca2+ and Mg2+ increased after biochar addition. Consistently, a significant increase in Ca2+ and Mg2+ concentrations and a decrease in the Na+/Ca2+ ratio (Table 3) in highly saline-sodic paddy field were found in all biochar treatments in both years, suggesting that divalent cations, such as Ca2+ and Mg2+, can ameliorate the soil physical properties through a significant reduction in the proportion of sodium ions in the soil exchange complex in the process of saline-sodic paddy soil reclamation. A high Na+ content can restrict K+ uptake by crops (Cakmak 2005). Wakeel (2013) discovered that maintaining an optimal potassium content in plant cells is beneficial to plant growth and yield in salt-affected stress soils. An increase in the K+ content in salted-affected soils due to biochar is a crucial mechanism for promoting crop growth in salt-affected soil (Akhtar et al. 2015; Ran et al. 2020). In this study, a peanut shell biochar application significantly increased the K+ concentration and decreased the Na+/K+ ratio (Table 3). The results of the current study indicated that biochar reduced the Na+/K+ ratio by increasing potassium availability, which is an effective means to ameliorate the soil physical structure (Fig. 1) and increase the rice yield (Fig. 2) in highly saline-sodic paddy soils. A similar result was observed by Huang et al. (2018) in sweet corn planted in saline-sodic upland soil. In saline-sodic soil of the western Songnen Plain, sodium ions are usually combined with CO32- and HCO3–; hence Na2CO3 and NaHCO3 are deemed to be the major sodic salt components of this soil. A high quantity of CO32- and HCO3– emerged due to the hydrolysis of inherent Na2CO3 and NaHCO3, which could promote sodium saturation in the soil, accompanied by the enhancement of soil pH (Gupta and Abrol 1990). In this field experiment, a significant decrease in CO32-, HCO3–, and SO42- was revealed in both years, and these reductions were directly related to the biochar application rate (Table 4). Recent research showed that NaHCO3 stress is primarily dependent on the specificities of weak acid ions (HCO3–), rather than high pH (Chen et al. 2021). This may be one of the important ways for peanut shell biochar to decrease saline-sodic stress in saline-sodic paddy fields.
Abrishamkesh et al. (2015) and Liu et al. (2020) found that the soil pH was significantly increased after biochar addition to salt-affected soils compared with saline soil without biochar. In contrast, many researchers observed a significant reduction in the pH of salt-affected soils in response to biochar application, and the decrease in soil ESP with biochar was one of the reasons for the reduction in the soil pH value (Wu et al. 2014; Lashari et al. 2015; Kim et al. 2016; Sun et al. 2017). Unlike the above conclusions, the authors discovered that planting year and peanut shell biochar treatment (pH = 7.94) had no significant effect on pH (Table 5) although the pH of biochar was alkaline. Sun et al. (2016) also revealed that the soil pH remained unchanged after peanut shell biochar (pH = 7.7) was added to salt-affected soil (pH = 8.6). The underlying explanation for the unchanged pH in the highly saline-sodic paddy field may be that (I) biochar application can increase Tp and Ks (Fig. 1), which can help leach Na+ from the soil profile (Table 3) and decrease ESP (Table 5); (II) H+ is dissociated from the ion exchange complex via K+, Ca2+, and Mg2+ (Table 3) through peanut shell biochar (Wang et al. 2015); (III) biochar significantly decreased the concentrations of CO32- and HCO3– (Table 4), which are main factors resulting in the high pH in saline-sodic soil; and (IV) the improvement of soil physical and chemical properties after biochar application promoted the growth of roots and increased the secretion of organic acids in roots (Li et al. 2022). In addition, prior studies also showed that the initial pH of biochar may play a marked role in biochar-induced changes in the pH of salt-affected soils (Liu and Zhang 2012). Sun et al. (2016) reported that despite a marked decrease in ESP, the soil pH was unchanged after amendment with maize stalk biochar (pH 8.0) and peanut shell biochar (pH 7.7) in salt-affected soil (pH 8.6). The pH value of the peanut shell biochar used in the current research was 7.94 (Table 2). Therefore, the unchanged pH under biochar applied to the highly saline-sodic paddy field was related to the low pH value of this biochar.
Many studies indicated that biochar decreases soil EC through the improved leaching of soluble salts (Liu and Zhang 2012; Lashari et al. 2015; Zhao et al. 2020). In a recent study, Zhang et al. (2020) indicated that biochar significantly decreased soil EC in saline-sodic soil. Similarly, the ECe of saline-alkali upland soil was reduced 42% in a two-year field trial compared to non-biochar treatment (Lashari et al. 2013). Consistent with these findings, the authors observed that peanut shell biochar application significantly decreased the soil ECe in the highly saline-sodic paddy field over at least two years and decreased with an increasing rate of biochar application. In addition, year and biochar treatment also exhibited an obvious interactive effect (Table 5). The amelioration of soil porosity and hydraulic conductivity (Fig. 1) after the biochar addition accelerated the leaching of salts and could have caused the reduction in ECe.
The CEC is one of the important properties of soil that impacts the uptake of mineral nutrients, such as Ca2+, Mg2+, K+, Na+, and NH4+, and soil fertility. Zhang et al. (2017a) found that soil CEC significantly increased after biochar application. Likewise, EI-Naggar et al. (2018) showed that CEC increased 906% after rice straw biochar was applied to sandy soil. In this study, the soil CEC values of saline-alkali soil increased markedly (P < 0.05) with an increased rate of peanut shell biochar applied during both years (Table 5). The underlying mechanism for the CEC increase under a saline-alkali paddy field condition is the presence of strong carboxylic and phenolic functional groups with negative charges on the surface of biochar particles (Tian et al. 2017; Palansooriya et al. 2019). The authors’ results indicated that under saline-alkali paddy field conditions, peanut shell biochar can increase the absorption of K+, Ca2+, and Mg2+ (Table 3), decrease soil ESP (Table 5), retain more nutrients, and reduce nutrient leaching (Yao et al. 2021) through increasing the soil CEC value. Similar results were observed by Mehdizadeh et al. (2020), who applied biochar (2% of total pot mass) to salt-affected soil, which increased the CEC 8.2% in a pot experiment in a greenhouse.
The ESP is one of the most important indicators for salt-affected soil and needs to be reduced for crop growth. The initial ESP of soil in this study was 24.1% (Table 1), which is known to inhibit rice growth. Many studies have shown the beneficial effect of biochar on decreasing ESP in salt-affected soil (Chaganti and Crohn 2015; Drake et al. 2016; Kim et al. 2016; Sun et al. 2017). However, these studies were mostly conducted in short-term laboratory incubations or greenhouses. Here, biochar applied to saline-alkali paddy soil clearly reduced the ESP over two years (Table 5), especially at higher application levels. A decrease in ESP due to the peanut shell biochar addition may be attributed to the following two aspects. First, biochar may decrease ESP by providing exchangeable Ca2+, K+, and Mg2+ to replace Na+ on the soil colloids; and second, by decreasing the Na+/K+ ratio and Na+/Ca2+ ratio (Table 3) and increasing the CEC (Table 5). Biochar application significantly decreased BD and increased Tp and Ks (Fig. 1), which may facilitate Na+ leaching from the soil profile, and then reduce ESP (Kim et al. 2016).
Because soil SARe depends on the relative proportions of Na+, Ca2+, and Mg2+ in soil solution and the contents of Na+, Ca2+, and Mg2+ may vary with biochar type, the usage level and types of biochar are the two most important factors controlling the impact of biochar on SAR in salt-affected soils (Luo et al. 2017; Zheng et al. 2018; Mehdizadeh et al. 2020). In this two-year field experiment, the SARe significantly decreased after peanut shell biochar application and decreased with the biochar application rate (Table 5). The reason for the reduction in SARe might be that peanut shell biochar significantly improved the soil physical properties (Fig. 1), increased the utilization of positive ions, such as Ca2+, K+, and Mg2+, and decreased soil Na+ from the exchange point of soil colloids (Table 3).
Role of Biochar in Improving the Rice Yield in the Highly Saline-sodic Paddy Field
Crop growth and yield in saline-sodic soils are inhibited due to (i) the presence of toxic of excessive Na+ (Al-Karaki 1997), (ii) reduced availability of water to crops caused by high osmotic pressure of the soil solution (Naidu and Rengasamy 1993), (iii) restrained absorption of indispensable nutrients (K, Ca, P, etc.) caused by a high concentration of Na+ (Chaganti and Crohn 2015), (iv) limited root growth caused by poor physical characteristics (Sumner 1993), and (v) low levels of soil fertility and soil microbial activity (Wong et al. 2010). Laboratory and field studies have shown that biochar applications can modify the soil physicochemical properties (Wong et al. 2010; Kim et al. 2016; Zhang et al. 2020), improve the soil nutrient status (Thomas et al. 2013; Yao et al. 2021), optimize the morphology and physiological characteristics of roots, and thus increase crop yields (Akhtar et al. 2015; Drake et al. 2016; Sun et al. 2017; Zhao et al. 2020). Additionally, some studies have shown that the effect of biochar on crop growth is primarily related to the biochar type, biochar application level, and soil texture (Zhang et al. 2017b; Santos et al. 2021). In this 2-year field experiment, the grain and biological yield of rice were significantly promoted with biochar under saline-sodic field conditions. The treatment with biochar applied at 67.50 t ha−1 had the most positive effect on yield performance in the authors’ field experiment (Fig. 2). However, there were no significant differences among 33.75, 67.50, and 101.25 t ha−1 biochar in terms of grain yield. The mechanism for the yield increase after the biochar application can be attributed to the following four aspects. First, peanut shell biochar significantly improved BD, TP, and Ks (Fig. 1), which can benefit root growth (Drake et al. 2016; Sun et al. 2017; Zhao et al. 2020; Li et al. 2022). In addition, the peanut shell biochar applied to the highly saline-sodic paddy field can reduce saline-sodic stress through significant decreases in ECe, ESP, SARe, CO32-, and HCO3– (Tables 4 and 5). Furthermore, the authors’ prior study showed that peanut shell biochar effectively enhanced the availability of soil total nitrogen, available phosphorus, available potassium, soil organic matter (Yao et al. 2021) and soil CEC (Table 5), thus facilitating nutrient absorption and promoting rice growth. Finally, peanut shell biochar decreased the soil Na+, Na+/K+ ratio, and Na+/Ca2+ ratio (Table 3) by transient Na+ binding due to its high adsorption capacity and by releasing mineral nutrients into the soil solution (Chaganti and Crohn 2015; Huang et al. 2019; Mehdizadeh et al. 2020). A similar result was also found by Zhao et al. (2020), who added corn straw biochar to saline-sodic upland soils, which significantly increased corn yield 50% under the 20 t ha−1 biochar application level. Peanut shell biochar greatly increased the harvest index (HI) in the current study (Fig. 2), which led to an increase in the conversion rate of photosynthetic C source to rice grain. This may be another reason for the increase in grain yield after peanut shell biochar was applied to the highly saline-sodic paddy field. The average biomass and grain yield in second rice growing season (in 2018) were higher than that in first rice growing season (in 2017), and there was no significant interaction between year and treatment on the rice yield. This may be due to the long-term effects of biochar on soils (Saifullah et al. 2018).
CONCLUSIONS
- Through a two-year field experiment, it was found that peanut shell biochar had beneficial effects on soil bulk density (BD), total porosity (TP), and saturated hydraulic conductivity (Ks) compared to B0, which greatly benefits root growth.
- Peanut shell biochar applications significantly reduced the soil Na+, Na+/K+ ratio, Na+/Ca2+ ratio, HCO3–, CO32-, electrical conductivity of soil saturation extract (ECe), exchangeable sodium percentage (ESP), and sodium adsorption ratio (SARe), and markedly increased the soil cation exchange capacity (CEC), primarily by transient Na+ binding due to its high adsorption capacity and by releasing mineral nutrients (such as K+, Ca2+, and Mg2+) into the soil solution.
- Planting year and peanut shell biochar treatment had no significant effect on pH in the highly saline-sodic paddy field for both years.
- Peanut shell biochar application significantly increased the rice biomass yield, grain yield, and harvest index, but there was no significant difference among the biochar addition levels (33.75, 67.50, and 101.25 t ha−1). This clearly showed the potential of peanut shell biochar to promote rice productivity in highly saline-sodic paddy soils, and the most economical application level of biochar was 33.75 t ha−1.
ACKNOWLEDGEMENTS
This study was funded by the National Natural Science Foundation of China (No.32071951) and Jilin Province Education Department Planning Project (No. JJKH20200340KJ).
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Article submitted: May 6, 2022; Peer review completed: August 21, 2022; Revised version received: August 22, 2022; Accepted: August 23, 2022; Published: September 2, 2022.
DOI: 10.15376/biores.17.4.5936-5957