Silicon Combined with Trichoderma harzianum and Organic Matter as an Environmental Friendly Strategy for Mitigating Salt Stress in Quinoa (Chenopodium quinoa Willd.)

Water stress harms quinoa, decreasing plant biomass. Maestro-Gaitán et al. [29] studied four quinoa cultivars, and they found that when quinoa was cultivated with 75% soil water available, fresh mass production was more significant than when grown with 35% soil water available, where the reductions in leaf biomass, stems, and seeds, also demonstrated this effect. Manaa et al. [30] also found reduced quinoa growth under water stress.

Losses in quinoa growth occur due to water stress, which promotes decreases in chlorophyll content, leaf osmotic, and water potential and induces significant changes in the chloroplast organization. The stress decreases the growth of the quinoa mainly by reducing leaf expansion and net photosynthesis. However, it is noteworthy that when subjected to stress recovery irrigation, the loss of structural integrity and chloroplast arrangement disorder are quickly recovered, suggesting that quinoa can withstand drought conditions well [30].

Under adequate soil moisture conditions (80% AWC), saline stress attenuators increased the fresh mass production in quinoa, mainly with the organic matter application (Si + OM). Rekaby et al. [31] studied organic compost and humic acid application in quinoa, and they also observed that plant growth variables increased. The authors justify that the organic matter addition mitigates the harm of salts as a direct effect and that these compounds help the plant to tolerate salts through metabolic changes. The organic compounds added to salt-affected soils reduce salt stress and improve plant growth by increasing nutrient availability, water retention, soil structure, and microorganism bioactivity [32,33]. The Trichoderma applicationd (Si + T and Si + OM + T) increased fresh mass, which may be associated with the fungus’s ability to improve soil nutrient availability by secreting organic acids into the rhizosphere [34].

The toxic ion accumulation in the tissues under salt stress can affect the total plant biomass. What can be observed in this work is that the treatments that promoted a more significant production of fresh mass (Si + OM, Si + T, and Si + OM + T) are the same ones that favored a lower toxic ions absorption—Na⁺ and Cl⁻. At the same time, the lower fresh mass production (control) is due to higher absorptions of Na⁺ and Cl⁻ instead of K⁺. It is essential to realize that in quinoa grown at 80% AWC, there was a higher salt entry into the soil, mainly Na⁺ and Cl⁻, due to the irrigation water being rich in these ions and having the capacity to salinize the soil. In addition, even with a higher soil salt concentration, the salinity attenuators effectively controlled the salt effect stress, decreasing the plants absorption of Na⁺ and Cl⁻. In other research, Silva et al. [35], by testing the same attenuators and combinations as this study in sorghum (Si + OM, Si + T, and Si + OM + T), showed that the Si combinations reduced toxic ion absorption (Na⁺ and Cl⁻) and favored nutrient ion absorption (K⁺), resulting in significant gains in biomass.

Potassium is a many-cytoplasmic enzyme activator necessary for photosynthesis and respiration. Still, as Na⁺ and K⁺ ions have similar physicochemical properties, they compete for the primary binding sites in these essential metabolic processes in the cytoplasm [36]. Therefore, K⁺ deficiency suppresses photosynthesis and reduces plant growth. Increasing K⁺ uptake alleviates the harmful effects of salinity [37]. In addition, K⁺ is an essential ion for maintaining and creating turgor pressure and adjusting the plant’s water balance [36]. Even when quinoa is cultivated with low soil available water content (30% AWC), salt stress attenuators effectively decreased the Na⁺ and Cl⁻ absorption and increased the K⁺ absorption of the quinoa plants, with a low fresh mass production associated with multiple stress occurrences (water + salt).

Terletskaya et al. [38] evaluated the influence of osmotic and saline stress on quinoa growth and found that a saline level between 100 and 200 mM NaCl was not crucial for its development. Still, with increasing salinity (300 mM NaCl), biomass was reduced due to higher Na⁺ absorption in the aerial part of the plant. This corroborates the results found in this work, as quinoa cultivated without adding salt stress attenuator (control) absorbed more Na⁺ and reduced its fresh mass production. The Na⁺ abundance in a saline environment results in its greater root uptake, increasing its concentration and reducing the other ions concentration, mainly K⁺, in the plant [36].

Regarding elements’ content in the soil (soluble and exchangeable), the reduction in soluble K⁺ in the highest available water content occurred as a consequence of the increase in soluble Na⁺, which presented higher levels for the highest available water content (80% AWC), due to the water quality used in the experiment, which had a high range of this element (Table 2). As these elements present antagonistic interactions on soil [39], increasing soluble Na⁺ reduces soluble K⁺. Although no differences were verified for Cl⁻, according to the data obtained in our study, it was evident that greater amounts of this saline water tend to increase Cl⁻ accumulation. As there was no drainage in the experiment, this contributed to the fact that the application of this water in more significant amounts resulted in higher contents of soluble Na⁺ and Cl⁻.

In this study, we verified that the exchangeable Na⁺ content also increased significantly for the highest available soil water content (80%) due to the application of higher saline irrigation depths. Na⁺ in saline water passes to the soil solution, increasing its levels. Increasing the concentration of this element in the soil solution promotes its passage to the exchangeable phase of the soil, thus increasing the levels of exchangeable Na⁺ [40].

The highest levels of elements verified for the highest available water content in the soil (80%) increased the soil pH values, electrical conductivity, and percentage of exchangeable sodium. According to Black and Campbell [41], more ions in the soil solution increase the soil solution’s ionic strength and thus its electrical conductivity values—soil salinization. On the other hand, as exchangeable Na⁺ is directly related to the percentage of exchangeable sodium [42], increasing its exchangeable levels, promoted by 80% AWC, resulted in higher ESP values. Increases in soil pH through saline irrigation occur because of the imbalance in the ionic composition due to sodium addition [43], and the predominance of carbonate and bicarbonate ions and hydroxide compounds, contributing to the increase in the soil pH [44]. Thus, the increase in pH observed at 80% AWC also occurred due to a greater contribution of HCO₃²⁻ ions, which is present in the composition of saline water used for irrigation (Table 2).

In the Brazilian semiarid region, saline waters with high levels of Na⁺ and Cl⁻ are commonly used to irrigate crops grown in soils without drainage systems [5]. As a consequence, increasing soil pH and electrical conductivity values and increasing exchangeable Na⁺ and ESP tend to deteriorate soil physical properties, such as its structure and permeability [18]. This represents the need for studies that assess plant growth and the effects on soil properties by using saline water for irrigation to find sustainable management strategies for using these waters.

Applying saline stress attenuators effectively increased the soluble and exchangeable K⁺ levels compared to the control, where no source of K⁺ was used. As there was a combined application of potassium silicate through the soil and application of organic matter, which were both sources of K⁺, this contributed to the increases in its soluble and exchangeable contents. In addition, Trichoderma is efficient in solubilizing nutrients and improving soil fertility and nutrient utilization efficiency of plants [45]. Significant reductions verified for the exchangeable Na⁺ contents indicate that Si, organic matter, and Trichoderma applications tend to regulate the Na⁺ balance in the soil, attenuating its adsorption in the exchange complex in higher available water contents (80% AWC), and consequently reducing the ESP values.

Our results show, notably, that where saline stress attenuators—isolated or combined with silicon—were applied, soil pH increased, and significant reductions in electrical conductivity and the percentage of exchangeable sodium occurred. Some studies have shown increases in soil pH values due to silicon source applications [46,47,48,49]. According to Haynes [50], OH⁻ ions are released during the dissolution of silicate fertilizers, contributing to the increase in soil pH.

The most significant differences for the variable electrical conductivity, verified with a higher content of available water in the soil (80% AWC), indicate a greater effectiveness of the saline stress attenuators in reducing soil salinity under conditions of a higher soil moisture content. The results verified in our study for EC values also suggest that saline stress attenuators are ineffective in promoting reductions in soil salinity at low soil available water content (30% AWC). Khan et al. [51] stated that exogenous silicon regulates electrical conductivity. In the present study, the salt stress attenuators were applied with silicon, alone or in combination. Thus, our findings are in agreement with those of previous studies [52,53], which have also found that exogenous silicon application reduces soil electrical conductivity and favors crop growth.