Blue Panicum-Alfalfa Mixture Buffers against Effects of Soil Salinity on Forage Productivity ^†

Abstract

Soil salinity limits forage productivity in ~1125 million ha of particularly arid lands across continents. Grass-legume mixtures are known to enhance soil nutrient cycling and forage productivity in non-saline conditions. We tested if these benefits can be replicated in the saline soils (EC_e > 14) of Laayoune-Morocco during the year 2021, based on a randomized complete block design (RCBD) experiment with 50–50 and 30–70% blue panicum grass (Panicum antidotales Reitz), alfalfa (Medicago sativa L.) mixtures compared to their monocrops. There was no significant variation (p = 0.36) in forage accumulation across treatment, but the blue panicum grass and alfalfa mixture had slightly greater forage yield compared to each in monocrop (13% and 5% Mg ha⁻¹). Blue panicum grass proportionate dry matter exceeded alfalfa in mixtures (62 and 54%). During establishment, blue panicum grass buffers against effects of high salinity on forage productivity in mixtures with alfalfa.

1. Introduction

Soil salinity is a major problem limiting crop production in arid and semi-arid lands across the globe including Morocco where, an estimated 3.6 million ha is affected [1]. This problem is triggered by intrusion of saline water from the Atlantic Ocean and persistent use of saline irrigation water, which deposits toxic sodium (Na⁺) ions in the soil. Therefore, soil EC levels are moderate (4 ds m⁻¹) to extreme (18 ds m⁻¹), which adversely affects forage crops. Recent reports indicate that alfalfa suffers yield losses of up to 26% due to salt stress in Morocco [2,3]. Sodium toxicity suppresses seed germination [4] and impairs plant osmotic potential, photosynthesis and cell elongation, ultimately suppressing forage yields [5]. Salt tolerant crops, including blue panicum grass, can maintain net photosynthesis, water potential and dry matter accumulation hence mitigate against increasing salinity [6]. Nevertheless, ref [7] have found blue panicum grass yields to an average maximum of 15 tons ha⁻¹ in saline conditions of southern Morocco, compared to the potential 60 Mg ha⁻¹ [8]. This study explores measures to alleviate effects of soil salinity by leveraging adaptive features of alfalfa and blue panicum grass in mixtures.

Perennial grass-legume mixtures are renowned for their complementarity in carbon (C) sequestration, nitrogen (N) acquisition, weed suppression and overyielding monocrops [9,10,11]. In southern Morocco, where salinity compounds temporal variation of temperatures, yields of alfalfa and blue panicum monocrops slump because of their C₃ and C₄ photosynthetic mechanisms that cannot withstand extremely hot summers and cold winters, respectively. Consequently, this research explores mechanisms of blue panicum grass-alfalfa mixtures to stabilize yields across seasons, when persistently exposed to saline growing conditions. We sought to understand if a grass-legume mixture complementarity can buffer against soil salinity and enhance forage productivity compared to monocrops. Hence the objective of this study was to evaluate correlations between forage yield and soil salinity and to test for species complementarity in overall forage productivity. We hypothesized that the halophytic nature of blue panicum grass can alleviate salinity and complement alfalfa growth and hence increase forage productivity of their mixtures unlike monocrops.

2. Materials and Methods

RCBD experiments were established in two sites in Foum El Oued-Laayoune with moderate and high soil salinity in April 2021. The study had four treatments, including blue panicum grass and alfalfa monocrops and their combination in 50–50 and 30–70% seeding proportions in four replicates. Crops were established by adjusting seeding rates up, to account for pure live seed estimates of germination and purity shortfalls from 100%. Soil salinity regimes were maintained by irrigation using moderate and highly saline water in drip irrigation according to crop evapotranspiration. Forage was harvested at approximately 10% flowering stage of alfalfa and oven dried at 60 °C for 72 h to determine forage productivity ha⁻¹ along with soil EC. The standard deviation of forage dry matter across different harvests of each treatment were divided by the corresponding means, to determine temporal stability of forage production. Data were subjected to ANOVA (stat package of R) to determine site × treatment, site or treatment effects on forage productivity, temporal stability and soil salinity. Mean separation was conducted using Fisher’s protected LSD.

3. Results

3.1. Initial Soil and Water Physical and Chemical Properties

Initial soil properties indicated that both sites had sandy loam soil texture, see

Table 1. Initial soil texture and soil and water salinity in sites of medium and high salinity in Laayoune-Morocco during April 2021.

Soil Characteristic	Salinity
	Medium	High
Textural class	Sandy loam	Sandy loam
Ratio of sand, loam and clay (%)	68:20:12	60:26:14
Soil ECe (dS m−1)	7.8	52.5
Water EC (dS m−1)	8	17

. There was greater dominance of sand in medium (68%) compared to highly saline soils (60%) coinciding with soil EC_e values of 7.9 and 52.5 dS m⁻¹ and water EC of 8 and 17 dS m⁻¹.

3.2. Forage Acumulation

There was no significant (p = 0.3) site × treatment interaction, but each of these factors singly (p < 0.05) influenced accumulated forage dry biomass. The 50–50 mixture of blue panicum grass and alfalfa accumulated the greatest amount of forage (10.4 Mg ha⁻¹), slightly exceeding that of alfalfa monocrop (10.2 Mg ha⁻¹) and the 30–70 m mixture (9.6 Mg ha⁻¹); however, it surpassed that of blue panicum grass monocrop (8.1 Mg ha⁻¹) based on averages across sites (

Table 2. Cumulative forage dry biomass of blue panicum (BP) and alfalfa (Alf) monocrops and their 50–50 and 30–70% mixtures in medium and high salinity from June to December 2021.

Treatment	Cumulative Forage Dry Matter
	Medium Salinity	High Salinity	Mean
	Mg ha−1
100BP	10.2a	6.0a	8.1b
100Alf	13.6a	6.8a	10.2a
50BP50Alf	13.6a	7.2a	10.4a
30BP70Alf	12.4a	6.8a	9.6ab
Mean	12.4A	6.7B

Value within column followed by similar letters in lower case and means across salinity followed by similar letters in upper case are not significantly different (LSD, p > 0.05).

). The cumulative forage biomass from medium salinity (12.4 Mg ha⁻¹) was almost double the mean yield from high salinity 6.7 Mg ha⁻¹. Cumulative forage dry biomass across treatments ranged between 10.2 and 13.6 Mg ha⁻¹ in moderate salinity and 6 to 7.2 Mg ha⁻¹ in high salinity.

3.3. Temporal Stability

There was no significant (p = 0.3) site × treatment interaction, but their main effects against dry matter temporal stability were significant (p < 0.05). Blue panicum had the highest mean temporal stability across sites, comparable to alfalfa monocrop and the 50–50 mixture (0.36), but greater than that of the 30–70 blue panicum alfalfa mixture (0.34) (

Table 3. Temporal stability of 50–50 and 30–70% blue panicum grass (BP), alfalfa (Alf) mixtures and monocrop dry matter in medium and high salinity from June to December 2021.

Treatment	Temporal Stability
	Medium Salinity	High Salinity	Mean
100BP	0.60a	0.45a	0.53a
100Alf	0.37a	0.35a	0.36ab
50BP50Alf	0.35a	0.38a	0.36ab
30BP70Alf	0.45a	0.20a	0.34b
Mean	0.44A	0.35B

Value within column followed by similar letters in lower case and means across salinity followed by similar letters in upper case are not significantly different (LSD, p > 0.05).

). Crops had greater mean temporal stability in medium (0.44) compared to high salinity (0.35). Blue panicum grass dry biomass dominated in mixtures established in high soil salinity (62 and 54%). In medium salinity, blue panicum was more abundant (71%) than alfalfa in the 50–50 mixture, unlike the 30–70 mixture where alfalfa dominated (61%).

4. Discussion

The inability of blue panicum grass to alleviate soil salinity contrary to our hypothesis might be explained by inadequate establishment of blue panicum grass roots to perform phytoremediation. Grasses alleviate salinity through root mechanisms of degrading calcite and releasing Ca²⁺ which replace Na⁺ from the soil solution [12] for exclusion into plant roots and shoots. Across sites, the 50–50% blue panicum grass alfalfa mixture slightly enhanced yields compared to monocrops. Substantial complementary advantages of mixtures usually manifest in years following establishment, particularly attributed to enhanced deposit of crop residues and turnover of soil N and C [13]. Apparently, dominance and stability of blue panicum grass shoot biomass might buffer the productivity of mixtures despite persistently saline conditions. In non-saline and warm conditions, blue panicum grass is expected to thrive via the C₄ photosynthetic machinery compared to the C₃ photosynthesis of alfalfa [14]. Temporal stability of biomass affirmed blue panicum grass tolerance to salinity, usually associated with its ability to maintain favorable potassium (K)/Na ratio, photosynthesis and proline mediated osmotic balance [15].

5. Conclusions

During establishment, blue panicum grass does not alleviate soil salinity, but its biomass stability and dominance buffers productivity of its mixture with alfalfa. Apparently, complementary benefits of grass–legume mixtures to increase forage productivity might extend to conditions of moderate to high soil salinity. Equitable blue panicum grass and alfalfa forage biomass persistence against increasing salinity can help reduce the pitfalls in forage yields due to salt injury and varying temperatures in arid conditions.