Biochar Amends Saline Soil and Enhances Maize Growth: Three-Year Field Experiment Findings

Yue, Yan; Lin, Qimei; Li, Guitong; Zhao, Xiaorong; Chen, Hao

doi:10.3390/agronomy13041111

Open AccessArticle

Biochar Amends Saline Soil and Enhances Maize Growth: Three-Year Field Experiment Findings

¹

Engineering & Technology Center of Electrochemistry, School of Chemistry and Chemical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China

²

College of Land Science and Technology, China Agricultural University, Beijing 100193, China

³

Institute of Agricultural Resources and Environment, Guangdong Academy of Agricultural Sciences, Guangzhou 510640, China

⁴

School of Agriculture, Sun Yat-Sen University, Guangzhou 510275, China

^*

Author to whom correspondence should be addressed.

Agronomy 2023, 13(4), 1111; https://doi.org/10.3390/agronomy13041111

Submission received: 21 March 2023 / Revised: 11 April 2023 / Accepted: 11 April 2023 / Published: 13 April 2023

(This article belongs to the Special Issue Effects of Tillage, Cover Crop and Crop Rotation on Soil)

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Abstract

:

Soil salinization is a significant obstacle to agricultural development in arid and semiarid regions. While short-term experiments have demonstrated the effective improvement of saline soils through biochar amendment, the long-term efficacy in sustainably ameliorating such soils remains uncertain. Addressing this knowledge gap, this study investigated the long-term effects of biochar amendment in a field setting by applying different rates of biochar to a salt-affected soil and cultivating silage maize for three consecutive years. The comprehensive assessment includes not only maize growth but also changes in soil physical and chemical properties over the study period. The results reveal a notable elevation in maize above-ground dry matter, directly correlated to the enhanced uptake of nitrogen, phosphorous, and potassium. Additionally, biochar application improves saline soil physical properties, including reduced bulk density (1–23%), increased soil large pores (0.7–12%), and macroaggregates (24–141%), and chemical properties, including a decrease in exchangeable sodium percentage (35–48%), and an increase in soil total organic carbon (112–857%), total nitrogen (9–198%), available nitrogen (12–49%), phosphorus (141–538%) and potassium (57–895%). These improvements ultimately resulted in better maize growth. However, the amelioration effect of biochar on these soil properties gradually diminished over the three-year study. Consequently, this study suggests that biochar is a promising soil amendment that can enhance maize growth in saline soil for at least three years in a field experiment, providing valuable insights for sustainable agricultural practices in salt-affected regions.

Keywords:

biochar amendment; maize growth; saline soil; three years

1. Introduction

In China, there are approximately 1 million hectares of salt-affected arable land, accounting for nearly 7% of the total arable land [1]. Soil salinity, poor soil structure, and nutrient deficiencies greatly limit crop growth, leading to decreased agricultural productivity [2]. Given the substantial population pressure and the occupation of arable land by infrastructure projects, it is imperative to improve saline soil to ensure food security in China [3]. Various measures, including hydraulic engineering, organic and inorganic amendments, and salt-tolerant crop breeding, have been tested to ameliorate the saline soil [4,5]. Among these measures, the application of organic materials, such as manure, humic acid, and polymer materials, has shown much potential to enhance crop productivity by improving the physiochemical, nutritional, and biological properties of saline soil [6,7,8].

Biochar, even with high acidity and salinity, may offer a novel and sustainable alternative for its high stability and long mean residence time [9,10]. The influence of biochar on soil physiochemical properties can be attributed to several factors. Its porous structure and abundant functional groups improve soil aggregation, water-holding capacity, and aeration, thus ameliorating soil structure and reducing soil compaction [11,12]. Additionally, biochar has a high cation exchange capacity, which enables it to adsorb and exchange cations in the soil, potentially mitigating the adverse effects of soil salinity [13]. The high carbon content of biochar also promotes the formation of stable soil organic matter, further contributing to improved soil structure and nutrient retention [14].

Recent studies suggest that biochar amendment has positive effects on both salt-affected soil properties and plant growth [3,5]. For example, biochar application has effectively promoted Miscanthus growth by alleviating salt stress in saline-alkali soil [13]. Similarly, biochar has enhanced rice biomass by modifying soil properties and regulating bacterial abundance and community structure [15]. Although Alessandrino et al. [16] reported an increase in electrical conductivity of leaching water with biochar application, our previous column experiment on the saline soil demonstrated that sunflower straw biochar amendment accelerated salt leaching [17]. It should be noted that short-term trials and leaching experiments may not provide comprehensive understanding of the impacts of biochar amendment on saline soil.

In the Hetao Irrigation area, maize (Zea mays L.) is of paramount importance to the local agricultural economy and food security, serving as both a widely cultivated food crop and a primary source of feed for livestock. Exhibiting mild salt tolerance, maize presents an ideal model for exploring the effects of biochar on saline soils [18]. Developing effective and sustainable strategies to enhance maize productivity in salt-affected areas is vital for ensuring food security and improving the livelihoods of farmers in these regions.

This study investigated the long-term impacts of biochar amendment on maize growth and productivity in saline soils, providing a comprehensive understanding of biochar’s effects beyond short-term trials. Focusing specifically on the Hetao Irrigation area, where maize plays a crucial role in the local agricultural economy and food security, this study aimed to (1) evaluate the effects of biochar amendment on soil physiochemical properties over time and (2) assess the long-term impacts of biochar amendment on silage maize grown in saline soil. We hypothesized that the incorporation of biochar would enhance maize dry matter by improving saline soil fertility—specifically by reducing salt content and exchangeable sodium percentage—and by boosting maize plant mineral nutrition through nutrient input from biochar addition.

2. Materials and Methods

2.1. Soil

The soil had a loamy texture, with a pH of 8.98, an electrical conductivity (EC) of 1955 μS cm⁻¹, an exchangeable sodium percentage (ESP) of 28%, and contained salt ions including CO₃²⁻ (0.05 cmol kg⁻¹), HCO₃⁻ (0.58 cmol kg⁻¹), Cl⁻ (0.89 cmol kg⁻¹), SO₄²⁻ (3.89 cmol kg⁻¹), K⁺ (0.06 cmol kg⁻¹), Na⁺ (7.98 cmol kg⁻¹), Ca²⁺ (0.37 cmol kg⁻¹), and Mg²⁺ (0.46 cmol kg⁻¹). Additionally, it contained 18.20 mg kg⁻¹ of available nitrogen (N), 1.08 mg kg⁻¹ of available phosphorus (P), and 92.01 mg kg⁻¹ of available potassium (K). Its bulk density was measured to be 1.38 g cm⁻³.

2.2. Biochar

The biochar (<2 mm) utilized in this study was derived from a mixture of cotton straw, peanut shell, and sawdust (90:5:5, w/w/w), and pyrolyzed at 400–600 °C for 2 h. The resulting biochar had a carbon content of 479.7 g C kg⁻¹, hydrogen content of 17.4 g H kg⁻¹, and nitrogen content of 12.5 g N kg⁻¹, with a pH of 9.25, EC of 1185 μS cm⁻¹, and contained ions such as HCO₃⁻ (1.29 cmol kg⁻¹), Cl⁻ (1.87 cmol kg⁻¹), SO₄²⁻ (5.07 cmol kg⁻¹), K⁺ (11.67 cmol kg⁻¹), Na⁺ (0.62 cmol kg⁻¹), Ca²⁺ (0.31 cmol kg⁻¹), and Mg²⁺ (0.19 cmol kg⁻¹).

2.3. Field Experiment

The field experiment was conducted in April 2014 in the Hetao Band of Inner Mongolia (40°14′58.66″ N, 110°51′0.97″ E). Metrological data are presented in Table S1. Four rates of biochar, namely, 0, 30, 75, and 150 t hm⁻², denoted as B0, B30, B75, and B150, were applied once on the soil surface and then mixed with the 0–15 cm soil layer using a rotocultivator. In May of each year, a volume of 200 mm of Yellow River water with a pH of 7.82 and a mineralization degree of 0.5 g L⁻¹ was used to flood all plots. Each treatment was replicated three times, and each plot was 20 m² (4 m × 5 m). A randomized block design was used to arrange the four treatments in the field. After the soil had dried off, compound fertilizer (N: P: K = 15%:15%:10%) was applied at a rate of 112.5 kg N hm⁻², and silage maize was sowed at a plant spacing of 30 cm and a row spacing of 60 cm. In the 2015 season, miko (Panicum miliaceum L.) was initially planted, but its low germination rate seriously affected the validity of the experiment. Consequently, we replanted the area with maize. At harvest time, plant samples were collected from each plot and the above-ground dry matter of maize and uptake of N, P, and K were measured. Undisturbed soil was correspondingly sampled to determine bulk density, porosity, water-stable aggregates, pH, EC, salt ions, exchangeable sodium (Na), cation exchange capacity (CEC), total organic carbon (TOC), total N (TN), and available N, P, and K.

2.4. Analysis

Various methods from the book Soil Agrochemical Analysis were employed to determine maize plant nutrient uptake and soil properties [19]. The Kjeldahl method, ascorbic molybdate blue spectrometry, and flame photometry were used to measure total N, P, and K in maize plants, respectively, after digestion with H₂SO₄-H₂O₂. Soil bulk density was measured using a steel ring method, while soil porosity was calculated based on the soil moisture retention curve using a sandbox at suctions of 0.5, 1, 2, 4, 6, and 9 kPa, corresponding to pore diameters of 0.6, 0.3, 0.15, 0.075, 0.05, 0.0375, and 0.0333 mm, respectively. Water-stable aggregates were measured using a modified method of Cambardella and Elliott [20]. Air-dried soil samples were sieved using a sieve shaker with a column of sieves at mesh sizes of 1000, 500, 250, and 53 μm. The soil samples were soaked and shaken to obtain aggregates with diameters >1000 μm. The residual soil samples were then shaken to obtain other aggregates. The obtained aggregates were categorized into macro-aggregates (>250 μm) and micro-aggregates (<250 μm) based on their size. Soil pH and EC (at a 1:5 ratio of soil to water, w/v) were measured using a pH meter and electrical conductivity meter (DDS-307A), respectively. Salt ions were extracted with deionized water and estimated using a previously reported method [17]. Soil CEC and exchangeable Na was estimated using flame photometry, and ESP was calculated according to the ratio of exchangeable Na to CEC. Soil TOC and TN was determined via elemental analyzer after removing CaCO₃. Available N was determined using the Conway method, while available P and K were determined using the Olsen method and flame photometry, respectively.

2.5. Statistical Analysis

All data were expressed as the mean of three replicates on an oven-dried basis. Significant difference among the treatments was assessed by one-way analysis of variance, and then expressed as the least significant difference (LSD_0.05) with the SAS software package (version 8.1). Pearson’s correlations between maize productivity and soil properties and the principal component analysis (PCA) were implemented in R studio.

3. Results

3.1. Soil Physical Properties

3.1.1. Soil Bulk Density and Porosity

Biochar application resulted in a reduction in soil bulk density ranging from 1.34 to 22.82% over the three-year experimental period, depending on the amount of biochar added and the duration of the amendment (Table 1). The degree of reduction in bulk density was found to be positively correlated with the amount of biochar applied. For instance, the bulk density in B150 decreased by 22.82% in the first year, and by 10.37% and 9.35% in the second and third year, respectively, compared to the control soil (B0). Biochar application also led to an increase in soil volumetric water content by 0.74–11.99% compared to B0 (Figure 1). The increase in soil volumetric water content was more pronounced with higher biochar application rates. Specifically, in B150, the soil volumetric water content reached up to 0.55 cm³ cm⁻³ at a matric potential of 6 kPa, an increase of 11.99% and 7.72% in 2014 and 2015, respectively, compared to B0. The improvement in soil water retention was sustained for two years but decreased in the second year.

3.1.2. Soil Water-Stable Aggregates

Application of a low dose of biochar, at a rate of 30 t hm⁻² (B30), did not result in significant changes in the mass fractions of macro- or micro-aggregates (Figure 2). However, higher biochar application rates caused a marked reduction in the mass fraction of micro-aggregates and a significant increase in that of macro-aggregates. For example, in B150, the mass fraction of micro-aggregates decreased by 23.95%, 18.26%, and 9.95% in the first, second, and third year, respectively, compared to that of B0.

3.2. Soil Chemical Properties

3.2.1. pH, EC, Exchangeable Sodium Percentage (ESP), and Salt Ions

All soils had similar values of pH, EC, and salt ions, indicating that biochar amendment did not induce significant changes in these parameters over the three-year period (Table 2 and Table 3). However, significant reductions in both Na⁺ and ESP values were observed in the first, second, and third year in B150, resulting in decreases of 36.07%, 10.11%, and 2.35% for Na⁺, and 47.48%, 43.07%, and 42.97% for ESP, respectively. These effects were most pronounced in the first year and gradually decreased in the following two years.

3.2.2. Soil Nutrients

Biochar-amended soils (B30, B75, and B150) exhibited significantly higher values of total organic carbon (TOC), total nitrogen (TN) and available nitrogen (N), phosphorous (P) and potassium (K), and cation exchange capacity (CEC) compared to the control (B0) (Table 4). The incorporation of biochar resulted in an average increase of 489.28%, 313.95%, and 302.67% in TOC, and 114.18%, 52.98%, and 47.27% in TN, during the first, second, and third year, respectively. Moreover, significant increases were observed for available N (12.53% to 48.90%), available P (141.26% to 537.93%), and available K (57.74% to 894.44%) with biochar application at a rate exceeding 30 t hm⁻², compared to soils without biochar amendment (B0). Furthermore, a positive correlation was found between the amount of biochar applied and the increase in nutrient availability. Additionally, biochar application at a rate of 150 t hm⁻² enhanced soil CEC in the first two years (Table 4).

3.3. Maize Dry Matter and Its Nitrogen (N), Phosphorus (P), and Potassium (K) Uptakes

Maize dry matter enhanced with the increasing dosage of biochar (Table 5), with the most noticeable increase in the first year. The lower maize dry matter in the second year was attributed to delayed sowing and a shorter growth period. Maize plants grown in soils amended with biochar (B30, B75, B150) showed significant increases in N (51.61–378.61%), P (46.36–418.58%), and K (23.32–430.29%) compared to the control (B0) (Table 5). Moreover, a positive correlation was observed between the quantity of biochar applied and the enhancement in nutrient uptake.

4. Discussion

4.1. Biochar Affected Saline Soil Physical Properties

Biochar application decreased soil bulk density in the saline soil due to both the dilute effect from the low bulk density of biochar and its impact on soil aggregation [11,14]. The high concentration of Na⁺ in saline soil often causes the dispersal of clay particles and disintegration of soil aggregates [21]. However, biochar with abundant organic functional groups and divalent cations can serve as a binder, promoting the aggregation of dispersed clay particles, which is consistent with the findings of Liang et al. [22]. The presence of hydrophilic functional groups on the surface and pores of biochar enhances soil water retention, thereby increasing soil volumetric water content after biochar application [23,24]. The increase in soil volumetric water content is consistent with the decrease in soil bulk density, which promotes the formation of large pores and enhances soil water holding capacity [25]. Laghari et al. [23] also found that rice husk biochar application increased soil porosity by decreasing bulk density and increased available water in a sandy clay loam soil. This increase in soil porosity and water content can improve air condition and alleviate water stress in saline soil and promote crop growth [26].

Our results indicate that biochar amendment significantly improved soil aggregation, and the observed changes suggest long-term improvements in soil aggregation. Similar results were also reported by Jien [12]. It is believed that biochar may act as an effective glue material for aggregating soil mineral particles into macro-aggregates due to its large surface and abundant functional groups [27,28]. Salt-affected soil is known to have a poor structure, with a high proportion of aggregates smaller than <10 μm, which readily leads to salt accumulation in the surface soil due to active capillary upward movement and slow downward movement of salts [29,30]. It is obvious that increasing aggregation to form large water-stable aggregates becomes one of the key measures for ameliorating salt-affected soils [31]. Our study suggests that adding biochar at a rate of more than 30 t hm⁻² could be an effective measure for improving mineral aggregation in the tested saline soil of this region.

4.2. Biochar Affected Saline Soil Chemical Properties

Biochars are widely known to have a liming effect on acidic soils, which can enhance soil pH depending on the type and amount of biochar [32,33,34]. Higher amounts of biochar with high ash content can result in greater pH increases in a sandy calcisol [16]. However, the effect on some alkaline soils, such as calcareous soils, and the salt-affected soils may be limited due to their high buffering capacity (Table 2) [3,35,36]. The reduction in exchangeable sodium percentage can be attributed to the adsorption of exchangeable Na⁺ ions onto biochar surfaces, as well as the replacement of Na⁺ ions by other cations (such as K⁺, Ca²⁺, and Mg²⁺) that were introduced into the soil through biochar addition (Table S2).

Biochar amendment significantly improved the saline soil fertility in this study, with positive impacts lasting for at least three years. These results are consistent with previous greenhouse and pot trials in saline soils [13,15]. The observed improvements can be attributed to the direct input of nutrients, as the biochar used in the study was rich in these elements [37]. In addition, the large and rapid increase in carbon pools resulting from biochar application is an effective method of carbon sequestration, as it resists chemical and biological decomposition [9,38]. Furthermore, the bioavailable carbon in biochar might support soil microorganism growth, which is important for soil nutrient dynamics [39]. Moreover, biochar application increased soil total nitrogen, but only a sufficient amount of biochar (over 30 t hm⁻²) could increase available nitrogen (Table 4). Similar results have been reported in other studies, where N availability increased in soils with high biochar application rates but decreased in soils with low biochar application rates, due to the adsorption of NO₃⁻ and NH₄⁺ by biochar [37,40]. Biochar also contains a certain amount of ash, including salt ions such as P and K, which can increase the availability of these nutrients [41]. This finding is consistent with previous studies by Noyce et al. [42] and DeLuca et al. [43], who found that biochar application increased available P and K in sandy soil.

Biochar increased saline soil CEC, one indicator of soil fertility. This finding is consistent with the results of Tomczyk et al. [44], who observed a 190% increase in soil CEC in an anthrosol following wood biochar application. The functional groups and rich surface charges present in biochar have been reported to enhance soil CEC [45,46]. Additionally, biochar can be oxidized in the soil environment, leading to an increase in oxygen-containing functional groups and surface negative charge, which further enhances soil CEC [47]. Therefore, prolonged biochar application could lead to a gradual increase in the CEC of the tested soil.

4.3. Biochar Affected Maize Plant Dry Matter and Its Nutrient Uptake

Previous studies have shown that crop yield is closely related to soil properties in non-saline soils [23,37]. To investigate the relationships between maize dry matter, nutrient uptake, and soil properties in the tested saline soil, Pearson’s correlation analysis was performed. The results showed that maize dry matter had a significantly positive correlation with the uptake of N, P, and K, macro-aggregate, TOC, TN, available N, P, and K, as well as CEC, and a negative correlation with bulk density, micro-aggregates, pH, and ESP (Figure 3). Therefore, the improvements in soil physical conditions, the decrease in ESP, and the increase in soil nutrients resulting from biochar amendment could provide more favorable conditions for maize growth.

The results obtained from principal component analysis (PCA) revealed that the uptake of N, P, and K by maize plants was highly correlated with the availability of N, P, and K in the soil, as well as the levels of total organic carbon (TOC) and total nitrogen (TN). The first principal component (PC1) and PC2 explained 70.7% and 12.5% of the total variation, respectively (Figure 4). This is in line with the findings of Kalu et al. [48], who reported that soil nutrient content affected the uptake of macro- and micro-nutrients. Clearly, the increased availability of N, P, and K in the biochar-amended soil provided a better nutrient source for maize plants, especially in poor saline soil [18,35]. Moreover, the bioavailable carbon and nitrogen in biochar could act as a nutrient source for soil microorganisms, which in turn facilitated the dynamics of N, P, and K and, thereby, provided more nutrients for maize growth in saline soil [37].

5. Conclusions

Overall, the effects of biochar on soil physiochemical properties and maize growth were investigated in a field experiment conducted on saline soil. The findings are summarized as follows:

Biochar application reduced soil bulk density, increased the number of large pores, and improved soil aggregation.
Biochar application decreased exchangeable sodium percentage (ESP) and enhanced soil nutrient availability and cation exchange capacity.
The increased maize productivity was due to the increased nutrient uptake, improved soil physical structure, and reduced ESP.
The amelioration effect of biochar amendment decreased gradually over time.

As a result, biochar could be used as a promising and sustainable soil amendment for at least three years in saline soils. This study provides a theoretical basis for the improvement of salt-affected soil with biochar amendment in the Hetao Irrigation area.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy13041111/s1, Table S1: Mean precipitation and air temperature in Hetao Irrigation area from 2014 to 2016; Table S2: Changes of exchangeable sodium in sulfate saline soils amended with biochar at various rates from 2014 to 2016 (cmol kg⁻¹). Different letters in the same column indicate the significant difference between different treatments (p < 0.05).

Author Contributions

Conceptualization, Q.L.; methodology, Y.Y.; validation, Q.L., G.L. and X.Z.; investigation, Y.Y.; data curation, Y.Y.; writing—original draft preparation, Y.Y.; writing—review and editing, Q.L., G.L. and X.Z.; funding acquisition, Y.Y., Q.L. and H.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Startup Foundation for Ph.D. of Qilu University of Technology (Shandong Academy of Sciences) (81110570), the National Natural Science Foundation of China (1371243 and 42007047), and the National Key Technology R & D Program (2013BAC02B06).

Institutional Review Board Statement

Not applicable, as no new experiments/measurements were conducted.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

This study was supported by the Startup Foundation for Ph.D. of Qilu University of Technology (Shandong Academy of Sciences) (81110570), the National Natural Science Foundation of China (1371243 and 42007047), and the National Key Technology R & D Program (2013BAC02B06). We also thank the anonymous reviewers and the editors for their suggestions which substantially improved the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Water retention curve under the low suctions in the soils amended with biochar at various rates.

Figure 2. Changes of mass fractions of macro-aggregate (>250 μm) and micro-aggregate (<250 μm) from 2014 to 2016 in the soils amended with biochar at various rates. Different letters represent significant difference among the treatments at the same sampling day (p < 0.05).

Figure 3. Correlation matrices between maize productivities and soil properties across all the treatments as revealed by Pearson’s rank correlation analysis. All squares represent p < 0.05.

Figure 4. Principal component analysis (PCA) of nutrient uptake and soil properties in saline soil. Cos2 represents the quality of variations in the PCA, and a higher value of cos2 indicates a larger contribution of variation to the principal component.

Table 1. Changes of bulk density in sulfate saline soils amended with biochar at various rates from 2014 to 2016 (g cm⁻³).

Treatments	2014	2015	2016
B0	1.49 a	1.49 a	1.52 a
B30	1.40 a	1.47 a	1.45 ab
B75	1.29 b	1.45 a	1.45 ab
B175	1.15 c	1.35 b	1.39 b