Soil Salinity Has Species-Specific Effects on the Growth and Nutrient Quality of Four Texas Grasses

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Abstract

Irrigation of farmlands in xeric areas can increase soil salinity, reducing their suitability for food and fiber crops. One way to repurpose these lands is to convert them for use in grazing. To choose the best forage species, it is important to understand the impact of soil salinity on the growth and nutritional quality of potential forage grasses. Here, we grew four perennial C4 grasses: blue grama (Bouteloua gracilis), sideoats grama (Bouteloua curtipendula), little bluestem (Schizachyrium scoparium), and bermudagrass (Cynodon dactylon) in soil treated with four different concentrations (0, 8, 16, and 24 dS/m) of sodium chloride salt (NaCl). We then determined the effects of soil salinity on germination, biomass production, and plant tissue nitrogen content (an indicator of nutritional quality). We found a high degree of variability in salinity responses among species. S. scoparium performed poorly relative to the other species across all metrics. C. dactylon showed high biomass and low sensitivity to soil salinity for each index but had the lowest shoot nitrogen concentration of all species tested. This indicated a tradeoff of tissue quality for quantity. On the other hand, the two Bouteloua species showed opposite results, falling on the shoot quality end of the quantity-quality spectrum and even showing increased nitrogen concentration with increasing soil salinity. Given their complimentary traits, C. dactylon and Bouteloua spp. may be good candidates for interseeding on saline lands. These results indicate that species choice can help mitigate negative impacts of soil salinity on forage production and quality and should be carefully considered by land managers.

Introduction

Farmlands in xeric areas of the southwestern United States are often heavily irrigated. This surface irrigation can increase soil salinity and eventually make the soil unsuitable for some food and fiber crops (Isidoro and Grattan 2011). As a result, landowners may opt to implement alternative land management practices when soils become increasingly saline. If palatable grasses for forage can grow in high-salinity soils while maintaining their nutritive value, they would prove to be a valuable land management practice to repurpose saline land unusable for food and fiber agriculture (Masters et al. 2007).

Salinity stress alters plant processes. For instance, high soil salinity can alter cellular functioning, including nutrient uptake and concentration, protein synthesis, and osmotic activity (Hamilton III et al. 2001; Masters et al. 2007; Uddin and Juraimi 2013). For most plants not adapted to saline conditions, exposure to saline soil may cause reductions in cell turgor, which would reduce the amount of water that can be brought in by the plant from its surrounding environment and negatively affect its growth (Masters et al. 2007; Flowers and Colmer 2008). In addition, plants can face ion toxicity and nutrient imbalances if ion uptake and retention from the soil is too great (Uddin and Juraimi 2013). These effects act to reduce germination (Dai et al. 2009; Guo et al. 2010) and plant growth (Wang et al. 2002; Marcum and Pessarakli 2006; Dai et al. 2009).

Plant responses to environmental stressors can vary by species (Wang and Frei 2011). This is often the case for studies examining plant growth responses to salinity. For instance, the impact of salinity on germination (e.g., Bayuelo-Jiménez et al., 2002; Zhang et al. 2011; Janousek and Folger 2013; Dudley et al. 2014) and biomass (e.g., Marcum and Murdoch 1994; Marcum 1999; Hamilton III et al. 2001; Harris et al. 2010; Zhang et al. 2011; Defalco et al. 2017) can vary widely between species. Roots are also one of the most sensitive plant organs to salinity and will usually be the first to show signs of salt stress (Muscolo et al. 2003). However, the response of roots to salinity stress has also been shown to be species specific (e.g., Hamilton III et al. 2001; Dai et al. 2009).

Although many prior studies have been conducted to understand the effects of salt on warm-season grass germination and growth, few have examined the effects of soil salinity on indicators of plant nutritive value, such as leaf nitrogen content. Forage selection by grazers depends on tissue nutrient quality (Westoby 1974), and plants with high shoot nitrogen content are preferred by grazers (Pauler et al. 2020). Still, it is unclear how salinity might impact forage quality, as previous findings concerning the nutritive value of salt-tolerant plants show inconsistent results (Masters et al. 2007). It was also reported that the response of the content of crude proteins in forage grasses to increasing salinity were inconsistent, although nutritive value may generally increase with increasing salinity (Robinson et al. 2004). However, if high salinity interferes with a plant's ability to take up nitrogen (e.g., Hawkins and Lewis 1993), then introducing forage grasses to saline environments could compromise grazer nutrition.

Here, we tested the effects of soil salinity concentration on the germination, biomass, and nitrogen content of four perennial C4 grasses that are used for forage in West Texas (Texas A&M AgriLife Extension 2019). We hypothesized that increasing salinity would decrease germination, shoot and root biomass, and foliar nitrogen content due to impaired plant-water relations. While we expected these general patterns to hold across all species, our goal was to quantify species-specific responses to identify characteristics that could best inform land manager choice of species for growth as forage on saline lands. In the experiment, we chose species previously identified as salt tolerant (see “Methods” later). However, we expected the non-native bermudagrass to be more salt tolerant than the native species because of its previous use on saline lands and reported higher salinity tolerance than other turfgrasses (Marcum and Murdoch 1994; Uddin and Juraimi 2013). Within the native species, we expected little bluestem to be more salt tolerant than the Bouteloa species because of its wide native range.

Section snippets

Species Selection

The species selected consisted of three native species: little bluestem (Schizachyrium scoparium [Michx.] Nash), blue grama (Bouteloua gracilis [Willd. Ex Kunth] Lag. Ex Griffiths), and sideoats grama (Bouteloua curtipendula [Michx.] Torr.) and one non-native species, bermuda grass (Cynodon dactylon [L.] Pers.) (USDA NRCS 2020). These grasses are referenced to be fair forage for livestock (Texas A&M AgriLife Extension 2019) and good turfgrasses for erosion control (USDA NRCS 2020). Seeds of

Germination

A significant interaction between species and salinity treatment (P < 0.05; Table 1; Fig. 1) indicated that response of germination to salinity was species specific. Post-hoc analyses indicated that B. gracilis showed a significant (i.e., nonzero) reduction in germination probability with increasing salinity (P < 0.05; Table 2; see Fig. 1). However, the slope of the response was not significantly different from 0 for B. curtipendula, C. dactylon, and S. scoparium (P > 0.05 in all cases; see

Discussion

Saline soils can be detrimental to the growth and palatability of forage grasses (Hawkins and Lewis 1993; Wang et al. 2002; Marcum and Pessarakli 2006; Dai et al. 2009; Pauler et al. 2020). Here, we confirmed our hypothesis that sensitivity to saline soil varies by species, even within a group of salt-tolerant grasses. This confirms the results of previous studies (e.g., Marcum and Murdoch 1994; Marcum 1999; Hamilton III et al. 2001; Bayuelo-Jiménez et al., 2002; Harris et al. 2010; Wang and

Implications

Overall, our results indicate that saline soil can impact growth and nutritional quality of forage grasses but species responses vary widely, even within salt-tolerant grasses. This implies that species choice can be used to improve the suitability of saline lands for grazing. S. scoparium possessed neither high biomass nor high salinity tolerance; these traits, combined with a lack of positive response of tissue quality to soil salinity, likely make this species a poor choice for forage on

Data Availability

All data can be accessed via a GitHub repository at https://github.com/DogtoothTrail/texas_saltygrass (doi: 10.5281/zenodo.3923044).

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.

Acknowledgments

The authors thank Risa McNellis, Evan Perkowski, Morgan Long, and Dr. Dylan Schwilk for their help with data collection and for their input on aspects of earlier versions of the manuscript.

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    This work was funded by grants from Texas Tech University Honors College, Beta Beta Beta Biological Honors Society, and Sigma Xi to AB. NGS acknowledges additional support from Texas Tech University.

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