Green waste biochar and plant growth-promoting bacteria enhance tomato growth under combined nutrient deficiency and salinity stress

Abstract

This study characterized a green-waste-derived biochar from date palms and ghaf trees and investigated its potential as a soil amendment with halotolerant Bacillus spp. to improve tomato seedling quality under dual stress of salinity and nutrient deficiency. Biochar was produced through pyrolysis at 450 °C and then characterized for yield, pH, electrical conductivity, proximate analysis, surface morphology, energy-dispersive X-ray spectroscopy, and heavy-metal content. Its effectiveness was tested both alone and in combination with a Bacillus sp. mix, using a completely randomized design with varying NPK fertilizer levels and saline irrigation. Tomato seedlings were evaluated 45 days after planting for various vegetative, morphological, physiological, and nutrient content indicators. Under normal conditions, applying biochar combined with a Bacillus mix at 0% NPK greatly enhanced all measured parameters, often exceeding values observed with 100% NPK fertilization. This approach was especially effective under saline irrigation, resulting in significant increases in morphological parameters (40–150%), physiological parameters (51–94%), and nutrient content (34–63%) compared to control plants that received 100% NPK. Additionally, this treatment resulted in a 42% decrease in sodium accumulation. Using the biochar with the Bacillus mix effectively replaces chemical fertilizers and enhances salinity tolerance, supporting sustainable farming through waste recycling and less dependence on fertilizers.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-32140-5.

Introduction

Climate change, rising populations, and the ongoing decline in soil fertility present significant challenges for the agricultural sector. Climate change is expected to exacerbate freshwater scarcity and increase the frequency of salinity issues in various regions worldwide, particularly in arid and hyper-arid areas such as the United Arab Emirates. Additionally, urbanization, along with the prevalence of intensive agricultural practices, has caused several environmental problems. First, managing and disposing of the increased woody waste has become a significant challenge¹. Second, mismanagement practices, such as the overapplication of chemical fertilizers, continue to reduce soil fertility and create environmental hazards^2,3. Finally, nutrient runoff from agricultural land into ponds poses a significant threat to aquatic ecosystems and water quality⁴.

With the growing interest in the circular economy and bioeconomy, it is essential to recycle all waste as sustainably and profitably as possible⁵. Converting green waste into beneficial biochar is a relevant strategy for achieving Sustainable Development Goal⁶. In fact, biochar, as a carbon-rich product, plays a significant role in climate change mitigation by sequestering carbon in the soil^7,8, making it a viable technology for reducing the effects of global warming. Biochar has emerged as an effective soil amendment to enhance soil health and crop productivity⁹ by improving nutrient retention, increasing water-holding capacity, and promoting microbial activity^10–12, while also reducing dependency on chemical fertilizers¹³. Moreover, biochar, often referred to as the “black gold of agriculture,” has garnered significant attention recently for its potential to mitigate the adverse effects of drought stress^14,15 and salt stress¹⁵. Indeed, the beneficial impact of biochar on soil fertility depends on several factors, including the forms of nutrients¹⁶, the duration of biochar presence in the soil¹³, the types of co-applied fertilizers^17,18, as well as the soil’s characteristics ¹⁹.

The use of bacterial inoculants represents an innovative approach to enhancing crop growth and productivity in a sustainable manner. Plant growth-promoting bacteria (PGPB) directly contribute to plant growth by increasing nutrient availability through processes such as nitrogen fixation and ammonium production, as well as solubilizing phosphorus and potassium. Additionally, PGPB produce phytohormones, including indole-3-acetic acid (IAA) and gibberellins, which regulate plant cell division, differentiation, and root and shoot elongation. Siderophore-producing PGPB also play a vital role in making iron more accessible to plants, thereby fostering rapid growth²⁰. Furthermore, PGPB support plants indirectly by producing antibiotics that combat pathogens, releasing hydrogen cyanide (HCN), and inducing systemic acquired resistance²¹. They also help produce enzymes that mitigate abiotic stresses, such as drought and salt stress²² There is growing interest in using PGPB inoculants as an alternative to chemical fertilizers and pesticides to promote plant growth and improve tolerance to abiotic stresses^22,23.

Considering the potential benefits of biochar and PGPB for enhancing plant growth, reducing dependence on chemical fertilizers, and alleviating salt stress, it is reasonable to hypothesize that their combined application will produce even greater positive effects. This combination could serve as a more effective alternative to traditional NPK fertilization, helping to reduce salt stress while improving tomato seedling quality. Previous studies have documented the individual and synergistic advantages of biochar and PGPB under both normal and stressful conditions. However, most of these studies have still relied on NPK supplementation and have mainly focused on single stress factors. There is a lack of comprehensive studies on the combined effects of these factors under dual stress conditions, specifically nutrient deficiency and saline irrigation, which closely mimic real-world challenges in arid and hyper-arid agriculture. This research is the first to address this gap by exploring the synergistic effects of co-applying two native Bacillus strains with biochar to develop a more sustainable method for tomato seedling production. By filling this knowledge gap, the study contributes to the development of innovative strategies to improve soil fertility, promote plant growth, and reduce the environmental impact of tomato farming.

This study aimed to achieve the following objectives: (i) characterize the potential of biochar derived from green waste as a soil amendment used with 50% NPK chemical fertilizer or a PGPB mix; (ii) explore the potential of biochar to replace NPK chemical fertilizers when combined with a PGPB mix; and (iii) assess its effectiveness in improving the morphological and physiological traits of tomato seedlings exposed to salt stress during the early vegetative stage.

Results

Biochar characterization

The proximate composition of biomass and its derived biochar, along with the pyrolysis yield, is presented in Table 1. The biochar yield, expressed as a percentage of the original biomass material, was approximately 45.74%. This indicates that nearly 50% of the mass was lost during the conversion of green waste into biochar. The study’s results showed that the pyrolysis process increased the pH level from 6.45 in the initial biomass to 9.05 in the produced biochar. Similarly, the electrical conductivity increased from 0.42 dS/cm^-1 to 0.63 dS/cm^-1 for the initial biomass and its derived biochar, respectively.

Table 1.

Proximate analysis of green waste residues and their derived biochar.

	Biomass	Biochar
Yield (%)	–	45.74 ± 0.98
pH	6.45 ± 0.05	9.05 ± 0.05
Electric conductivity (dS/cm−1)	0.42 ± 0.02	0.63 ± 0.01
Moisture (%)	5.63 ± 0.08	2.52 ± 0.92
Volatile matter (%)	16.31 ± 0.61	10.79 ± 1.0
Ash (%)	15.07 ± 0.92	27.94 ± 0.08
Organic matter (%)	84.93 ± 0.92	72.40 ± 0.07
Organic carbon (%)	47.18 ± 0.51	40.22 ± 0.04
Fixed carbon (%)	11.75 ± 1.26	61.27 ± 0.97

The mean ± standard deviation for three determinations.

The pyrolysis process resulted in substantial increases in ash and fixed carbon, with rises of 85.40% and 421%, respectively, compared to their levels in the original biomass. In contrast, the levels of moisture, volatile matter, and organic matter experienced significant decreases of 55.24%, 33.84%, and 14.75%, respectively, when compared to the initial biomass.

The scanning electron microscopy (SEM) images of the biochar produced are shown in Fig. 1. The images show that the surface morphology of the biochar exhibits various types of pores, including cylindrical and polygonal shapes, as well as several larger pores. These well-developed pores are arranged in a honeycomb-like pattern.

Fig. 1.

Scanning electron microscopy (SEM) images of green waste biochar illustrating its structural features at varying magnifications.

The energy dispersion X-ray spectroscopy (EDS) analysis of the collected green waste and its derived biochar samples revealed the following findings: compared to the initial biomass, the pyrolysis process led to an increase in the atomic percentages of several elements: carbon rose by 24.36%, nitrogen by 4.39%, phosphorus by 0.13%, potassium by 0.18%, sodium by 0.42%, magnesium by 0.32%, calcium by 1%, manganese by 0.12%, iron by 0.05%, copper by 0.02%, and bromine by 0.13%. However, the oxygen content dropped from 39.12% in the green waste to 15.93% in the biochar. Zinc was absent in the biochar. Additionally, the carbon-to-oxygen (C/O) ratio increased from 1.14 in the green waste to 4.34 in the produced biochar.

The analysis of heavy metal content in green waste and its derived biochar revealed that among thirteen metals examined, only Bi and In were detected at the same levels in both the green waste and the resulting biochar (Table S1). The results suggest that the pyrolysis process did not increase the concentrations of Bi and In (Table 2).

Table 2.

Atomic percentage of carbon (C), oxygen (O), and other nutrients in the green waste and its derived biochar as determined by EDS spectroscopy.

	C	O	C/O	N	P	K	Na	Mg	Ca	Mn	Fe	Cu	Zn
Green waste	44.72	39.12	1.14	7.35	0.03	0.14	0.09	0.15	0.43	0	0.04	0.11	0.03
Biochar	69.08	15.93	4.34	11.74	0.16	0.32	0.51	0.47	1.43	0.12	0.09	0.13	ND

N: Nitrogen, P: Phosphorus, K: Potassium, Na: Sodium, Mg: Magnesium, Ca: Calcium, Mn: Manganese, Fe: Iron, Cu: Copper, Zn: Zinc, Br: Bromine, ND: not detected.

Pot experiment

Vegetative indices and morphological parameters

The analysis of variance revealed that the various treatments and salinity levels, along with their interactions, had a significant influence on DB, LA, and NDVI, except for GA, which remained unaffected by these interactions (Table 3, Table S2). The results show that tomato plants treated with either the bacterial mix M or a combination of biochar and the bacterial mix grew significantly similarly to or even better than the control plants C-100%NPK. This was observed under both non-saline and saline conditions (Table 3). Using saline water for irrigation resulted in a significant decrease of 42% in DB, 38% in LA, 24% in GA, and 15% in NDVI compared to irrigation with non-saline water in control plants (C-100%NPK). However, applying the bacterial mix significantly increased DB by 63%, LA by 88%, GA by 29%, and NDVI by 49% compared to C-100%NPK under saline conditions. The results revealed that under saline conditions, plants treated with BM exhibited the highest increases in several of the measured parameters: DB (81%), LA (98%), GA (32%), and NDVI (64%) compared to C-100%NPK. Notably, the soil amendment with BM resulted in the highest growth of DB, LA, GA, and NDVI under saline conditions, achieving values that were either significantly similar to or higher than those observed in plants treated with C-100%NPK and irrigated with non-saline water.

Table 3.

Effects of the different treatments on digital biomass (DB), leaf area (LA), greenness average (GA), and the normalized difference vegetation index (NDVI) of tomato plants at 45 DAS.

Treatments	DB	LA	GA	NDVI
	(103 mm3/plant)	(mm2/plant)
C-100%NPK-N	397.69a ± 58.23	22,196.80c ± 2445.70	0.41a ± 0.04	0.67b ± 0.03
C-50%NPK-N	157.63d ± 49.20	10,903.52 g ± 786.91	0.26d ± 0.05	0.46e ± 0.02
M-50%NPK-N	390.11a ± 33.87	24,031.30bc ± 527.71	0.42a ± 0.01	0.66b ± 0.01
B-50%NPK-N	322.20b ± 5.13	17,252.96d ± 1118.59	0.35b ± 0.05	0.56d ± 0.30
BM-0%NPK-N	392.81a ± 10.28	24,924.57b ± 1070.27	0.44a ± 0.02	0.66bc ± 0.04
C-100%NPK-S	233.86c ± 23.41	13,743.94ef ± 1277.02	0.31bc ± 0.04	0.45e ± 0.04
C-50%NPK-S	81.10e ± 4.71	11,922.37 fg ± 563.87	0.19e ± 0.02	0.39f. ± 0.04
M-50%NPK-S	382.04a ± 17.07	25,807.63ab ± 554.43	0.40a ± 0.01	0.67b ± 0.02
B-50%NPK-S	268.74bc ± 60.19	14,862.60e ± 86.29	0.30 cd ± 0.02	0.61c ± 0.01
BM-0%NPK-S	423.20a ± 24.35	27,262.32a ± 1176.22	0.41a ± 0.02	0.74a ± 0.02
Treatments (TRT)	***	*	***	**
Salinity (S)	***	***	***	***
TRT*S	**	***	Ns	***

C: uninoculated plants, M: plants inoculated with a mix of Bacillus sp. Each value is the mean of three replicates. Different letters indicate significant differences according to the LSD test. Significance: ns = not significant; * significant at p < 0.05; ** significant at p < 0.01; *** significant at p < 0.001.

For their part, the PH, LN, RL, SD, SDW, and RDW of tomato plants measured at harvest were significantly and positively affected by the different treatments, as well as their interactions with salinity (Table 4, Table S3). Under non-stressed conditions, the soil amendment with BM induced the highest PH, LN, RL, SD, SDW, and RDW values, which were similar to those of plants in treatment C-100%NPK. Notably, this treatment was even more effective when the tomato plants were irrigated with saline water, resulting in substantial increases in PH (+ 42%), LN (+ 67%), RL (+ 93%), SD (+ 150%), SDW (+ 58%), and RDW (+ 40%) compared to control plants grown in non-saline conditions. Interestingly, at this level of treatment, the values for PH, LN, RL, SD, SDW, and RDW were statistically similar to those observed in treatment C-100%NPK under normal conditions.

Table 4.

Effects of the different treatments on plant height (PH), leaf number (LN), root length (RL), stem diameter (SD), shoot dry weight (SDW), and root dry weight (RDW) of tomato plants at 45 DAS.

Treatments	PH (cm/plant)	LN	RL (cm/plant)	SD (mm/plant)	SDW (g/pant)	RDW (g/plant)
C-100%NPK-N	28.50a ± 1.32	6.50ab ± 05	19.70ab ± 2.25	5.33a ± 0.29	0.89bc ± 0.01	0.34b ± 0.03
C-50%NPK-N	20.00d ± 0.50	4.00c ± 00	12.53 cd ± 1.00	2.50d ± 0.50	0.47f. ± 0.03	0.15e ± 0.01
M-50%NPK-N	27.00abc ± 3.00	6.00b ± 00	18.26b ± 2.42	5.40a ± 0.36	0.92ab ± 0.04	0.38a ± 0.02
B-50%NPK-N	25.00c ± 1.00	5.00c ± 00	14.60c ± 0.56	3.33c ± 0.58	0.72d ± 0.02	0.19d ± 0.01
BM-0%NPK-N	28.00ab ± 1.00	6.50ab ± 0.50	21.33a ± 2.22	5.67a ± 0.29	0.95a ± 0.03	0.38a ± 0.03
C-100%NPK-S	18.83d ± 0.76	4.00c ± 00	10.40de ± 0.96	2.33d ± 0,29	0.57e ± 0.05	0.24f. ± 0.00
C-50%NPK-S	8.50e ± 0.50	3.33e ± 0.57	7.75e ± 0.25	1.00e ± 0.00	0.35 g ± 0.01	0.11c ± 0.01
M-50%NPK-S	26.00bc ± 1.00	6.33ab ± 0.57	20.00ab ± 3.00	6.00a ± 0.50	0.86c ± 0.03	0.35ab ± 0.02
B-50%NPK-S	18.17d ± 0.76	5.00c ± 00	15.07c ± 0.40	4.17b ± 0.29	0.68d ± 0.02	0.25c ± 0.01
BM-0%NPK-S	26.67abc ± 2.08	6.67a ± 0.57	20.10ab ± 1.65	5.83a ± 0.76	0.90bc ± 0.01	0.34b ± 0.02
Treatments (TRT)	***	**	***	***	***	***
Salinity (S)	***	***	***	***	***	***
TRT x S	***	***	***	***	***	***

C: uninoculated plants, M: plants inoculated with a mix of Bacillus sp. Each value is the mean of three replicates. Different letters indicate significant differences according to the LSD test. Significance: ns = not significant; * significant at p < 0.05; ** significant at p < 0.01; *** significant at p < 0.001.

Physiological parameters

The results indicated that photosynthetic pigments, including Chl a, Chl b, and ChlTot concentrations, were significantly influenced by various treatments (p < 0.001), salinity (p < 0.001), and their interactions (p < 0.001) (Table 5, Table S5). In the control plants treated with C-100%NPK, irrigation with saline water caused significant reductions in the contents of Chl a, Chl b, and ChlTot, with decreases of 33%, 30%, and 31%, respectively. Soil amendment with M and BM led to significant increases in Chl a, with increases of 53% and 51%, respectively; in Chl b, with increases of 54% and 63%, respectively; and in ChlTot, with increases of 54% and 59%, respectively, compared to plants in C-100%NPK. The BM-treated plants exhibited the highest values under a salt regime, which were significantly similar to or exceeded those observed in control plants (C-100%NPK) under normal conditions.

Table 5.

Effects of the different treatments and salinity on chlorophyll pigments and leaf relative water content of tomato plants at 45 DAS.

Treatments	Chlorophyll a (mg/g FW)	Chlorophyll b (mg/g FW)	Chlorophyll Tot (mg/g FW)	Leaf RWC (%)
C-100%NPK-N	1.64a ± 0.13	2.86c ± 0.07	4.50d ± 0.06	83.71b ± 4.17
C-50%NPK-N	1.25b ± 0.03	1.95e ± 0.02	3.20g ± 0.53	66.56d ± 5.63
M-50%NPK-N	1.58a ± 0.03	3.06b ± 0.04	4.64c ± 0.02	88.03ab ± 1.18
B-50%NPK-N	1.62a ± 0.07	2.57d ± 0.05	4.20e ± 0.13	78.03c ± 1.27
BM-0%NPK-N	1.61a ± 0.00	2.93c ± 0.02	4.54d ± 0.01	89.52a ± 1.19
C-100%NPK-S	1.09c ± 0.02	2.01e ± 0.04	3.09h ± 0.02	43.23f. ± 2.69
C-50%NPK-S	0.60d ± 0.06	0.59e ± 0.05	1.201i ± 0.01	38.29f. ± 4.31
M-50%NPK-S	1.67a ± 0.04	3.10b ± 0.07	4.77b ± 0.03	77.95c ± 2.22
B-50%NPK-S	1.29b ± 0.13	2.01e ± 0.06	3.30f. ± 0.08	59.89e ± 3.45
BM-0%NPK-S	1.64a ± 0.03	3.26a ± 0.07	4.91a ± 0.04	83.85b ± 2.80
Treatments (TRT)	***	***	***	***
Salinity (S)	***	***	***	***
TRT x S	***	***	***	***

C: uninoculated plants, M: plants inoculated with a mix of Bacillus sp, B: plants treated with biochar., BM: plants treated with a combination of Bacillus sp and biochar. Each value is the mean of three replicates. Different letters indicate significant differences according to the LSD test. Significance: ns = not significant; * significant at p < 0.05; ** significant at p < 0.01; *** significant at p < 0.001.

The relative water content (RWC) of the leaves was significantly influenced by various treatments (p < 0.001), salinity levels (p < 0.001), and the interaction between these two factors (p < 0.001) (Table 5, Table S4). The RWC in leaves decreased significantly by 48% in response to salt stress compared to C-100%NPK irrigated with non-saline water (Table 5). Furthermore, the application of BM notably improved the leaf RWC by 94% compared to control plants exposed to salt stress, reaching a leaf RWC value of 84%, comparable to that of control plants irrigated with non-saline water.

Micro and macronutrient content of tomato shoots

The nutrient content of tomato shoots, specifically micro and macronutrients such as N, P, K, Ca, Mg, Na, Fe, and Zn, is summarized in Table 6. The analysis of variance revealed that the different treatments and salinity levels, along with their interactions, had a significant effect on the accumulation of all the nutrients studied. However, the treatments did not influence Mg, and K did not show any effect from the interaction of the two factors (Table 6; TableS5).

Table 6.

Effect of different treatments on nitrogen (N), phosphorus (P), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), copper (Cu), iron (Fe), and zinc (Zn) contents of tomato shoots.

Treatments	N (mg/g DM)	P (mg/g DM)	K (mg/g DM)	Ca (mg/g DM)	Mg (mg/g DM)	Na (mg/g DM)	Fe (µg/g DM)	Zn (µg/g DM)
C-100%NPK-N	5.03ab ± 0.17	4.52a ± 0.18	77.15ab ± 3.49	37.16b ± 1.76	43.23b ± 5.89	8.71bc ± 0.08	70.76 c ± 1.78	137.96b ± 10.36
C-50%NPK-N	2.33e ± 0.10	2.57d ± 0.15	51.10e ± 5.45	21.09e ± 0.60	28.37 g ± 4.86	8.31c ± 0.58	39.89 f. ± 1.41	88.81f. ± 5.31
M-50%NPK-N	4.92ab ± 0.10	4.38a ± 0.07	79.76ab ± 2.73	38.07b ± 0.81	40.44bcd ± 0.63	8.60bc ± 0.30	85.38 a ± 4.06	145.52ab ± 11.64
B-50%NPK-N	3.35d ± 0.20	3.22c ± 0.20	60.27 cd ± 7.01	25.89d ± 0.86	36.88cde ± 1.00	8.14 cd ± 0.37	61.67 d ± 0.65	114.62c ± 1.71
BM-0%NPK-N	5.21a ± 0.95	4.53a ± 0.13	81.74a ± 7.07	46.43a ± 0.13	41.20bc ± 0.19	7.14d ± 0.07	88.97 a ± 6.94	153.56a ± 6.31
C-100%NPK-S	3.41d ± 0.16	2.73d ± 0.40	54.04de ± 0.64	25.05d ± 1.46	35.53de ± 0.59	13.98a ± 0.69	48.95 e ± 0.54	100.36d ± 2.31
C-50%NPK-S	1.46 g ± 0.20	1.23e ± 0.15	35.30f. ± 2.25	16.14f. ± 0.71	22.51f. ± 1.39	13.13c ± 1.09	28.13 g ± 1.13	60.81e ± 8.25
M-50%NPK-S	4.31c ± 0.21	3.98b ± 0.13	63.58c ± 5.95	32.89c ± 1.29	43.11b ± 1.78	8.65bc ± 0.53	66.75 cd ± 3.28	120.20c ± 1.78
B-50%NPK-S	1.84f. ± 0.22	2.51d ± 0.19	47.81e ± 0.11	22.03e ± 3.16	31.87ef ± 4.86	9.59b ± 1.08	36.25 f. ± 0.53	109.90 cd ± 1.52
BM-0%NPK-S	4.80b ± 0.26	4.46a ± 0.25	72.87b ± 4.35	39.00b ± 0.88	53.07a ± 1.91	8.12 cd ± 0.39	78.35 b ± 7.99	140.89b ± 4.05
Treatments (TRT)	***	***	***	***	ns	***	***	***
Salinity (S)	***	***	***	***	***	***	***	***
TRT*S	***	***	ns	***	***	***	*	**

C: uninoculated plants, M: plants inoculated with a mix of Bacillus sp, B: plants treated with biochar., BM: plants treated with a combination of Bacillus sp and biochar. Each value is the mean of three replicates. Bars indicate the standard error of the mean. Different letters indicate significant differences according to the LSD test. Significance: ns = insignificant; * significant at p < 0.05; *** significant at p < 0.001.

Table 7.

Treatments used in the experiment, including combinations of mineral fertilization, biochar, bacterial inoculation, and salinity

T1: C-100%NPK-N	T6: C-100%NPK-S
T2: C-50%NPK-N	T7: C-50%NPK-S
T3: M-50%NPK-N	T8: M-50%NPK-S
T4: B-50%NPK-N	T9: B-50%NPK-S
T5: M-B-0%NPK-N	T10: M-B-0%NPK-S

C: uninoculated plants; M, plants inoculated with the mix of the two strains: E1 and E3; B: plants fertilized with biochar; N: plants grown under normal conditions; S: plants grown under saline conditions.

The results indicated that, under normal conditions, inoculated plants exhibited the highest nutrient uptake when treated with 50% NPK, particularly when used with biochar at 0% NPK. These treatments resulted in nutrient levels nearly equivalent to those of the control plant at 100% NPK. Conversely, irrigation with saline water significantly decreased the accumulation of N by 32.21%, P by 39.58%, K by 29.95%, Ca by 32.50%, Mg by 17.81%, Fe by 30.82%, and Zn by 27.25% compared to the control plants under normal conditions. Additionally, there was a significant increase of 60.51% in Na.

Interestingly, bacterial inoculation greatly enhanced the mineral content of the tomato plants’ shoots. When applied at 50% NPK, the increases were 26.39% for N, 45.63% for P, 17.65% for K, 31.32% for Ca, 21.33% for Mg, 36.36% for Fe, and 19.77% for Zn. The most significant increases were observed when applying bacterial inoculation with biochar at 0% NPK, recording increases of 40.76% for N, 63.30% for P, 34.85% for K, 55.71% for Ca, 49.37% for Mg, 60.06% for Fe, and 40.38% for Zn. These increases resulted in nutrient levels that were equal to or greater than those found in the control plants receiving 100% NPK with non-saline water. Both treatments, M-50% NPK and BM-0% NPK, effectively reduced the accumulation of Na in the shoots of tomato plants compared to the control group, which received 100% NPK and was irrigated with saline water.

Relationship between the different treatments and th measured parameters under normal and saline conditions

A principal components analysis (PCA) was performed to explore the relationships between various treatments and the measured parameters under both normal and salt conditions. The first two components (PC1 and PC2) accounted for 92.3% of the total variation (Fig. 2). Specifically, the first component, PC1, explained 87.9% of the variation and was closely associated with the morphological and physiological parameters. Additionally, most mineral nutrients showed a strong positive correlation with PC1. The results indicated that PC2, which accounted for only 4.4% of the total variation, was significantly affected by the sodium content in the tomato shoot plants.

Fig. 2.

Principal component analysis (PCA) biplot illustrating the relationships between treatments and measured parameters under normal and saline conditions. PC1 (87.9%) captures variation in morphological, physiological, and nutrient traits, while PC2 (4.4%) is mainly influenced by sodium accumulation—those involving biochar and a Bacillus mix cluster closely with improved plant performance indicators, especially under stress conditions. C: uninoculated plants, M: plants inoculated with a mix of Bacillus sp, B: plants treated with biochar., BM: plants treated with a combination of Bacillus sp and biochar DB: digital biomass, LA: leaf area, GA: greenness average, NDVI: the normalized difference vegetation index, PSRI: plant senescence reflectance index, LN: leaf number, PH: plant height, RL: root length, SD: stem diameter, SDW: shoot dry weight, RDW: root dry weight, Chla: chlorophyll a, Chlb: chlorophyll b, Chltot: total chlorophyll, RWC: leaf relative water content, Bac: biomass microbienne, N: nitrogen, P: phosphorus, K: potassium, Na: sodium, Mg: magnesium, Ca: calcium, Cu: copper, Fe: iron, Zn: zinc.

The PCA indicated that the ten different treatments were categorized into three groups based on the measured parameters. The first group, located in the top left quadrant, consists of two control treatments: one with a half-dose and one with a full-dose of NPK chemical fertilizer under saline conditions. content. The sodium content positively influenced this group in the tomato shoot plants, but showed a negative correlation with all the other studied parameters. The second group, situated in the bottom left quadrant, includes three treatments: B-50%NPK-S, B-50%NPK-N, and C-50%NPK-N. This group exhibits a negative correlation with shoot sodium content and the other studied parameters. In contrast, the third group comprises five treatments: C-100% NPK-N, M-50% NPK-N, M-50% NPK-S, BM-50% NPK-N, and BM-50% NPK-S. These treatments are positively correlated with various morphological parameters, including DB, LA, GA, NDVI, PSRI, LN, PH, RL, SD, SDW, RDW, and physiological parameters such as chlorophyll pigments and LRWC. However, sodium content remains negatively correlated within this group. Additionally, the treatments in this group are positively correlated with the soil bacterial population.

Discussion

Producing biochar from green waste for use as a soil amendment aligns with zero-waste principles within a circular economy framework¹⁶. However, the effectiveness of biochar as a soil amendment is significantly influenced by its characteristics, including its structure, chemical composition, and levels of contaminants^24,25. This study is one of the first to investigate the quality of biochar produced from a mixture of Date palm and Ghaf wastes. It also explores the potential of this biochar as a soil amendment to enhance the quality of tomato seedlings under conditions of combined nutrient deficiency and salt stress.

The characterization of green waste and its derived biochar revealed that pyrolysis induced a biochar yield of 45.74% (Table 2). This yield aligns with previous research on other woody wastes, such as Acacia nilotica bark²⁶ and date palm residues²⁵. The proximate characterization of green waste and its derived biochar (Table 2) indicates that the pyrolysis process significantly raises pH levels by reducing acidic functional groups while enhancing basic functional groups²⁷. Pyrolysis also increased electrical conductivity, supporting the findings of Choudhary et al.²⁸ and Rehali et al.²⁵. The observed increase is likely due to a higher ash content, which exceeds that of biochar produced solely from date palms^25,29–31. Additionally, the rise in fixed carbon content in biochar and the decrease in its volatile matter influence both its stability and aromaticity³². Specifically, the recorded high fixed carbon content of 61% indicates that biochar is stable and resistant to decomposition, making it a valuable long-term carbon sink in soils. This enhances soil organic matter and supports carbon sequestration, a crucial aspect of mitigating climate change.

The EDS spectroscopy analysis revealed a high concentration of carbon, along with essential macronutrients such as N, P, K, and Ca (Table 3). It also identified crucial micronutrients necessary for plant growth, including magnesium (Mg), iron (Fe), copper (Cu), and zinc (Zn). These findings underscore the potential of biochar to enhance soil nutrient content, thereby supporting both microbiological soil fertility¹¹ and crop productivity⁹. Notably, out of the thirteen analyzed heavy metals, only Bi and In were found in significantly low concentrations in green waste and the resulting biochar (Table S1). This confirms previous research indicating that plant-based biochars are safe for use as a soil amendment^24,25,33,34.

The visualization of biochar surface morphology using SEM revealed that pyrolysis leads to the formation of distinct pores with both cylindrical and globular features on the biochar surface (Fig. 1), resulting from the volatilization of organic compounds³⁵. The porous structure of the produced biochar provides an excellent habitat for soil microbes, including bacteria, fungi, and actinomycetes, ultimately enhancing soil health and plant performance³⁶. Moreover, biochar’s porosity enhances the soil’s ability to retain nutrients, absorb water, and maintain organic matter content. It also improves the physicochemical and biological properties of the soil, which collectively promote plant growth^36–38.

Based on the provided background, the produced biochar was evaluated as a soil amendment for tomato seedlings. This assessment was conducted in conjunction with NPK chemical fertilization and a mixture of PGP Bacillus strains under both normal and saline conditions. The results indicated that, under normal conditions, there was a significant improvement in all measured parameters when biochar was applied, compared to control plants receiving 50% NPK. This finding is consistent with previous studies demonstrating that biochar application can enhance the growth of various plants, including cowpea ³⁸, tomato ⁴⁴, cabbage ³⁹, and other crops ^49,50, 50. Previous reports suggest that co-applying biochar with chemical fertilizers may help reduce nutrient losses and improve nutrient use efficiency^39,40. In fact, biochar application effectively controls the release of nutrients from chemical fertilizers, potentially enhancing plant growth by 53%. However, biochar could not completely replace the loss of 50% NPK fertilizer (Tables 5 and 6). This result aligns with previous research on bread wheat, cotton, maize, and holy basil^40–42. According to An et al.¹³, the extent to which biochar can reduce NPK fertilizer usage depends on the duration of its application, as its effects tend to be limited in the initial years. Nonetheless, the economic benefits of using biochar to reduce the need for chemical fertilization and enhance fertilizer efficiency may become evident three years after its initial application¹³.

It is essential to highlight that using biochar with a PGP Bacillus mix at 0% NPK yielded the highest values for DB, LA, LN, PH, RL, and SD, similar to or even exceeding those recorded in control plants that received conventional fertilization (Tables 4 and 5). The observed highest growth is likely attributed to increased photosynthetic activity in plant leaves. Photosynthetic pigments play a crucial role in absorbing and converting light energy, which initiates the primary reactions involved in the photosynthetic processes of crops. It can be suggested that plants inoculated and fertilized with biochar may absorb more light energy, thereby enhancing photosynthesis. This effect is evident in the current study, which demonstrated higher values of GA, NDVI, and chlorophyll pigments (Table 4). Supporting the findings of Phares et al.⁴³, the increase in the chlorophyll content induced an increase in the accumulation of SDW and RDW.

Notably, earlier studies^43–45have documented the positive synergistic effect of PGP mixes and biochar when used in combination. However, this study is the first to demonstrate that the integration of biochar and Bacillus mix is a cost-effective strategy that can fully replace chemical NPK fertilizers without adversely affecting the vegetative growth of tomato seedlings. Previously, Tounsi-Hammami et al.^22,23 noted that PGPB mixes alone were insufficient to replace the conventional rate of chemical fertilizers. In contrast, this study showed that adding biochar to the Bacillus mix at a 0% NPK rate resulted in a significantly positive effect, likely due to the synergistic interaction that can match conventional NPK fertilization. The synergistic effect between biochar and the Bacillus mix primarily stems from the unique pore structure of biochar, which provides an ideal habitat for bacteria. Additionally, due to its physical properties, biochar can directly supply nutrients to meet the needs of both plants and PGP bacteria⁴⁶.

Interestingly, the same trend was observed when tomato seedlings were irrigated with saline water. In fact, the application of biochar and the Bacillus mix led to significant improvements in morphological parameters and vegetative indices compared to control plants receiving the full rate of NPK fertilization. These enhancements resulted in values that were closely similar to those recorded in control plants at 100% NPK fertilization under normal conditions. The beneficial effects of biochar and the PGP mix when applied together have been previously demonstrated to enhance crop growth and yield in both induced^47,48 and naturally saline soils⁴⁹. However, it is essential to note that most of these studies evaluated the co-application of biochar and PGPB under conventional NPK fertilization^42,43,45,49. In contrast, the present study was conducted without any NPK fertilization. This confirms that this treatment can effectively replace chemical fertilizers under normal conditions, as well as in saline conditions. The enhancement of photosynthetic activity and vegetative growth under saline conditions can be attributed to improved water and nutrient availability⁴⁸. This increased availability directly influences the water status of leaves and leads to a healthier photosynthetic system. In addition, the porous nature of biochar improves soil structure, particularly its aeration, while reducing bulk density⁵⁰, which allows for better root system extension. Similarly, the ability of the Bacillus strains to produce indole-3-acetic acid (IAA) promotes the growth of root cortical cells. This leads to an increase in the number of root tips and branches, enabling greater exploration of the soil and improved nutrient absorption²³. This study demonstrates that the combination of biochar and Bacillus sp. significantly enhances root length and biomass, indicating a healthier root system. This resulted in a significant increase in the availability and accumulation of essential nutrients, including N, P, K, Ca, Mg, Fe, and Zn. In contrast, the same treatment resulted in a significant decrease in sodium accumulation in the shoots of tomato seedlings. (Table 7). The detoxification of sodium ions (Na⁺) in the soil may be attributed to biochar, which facilitates mechanisms such as electrostatic attraction and cation retention^49,51,52. This process inhibits the movement of Na⁺ from saline soil into the tomato plants. Additionally, the used halotolerant Bacillus strains may trigger protective functions related to plant osmoregulation and antioxidant mechanisms, which help detoxify reactive oxygen species generated by sodium⁵³.

Biochar, while typically low in mineral nutrients, is rich in carbon, making it a valuable resource for soil microbial communities. It likely serves both as an energy source and a physical habitat for microorganisms⁵⁴. The presence of metabolically active labile carbon appears to play a crucial role in enhancing microbial biomass. By acting as a balanced carbon reservoir, biochar supports the growth and proliferation of microbes. Notably, treatments that combine biochar with a mix of Bacillus species have resulted in significantly higher soil microbial biomass under both normal and saline conditions (Figs. 2 and S1). This finding underscores a key indicator of soil health. The enhancement in microbial biomass can be attributed to the protective matrix provided by biochar, which extends the viability of soil microorganisms and shields them from environmental stressors.

It’s important to note that the benefits described are specific to the biochar produced and used in this study, as they depend on the conditions of its production and the origin of the feedstock. Different types of biochar may have varying effects on soil properties and microbial activity. As discussed by Ghorbani et al.⁵¹, not all biochar will provide the same advantages. While the results of this study are promising, several limitations warrant consideration. First, the experiments were conducted under controlled conditions in a growth chamber using pots, which may not fully represent field scenarios. To assess the scalability and real-world applicability of the findings, field trials under diverse climatic and soil conditions are essential. Second, measurements were limited to 45 days after sowing, capturing only the early vegetative stage of growth. The long-term effects on flowering, fruit yield, and overall crop productivity remain unexplored. Additionally, the biochar used was derived from a specific feedstock blend of Date palm and Ghaf trees; since biochar properties vary significantly with feedstock type and pyrolysis conditions, the generalizability of the results may be constrained. Although the PGP bacteria demonstrated short-term efficacy, their long-term persistence, colonization potential, and interactions with native soil microbiota were not investigated. Addressing these limitations in future studies will be critical to fully validate and extend the applicability of the current findings. This study revealed that a combined strategy using biochar and PGPB has a significant impact on nutrient uptake and plant respiration under saline stress conditions; however, the molecular or cellular mechanisms underlying these effects remain to be elucidated. Future studies should adopt approaches such as transcriptomics and metabolomics to better understand the signalling pathway and physiological responses triggered by biochar-microbe interactions.

Conclusion

This study demonstrates that using biochar from green waste combined with halotolerant Bacillus spp. effectively enhances tomato seedling growth in nutrient-poor and salty conditions. This method can replace chemical NPK fertilizers while maintaining tomato plants healthy and supporting nutrient absorption. It also encourages a circular bioeconomy by utilizing local biomass and reducing dependence on synthetic inputs. The findings highlight the potential of combining microbial biotechnology with organic amendments to develop climate-resilient agricultural systems in arid and hyper-arid regions. Future research should examine long-term effects, impacts on soil microbiota, and economic viability across various farming systems.

Methods

Biochar production

Green wastes from Date palm (Phoenix dactylifera) and Al Ghaf trees (Prosopis cineraria) were collected from Zayed University, Dubai Campus, United Arab Emirates. The collected materials were dried in the sunlight and then chopped into small pieces. Samples of date palm and Al Ghaf wastes were thoroughly mixed to create a homogenized mixture (w/w). The chopped mix was dried in an air-forced oven at 60 °C for 24 h to extract moisture. A known mass of raw material was placed in pre-weighed ceramic crucibles with lids and subjected to pyrolysis at a temperature of 450 °C for 2 h, according to Sarfaraz et al.⁵⁵. A pyrolysis heating rate of 10 °C min^–1 was employed. Finally, the biochar was left in the furnace to cool down to room temperature.

Biochar characterization

After pyrolysis, the crucibles containing biochar were removed and weighed to assess the biochar yield. Moisture content was analyzed by placing a known mass of biochar in an oven at 105 °C for 24 h. The loss on ignition method at 550 °C for 10 min was used to determine the volatile matter, and for 4 h to analyze the ash and carbon content⁵⁶. Biochar pH and electrical conductivity (EC) were measured in distilled water at a 1:20 ratio using pH and EC meters, respectively⁵⁷. Additionally, heavy metal concentrations were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES; Model no. 700 series, Agilent Technology, Santa Clara, CA, USA) following dry digestion. SEM coupled with Energy Dispersive X-ray Spectroscopy (SEM–EDS) was assessed with INCAx-act (Oxford Instruments)⁸.

Pot experiment

PGP strains and inoculant preparation

Two compatible native strains of Bacillus sp., E1 and E3, previously isolated from mangrove trees, were selected based on their ability to solubilize phosphate and potassium, as well as produce ammonia, siderophores, and IAA²³. First, the strains were grown individually in glass tubes containing liquid nutrient broth medium on a rotating shaker (150 rpm) for 48 h at 28°C, until they reached the late exponential phase, to achieve a final concentration of 10⁹ CFU mL^-1. Then, a bacterial consortium was prepared by combining equal volumes of the individual bacterial suspensions before application, resulting in a final concentration of 10⁹ CFU mL^-1.

Experimental setup

The tomato (Solanum lycopersicum; cv. Shourouq) was selected as the model crop for this experiment. A substrate composed of a 1:1 mixture of vermiculite and peat was used. Biochar was incorporated into the substrate at a rate of 2% (w/w), and the substrate was thoroughly mixed with biochar before filling the 500 ml pots. Pots without biochar served as controls. The tomato seeds were surface sterilized by immersing them in 70% ethanol for 1 min, followed by soaking in a 25% sodium hypochlorite solution for 1 min²³. They were then rinsed ten times with sterile distilled water and soaked for an additional 10 min in sterile distilled water to remove any remaining traces of disinfectant. The effectiveness of this disinfection was confirmed by incubating ten seeds on nutrient broth agar plates at 28 °C for 72 h and examining for any signs of bacterial contamination.

Two seeds were sown in each pot, and seven days after sowing (DAS), only one seedling per pot was retained. At this stage, inoculants were applied directly to the soil near the stem at a rate of 1 mL per seedling²². Uninoculated plants served as controls (C). Three levels of NPK synthetic fertilization were applied: 0%, 50% and 100% of the recommended dose. The full dose of NPK synthetic fertilization was achieved using the commercial fertilizer Macro Greenmix 20:20:20 (N:P:K) at a rate of 3 g/l, as recommended by the manufacturer. Half of the pots were irrigated with non-saline water, while the remaining pots were watered with saline water equivalent to 100 mM throughout the experiment. In total, 10 treatments were established in three replications, arranged in a completely randomized design (Table 7).

The experiment was conducted in a growth chamber at a day/night temperature of 25/22 °C, featuring a light/dark cycle of 16/8 h and an average light intensity of 250 µmol m^{−2 s−1}, measured with a handheld spectrometer (LICOR, USA, model LI-180). The pots were rotated nearly every day to minimize positional effects.

Plant and soil measurements

Morphological and vegetative indices

The effects of biochar, mineral fertilization, and PGPB inoculation under normal and saline conditions on the morphological and vegetative characteristics of tomato plants were evaluated at 45 DAS using the Phenospex Scan device (PlantEye F500, Phenospex, Heerlen, The Netherlands). Various traits were measured, including digital biomass (mm³/plant) (DB), projected leaf area (mm²/plant), average leaf greenness, and the normalized difference vegetation index (NDVI).

For each treatment, plant height (PH) was measured from the soil surface to the tip of the main stem. The number of leaves (LN) was counted for each plant. Then, seedlings were removed from the potting mix and carefully washed to measure stem diameter (SD) and root length (RL). Afterward, the plants were dried in an oven at 65 °C until a constant weight was achieved, allowing for the determination of their corresponding shoot dry weight (SDW) and root dry weight (RDW).

Physiological parameters

Leaf chlorophyll concentration was analyzed by estimating the content of chlorophyll a (Chla), chlorophyll b (Chlb), and carotenoids (Car) using spectrophotometry at 645 nm, 663 nm, and 470 nm, following the method of Torrecillas et al.⁵⁸.

The relative water content (RWC) was determined according to the method described by Turner⁵⁹. Leaf tissues were collected from the most expanded leaf of each treatment and immediately weighed to determine the fresh weight. The leaves were then immersed in distilled water at 10 °C in the dark for 24 h, after which the saturated weight (turgid weight) was measured. Following this, the leaves were dried in an oven at 80 °C for 48 h to obtain their dry weight. The RWC was calculated using the following equation:

Micro and macro-nutrient content

Dry shoots of tomato plants were grounded using a blade mill and then passed through a 40-mesh sieve. Plant tissue samples were utilized for mineral analysis. Total nitrogen (N) was determined by the Kjeldahl method⁶⁰. The phosphorus concentration was assessed using a colorimetric method at 840 nm⁶¹. Additionally, the concentrations of potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), copper (Cu), iron (Fe), and zinc (Zn) were analyzed through inductively coupled plasma optical emission spectrometry—ICP-OES (Model no. 700 series, Agilent Technology, Santa Clara, CA, USA)⁶².

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Soumaya Tounsi-Hammami: methodology, investigation, statistical analysis, writing-original draft preparation, review, and editing. Munawwar Ali Khan: Project administration, review, editing, and validation. Mahra Alqemzi: formal analysis, investigation. Salama Ali Almehairi: formal analysis, investigation. Aneesa Rasheed Anwar: formal analysis, investigation. All the authors have read, reviewed, edited, and agreed to publish this version of the manuscript.

Funding

This research was financially supported by the Zayed University Research Incentive Fund (RIF), Project No. 23075.

Data availability

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.