Effects of soil salinity on nodulation and growth of invasive and native Prosopis seedlings in arid deserts

doi:10.21203/rs.3.rs-4814708/v1

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Effects of soil salinity on nodulation and growth of invasive and native Prosopis seedlings in arid deserts

https://doi.org/10.21203/rs.3.rs-4814708/v1

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Soil salinity is a major environmental stressor that significantly affects nodule formation and the growth of both exotic and native plant species. This study investigates the effects of soil salinity and the canopy cover of Prosopis juliflora on the physico-chemical properties of soil and the nodulation and growth of two exotic (P. juliflora and P. pallida) and one native (P. cineraria) Prosopis species in the arid deserts of the UAE. The results reveal significant variations in soil nutrient content and physical properties under and away from the canopies in both salty and non-salty habitats. Soils under the P. juliflora canopy, particularly in non-salty habitats, exhibited higher levels of essential nutrients and improved physical conditions compared to soils away from the canopy. These favorable conditions led to enhanced nodule formation and biomass production. Prosopis juliflora and P. pallida showed superior growth and nodulation compared to P. cineraria, suggesting a better adaptation to the modified soil environment under the P. juliflora canopy. Despite the study finding that soil salinity negatively impacted soil microbial communities, nutrient availability, and plant growth, P. juliflora demonstrated high salinity tolerance. It maintained robust nodulation and growth, indicating its potential for invading and even rehabilitating degraded saline lands. These findings underscore the importance of canopy cover in mitigating salinity, enhancing soil fertility, and supporting plant growth. The study provides valuable insights into the invasive ability of different Prosopis species, and ecosystem management in arid regions.

Invasive Biology

Nodulation Capacity

Prosopis Species

Salinity Tolerance

Soil Fertility

The impact of soil environmental stresses on both native and exotic plant species is a topic of great importance in ecological research and agriculture. Environmental factors such as soil salinity, water availability, temperature extremes, and nutrient levels significantly influence the distribution of both native and exotic plants (Bui et al. 2013). These factors can limit the naturalization and invasion of exotic species by creating conditions that only highly adaptable or specifically tolerant plants can withstand (Zefferman et al. 2015). Therefore, the ability of exotic plants to invade new areas often depends on their capacity to cope with these environmental stresses, which also determines their impact on native plant communities (Pyšek et al. 2012).

Salinity affects almost all aspects of plant development, including germination, vegetative growth, and reproductive development. Soil salinity leads to ion toxicity, oxidative stress, nutrient (e.g., N, Ca, K, P, Fe, Zn) deficiency, and osmotic stress on plants, consequently reducing water uptake from the soil (Bano and Fatima 2009). Salinity may also disrupt the nutrient balance in the soil, impacting plant growth and overall ecosystem health. High salt levels in the soil present a significant obstacle to the growth and survival of indigenous and non-native plant species (Bui et al. 2013). While some native halophytes have evolved mechanisms to cope with high salinity, many native non-halophytes are adversely affected, leading to shifts in ecosystem composition and health (Al Hassan et al. 2016). Exotic species, particularly invasive ones, can exploit saline environments to outcompete native plants, further altering ecosystems. Studying how different plant species, both native and invasive, adapt to and manage soil salinity is crucial for understanding the mechanisms behind alien plant invasion in saline environments.

Prosopis L. is a genus of flowering plants in the family Fabaceae, Mimosoideae, comprising about 45 species of spiny trees and shrubs common in subtropical and tropical regions (Catalano et al. 2008). Prosopis juliflora (Sw.) D.C., native to Central America, was introduced to several deserts in tropical and subtropical regions, including the Arab Gulf, for greening landscapes and controlling sand and desertification (Hussain et al. 2021). In the UAE, P. juliflora has escaped forested zones and occupied both natural and managed habitats, including farms, leading to habitat degradation and land abandonment. This highly competitive species often displaces native vegetation (Tiwari, 1999; El-Keblawy 2002). Similarly, Prosopis pallida (Humb. & Bonpl. ex Willd.) Kunth, another fast-growth invasive species, has been found in the UAE. Few studies have assessed the impact of P. pallida in the Arab Gulf regions, but its nearly exclusive occurrence in high density indicates the plant's aggressive ability to displace native flora. On the other hand, Prosopis cineraria (L.) Druce, a slow-growing tree species native to Arabia and western Asia, supports the growth of native species. It is a significant cultural symbol in the region and is not considered a weed (Barnes and Archer 1999; Kaur et al. 2012; Sharifian et al. 2021). Field observations in the UAE indicate that both exotic species (P. Juliflora and P. pallida) could be present in both salty and non-salty habitats, while P. cineraria was only found in non-salty habitats.

Prosopis species, including P. juliflora, P. pallida, and P. chilensis, have demonstrated significant salinity tolerance, making them suitable for reclamation saline soils in several regions, such as Australia (Le Houerou 2003) and India (Stave et al. 2003; Tomar et al. 2003). Salinity tolerance enables Prosopis species to invade and thrive in saline environments (Bui et al. 2013; El-Keblawy et al. 2021; Hussain et al. 2021; Singh 2022). This salinity tolerance, particularly at the seedling recruitment stage, allows these species to exploit niche opportunities in saline-stressed environments (Bui et al. 2013). In arid hot regions, such as the UAE, P. juliflora and P. pallida have successfully spread beyond their introduced zones, occupying both natural and managed habitats, including farms, and often outcompeting native vegetation (Slate et al. 2020; Aljasmi et al. 2021). Its ability to germinate during rainy seasons, when soil salinity is typically lower, further facilitates its invasion and establishment in saline environments (El-Keblawy and Al-Rawai 2005; El-Keblawy and Abdelfatah 2013; Abdul Hameed et al. 2021; El-Keblawy et al. 2021; Aljasmi et al. 2021). This resilience to salinity, coupled with its aggressive growth and adaptability, underscores the importance of understanding the invasion dynamics of Prosopis species. This would help mitigate the impacts of the invasive Prosopis species on local ecosystems and prevent habitat degradation.

Legume plants have a unique ability to develop nodules on their roots, which accommodate nitrogen-fixing bacteria. However, the formation of these nodules is influenced mainly by the quality of the soil in which they are cultivated. Soil characteristics such as nutrient levels and salinity play a crucial role in nodule formation (Bordeleau and Prévost, 1994; Bruning and Rozema 2013). Research indicates that the response of legume plants to salt stress can vary across different species, soil properties, and developmental stages, affecting nodule formation (Zahran and Sprent 1986; Etesami and Adl 2020; Li et al. 2021). Other studies have shown that legume plants' growth and symbiotic interactions are adversely affected by high salinity, with shoot development being more sensitive than roots (Tejera et al. 2004; López et al. 2008). For instance, Ben Salah et al. (2009) investigation revealed that Medicago ciliaris lines with varying salinity tolerances exhibited significant growth inhibition and depression in N2 fixation under salt stress. Notably, it has been reported that leguminous nodulating bacteria are vital in nitrogen fixation and salinity tolerance of P. juliflora (Diouf et al. 2015; Fall et al. 2019). Assessing soil physical and chemical properties is essential to evaluate the growth and invasion ability of Prosopis species in salt-affected habitats. The study aims to analyze how soil salinity under the invasive P. juliflora impacts the physical and chemical properties of the soil and subsequently affects nodule formation and plant growth in the invasive P. juliflora and P. pallida compared to the native P. cineraria. A thorough understanding of how soil salinity affects soil properties (e.g., nutrient content, pH, EC) and how these changes influence the growth and nodulation of Prosopis species could identify key factors that enable exotic Prosopis species to invade and thrive in saline environments, highlighting their competitive advantages over native species. We further anticipate that alien Prosopis species (i.e., P. juliflora and P. pallida) are more efficient in establishing mutualistic interactions with rhizobia than the native P. cineraria. It is expected that the study's results will help understand how different levels of soil salinity affect the nodulation process in native and exotic Prosopis species.

Seed collection

The study utilized seeds from three Prosopis species: the native P. cineraria and the exotics P. juliflora and P. pallida. These species are widespread in non-salty soils across various regions of the UAE, with P. juliflora also observed in salty habitats. Seeds were collected from three non-saline habitats: P. cineraria seeds from a site near Sharjah City (25°22'13" N 55°41'43" E), P. juliflora seeds from a site near Fujairah City (25°14'30" N 56°21'22" E), and P. pallida seeds from a site near Sharjah Airport (25°20'28" N 55°31'19" E). Pods were collected from at least thirty trees in each population to capture genetic diversity within each population. The seeds were then extracted from the pods using a sharp knife and stored in brown paper bags.

Soil collection and analysis

Soil samples were collected from under the canopies of P. juliflora of the non-salty habitat near Fujairah City and the salty habitat near Kalba City on the northern coast of the UAE. Samples were collected from beneath a minimum of five Prosopis trees per studied population, targeting a depth of up to 20 cm. Large particles were removed, soil samples were pulverized, air-dried, mixed thoroughly to ensure homogeneity, and sifted through a 2-mm sieve.

The soil's pH and electrical conductivity (EC) were measured using a 1:2.5 soil-to-water suspension, as outlined by Dahnke & Whitney (1988). Measurements were conducted with a Thermo Scientific Orion Star A211 pH Bench-top Meter after 24 hours of agitation. Soil texture was determined using the hydrometer method (Gee and Or 2002). The total organic matter content was calculated using loss-on-ignition at 550°C for two hours (Wilke 2005). Total nitrogen content was extracted using 2 M KCl and quantified via the micro-Kjeldahl method. Available phosphorus was determined using sodium bicarbonate from Olsen's solution as an extracting agent, and flame photometry was used to estimate Na, Ca, K, and Mg concentrations. These methods, detailed in Black (1965) ensured a comprehensive assessment of soil physicochemical properties.

Effects of Soil Salinity on Nodulation and Growth of Prosopis Species

This experiment tested the ability of two exotic and one native Prosopis seedlings from plants of non-salty soil to nodulate and grow in both salty and non-salty soils. Seeds from the three Prosopis species collected from non-salty habitats were grown in pots with soils collected from beneath P. juliflora of the salty and non-salty habitats.

After six months of storage, seeds of the three species received a 10-minute treatment with concentrated sulfuric acid. Subsequently, seeds were thoroughly washed with running tap water, rinsed in distilled water, and then soaked in distilled water for 24 hours for hydropriming. Following hydropriming, seed samples from each Prosopis species were placed in fine-mesh cloth packets and stored in zip-lock plastic bags along with damp paper towels until radical emergence. Most seedlings emerged within a week. Equal-sized seedlings from each Prosopis species were then transferred into free-draining plastic pots with saucers (10 cm in diameter) filled with salty or non-salty soils of P. juliflora. Five replicates, each with one seedling, were used for both soil types. The pots were arranged in a completely randomized block design (CRBD) in a growth chamber, with sixty pots in total: five replicates for under and away of the three species in both salty and non-salty habitats. Plants were watered daily without additional fertilizer. Growth parameters were evaluated, including root dry weight, shoot dry weight, root dry weight to shoot dry weight ratio, and root length.

The plants were harvested after 60 days. The soil was rinsed away from the roots with care, and each plant was then divided into root and shoot tissues. Nodules were gently extracted using forceps, counted, and weighed with a precise scale accurate to 0.01 mg. The shoots and roots were segregated from each plot, dried at 70°C for 72 hours till constant weight, and then weighed.

Statistical analysis

The impacts of P. juliflora habitat (salty and non-salty) and position from the canopy (under and away) on chemical and physical characteristics and growth and nodule formation attributes were assessed using two-way ANOVAs. Tukey's test evaluated differences between the means at P ≤ 0.05. The statistical software SYSTAT, version 13.0, was used for all studies.

1. Physico-chemical properties of soils

Table (1) shows the chemical and physical properties of soils collected from beneath and away from Prosopis juliflora canopies in both salty and non-salty habitats. The chemical properties exhibited significant variations, with noticeable trends observed in the concentrations of calcium (Ca), magnesium (Mg), and potassium (K). Under the canopies in the salty habitat, both Ca and Mg concentrations were higher compared to away from the canopies, showing decreases of 14.77% and 58.19%, respectively. Similarly, potassium (K) showed a reduction of 31.25% when moving away from the canopy. In the non-salty habitat, the trends were consistent, with Ca, Mg, and K concentrations decreasing by 39.25%, 61.46%, and 53.91%, respectively, when moving away from the canopy. Comparatively, Ca and Mg were higher under non-salty canopies than under salty ones, indicating increases of 81.82% and 38.98%, respectively. In contrast, K showed a 21.88% increase under non-salty canopies compared to salty ones. The variation in N and P concentrations showed similar patterns. Under the canopies in the salty habitat, N and P were higher, decreasing by 43.48% and 25.20%, respectively, when moving away from the canopies. In the non-salty habitat, these reductions were 52.7% for N and 30.48% for P. Comparing between habitats, N increased by 29% and 8% under and away from the canopies, respectively, in non-salty habitats, while P showed increases of 15.75% under canopies and 7.58% away from canopies in non-salty habitats (Table 1a).

Sodium (Na) and electrical conductivity (EC) exhibited distinct trends. Na and EC were slightly higher under the canopies in the salty habitat, decreasing by 12.13% and 19.43%, respectively, away from the canopies. In the non-salty habitat, both Na and EC decreased more dramatically, by 21.94% and 74.68%, respectively. When comparing salty to non-salty habitats, Na decreased by 37.88% under canopies and 44.79% away from canopies, while EC showed an even more substantial decrease, with 59.89% under canopies and 87.39% away from canopies (Table 1a).

The physical properties of the soil also demonstrated clear trends. Organic matter content was higher under the canopies, decreasing by 30.30% and 49.29% in salty and non-salty habitats, respectively, when moving away from the canopies. Additionally, the clay content decreased by 49.02% in salty habitats and 57.73% in non-salty habitats away from the canopies. Silt content followed a similar pattern, with significant decreases noted. Conversely, sand content increased away from the canopies in both habitats, reflecting the more sandy nature of soils further from the Prosopis juliflora canopies (Table 1b). The overall results indicate that the canopies generally enhance soil nutrient content and organic matter while reducing the sandy texture away from the canopy.

Table 1

Chemical and physical properties of soils collected from under and away from *Prosopis juliflora* canopies in salty and non-salty habitats. Values are means ± SE. Means with the same letter are not significantly different at P ≤ 0.05. (a) Chemical Soil Properties
	Under P. juliflora canopy		Away from P. juliflora canopy
Property	Salty	Non-salty	Salty	Non-salty
Ca (meq/l)	26.4 ± 5.2b	48.0 ± 10.9a	22.5 ± 3.1b	29.16 ± 5.21b
Mg (meq/l)	17.7 ± 2.1b	24.6 ± 2.9a	7.4 ± 2.0c	9.48 ± 2.11c
K (meq/l)	12.8 ± 1.2a	15.6 ± 1.9a	8.8 ± 1.2b	7.19 ± 0.85b
Na (meq/l)	80.0 ± 5.6a	49.7 ± 4.9c	70.3 ± 5.2b	38.81 ± 2.95d
N (mg/kg)	6.9 ± 1.3a	8.9 ± 1.4a	3.9 ± 1.0b	4.21 ± 1.11b
P (mg/kg)	12.7 ± 1.0ab	14.7 ± 1.8a	9.5 ± 1.1b	10.22 ± 2.13b
pH	7.50 ± 0.19b	7.56 ± 0.42a	8.10 ± 0.21a	7.82 ± 1.02a
EC (dS/m)	87.5 ± 10.3a	35.1 ± 8.2c	70.5 ± 5.3b	8.89 ± 2.32d

(b) Physical Soil Properties

Property	Under - salty	Under – non-salty	Away - salty	Away – non-salty
Organic matter (%)	3.3 ± 0.19a	4.2 ± 0.32a	2.3 ± 0.18b	2.13 ± 0.25b
Clay (%)	10.2 ± 1.61a	8.8 ± 2.1a	5.2 ± 1.61ab	3.72 ± 1.78b
Silt (%)	17.2 ± 3.9a	20.9 ± 4.4a	10.2 ± 2.5b	6.43 ± 1.21c
Sand (%)	72.6 ± 4.4b	70.3 ± 4.9b	86.4.6 ± 4.4a	89.89 ± 5.12a

2. Nodule formation

The two-way ANOVA results demonstrated significant effects of species, position, and their interaction on both the number of nodules and the nodule weight (P < 0.05, but not for soil salinity (P > 0.05, Table 2). Additionally, the interaction between species and position was significant (P < 0.001), suggesting that the influence of species on the number of nodules varies with position. No nodulation was observed in seedlings grown in soil collected away from the Prosopis species.

The results revealed significant differences in both the number and weight of nodules among the three species in salty and non-salty soils collected from under the canopy of P. juliflora. For P. juliflora, the average number of nodules was markedly high in both non-salty (46.25) and salty (46.83) soils, indicating a strong ability to form nodules under the canopy regardless of soil salinity. Similarly, the average weight of 100 nodules for P. juliflora was substantial in non-salty (0.20 g) and salty (0.22 g) soils. Prosopis pallida also demonstrated significant nodule formation under the canopy, with an average of 22.00 nodules in non-salty soil, 19.20 in salty soil, and a consistent average nodule weight of 0.11 g in both conditions. In contrast, P. cineraria showed fewer nodules under the canopy, with average of 5.75 in non-salty soil and 3.00 in salty soil. The weight of 100 nodules for P. cineraria was 0.11 g in non-salty soil and 0.12 g in salty soil. These findings highlighted the species-specific responses to soil salinity under the canopy, with P. juliflora and P. pallida exhibiting more robust nodule formation and higher nodule weights than P. cineraria Fig. 1).

3. Growth parameters

Shoot and root dry weight

There were significant effects for the Prosopis species, soil salinity, and position from P. juliflora crown on both root and shoot dry weight (P < 0.01, Table 3. In addition, the interaction between Prosopis species and position from P. juliflora canopy on the two variables was significant (P < 0.05). Furthermore, the interaction between Prosopis species and position from P. juliflora canopy on the root dry weight was significant (P < 0.05, Table 2).

The shoot dry weight varied significantly across Prosopis species, soil salinity, and their position relative to the P. juliflora canopy. On average, P. juliflora exhibited the highest shoot dry weight, especially in soil from under the canopy in non-salty conditions (0.37 g), while P. cineraria showed the lowest shoot dry weight in salty soils away from the canopy (0.08 g). Non-salty soils generally supported higher shoot dry weights for all species than salty soils. For example, P. pallida had an average shoot dry weight of 0.23 g in non-salty soils and 0.17 g in salty soils. The canopy effect was evident, as all species displayed higher shoot dry weights in soils from under the canopy than soils away from it, with P. juliflora showing the most significant increase (Fig. 2a).

Root dry weight varied significantly based on species, soil salinity, and canopy position. P. pallida had the highest root dry weight in soils from under the canopy in non-salty conditions (0.22 g) compared to other species. In contrast, P. cineraria had the lowest root dry weight in salty soils (0.10 g). Non-salty conditions favored higher root dry weights for all species. The canopy's effect was notable, as root dry weights were higher in soils from under the canopy for all species, particularly in non-salty soils, highlighting the importance of P. juliflora canopy in mitigating salinity stress and enhancing root growth (Fig. 2b).

Root length

The two-way ANOVA showed significant effects for the Prosopis species, soil salinity, and position from P. juliflora crown, and the interaction between Prosopis species and soil salinity on root length P < 0.05, Table 3). Prosopis juliflora exhibited the longest roots in both non-salty (17.00 cm) and salty soils (18.67 cm) away from the canopy. In soils from under the canopy, its root lengths were slightly shorter, measuring 16.88 cm in non-salty soil and 13.70 cm in salty soil. Prosopis pallida also showed longer roots in soils away from the canopy, with root lengths of 17.00 cm in non-salty soil and 15.00 cm in salty soil, compared to 13.40 cm and 12.34 cm, respectively, in soils under the canopy. In contrast, P. cineraria displayed a different pattern, with shorter root lengths overall and minimal differences between positions; root lengths were 15.88 cm in non-salty soil and 9.75 cm in salty soil away from the canopy, and 12.18 cm and 9.83 cm in soils from under the canopy, respectively.

Table 2

Two-way ANOVAs (F-values) for the effects of *Prosopis* species (three species) grown in two types of soils (highly saline and non-saline) from under the canopy of *P. juliflora* on the number and weight of nodules and average weight of 100 nodules of each of the three species.
Source	df	Mean Squares	F-Ratio	p-Value
Number of nodules
Species (Sp)	2	18.619	81.447	0.000
Position (P)	1	223.855	979.237	0.000
Soil salinity (SS)	1	0.292	1.279	0.265
Sp x P	2	23.063	100.888	0.000
Sp x SS	2	0.117	0.510	0.605
P x SS	1	0.292	1.279	0.265
Sp x P x SS	2	0.117	0.510	0.605
Error	48	0.229
Average nodule weight per seedling
Species (Sp)	2	0.008	104.152	0.000
Position (P)	1	0.020	245.277	0.000
Soil salinity(SS)	1	0.000	0.034	0.856
Sp x P	2	0.008	104.953	0.000
Sp x SS	2	0.000	0.627	0.540
P x SS	1	0.000	0.034	0.856
Sp x P x SS	2	0.000	0.627	0.540
Error	37	0.000
Weight of 100 Nodules
Species (Sp)	2	0.009	12.159	0.000
Position (P)	1	0.222	292.205	0.000
Soil salinity(SS)	1	0.000	0.208	0.651
Sp x P	2	0.014	18.624	0.000
Sp x SS	2	0.000	0.109	0.897
P x SS	1	0.000	0.208	0.651
Sp x P x SS	2	0.000	0.109	0.897
Error	37	0.001

Table 3

Two-way ANOVAs (F-values) for the effects of *Prosopis* species (three species) grown in two types of soils (highly saline and non-saline) from under the canopy of *P. juliflora* on growth parameters of each of the three species.
Source	df	Mean Squares	F-Ratio	p-Value
Shoot dry weight
Species (Sp)	2	0.044	91.446	0.000
Position (P)	1	0.177	368.927	0.000
Soil salinity(SS)	1	0.029	59.542	0.000
Sp x P	2	0.014	29.279	0.000
Sp x SS	2	0.001	2.758	0.076
P x SS	1	0.013	27.713	0.000
Sp x P x SS	2	0.001	2.209	0.124
Error	37	0.000
Roots dry weight
Species (Sp)	2	0.007	10.058	0.000
Position (P)	1	0.012	16.559	0.000
Soil salinity(SS)	1	0.007	9.341	0.004
Sp x P	2	0.005	6.467	0.004
Sp x SS	2	0.003	4.090	0.025
P x SS	1	0.000	0.313	0.579
Sp x P x SS	2	0.000	0.297	0.745
Error	37	0.001
Root length
Species (Sp)	2	80.061	21.974	0.000
Position (P)	1	74.433	20.430	0.000
Soil salinity(SS)	1	56.602	15.536	0.000
Sp x P	2	1.954	0.536	0.589
Sp x SS	2	12.282	3.371	0.045
P x SS	1	0.022	0.006	0.939
Sp x P x SS	2	17.780	4.880	0.013
Error	37	3.643
Shoot : root ratio
Species (Sp)	2	1.403	20.434	0.000
Position (P)	1	4.190	61.023	0.000
Soil salinity(SS)	1	0.226	3.293	0.078
Sp x P	2	0.834	12.148	0.000
Sp x SS	2	0.087	1.271	0.293
P x SS	1	0.201	2.930	0.095
Sp x P x SS	2	0.047	0.681	0.512
Error	37	0.069

The interaction between species and soil salinity significantly influenced root length. P. juliflora showed increased root length in salty conditions away from the canopy, while P. cineraria and P. pallida exhibited reduced root length in salty soil, with more pronounced decreases under the canopy. These results highlight the complex interplay between species, soil salinity, and canopy position. They indicate that P. juliflora has a better capacity to maintain longer roots under saline conditions, whereas P. cineraria and P. pallida are more negatively affected by soil salinity and canopy position (Fig. 2c).

Shoot-to-root ratio

The shoot/root ratio was also significantly affected by species, soil salinity, and canopy position. P. juliflora in soils under the canopy from non-salty conditions exhibited the highest shoot/root ratio (2.39), indicating a significant allocation of biomass to shoots. In contrast, P. cineraria in soils from away from the canopy in both salty and non-salty conditions had the lowest shoot/root ratios (0.86 and 0.81, respectively). The interaction between Prosopis species and canopy position highlighted that all species had higher shoot/root ratios under the canopy, particularly in non-salty conditions, suggesting that the canopy mitigates the adverse effects of salinity on biomass allocation (Fig. 2d).

This study examined the impact of soil salinity and the canopy cover of P. juliflora on the physico-chemical properties of soil and the resulting effects on nodule formation, shoot and root dry weight, root length, and shoot-to-root ratio among two exotic and one native Prosopis species. Our findings reveal significant variations in soil nutrient content and physical properties in soils under and away from the canopies in both salty and non-salty habitats, which in turn influence plant growth and development. Specifically, the soil under the P. juliflora canopy in non-salty habitats showed higher levels of essential nutrients and better physical conditions than soils away from the canopy, contributing to enhanced nodule formation and biomass production. Prosopis trees typically form "fertility islands" in dry environments, increasing soil nutrient levels and microbial functions under their canopies, thereby enhancing microbial activity and plant growth (Salazar et al. 2019; García-Sánchez et al. 2012). Vallejo et al. (2012) attributed soil's improved physical and chemical properties under P. juliflora to the buildup of organic matter, heightened biological activity, and better soil aggregation and porosity. The results also showed that P. juliflora and P. pallida have higher ability, compared to P. cineraria, to form more nodules and maintain higher shoot and root biomass in both salty and non-salty soils from under P. juliflora’s canopy. This suggests that these exotic species are better adapted to the modified soil environment under the P. juliflora canopy, indicating their higher invasive potential to take over lands previously dominated by P. juliflora.

Soil salinity negatively impacts soil microbial communities, including those involved in nitrogen fixation. The association between legumes and rhizobia is greatly affected by salinity, impacting nodule development and nitrogen fixation (Kirova and Kocheva, 2021; Chakraborty and Harris 2022). The higher sodium and salinity (i.e., higher EC) and lower concentrations of essential nutrients in salty than in non-salty soils from under the P. juliflora canopy (Table 1) could disrupt the formation of nodules and reduce plant growth. Li et al. (2021) demonstrated that high salinity significantly reduced bacterial diversity, altered community composition, and destabilized the bacterial network. Furthermore, high salinity can disrupt the availability and uptake of essential nutrients (Syed et al. 2021; Balasubramaniam et al. 2023). Salts, particularly sodium (Na+) and chloride (Cl-), can accumulate in plant tissues, causing ion toxicity. These ions can interfere with essential nutrient uptake and metabolism, leading to nutrient imbalances and impaired physiological functions. This imbalance in nutrient availability caused by soil salinity impacts various metabolic processes, which in turn affects nodule formation and plant growth (Etesami and Adl 2020). Additionally, salinity can deteriorate soil structure by dispersing soil particles, reducing porosity, and impairing water infiltration and aeration. Poor soil structure adversely affects plant growth and microbial activity, which are crucial for nutrient cycling and plant health (Kumar and Karthika 2020). Understanding the impact of salinity on soil bacterial communities and nutrient dynamics is crucial for managing saline soils and maintaining a healthy ecosystem.

The ability of P. juliflora to form more nodules and maintain higher shoot and root biomass under salty conditions suggests a better adaptation of this exotic species to the saline environment. In contrast, P. cineraria and P. pallida showed less nodulation and lower biomass, indicating weaker tolerance to soil salinity. This differential nodulation capacity among the species could reflect evolutionary adaptations to environmental stressors like salinity, where P. juliflora has developed mechanisms to maintain or even enhance nodulation efficiency under salt stress, unlike their congeners. This distinction underlines the adaptive advantages of P. juliflora, enabling it to thrive in a broader range of soil conditions and potentially altering local biodiversity by outcompeting native species like P. cineraria (Dagar and Gupta 2020; Ravi and Hiremath 2024).

Despite the statistically insignificant effect of salinity on the nodule formation ability of the three Prosopis species, salinity significantly reduced their growth parameters, indicating that physiological and biochemical constraints, other than nitrogen fixation, limit the growth of these species in salt-affected soils. Field observations in the study area indicate the absence of both P. cineraria and P. pallida in salt-affected soils in the UAE (Ali El-Keblawy, unpublished data). Elsheikh (1998) concluded that rhizobia tolerate salinity more than their leguminous hosts. For example, Ventorino et al. (2012 showed that natural rhizobial strains (i.e., halo-tolerant rhizobia) were generally higher in salinity tolerance than their leguminous hosts (common vetch and broad bean). Rhizobia strains employ different strategies to adjust to salt stress, including the buildup of small organic solutes within the cells (Irshad et al. 2021; Bertrand et al. 2020) or alterations in cell shape and size, as well as adjustments in the structure of outer polysaccharides (Soussi et al. 2001; Singh et al. 2021). Therefore, the tolerance of leguminous hosts to salt is the most critical factor in determining their success in highly saline soil. However, the significantly greater shoot dry weight of P. juliflora in saline indicates the ability of this species to invade and rehabilitate degraded salt-affected lands (Singh et al. 1994; Fall et al. 2018).

The successful establishment of introduced legumes, such as Prosopis species, is significantly influenced by the ability to form symbiotic relationships with compatible rhizobia in new environments, a process affected by factors like the co-introduction of rhizobia, the presence of native legumes, and soil conditions including pH, salinity, and nutrient content (Le Roux et al. 2016). Our study shows enhanced nodulation in P. juliflora seedlings grown in soil from under P. juliflora grown in saline and non-saline soils, suggesting the presence of compatible rhizobia under these trees, even under salinity stress. Prosopis juliflora is known for improving soil quality and increasing soil nutrient concentration and diversity of microbial activity under its canopy (Abril et al. 2009; Vallejo et al. 2012). Reyes-Reyes et al. (2002) and Perroni-Ventura et al. (2010) reported that P. juliflora trees can establish a symbiotic relationship with N-fixing Rhizobium bacteria, increasing the levels of nitrogen in the rhizosphere due to root and nodule turnover. Additionally, Herrera-Arreola et al. (2007) indicated that litter accumulations under P. juliflora canopies enhanced mineralization and increased the content of essential elements. The results of these studies elucidate the observed enhancements in soil quality under P. juliflora canopies, characterized by elevated levels of organic carbon, total nitrogen, and available phosphorus, alongside a high proportion of silt and clay, compared to the soil located away from the canopies (Table 1). We also noticed that the improved nodulation was correlated with higher levels of total nitrogen, organic matter, and finer soil particles in the soils collected under P. juliflora, indicating the role of soil physical and chemical properties in supporting rhizobial compatibility and activity (Lagunas et al. 2023).

The effect of species, but not salinity, on the shoot-to-root dry weight ratio was significant, with P. juliflora allocating significantly more biomass to shoots than the other two species. This higher shoot-to-root ratio in P. juliflora highlights its growth strategy of prioritizing above-ground development regardless of soil salinity. Allocation of resources is a fundamental concept in modern ecology and forms the basis for different growth strategies (Weiner 2004). According to the optimal allocation theory, derived from economics (Bloom et al. 1985), plants should allocate resources to enhance the uptake of the most limiting resource to growth. Optimal behavior is achieved when all resources are equally limiting. From an allometric perspective, allocation is a size-dependent process, with allometry describing the quantitative relationship between growth and resource allocation (Weiner 2004). Therefore, allocation questions should be framed allometrically rather than as simple ratios or proportions. Plants develop allometric patterns in response to various selection pressures and limitations, which explain many behaviors in plant populations. Consequently, P. juliflora, more than the other two species, allocates available resources to maximize vegetative growth.

The ability of the three Prosopis species to nodulate well in both salty and non-salty soils indicates that their compatible rhizobia are salt-tolerant. This is consistent with findings from various studies on Prosopis species. For example, Jenkins (2003) reported that several species of rhizobia isolated from P. glandulosa were capable of forming nodules even at 300 mM NaCl, with some strains remaining viable at 500 mM NaCl. Similarly, Otieno et al. (2017) isolated 15 nitrogen-fixing microsymbionts from root nodules of P. juliflora that exhibited high salt tolerance, tolerating up to 5% NaCl. Additionally, Baker et al. (1995) found that P. juliflora plants could nodulate and grow well in environments with 300 mM NaCl, demonstrating both the plant’s and rhizobia's high salinity tolerance. These findings underscore the adaptability of Prosopis species to saline environments, facilitated by their association with highly salt-tolerant rhizobia, which enables them to maintain nitrogen fixation and growth under challenging conditions. This adaptability is crucial for invading Prosopis species, particularly P. juliflora, in arid and saline environments.

The saline soils used in this study were taken from the rhizosphere of P. juliflora growing in a highly saline soil in UAE (87.5 dS m^− 1), indicating that this invasive species is extremely salt tolerant. In addition, the salinity of the salty area away from the P. juliflora canopy was lower than underneath it, indicating the ability of this species to accumulate more salts under the canopy, making the environment harsher for both native halophytes and seedlings of this species. However, field studies showed that only seedlings of P. juliflora were recorded under the canopy of this tree, indicating the high ability of P. juliflora to exclude natives even under harsh saline conditions (El-Keblawy and Al-Rawai 2007). In this study, salinity significantly reduced shoot and root dry weights as well as root length of seedlings of all species compared with the control.

This study highlights the significant influence of soil salinity and P. juliflora canopy cover on soil properties, nodulation, and growth of Prosopis species. Soils under P. juliflora canopies, especially in non-salty habitats, exhibited enhanced nutrient levels and improved physical conditions, which promoted nodule formation and biomass production. The invasive species, P. juliflora, and P. pallida, demonstrated superior growth and nodulation under these conditions compared to the native P. cineraria. Despite the negative impact of salinity on nodulation, nutrient availability, and plant growth, P. juliflora exhibited high salinity tolerance, suggesting its potential for rehabilitating degraded saline lands. These findings underscore the importance of canopy cover in mitigating salinity, enhancing soil fertility, and supporting plant growth, as well as informing sustainable agricultural practices and ecosystem management in arid regions. Understanding the dynamics of soil salinity and its impact on soil properties and plant growth is crucial for developing effective management strategies for invasive species and preserving native biodiversity in arid and semi-arid ecosystems.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

A.E.K., E.T.E.K., and S.K.S. conceptualized the study. A.E.K. contributed to data curation, formal analysis, funding acquisition, methodology, project administration, writing the original draft, and review & editing. E.T.E.K. contributed to data curation, methodology, writing the original draft, and review & editing. S.K.S. contributed to data curation, formal analysis, writing the original draft, and review & editing. All authors reviewed the manuscript.

Author Contribution

Acknowledgement

The authors thank Dr. Francois Mitterand Tsombou, Fujairah Research Center, and Dr. Attiat Elnaggar, University of Sharjah, for their valuable contribution to data collection.

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Effects of soil salinity on nodulation and growth of invasive and native Prosopis seedlings in arid deserts

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Abstract

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Introduction

Materials and Methods

Seed collection

Soil collection and analysis

Statistical analysis

Results

1. Physico-chemical properties of soils

(b) Physical Soil Properties

2. Nodule formation

3. Growth parameters

Shoot and root dry weight

Root length

Shoot-to-root ratio

Discussion

Conclusion

Declarations

Declaration of competing interest

CRediT authorship contribution statement

Author Contribution

Acknowledgement

References

Additional Declarations

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