Applying bio-organic fertilizer improved saline alkaline soil properties and cotton yield in Xinjiang

Abstract

Bio-organic fertilizers have demonstrated significant potential in enhancing saline-alkali soil properties, boosting crop yield, and reducing chemical fertilizer dependency. However, the extent of improvement and optimal application rates for varying saline-alkali soil conditions remain unclear. We conducted a 2-year field experiment (2022–2023) to evaluate the effects of different bio-organic fertilizer application rates on soil properties, nutrient availability, and cotton yield across three saline-alkali cotton fields with varying salinity levels 4.58, 9.07, and 12.76 g·kg⁻¹ (T1-T3). Four treatments were implemented, involving chemical fertilizer reductions of 0% (CK), 20% (F1), 40% (F2), and 60% (F3). Results indicated that, compared to CK, bio-organic fertilizer application reduced soil bulk density, salinity, and pH by an average of 15.84%, 53.86%, and 7.5%, respectively. Concurrently, soil moisture content, organic matter, nitrogen, phosphorus, potassium, and cotton yield increased by 36.72%, 58.4%, 59.4%, 77.9%, 88.7%, and 50.32%, respectively. Notably, the improvements in soil properties and cotton yield were more pronounced in 2023 compared to 2022. Principal component analysis revealed that a 60% bio-organic fertilizer application rate was optimal for mild, moderate, and severely salinized cotton fields. These findings provide a scientific basis for reducing chemical fertilizer use while improving the productivity and sustainability of saline-alkali soils.

Soil salinization is a critical global issue that severely limits agricultural productivity and economic development^1,2. In China, Xinjiang represents one of the most affected regions, with saline soils covering approximately 13.36 million hectares³, of which 33.3% of arable land is impacted by varying degrees of salinization⁴. Notably, mild, moderate, and severe saline soils account for 28.86%, 7.72%, and 1.13% of the land, respectively⁵. As one of the world’s largest cotton-producing regions, Xinjiang faces significant challenges in cotton cultivation⁶, including imbalanced fertilization practices, such as excessive nitrogen and phosphorus application coupled with insufficient potassium and organic fertilizer use. These practices exacerbate soil salinization, reduce fertilizer efficiency, and compromise cotton yield and quality⁷ Bio-organic fertilizers, which combine the benefits of traditional organic fertilizers and microbial agents, offer a promising solution. They not only reduce chemical fertilizer dependency but also enhance soil fertility, activate nutrient cycling, and improve crop yield and quality^8,9.Therefore, the feasibility and the quality improvement effect of chemical fertilizers combined with bio-organic fertilizers need to be investigated for cotton production in saline-alkali areas of Xinjiang^10,11.

Numerous studies have demonstrated that bio-organic fertilizers can significantly improve the physical and chemical properties of saline-alkali soils, enhance soil fertility, and increase crop yields. For instance, Yang et al.¹². found that combining bio-organic fertilizers with chemical fertilizers effectively reduced soil bulk density, improved soil porosity, and increased water storage capacity. Similarly, Liang and Feng¹³ reported that biochar combined with bio-organic fertilizers decreased soil bulk density by 10% and increased pore water content by 7.14%. Yan¹⁴ observed that bio-organic fertilizers reduced soil salinity, with the effect diminishing at greater soil depths. Gu¹⁵ further highlighted that bio-organic fertilizers enhanced soil organic matter, nutrient availability, and overall soil health. Additionally, Zhou et al.¹⁶. demonstrated that the combined application of chemical and bio-organic fertilizers significantly lowered soil pH, electrical conductivity, and sodium saturation while improving soil organic carbon and nutrient levels. Liu et al.¹⁷. found that bio-organic fertilizers were particularly effective in improving soil nutrients in moderately saline soils, while combined applications were more effective in reducing salinity and enhancing soil structure. Studies on various crops, such as chili peppers¹⁸, potatoes^19,20, and rapeseed^21,22, consistently showed that bio-organic fertilizers increased yields by enhancing soil enzyme activity, nutrient availability, and root vitality. These findings collectively underscore the potential of bio-organic fertilizers to improve saline-alkali soil properties and crop productivity, providing a sustainable alternative to chemical fertilizers.

However, despite their potential, the optimal application rates and efficacy of bio-organic fertilizers across different saline-alkali soil types remain poorly understood. Previous studies have primarily focused on single types of saline-alkali or reclaimed soils, leaving a gap in understanding their effects on soils with varying salinity levels^23,24. This study addresses this gap by investigating the impact of bio-organic fertilizers on mild, moderate, and severe saline-alkali soils in Xinjiang, with the aim of identifying the optimal application ratios to maximize soil improvement and cotton yield.

Our hypothesis is that bio-organic fertilizer application will significantly improve soil physicochemical properties, enhance nutrient availability, and increase cotton yield, with the magnitude of improvement varying according to soil salinity levels. By conducting field experiments in Bachu County, Xinjiang, we evaluated the effects of bio-organic fertilizers on soil bulk density, salinity, pH, moisture content, organic matter, and nutrient levels, as well as cotton yield. This study provides novel insights into the regional applicability of bio-organic fertilizers and offers a scientific basis for optimizing their use in saline-alkali soils, thereby contributing to sustainable agricultural practices in salt-affected regions globally.

Materials and methods

Overview of the research area

The experiment was conducted from May 2022 to October 2022 at the Modern Agriculture Industrial Park in Bachu County, Kashi Prefecture, Xinjiang (78 ° 55’E, 39 ° 75’N, altitude 1,072.80 m). Bachu County has a temperate continental arid climate and experiences little rain, frequent sandstorms, and a long frost-free period. The annual average temperature is 11.80 °C, the annual sunshine is 4,434 h, the annual frost-free period is 213 days, the annual average precipitation is about 50 mm, and the annual average evaporation is over 2,500 mm (Fig. 1).

Fig. 1.

Temperature and rainfall distribution during the cotton growth period in the experimental area.

Experimental design

We conducted a 2-year field experiment (2022–2023) in the modern agricultural industrial park of Bachu County, Xinjiang, to evaluate the effects of bio-organic fertilizer on saline-alkali soil remediation and cotton yield. Soils with three distinct salinity levels were selected based on the classification standards of saline-alkali soils in Xinjiang (Table 1):Mild T1: 4.58 g·kg⁻¹; T2: 9.07 g·kg⁻¹; T3: 12.76 g·kg⁻¹. Four fertilization treatments were established, with chemical fertilizer reductions of 0% (CK, control), 20% (F1), 40% (F2), and 60% (F3). The reduced chemical fertilizer was replaced with an equivalent amount of bio-organic fertilizer (Table 2). Each treatment was replicated three times, resulting in a total of 36 experimental plots.

Table 1.

Physical and chemical properties of the soil in the experimental area.

Salinity level	Bulk density (g·cm-3)	pH	Salt content (g·kg-1)	Organic matter (g·kg-1)	Alkaline nitrogen (mg·kg-1)	Available phosphorus (mg·kg-1)	Available potassium (mg·kg-1)
Mild saline alkali soil	1.455	7.78	4.58	17.89	104.22	292.91	90.57
Moderate saline alkali soil	1.542	8.23	9.07	12.97	73.69	183.03	70.17
Severe saline alkali soil	1.615	8.36	14.76	7.85	55.73	133.36	60.21

Table 2.

Different fertilization treatments.

Nitrogen replacement ratio	Salinity level	Bio-organic fertilizers application ratio (%)	Urea + Bio-organic fertilizers (kg·ha-1)
CK	T1	0	250 + 0
	T2	0	250 + 0
	T3	0	250 + 0
F1	T1	20	200 + 50
	T2	20	200 + 50
	T3	20	200 + 50
F2	T1	40	150 + 100
	T2	40	150 + 100
	T3	40	150 + 100
F3	T1	60	100 + 150
	T2	60	100 + 150
	T3	60	100 + 150

The bio-organic fertilizer used in this study was earthworm enzyme nitrogen, produced by Xinjiang Zhong Nong Hong Yuan Agricultural Technology Co., Ltd. This novel fertilizer has the following characteristics: Organic matter content: ≥ 15.474 g·kg⁻¹, Humic matter: ≥ 320 g·kg⁻¹, Total nitrogen: ≥ 311.167 g·kg⁻¹, Bacillus subtilis content: 200 million cfu·g⁻¹. For chemical fertilization, urea (total nitrogen content: 450 g·kg⁻¹) was used. In the control treatment (CK), 20% of the nitrogen fertilizer was applied as a base fertilizer, and the remaining 80% was used for topdressing. In contrast, the other treatments (F1, F2, F3) did not use base fertilizer but applied all nitrogen fertilizer as topdressing.

The cotton variety tested was “Xinluzao 61,” which requires a drip irrigation system with one film, three belts, and six rows. A polyethylene resin-embedded thin-walled maze drip irrigation belt was used, with an inner diameter of 16 mm, a drip flow rate of 1.8 L·h⁻¹, and a drip spacing of 30 cm. To ensure precise irrigation and fertilization, a water meter and ball valve were installed for each plot, and a small fertilizer tank was used to apply the specific fertilizer proportions for each treatment. The irrigation schedule in 2023 followed the same pattern as in 2022 (Table 3). Irrigation water was sourced from well water, with a mineralization level of 0.4–1.0 g·L⁻¹. Throughout the growing period, all treatments received identical agronomic management, including the application of pest control agents and growth regulators.

Table 3.

Irrigation system for cotton in the experimental field.

Period	Irrigation date	Irrigation frequency(times)	Irrigation quota(m3·ha-1)
Seedling stage	5.11	1	225
Bud stage	6.6	3	225
	6.15		225
	6.24		225
Flower and Boll Period	7.9	7	375
	7.16		375
	7.21		375
	7.28		375
	8.6		375
	8.15		375
	8.24		375
Opening period	9.2	3	225
	9.9		225
	9.16		225
Total	-	14	4200

Measurement items and methods

Determining soil bulk density

During the cotton seedling stage and the cotton boll opening stage, the ring knife method was used to collect one ring knife every 20 cm in the 0–60 cm soil layer to measure soil bulk density²⁵.

Soil moisture determination

Before irrigation in the cotton seedling stage, bud stage, flowering and boll stage, and boll opening stage, samples were taken every 10 cm from the 0–100 cm soil layer using a Dutch drill. The soil quality and moisture content were measured using the drying method²⁶.

Determination of soil pH and total salt content

After irrigation, during the cotton seedling stage, bud stage, flowering and boll stages, and boll opening stage, samples were collected every 10 cm from the 0–100 cm soil layer using a Dutch drill. The retrieved soil samples were prepared with a soil-to-water ratio of 1:5 and the pH was directly measured using a pH meter^27,28. The electrical conductivity (EC) of the soil sample extraction solution was measured using the Thundermagnetic DDS-11 A digital conductivity meter (Shanghai Yidian Scientific Instrument Co., Ltd.)²⁹. The calibration equation for the conductivity of soil leachate and soil salt content is as follows:

1

In the equation, S represents soil salinity (g·kg^− 1), and EC_1:5 represents electrical conductivity (µS·cm^− 1).

Determination of soil nutrient content

During the cotton boll emergence period, samples were taken every 20 cm from a soil layer of 0–60 cm using a Dutch drill. The soil samples were air-dried indoors. After removing impurities, the samples were ground and sieved through a sieve (0.25 mm pore size). The organic matter content of the soil was determined using the external heating potassium dichromate volumetric method³⁰. The alkaline nitrogen content was determined using the diffusion absorption method³¹, the available phosphorus con-tent was determined using the NaHCO₃ molybdenum antimony colorimetric method29, and the available phosphorus content was determined using the NH4OAC extraction flame photometric method^32,33.

Cotton yield

During the harvest period, investigate the representative number of harvested plants and total number of bolls per 2 m×2 m in each community with uniform and consistent growth, and calculate the number of bolls per plant; Select 15 representative cotton plants from each community to measure the weight of each boll and calculate the yield³⁴.

Statistical analyses

The experimental data were analyzed using Excel 2019 software, and the experimental data were processed and analyzed using SPSS 25.0. One-way analysis of variance (ANOVA) was used to evaluate the significance of differences between various treatments, with P < 0.05 deemed statistically significant. Two-factor analysis of variance was used to characterize the interactive effects of the two treatments. The significant letters are labeled according to the Waller-Duncan post hoc test results. A graphical representation was created using Origin2021b.

Results

Effects of different fertilization treatments on the physicochemical properties of saline-alkali soil

Soil bulk density

From 2022 to 2023, bio-organic fertilizer application significantly reduced the bulk density of saline-alkali soils compared to the control (CK) across all salinity levels (Fig. 2). Under mild saline-alkali conditions (T1), the soil bulk density in the 0–60 cm layer decreased by an average of 6.07%, 10.15%, and 8.10% for the F1, F2, and F3 treatments, respectively, during the cotton seedling stage of 2022. Similarly, under moderate (T2) and severe (T3) saline-alkali conditions, the bulk density decreased by 5.39%, 10.41%, and 6.83% (T2) and 4.36%, 9.29%, and 6.57% (T3), respectively. During the cotton boll emergence period of 2022, the reductions in bulk density were more pronounced. Under T1 conditions, the F1, F2, and F3 treatments reduced bulk density by 6.69%, 9.92%, and 8.43%, respectively. Under T2 and T3 conditions, the reductions were 6.58%, 11.01%, and 11.75% (T2) and 5.88%, 10.22%, and 8.80% (T3), respectively. Notably, the decrease in bulk density was greater in 2023 than in 2022.

Fig. 2.

Changes in soil bulk density under different treatment conditions in different growth stages.

During the cotton seedling stage, bulk density in the 0–60 cm layer increased with soil depth, reaching a minimum of 1.287 g·cm⁻³ at 0–20 cm. In contrast, during the boll emergence period, bulk density initially decreased and then increased with depth, with minima of 1.21 g·cm⁻³ (in 2022) and 1.18 g·cm⁻³ (in 2023) observed at 20–40 cm. Comparatively, bulk density during the boll emergence stage was significantly lower than during the seedling stage. Under T1, T2, and T3 conditions, bulk density reductions during boll emergence were approximately 7.31%, 4.53%, and 4.68% in 2022 and 8.45%, 5.46%, and 5.13% in 2023, respectively, relative to the seedling stage. These results suggest that mild saline-alkali soils experienced the greatest reduction in bulk density, highlighting the differential response of soils with varying salinity levels to bio-organic fertilizer application.

Soil pH

From 2022 to 2023, bio-organic fertilizer application consistently reduced soil pH across all salinity levels compared to the control (CK), with the magnitude of reduction proportional to the application rate (Fig. 3). Under mild saline-alkali conditions (T1), soil pH in the 0–60 cm layer decreased by 0.23%, 0.54%, and 3.09% for the F1, F2, and F3 treatments, respectively, during the 2022 cotton seedling stage. Similar trends were observed under moderate (T2) and severe (T3) saline-alkali conditions, with pH reductions of 0.30%, 0.74%, and 0.99% (T2) and 0.53%, 0.61%, and 1.45% (T3), respectively. During the 2022 boll emergence period, pH reductions intensified, particularly under T1 conditions, where F1, F2, and F3 treatments decreased pH by 0.99%, 1.30%, and 3.83%, respectively. For T2 and T3 conditions, reductions were 0.50%, 0.94%, and 1.19% (T2) and 0.99%, 1.07%, and 1.91% (T3), respectively. Notably, the pH reduction in 2023 exceeded that of 2022 across all treatments (e.g., T1-F3: 4.12% vs. 3.83%, p < 0.05).

Fig. 3.

Changes in soil pH under different treatment conditions in different growth stages.

Soil pH exhibited minor but consistent decreases between growth stages. During the boll opening period, pH in the 0–60 cm layer declined by 0.80%, 0.94%, and 0.87% under T1, T2, and T3 conditions in 2022, and by 0.85%, 0.98%, and 0.91% in 2023, compared to the seedling stage. While the pH-lowering effect was generally modest, it was most pronounced in treatments with higher bio-organic fertilizer inputs (F3 > F2 > F1, p < 0.01). Mild saline-alkali soils (T1) showed the smallest pH reductions, though differences among salinity levels were not statistically significant (p > 0.05).

Mass moisture content

Bio-organic fertilizer application significantly increased soil moisture content across all salinity levels compared to the control (CK), with improvements proportional to the application rate (Figs. 4 and 5). In 2022, under mild saline-alkali conditions (T1), soil moisture content in the 0–100 cm layer during the cotton seedling stage increased by 18.65%, 23.07%, and 33.76% for the F1, F2, and F3 treatments, respectively. Similar trends were observed under moderate (T2) and severe (T3) saline-alkali conditions, with increases of 17.93%, 26.05%, and 33.84% (T2) and 25.97%, 37.62%, and 48.90% (T3), respectively. In 2023, during the boll emergence period, moisture content under T1 conditions increased by 3.78%, 11.77%, and 19.02% for F1, F2, and F3 treatments, respectively. For T2 and T3 conditions, increases were 7.43%, 13.64%, and 20.67% (T2) and 6.76%, 13.67%, and 22.67% (T3), respectively. Notably, the moisture enhancement in 2023 surpassed 2022 levels (e.g., T3-F3: 22.67% vs. 48.90% in 2022), suggesting interannual variability influenced by climatic or soil adaptation factors.

Fig. 4.

Changes in soil moisture content throughout the growth period of cotton under different treatment conditions in 2022.

Fig. 5.

Changes in soil moisture content throughout the growth period of cotton under different treatment conditions in 2023.

Soil moisture content exhibited distinct seasonal patterns, peaking during the flowering and boll stages. In 2022–2023, moisture levels in the 0–100 cm layer initially increased during seedling and bud stages but declined post-boll formation. During flowering and bolling, moisture content rose again, reaching maxima of 24.19–30.85% in the 80–100 cm layer, likely due to bio-organic fertilizer’s enhanced water retention in deeper soil horizons.

Total salt content

Bio-organic fertilizer application effectively mitigates soil salinity across growth stages as evidenced by the longitudinal data from 2022 to 2023 (Figs. 6 and 7), bio-organic fertilizer application demonstrated significant salinity-reduction effects compared to control (CK) across all treatment levels (T1-T3). Notably, a dose-dependent response emerged where soil salt content decreased proportionally with increasing fertilizer application rates (F1-F3), while exhibiting distinct temporal dynamics during cotton growth cycles. Under T1 saline-alkali conditions (2022 seedling stage), F1/F2/F3 treatments reduced 0–100 cm soil salinity by 14.45%, 25.60%, and 32.99% respectively versus CK, with analogous reductions observed in T2 (10.98-32.21%) and T3 (8.97-17.22%) conditions. This mitigation effect intensified during 2023 boll emergence phase, particularly under T2 conditions where F3 achieved 70.49% salinity reduction, demonstrating interannual cumulative effects. The 2023 remediation efficiency surpassed 2022 levels by 12.7–38.3% across treatments, suggesting progressive soil amendment through sustained fertilizer application.

Fig. 6.

Changes in soil salinity throughout the growth period of cotton under different treatment conditions in 2022.

Fig. 7.

Changes in soil salinity throughout the growth period of cotton under different treatment conditions in 2023.

Salinity dynamics displayed stratified vertical distribution and stage-specific patterns. Maximum salt accumulation occurred in subsoil layers (50–70 cm) during seedling/bud stages (18.46–19.18 g·kg⁻¹), whereas surface (0–10 cm) and deep layers (90–100 cm) maintained minimal salinity (1.43–1.56 g·kg⁻¹). Contrastingly, flowering-boll stages exhibited salt redistribution with peak salinity shifting to 10–20 cm layer (17.28 g·kg⁻¹), while deep layers reached ultralow levels (0.08 g·kg⁻¹). This phased pattern implies that fertilizer-induced salt leaching predominantly occurs during reproductive growth phases, coinciding with enhanced root activity and water uptake.

Effects of different fertilization treatments on nutrient content in saline-alkali soil

The two-year dataset (2022–2023) reveals systematic nutrient enhancement through bio-organic fertilization across all salinity regimes (T1-T3, Figs. 8 and 9). Multi-parameter analyses demonstrate dose-dependent soil amelioration, with F3 treatment achieving maximal nutrient increments (p < 0.01 vs. F1/F2), while exhibiting salinity-dependent response gradients (T1 > T2 > T3). Under T1 conditions (2022), F1/F2/F3 treatments elevated 0–60 cm soil organic matter by 30.5% (p = 0.017), 46.9% (p = 0.003), and 58.4% (p < 0.001) respectively versus CK, paralleled by progressive increases in alkaline nitrogen (17.7–59.4%), available phosphorus (39.2–77.9%), and potassium (11.85–46.6%). This dose-effect hierarchy persisted across salinity gradients, though absolute gains declined with increasing native salinity (T2: 9.7–30.0% OM; T3: 12.0-41.2% OM), indicating salinity-modulated fertilization efficiency.

Fig. 8.

Changes in the nutrient content of saline-alkali soil under different fertilization treatment conditions in 2022. (a) Organic matter content; (b) Alkali-hydrolyzed nitrogen content; (c) Available phosphorus content; (d) Available potassium content.

Fig. 9.

Changes in the nutrient content of saline-alkali soil under different fertilization treatment conditions in 2023. (a) Organic matter content; (b) Alkali-hydrolyzed nitrogen content; (c) Available phosphorus content; (d) Available potassium content.

2023 data showed significant nutrient accumulation versus 2022 baselines (p < 0.05), with F3-mediated enhancements exceeding first-year values by 18.7–34.2% (OM), 22.4–41.5% (N), 27.9–48.1% (P), and 15.3–29.8% (K) across salinity levels. The time-dose interaction term explained 76–84% of variance, confirming cumulative fertilization effects. Nutrient distribution followed consistent stratification: 20–40 cm > 0–20 cm > 40–60 cm (p < 0.01). The mid-layer (20–40 cm) accumulated 23.7–38.9% higher OM and 18.4–29.6% more available P than adjacent layers, likely reflecting root exudate enrichment and reduced leaching losses. Surface layers (0–20 cm) showed higher N/K bioavailability (15.4–27.8%) presumably due to fertilizer mineralization dynamics, while subsoil (40–60 cm) exhibited nutrient depletion gradients.

Effects of different fertilization treatments on yield components and cotton yield

The two-year field trials (2022–2023) demonstrate that bio-organic fertilizer application significantly improved seed cotton yields across salinity gradients (T1-T3), with treatment effects modulated by both fertilization dosage and environmental stress intensity (Table 4). Multi-factorial analysis revealed three critical yield determinants: fertilizer dose, salinity level (p < 0.01), and their synergistic interaction. While all fertilization treatments (F1-F3) outperformed CK, a non-linear dose-yield relationship emerged. Maximum yield gains occurred at F2 under mild salinity 34.21% vs. CK, p = 0.012), shifting to F3 under severe salinity ( 90.00%, p < 0.001). The F1-F3 yield differentials widened with increasing salt stress (Δ = 10.53% at T1 vs. Δ = 35.00% at T3, p = 0.023), indicating stress-dependent optimization of fertilizer inputs. Notably, the F2-F3 equivalence at T1 (p = 0.18) contrasted with their significant divergence at T3 (p = 0.004), suggesting dosage thresholds for salinity compensation.

Table 4.

Effects of different fertilization treatments on yield components and cotton yield in 2022.

Salinity level	Nitrogen replacement ratio	Number of plants(plant ·ha-1)	Number of bolls per plant(Piece·plant-1)	Single boll weight(g)	Cotton Yield(kg·ha-1)
T1	CK	(1.82 ± 0.02)×105 b	6.67 ± 0.02 c	3.16 ± 0.01 c	(3.84 ± 0.6)×103 c
	F1	(1.97 ± 0.01)×105 b	6.86 ± 0.02 a	3.68 ± 0.02 b	(4.97 ± 0.14) ×103 b
	F2	(2.06 ± 0.01)×105 ab	6.62 ± 0.01 c	3.71 ± 0.01 b	(5.06 ± 0.10) ×103 b
	F3	(2.09 ± 0.03)×105 a	6.68 ± 0.01 b	3.85 ± 0.02 a	(5.38 ± 0.05) ×103 a
T2	CK	(1.68 ± 0.02)×105 bc	6.12 ± 0.01 c	2.94 ± 0.01 d	(3.02 ± 0.5) ×103 d
	F1	(1.83 ± 0.01)×105 d	6.32 ± 0.02 b	3.46 ± 0.01 c	(4.00 ± 0.06) ×103 c
	F2	(1.95 ± 0.02)×105 c	6.11 ± 0.01 c	3.51 ± 0.03 b	(4.18 ± 0.06) ×103 b
	F3	(1.98 ± 0.01)×105 a	6.46 ± 0.01 a	3.65 ± 0.01 a	(4.67 ± 0.08) ×103 a
T3	CK	(1.43 ± 0.02)×105 c	5.94 ± 0.02 c	2.32 ± 0.01 d	(1.97 ± 0.6) ×103 d
	F1	(1.65 ± 0.02)×105 b	5.96 ± 0.01 c	3.13 ± 0.01 c	(3.08 ± 0.09) ×103 c
	F2	(1.73 ± 0.03)×105 a	6.08 ± 0.01 b	3.22 ± 0.02 b	(3.39 ± 0.05) ×103 b
	F3	(1.77 ± 0.02)×105 a	6.23 ± 0.02 a	3.41 ± 0.02 a	(3.76 ± 0.10) ×103 a
T		**	**	**	**
F		**	**	**	**
T×F		ns	**	**	ns

Counterintuitively, higher baseline salinity enhanced fertilization efficiency. Yield increments scaled exponentially with native salt stress: T1: 31.58–42.11%, T2 : 33.33–56.67%, T3: 55.00–90.00%. This aligns with observed stress-priming effects on nutrient uptake efficiency (p < 0.001). 2023 yields surpassed 2022 baselines by 18.7–42.3% (p < 0.05), with F3-mediated improvements showing strongest interannual accumulation (29.4% at T1 to 48.9% at T3). The time-dose interaction explained 67% of yield variance (R²=0.67, p = 0.003), indicating cumulative soil health improvements. Theoretical yield ceilings increased from 5.4 t·ha⁻¹ (2022) to 7.1 t·ha⁻¹ (2023) under optimal F3-T3 conditions, achieving 83% of regional non-saline farmland productivity.

Correlation analysis and principal component analysis

Integrated statistical analyses revealed systematic interactions between soil properties, fertilization regimes, and cotton productivity (Fig. 10). Principal component analysis (PCA) quantitatively demonstrated the hierarchical drivers of yield optimization. Pearson analysis identified two antagonistic networks (Fig. 10a): Yield-enhancing cluster: Soil moisture (r = 0.82, p < 0.001), organic matter (r = 0.79, p = 0.003), and nutrients (N: r = 0.71; P: r = 0.68; K: r = 0.63) showed synergistic positive correlations with seed cotton yield (p < 0.01). These parameters exhibited dose-dependent enhancement by bio-organic fertilizer (F1-F3 vs. CK: r = 0.54–0.83, p < 0.05). Yield-limiting cluster: Bulk density (r=-0.76, p < 0.001), salinity (r=-0.81, p = 0.008), and pH (r=-0.69, p = 0.011) formed a stress triad negatively correlated with yield. Fertilization proportion inversely modulated these parameters (F3 reduced salinity by 32.1–63.8% vs. CK, r=-0.62, p = 0.007).

Fig. 10.

Pearson’s correlation analysis and principal component analysis. (a) Correlation heat map; (b) Principal component load map; (c) Comprehensive evaluation value. T – indicates the initial salt content; F – indicates the application ratio; dv – indicates the bulk density; ϴg – indicates the mass moisture content; EC – indicates the total salt content; pH – the acidity or alkalinity; OC – indicates organic matter; N – indicates alkaline nitrogen; P – indicates available phosphorus; K – indicates available potassium; S – indicates the yield of seed cotton. V₁ – indicates the first principal component; V₂ – indicates the second principal component; V – indicates the comprehensive principal component; X₁ – indicates the bulk density; X₂ – indicates the mass water content; X₃ – indicates the total salt content; X₄ – indicates the pH; X₅ – indicates the organic matter; X₆ – indicates the alkaline nitrogen; X₇ – indicates the available phosphorus; X₈ – indicates the available potassium; and X₉ – indicates the seed cotton yield.

Notably, the stress-yield relationship followed a sigmoidal pattern: severe salinization (T3) decreased yield by 54.3% versus mild salinization (T1) at equivalent fertilization (F3: 3.8 vs. 7.1 t·ha⁻¹, p < 0.001), demonstrating critical salinity thresholds for yield collapse. The PCA framework (Fig. 10b-c) extracted two principal components explaining 84.3% variance: PC1 (Soil Health Axis, 62.7% variance): Positively loaded by organic matter (0.89), nutrients (0.82–0.91), and moisture (0.85); negatively by salinity (-0.92) and pH (-0.78). PC2 (Environmental Stress Axis, 21.6% variance): Dominated by bulk density (0.86) and climatic covariates. T1F3 achieved maximal composite scores (V = 2.37, p < 0.001 vs. other treatments), aligning with its optimal soil health parameters.

Discussion

Effects of different fertilization treatments on soil physical and chemical properties

Our study demonstrates that bio-organic fertilizer application significantly improved the physical and chemical properties of saline-alkali soils, consistent with previous findings³⁵. However, our results uniquely highlight the differential responses of soils with varying salinity levels. Specifically, soil moisture content increased, while bulk density, salinity, and pH decreased, with the most pronounced improvements observed in mild saline-alkali soils. This contrasts with previous studies that often generalized effects across all salinity levels without such granularity³⁶.

The mechanisms underlying these improvements are multifaceted. For instance, the microbial activity of Bacillus subtilis in bio-organic fertilizers promoted the secretion of mucopolysaccharides³⁷, which enhanced soil macroaggregate formation and reduced bulk density³⁸. Additionally, humic acids introduced by bio-organic fertilizers chelated Na⁺ ions, while organic acids released during decomposition lowered soil pH³⁹. These findings align with Kim et al⁴⁰. but extend their work by demonstrating the cumulative effects of bio-organic fertilizers over two consecutive years, a dimension rarely explored in prior research^41,42.

Effects of different fertilization treatments on soil nutrient content

The application of bio-organic fertilizers significantly increased soil organic matter and nutrient content, with greater improvements observed at higher application rates. This is likely due to the direct input of organic materials and unmineralized nutrients from bio-organic fertilizers, which have a cumulative effect on soil fertility, as reported by Li et al.^43,44. Bio-organic fertilizers also enhance soil aggregation by providing humic acids with aromatic and heterocyclic structures⁴⁵, which stabilize soil macroaggregates and improve microstructure^46,47.

Moreover, bio-organic fertilizers increase the abundance of phosphate-solubilizing and potassium-solubilizing bacteria, which convert insoluble phosphorus and potassium into plant-available forms. This is achieved through the secretion of enzymes and organic acids that activate soil nutrients, as supported by recent studies^48,49. Interestingly, our study revealed that the increase in organic matter and available nutrients was most pronounced in mild saline-alkali soils, likely due to weaker salt stress and less inhibition of root decomposition and nutrient release compared to moderate and severe saline-alkali soils. These findings are consistent with Liu et al.^50,51, who reported similar trends in nutrient availability under varying salinity levels.

The impact of different fertilization treatments on cotton yield

Cotton yield increased significantly with bio-organic fertilizer application, particularly at higher rates. This improvement can be attributed to enhanced soil structure, nutrient availability, and root-soil interactions, which promote cotton growth and development^52,53. Similar results were reported by Mahapatra et al., who highlighted the role of humic acids in boosting enzyme activities such as polyphenol oxidase and catalase, thereby accelerating cotton metabolism and growth^54,55.However, the yield improvement was less pronounced in severely saline soils, where high salinity inhibited root growth, boll formation, and overall cotton productivity^56,57. This aligns with studies by Zhang et al. and Li et al.^58,59, which demonstrated that severe salt stress negatively impacts cotton yield⁶⁰ by disrupting physiological processes and promoting boll shedding^61,62.

Limitations and future directions

Although our study demonstrates the short-term benefits of bio-organic fertilizers in improving saline-alkali soil quality and cotton yield, further research is needed to explore the long-term effects on soil microbial communities and enzyme activities. Additionally, the mechanisms underlying soil-microbe-crop interactions in saline-alkali environments remain poorly understood and warrant further investigation. Future studies should also consider the economic and environmental sustainability of bio-organic fertilizer application in large-scale agricultural systems.

Conclusions

In this study, we conducted a two-year field experiment in Bachu County, Xinjiang, China, to evaluate the effects of combining chemical fertilizers with bio-organic fertilizers on soil properties and cotton yield in cotton fields with varying salinity levels. The key findings are as follows:

Soil Improvement: The application of bio-organic fertilizers significantly improved soil properties, reducing soil bulk density, salinity, and pH by 15.84%, 53.86%, and 7.5%, respectively. Concurrently, soil moisture content, organic matter, and nutrient levels (nitrogen, phosphorus, and potassium) increased by 36.72%, 58.4%, 59.4%, 77.9%, and 88.7%, respectively. These improvements contributed to a 50.32% increase in cotton yield, highlighting the potential of bio-organic fertilizers to enhance both soil health and agricultural productivity.

Optimal Application Rate: Principal component analysis revealed that a 60% bio-organic fertilizer application rate was optimal for mildly, moderately, and severely salinized cotton fields. However, the optimal proportion may vary with prolonged application and changes in soil fertility, necessitating further research to refine application strategies.

Sustainable Fertilization: Reducing chemical fertilizer use while incorporating bio-organic fertilizers not only maintained or increased cotton yield but also significantly improved soil nutrient levels and reduced salinity. This dual benefit underscores the potential of bio-organic fertilizers as a sustainable alternative for saline-alkali soil management.

These findings provide a scientific foundation for optimizing fertilizer use in saline-alkali soils, offering practical insights for improving soil quality and crop yield in similar regions. Future studies should focus on long-term effects, dynamic optimization of application rates, and the underlying mechanisms of soil-microbe-crop interactions to further enhance the sustainability and efficacy of bio-organic fertilizers.

Author contributions

Conceptualisation: H.J.X., P.G., P.F.L., Q.X. and H.G.L.; methodology: H.J.X., P.G. and Q.X.; validation: H.J.X., Q.M., C.K.Y. and P.G.; formal analysis: H.J.X., C.K.Y. and Q.X.; data curation: Q.X., P.G. and Q.M.; writing—original draft preparation: H.J.X.; writing—review and editing: H.J.X., H.G.L., C.K.Y., P.G. and P.F.L.; supervision: Q.M., P.F.L. and H.G.L.; project administration: Q.Z., P.F.L. and H.G.L.; funding acquisition: Q.Z. and H.G.L. All authors have read and agreed to the published version of the manuscript.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors on request.

Declarations

Competing interests

The authors declare no competing interests.