Increasing sunflower productivity by mitigating soil salt stress through biochar-based amendments

ABSTRACT

The utilization of biochar encountered numerous constraints in achieving increased production and addressing soil salinity issues when employed in isolation. It is essential to take specific measures to maximize its advantages. A two-season field experiment was conducted for planting sunflower using biochar (C, 15 t ha⁻¹) and fulvic acid (FA, 0.15 t ha⁻¹) amendment in the Hetao Irrigation Region in Inner Mongolia, China. The results indicated that the C+FA treatment led to a 28.9% reduction in soil salinity levels at a depth of 0–20 cm after the two-year sunflower cultivation when compared to CK. Analysis of ions revealed that the increase in the ratio of CO₃^2- and HCO₃⁻ led to soil pH rise. The C+FA treatment showed 22.64 g kg⁻¹ organic matter content at depth of 0–20 cm after harvested in the second year. The growth characteristics of sunflowers were enhanced, along with an improvement in the plant’s nitrogen nutrition. The C+FA treatment resulted in a seed yield of 8.5 t ha⁻¹, indicating a twofold increase in comparison to the CK treatment. The research indicates that utilizing biochar-based amendments offers practical benefits for promoting saline agricultural development in arid and semi-arid lands.

GRAPHICAL ABSTRACT

KEYWORDS:

• Biochar
• fulvic acid
• amendment
• saline soil
• pH

Introduction

Soil salinity is a widespread issue affecting over 100 countries worldwide, with no continent being entirely exempt from its presence (Singh 2022). There are over 1 billion hectares of saline soils around the world, which accounts for approximately 7% of the Earth’s land surface (Hopmans et al. 2021). Soil salinization is on the rise globally, with a particular increase in developing nations. The primary cause of rising salinity levels is the result of the excessive intensification of agriculture practices, which prioritizes short-term benefits over the sustainable management of soil and water resources necessary to meet food production needs (Negacz et al. 2022). However, the global population will grow up to 11.2 billion by 2100 (Vollset et al. 2020). The significant demand for increased food consumption poses a major challenge for society as a whole. The saline soils cultivated for saline agriculture, involving soil remediation, water control and planting salt-tolerant crops could be taken into account for increasing food supply (Sun et al. 2020). There are large areas of saline farmlands existed in China, and the saline soils encountered various hindrances in coastal and inland regions. It also has enough rain but the saline water intrusion affected consistently in the coastal lands, and the rain was little with the intense evaporation in the arid and semi-arid lands (Zhou et al. 2021). Despite the scarcity of water, the arid inland soil poses challenges for achieving high crop production, but its scientific exploration never stopped. In the Inner Mongolia of China, the development of saline agriculture development has persisted for hundred years and becoming an important grain production base (Li et al. 2022).

The sunflower (Helianthus annuus L.) was the main oil crop in the Inner Mongolia, its planting area occupied 40% of the whole cultivated area in China. However, for the continue affecting of saline in soils, it was hard to increase its production for a long time (Chang et al. 2023). Sunflower is a plant that exhibits moderate tolerance to salt, and the capacity of sunflowers to thrive and develop in salty environments is contingent upon the specific varieties of the plant. Many measures have been used to maintain and improve the sunflower yields. Such as cover mulch with straws (Selolo et al. 2023), drip irrigation (Sher et al. 2022), adding fertilizers (Aryafar et al. 2023), or applying organic amendments (Li et al. 2022). Organic-based amendments were regarded as the most acceptable among all practical measures. Utilizing alternative techniques not only depletes additional resources but also contributes to the spread of diseases or exacerbates soil salinity (Kiani et al. 2016; Ren et al. 2015). The lack of organic matter and pure water was the most obstacle in the Inner Mongolia, and the extended duration of sunlight in this region proved to be the greatest benefit in ensuring sufficient photosynthesis (Mei et al. 2022). However, the intense transpiration will speed up the process of soil salt moving from the deeper layers to the shallow layers (Lei et al. 2020). Plants have the potential to produce organic amendments that can be beneficial in regulating soil water content (She et al. 2018), salt (Drake et al. 2016), and nutrition (Taheri et al. 2023) and so on. In recent years, biochar derived from various plant sources has emerged as a highly debated material for agricultural purposes (Arshad et al. 2024).

Biochar (C) used in agriculture refers to the C-rich residues of agricultural biomass pyrolyzed under oxygen limited conditions and relatively low temperatures (<700 °C). It has porous structure, large specific surface area, and abundant surface functional groups with redox property (Aller 2016). It can additionally retain its physical characteristics in the soil over an extended period, thereby contributing to carbon storage and altering soil compositions (Jing et al. 2022). Numerous studies have demonstrated that the application of biochar with appropriate dosage in saline soils can effectively reduce bulk density, increase soil porosity (Liang et al. 2021), reduce salt toxicity (Wang et al. 2023), and thus enhance agricultural productivity. Nevertheless, the biochar production process consistently results in minimal organic matter remaining, which in turn speeds up the consumption of nutrients in coastal saline farmlands (Sun et al. 2019). Furthermore, the fulvic acid (FA) is one of the two classes of natural acidic organic matter that can be directly extracted from humic acid in soil (Ukalska-Jaruga et al. 2018). The FA proved to be the primary active ingredient in enhancing microbial activity within the soil, achieved through a process of microbial fermentation utilizing various agricultural residues as raw materials (Lv et al. 2022). The oxygen-to-carbon ratio is greater than 0.5:1, indicating its more acidic character than humic acid. It has the ability to create robust complexes with mineral elements, enhancing their availability in saline soils (Liu et al. 2022). Despite numerous reports on the use of these two materials for saline soil remediation (Sun et al. 2019; Lv et al. 2022), there is still a need for further supplementation when it comes to their application in sunflower cultivation over extended periods in arid lands.

In the Hetao Irrigation District of Inner Mongolia, China, the sunflower planting faced the challenges that soil salinity increase, water lack, and nutrition deficiency. It is imperative to investigate the establishment of an ideal growth environment for sunflowers through environmentally sustainable practices. Furthermore, the expansion of biochar utilization in inland regions is necessary to enhance carbon storage. The use of biochar-based natural materials remains relatively unfamiliar in drylands where sunflowers are cultivated. So, our research aimed to examine the impact of adding biochar and fulvic acid on saline soil and sunflower yields during a two season’s field experiment within 2 years. The composition of ions in soil and agricultural growth characteristics of sunflower was analyzed, revealing significant changes following the application of biochar-based materials. Utilizing organic materials derived from plants with varying physical and biological properties in a creative manner can result in more beneficial effects when combined, as opposed to when used individually. We hypothesis that the combination of fulvic acid could improve biochar effects on ameliorating soil properties and yields increase. This research would consolidate and expand the biochar application in agriculture.

Materials and methods

Experiment site

The field experiment was conducted on Dengni Village (40°49´42.7″ N, 106°55′28.5″ E), Sandaoqiao town, Bayannaoer City, Inner Mongolia, China (Figure 1). The region’s climate falls within the warm temperate zone, and the mean altitude was 1041 m. The mean yearly precipitation, evaporation, and frost-free duration in this region were 138.2 mm, 2094.4 mm, and 135 days, respectively. The irrigation water originated from the Yellow River, with a water salinity level of approximately 0.59 g L⁻¹. The flooding irrigation was conducted before sunflower seedling season annually, resulting in the underground water level fluctuating between 1 and 3 m during the subsequent period. The underground water had a salinity level of approximately 3.25 g L⁻¹. Sunflower, maize, and spring wheat are the main crops grown in this region, with a yearly harvest. The soil’s fundamental properties at the research site were outlined in Table 1.

Figure 1. Field experimental site and design.

Table 1. Basic properties of the soil (0–20 cm), biochar and fulvic acid before the experiment.

Soil		Biochar		Fulvic acid
Salt (g kg^−1)	7.06	Salt (g kg^−1)	0.82	Salt (g kg^−1)	0.11
pH	8.19	pH	8.02	pH	4.50
Bulk density (g cm^−3)	1.49	Density (g cm^−3)	0.42	Organic matter (g kg^−1)	517.20
Organic matter (g kg^−1)	16.99	Organic matter (g kg^−1)	493.10	Total N (g kg^−1)	0.67
Total N (g kg^−1)	0.85	Total N (g kg^−1)	4.00	Purity (%)	99.6
Cl^− (g kg^−1)	1.10	Raw material	Maize straw	H (%)	2.33
SO4^2- (g kg^−1)	3.17	Pyrolysis temperature (°C)	400–500	S (%)	2.16
HCO3^− (g kg^−1)	0.68	Total ash (%)	23.47	O (%)	38.80
Ca^2+ (g kg^−1)	0.15	Ca (mg kg ^−1)	10.20	H/C (%)	0.49
Mg^2+ (g kg^−1)	0.14	Mg (mg kg^−1)	6.12	O/C (%)	0.48
Na^+ (g kg^−1)	1.59	CEC (cmol kg^−1)	30.20	N/C (%)	0.01
K^+ (g kg^−1)	0.24	Raw material	Straw	Acidic groups (mmol g^−1)	9.38
Silt (%)	78.97			Density (g cm^−3)	0.39
Sand (%)	5.30			Raw material	Straw
Clay (%)	15.73

Experimental design

The biochar used in the study was bought from Tairan Organic Fertilizer Company in Shandong province. The biochar was produced through the pyrolysis technique, where air combustion was completely absent, at a temperature of around 600 °C. The fulvic acid (FA) was purchased from Tianxinyuan Bioengineer Company in Ningxia province. The fulvic acid is generated through microbial fermentation using straw as the primary source material. The basic physical and chemical properties of the biochar and FA are listed in Table 1. The study took place between May 2018 and September 2019. The biochar was applied a single time, while the FA was added prior to every sawing phase. Four treatments were designed with three replicates. The CK (no C or FA), biochar (C) 1.5 t ha⁻¹, fulvic acid (FA) 0.15 t ha⁻¹, and biochar 1.5 t ha⁻¹+ fulvic acid 0.15 t ha⁻¹ (C+FA) treatments were completely random distributed in each plot with size of 8 m × 4.2 m. Each plot was divided by a 50 cm width aisle. The soil in the relevant plots was enriched with a mixture of biochar and fulvic acid, which was incorporated at a depth of 0–20 cm. The sunflower variety was 361. Before sawing the seed, the base nitrogen fertilizer (urea, 375 kg ha⁻¹, nitrogen content 45%) was added to soil, then applied 240 kg ha⁻¹ and 100 kg ha⁻¹ urea at squaring stage (45 days after sowing) and flowering (60 days after sowing) stage, respectively. The seed was planted at a depth of 5 cm in the soil, and a film mulch was applied on the surface. The spacing between each film was 1.30 m. A total of 3000 m³ ha⁻¹ of water were used for flooding irrigation before the sowing process began. The line space and row space of the sunflower was 60 and 50 cm, with the planting density of 33,000 plants ha⁻¹. Two years were dedicated to the cultivation of sunflowers, encompassing two planting seasons. About 10 m³ water was irrigated twice in each plot before planting to leach soil salts on May 1 and May 18 in 5 May 2018 and May 20 in 2019.

Soil sampling was carried out between May and September at depths of 0–20 cm and 20–40 cm at seedling stage, squaring stage, flowering stage, and manure stage by soil auger during each planting period. The soil salinity and pH were measured at a depth of 0–40 cm following the flooding irrigation. The soil samples underwent air drying and were sieved to pass a 2 mm sieve in order to test the EC and pH values in soil and water solutions at a ratio of 1:5 (Seven Excellence Cond meter, Mettler Toledo, China). The calculation between salt content and EC refer to our previously published paper (Zhang et al. 2014). Soil samples were gathered from the central area between each row of plants. Soil samples were collected at depths of 0–100 cm, divided every 20 cm, before the experiment on 19 April 2018, and after harvesting the sunflowers on 29 September 2019. In each plot, three samples were obtained and subsequently air-dried to undergo the sieve test for analyzing the ions present in their water extraction. The potentiometric titration method was employed to measure the ion composition of Na⁺, K⁺, Ca²⁺, Mg²⁺, HCO₃⁻, CO₃^2-, Cl⁻, and SO₄^2- in the soil samples collected at the start and conclusion of the planting phase (METTLER TOLEDO T70, New York, U.S.A.). The soil organic matter content in the 0–20 cm depth following each season was assessed using the potassium dichromate titration method (Sun et al. 2020). The soil total nitrogen content was tested using Kjeldahl method (Leici KDN-1, Shanghai, China). The sunflower plant height, leaf area, and stem diameter were measured at seedling stage, squaring stage, flowering stage and mature stage in each planting season using tape with three plants in each plot was recorded. The grain yield of sunflowers was determined by gathering 15 plants from each plot, measuring the diameter of the sunflower head, and subsequently air drying them for analysis of sterility rate, seed weight, and seed kernel weight. The nitrogen content of the plant was analyzed in a similar manner to that of the soil.

Statistical analysis

The Excel 2022 (Microsoft, Washington, U.S.A.) was used to analyze the data. All data were expressed as means ±SD. The graphical designs were carried out by Sigmaplot 12.5 (Systat Software Inc., California, U.S.A.). The means were compared by the variance analysis (ANOVA) and LSD test using SPSS software, version 23 (IBM, New York, U.S.A.) at a 0.05 significance level.

Results

Soil salt and pH

The biochar addition increased soil salt content at the 0–40 cm soil depth (Figure 2), and biochar + fulvic acid decreased soil salts at the end of the experiment. The soil salinity exhibited a consistent pattern of change within the depth range of 0–20 cm. However, in the deeper layers, there was no discernible trend observed. The EC (electrical conductivity, soil: water 1:5) in each plots within the 2-year cultivation were all higher than the first sampling values, they were all bigger than 2000 μS cm⁻¹. No clear pattern was observed in the difference between the treatments at various sampling times. On 22 September 2019, during the sunflower harvest, the soil salt content in the 0–20 cm soil depth was found to be the lowest in the C+FA. At the conclusion of the experiment, there was no significant variation in pH levels across all treatments. The electrical conductivity (EC) and pH levels experienced significant fluctuations at a depth of 20–40 cm throughout the duration of the experiment. Specifically, the EC levels at a depth of 20 cm were consistently higher compared to the values recorded in the 20–40 cm layer across all treatments during sampling on 22 September 2019. The pH of the soil decreased following the two seasons of field experimentation, showing a trend opposite to the changes in EC at a depth of 0–20 cm. The pH levels dropped below 8.0 in both soil layers during the final sampling period. The pH in the soil layer at a depth of 20–40 cm was slightly elevated compared to that of the layer at 0–20 cm. Throughout the experiment, the treated soil exhibited a reduced level of alkalinity.

Figure 2. Soil EC and pH at depth of 0–20 cm and 20–40 cm during the sunflower planting within two years.

In 2018, prior to sowing, the soil EC in deeper layers was observed to be lower than that of the surface, as illustrated in Figure 3. Following the 2018 harvest, there was an increase in soil salinity; however, the electrical conductivity (EC) values at a depth of 20–40 m experienced notable changes. And before sowing in 2019, the EC at depth of 0–40 varied greatly. The soil EC had similar change tendency during the 2-year sunflower growth. The experimental site demonstrated that the salt easily accumulated in the top layer of soil. The salt content experienced a notable increase within the soil depth of 0–100 cm throughout the year 2018. However, it underwent a drastic change just before the sowing period in 2019, and exhibited minimal variation after the harvest in the same year. The flooding treatment significantly decreased soil salt before each growth season, but the salt still remained or accumulated in soil after water infiltration.

Figure 3. Soil EC at depth of 0–100 cm before sowing and after harvest during the two years experiment.

Throughout the two seasons’ experiment, the soil pH remained above 7.5 consistently both prior to and following the planting of sunflowers (Figure 4). The pH level of the lower soil was higher than that of the upper soil layer. The C+FA application recorded the highest pH value in 2018; however, it declined in 2019. The pH distribution showed a negative correlation with the electrical conductivity, where higher electrical conductivity was associated with lower pH levels. After the 2019 harvest season, the C treatment exhibited the highest pH level compared to the other two treatments that were administered. The pH level in 2019 exhibited a decrease compared to 2018, indicating that all treatments facilitated soil improvement. The pH value of FA treatment decreased during the post-harvest period in every season. The fluctuation of pH indicating the existence of other affecting factors that influencing soil properties, such as tillage, climate, and varieties.

Figure 4. Soil pH at depth of 0–100 cm before and after the sunflower planting among two years.

The primary soluble ionic makeup following the sunflower harvest within the 2-year study is illustrated in Figure 5. Among all treatments, the content of SO₄^2- accounted for approximately 50% of the total ions present in all soil layers. In the year 2018, the Cl⁻ and Na⁺ ions held the second and third positions, while the Mg²⁺ ion showed significant improvement at a depth of 0–20 cm in 2019. The salinity in the soil layer between 40 and 60 cm depth was lower compared to that of the soil layer between 0 and 40 cm depth, however, the relative ratios of ions were balanced. In 2018, the ranking of ions was dominated by SO₄^2-> Cl⁻> Na⁺> Ca²⁺> Mg²⁺> K⁺> CO₃^2-+HCO₃⁻, and the sequence was SO₄^2-> Na⁺ > Cl⁻ > Mg²⁺ > Ca²⁺ > K⁺> CO₃^2-+HCO₃⁻ in 2019. The high levels of Ca²⁺ and Mg²⁺ proved advantageous for plants dealing with the effects of salt stress. It is crucial to take into account the matter of salt accumulation in the top layer of soil.

Figure 5. Main soluble ions composition at depth of 0–100 cm after sunflower harvest in 2018 and 2019.

Soil nitrogen and organic matter

In 2018, before sowing sunflowers, there was a minor difference in nitrogen content among all plots, whereas organic matter showed no significant variations (Table 2). Following one growth season, the nitrogen content declined in the C and FA treatment, while it rose in the C+FA treatment, reaching 1.04 g kg⁻¹ in 2018. In 2018, there was an increase in the organic matter content. However, no significant variations were observed among the different treatments, with an average value of approximately 20 g kg⁻¹. In 2019, there was a significant decline in nitrogen levels prior to sowing, as opposed to the amount observed during the harvest in 2018. The time gap between these two samplings was approximately 6 months, during which no agricultural activity took place. After harvest in 2019, the nitrogen increased in the C and FA treated plots. The organic matter increased in C+FA treatment and decreased in C and FA treatments. The organic matter increased, while the nitrogen content decreased when comparing the initial and final values. The C+FA had the highest nitrogen content and organic matter, which was 0.81 and 22.64 g kg⁻¹, respectively.

Table 2. The nitrogen content and organic matter (g kg⁻¹) before and after the sunflower cultivation in 2018 and 2019.

	Treatment	Before sowing 2018		After harvest 2018		Before sowing 2019		After harvest 2019
Nitrogen content		Average	SD	Average	SD	Average	SD	Average	SD
	CK	0.70 b	0.09	1.07	0.25	0.31 a	0.07	0.23 b	0.03
	C	1.20 a	0.13	0.86	0.01	0.12 b	0.06	0.28 b	0.07
	FA	1.04 a	0.03	1.01	0.01	0.17b	0.07	0.19 b	0.07
	C+FA	0.86 b	0.01	1.04	0.04	0.20 ab	0.01	0.81 a	0.31
Organic matter
	CK	18.22	2.67	21.64	0.67	23.24 a	0.75	18.65 b	1.40
	C	15.65	0.84	20.38	0.10	25.41 a	1.30	18.73 b	1.29
	FA	19.74	0.31	19.21	0.64	28.35 a	3.43	21.53 a	2.06
	C+FA	18.72	2.43	20.23	0.58	20.37 b	6.88	22.64 a	3.18

SD, standard deviation. Different letters after the mean value indicate the significant difference at 0.05 level. There is no significant difference between the treatments for the mean values without any letter. The same below.

Sunflower nutrition and growth characteristics

The sunflower nitrogen content was higher in 2018, C+FA had the highest value up to 1.84 g kg⁻¹, showing significant difference with CK (Figure 6). The sequence was C+FA > FA > C > CK in 2018, and C+FA > C > FA >CK in 2019. The nitrogen content in 2018 exceeded that of 2019 across all treatments. The soil N decreased in 2019, the plant N decreased as well. Despite applying the FA twice, there was still a significant reduction in plant N. Biochar-based amendments have enhanced plant nutrition, demonstrating superior effects compared to using biochar or fulvic acid individually.

Figure 6. Sunflower nitrogen content at mature stage.

The C+FA treatment resulted in the tallest sunflower plants in the field experiment (Figure 7). From the squaring stage to the mature stage, plant height increased up to 4 times, indicating the significance of this growth phase. The plant height at mature stage in 2018 was higher than 2019 in all treatments, they all grew more than 200 cm in 2018. The stem diameter was all bigger in 2018, the C+FA treatment got the biggest value of 33.04 mm, followed by FA, CK, and C treatment. In 2019, the higher stem diameter was obtain at flowering stage, and C+FA got a maximum value 27.95 mm. At the mature stage, the plant undergoes shrinkage, which inhibits water absorption when measuring parameter values. In contrast to the plant height and stem diameter, the leaf area in 2019 exceeds that of 2018 at the corresponding growth stage. The biggest leaf area was 13,090.44 cm² in C+FA treated plots in 2019. The maximum leaf area was got in C treatment in 2018 and showed no significant difference with C+FA treatment.

Figure 7. Plant growth characteristic during different growth stages in 2018 and 2019.

As shown in Table 3, sunflower seed was the most important production of the whole plant, the C+FA got the highest seed yield within the 2-year experiment. The seed and seed kernel following the sequence C+FA > C > FA >CK, but the discus width in C+FA was little than the C treatment. The C+FA exhibited a higher sterility rate, suggesting that it contained full grain and was capable of producing a greater seed kernel. The discus width in 2019 was bigger than 2018 in each treatment, which had the average 20.5 cm and 17.8 cm, respectively. The total seed kernel production in CK, C, FA, and C+FA treatment was 4.82, 7.04, 6.33, and 7.22 t ha⁻¹, respectively. The order of seed yield was C+FA > C >FA >CK, when comparing the total output of the sunflower in 2018 and 2019. During the planting period, the C+FA achieved a kernel production that was 1.5 times higher than that of the conventional planting method.

Table 3. Sunflower production characteristics in the years 2018 and 2019.

	Sterility rate (%)		Discus width (cm)		Seed (t ha^−1)		Seed Kernel (t ha^−1)
Treatment	Average	SD	Average	SD	Average	SD	Average	SD
2018
CK	6.34 b	2.34	17.83	1.89	3.33	0.15	2.83	0.13
C	6.20 b	1.05	17.80	1.98	4.60	0.02	3.91	0.02
FA	8.37 a	3.06	18.05	3.25	4.26	0.20	3.62	0.17
C+FA	8.22 a	0.31	17.28	0.85	4.71	0.09	4.00	0.07
2019
CK	6.23	1.50	20.20	1.50	2.34	0.17	1.99	0.15
C	8.49	1.16	20.40	1.82	3.68	0.18	3.13	0.15
FA	8.67	1.50	21.03	0.52	3.19	0.22	2.71	0.19
C+FA	8.37	0.35	20.13	1.43	3.79	0.16	3.22	0.14

Discussion

Soil salinity limited agricultural development all over the world. Adding biochar (C) for modulating this situation has been proven to be a practical method. Considering its defective function on improving direct organic nutrition in soils, biochar-based amendments may be a more reasonable measure for promotion. The 2-year field experiment in this study supplied detailed explanation of biochar, fulvic acid and the combination materials on soil EC, pH, ions composition, soil nutrition, and sunflower growth.

Effects of biochar based amendment on saline soil amelioration

Considering the properties of biochar (C), the high surface area and porous materials are the most obvious characteristics (Ding et al. 2016). The fulvic acid is easily soluble, the micro molecular organic acid making it efficient for absorbing by microorganism and roots (Borcioni et al. 2016). The biochar (C) has the ability to persist in saline soil for an extended period, whereas the maintenance of fulvic acid (FA) becomes challenging, despite the inhibitory effect of salt on organic matter decomposition (Wang et al. 2021). The experiment demonstrated that the combination of biochar and fulvic acid did not have a notable impact on reducing soil pH. However, it was found to be effective in reducing soil salinity during the growth of sunflowers (Figures 2, 3). Our research demonstrated that the combination of biochar and fulvic acid had a more positive impact on improving surface soil quality. Biochar (C) added alone increased soil salt content at depth of 0–60 cm after the field experiment, this may be caused by the huge absorption ability of biochar (Kapoor et al. 2022). As reported by (Saifullah et al. 2018), biochar has the potential to enhance the accumulation of salt at the surface, with its primary effects being influenced by the methods of application. During our study, the modifications were implemented in soils located at a depth of 0 to 20 cm, establishing a salinity environment conducive to plant growth. Our previous study showed that in the coastal saline soils, the 15 t ha⁻¹ and 1.5 t ha⁻¹ fulvic acid could got the best effects on soil salts leaching and crops improvement (Sun et al. 2020), but in this study we reduced the FA dosage for the dry climate in the study area. The biochar (C) and fulvic acid (FA) were all combined with the surface when applied to the plots, they could strongly absorbed with the soil particles. Prior to the amendments being added to the soil, irrigation had already leached the soil salts. Consequently, the soil retained a high water content when the amendments were applied, facilitating the release of their nutritional elements into the soil (Hossain et al. 2020; Kremer et al. 2021). The sunflower in seedling stage had short roots within 20 cm, the amendments built better growth substances for seedlings. Furthermore, the acid functional groups in FA could neutralize the alkalinity of biochar (C), preventing aggravation of alkalization problem of the saline-alkali soils (Bai et al. 2024). In our study, we found that as the soil EC decreases, the soil pH increases (Figure 3, 4). Comparing the primary soluble ions at depth of 0–100 cm after sunflower harvest, the SO₄^2-, Cl⁻ and Na⁺ occupied the most rates. However, the rate of CO₃^2- +HCO₃⁻ increased (Figure 5) when the total salts decreased, and this may be the major cause that lead to the pH rise (Sun et al. 2023). The pH at depth of 0–40 cm during the two year cultivation were all bigger than 7.5 (Figure 3). The issue of saline-alkali continues to persist in the Hetao irrigation region (Wang et al. 2019; Yang et al. 2018), despite the application of irrigation from the Yellow River implemented two or three times every year (Xu et al. 2009). During the transition from the 2018 harvest season to the 2019 sowing season, there was a noticeable rise in organic matter levels and a decrease in total nitrogen content in sunflower-planted soils (Table 2). Apart from the time allocated for planting, the soils remained fallow without any tillage or irrigation. During the period when plants are not being cultivated, the reduction in nitrogen levels can be attributed to the utilization of soil biology (Geisseler et al. 2010), and the organic matter improved possibly because the lower temperature promoted carbon storage (Hartley et al. 2021) or irrigation before sowing brought about more organic substances and soluble organic matter released from deeper soils (Yang et al. 2023). In contrast, if soluble salts are not addressed, they may cause soil particles to disperse. The reduction in salt levels observed after the amendment in our research would promote the formation of aggregates. Furthermore, the lower C/N ratios before sowing indicating that soil needs more carbon application, and FA addition provided the abundant carbon input and could increase soil organic matter stability by promoting mineral oxide complexation (Zhang et al. 2022).

Biochar based materials affects sunflower growth and yield

Biochar (C) and fulvic acid (FA) all have soluble ions and organic substances. Although their fertility effects were not enough compared to the chemical fertilizer, they can improve the fertilizer utilization efficiency (El-Naggar et al. 2019; Yao et al. 2021). The sunflower’s nitrogen content was greater in 2018 compared to 2019, which could be primarily attributed to the reduced temperature and rainfall experienced in 2019 (Gao 2020). Nitrogen is crucial for the growth of plants as it plays a vital role in the synthesis of chlorophyll molecules and serves as the main constituent of plant protoplasm. Biochar (C) combined with fulvic acid (FA) increased plant nitrogen content (Figure 6) and showed the best performance. The plant height and stem diameter were greater in 2018 compared to 2019; however, each treated plot exhibited a larger leaf area (Figure 7). The larger leaf area was important plant trait that reflects plant growth, health, and environmental affection (Jo and Shin 2020). During the experiment, it was observed that leaves with a greater surface area significantly enhanced the production of photosynthetic food. Notably, the C+FA exhibited the most optimal growth among all the variables tested. The sunflower leaf area had a linear relationship with yield that has been reported in many researches (Leite et al. 2006; Peixoto et al. 2022; Tunca et al. 2018). The C+FA treatment obtained the highest seed yield, followed by C and FA treatment alone. The higher sterility rate and lower discus width indicating that the biochar (C) based materials produced fuller seeds, increasing the production quality (Zeng et al. 2016). Our findings indicated that the utilization of biochar (C) based amendments enhanced the growth characteristics of sunflowers and resulted in a seed yield that was twice as large as that achieved through the conventional planting method. The application dosage in this study was changed according to our previous field experiment in coastal areas. The most appropriate application rate need further investigated, and its material costs and application input always need more refined verification (Pfister and Saha 2017). Nevertheless, it is imperative to acknowledge that the climate underwent annual fluctuations, which had a profound impact on the growth of sunflowers, as evidenced by this research. Therefore, the rational application control of biochar-based amendments need more in-depth research based on the climate variation, water supply, varieties, and planting area, etc.

Conclusions

The field experiment conducted in the Hetao Irrigation Region of Inner Mongolia, which spanned two seasons, provided a detailed account of the effects of biochar (C) and fulvic acid (FA) application in enhancing soil quality and increasing crop yield. The combined application of 15 t ha⁻¹ of biochar (C) and 0.15 t ha⁻¹ of fulvic acid (FA) demonstrated superior results in reducing soil salt levels, enhancing nutrient content and improving seed yield compared to individual additions. Despite unfavorable weather conditions in the given year, the C+FA treatment achieved the highest seed yield, providing strong support for ensuring grain production safety. The total seed yield in C+FA treatment plots got a two times higher yield than the conventional farming method. Biochar-based amendments have immense potential in enhancing and stimulating food production in arid and semi-arid saline lands.

Acknowledgments

The authors thank all the researchers who participated in the work of the experiment and the manuscript. We thank to the anonymous reviewers for the precious comments.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was funded by the Natural Science Foundation project of Inner Mongolia Autonomous Region [2023ZD28], the National Key Research & Development Program of China [2021YFD1900602; 2021YFC3201201}, and the Key Research & Development Program of Inner Mongolia Autonomous Region [NMKIXM202107].