Effects of saline water irrigation on growth, persistence and nutritional quality of Medicago arborea and Atriplex species grown on different soil types
Creators: name: Panta, SB
Publisher: name: University Of Tasmania
Year: 2023

description: Agricultural crop production relies heavily on irrigation. Due to the lack of sufficient fresh water, the usage of low quality (saline) water is gaining momentum in many arid and semi-arid regions of the world. However, there are some concerns that using saline irrigation water may be an environmentally non-sustainable practice with potential to cause irreversible soil degradation. In addition to this, there is a lack of information on physiological changes in the plant when irrigated with saline water over a long-period of time and the impact of long term saline water irrigation on soil chemical properties and nutritional qualities of plants grown under such conditions. The major aim of this work was to investigate the effect of different levels of salinity in irrigation water and a range of irrigation regimes on the performance of Atriplex halimus, Atriplex lentiformis and Medicago arborea grown in different soil types. This study also investigated the effect of saline water on selected soil chemical properties and nutritional qualities of these species. By doing this the following objectives were addressed: 1. To understand the effect of saline irrigation on changes in the physiological characteristics and ion accumulation in plants in the context of different growing media (soil types - clay, duplex (texture-contrast) and sandy loam soil) and the amount of irrigation water applied. 2. To monitor changes in plant physiological characteristics and ion accumulation patterns over time of exposure to salinity during the growing period. 3. To determine the effects of quality and quantity of saline water on plant biomass production on different soils. 4. To investigate the effect of saline water irrigation on soil salinity and some selected chemical properties in the soil profile. 5. To understand the impact of saline water on some selected nutritional qualities of plants and to explain nutritional quality difference between plant species. A series of glasshouse experiments were conducted investigating plant gas exchange and photosynthetic characteristics and relating them to plant tissue ionic profiles. The stomatal conductance (`g_s`) of M. arborea was found to be reduced by 41% and 74% of control at 8 and 16 dS/m salinity, respectively, whereas no such effect was apparent in Atriplex lentiformis. In both plant species leaf ions (`Na^+` and `Cl^‚ÄövÑv¶`) content was increased with increasing water salinity (ECw) but a significant effect was seen only in a sandy loam soil. This indicates that both osmotic effect and ionic toxicity impacted on the physiological performance of M. arborea but A. lentiformis plants were insensitive to both components of the salt stress. Plant growth in a sandy loam soil was poor compared to a clay soil, indicating that soil texture and structure may have a significant role in the cation exchange process under saline conditions. Monthly monitoring of plant physiological characteristics and relating them to soil physical and chemical characteristics gave some further insights into effects of soil types and saline irrigation regimes on the performance of both species. Both chlorophyll content and Fv/Fm chlorophyll fluorescence characteristics were stable and did not change with time for all soil types, salinity levels and irrigation regimes. A general decline in the stomatal conductance (`g_s`) was observed in both A. lentiformis and M. arborea as the stress progressed, with the decline being much less pronounced in A. lentiformis (i.e. 40 vs 55 % over the 4 months at 16 dS/m `EC_w` salinity). Plants grown in clay and duplex soils showed a greater reduction in `g_s` compared with a sandy loam soil in both species. In addition, the leaf `Na^+` content in A. lentiformis remained constant over the entire duration of the experiment (5 months) regardless of `EC_w` and irrigation levels, while in M. arborea a continuous accumulation of `Na^+` in leaves was observed. Leaf `K^+` content in A. lentiformis was steady for the clay soil but declined when grown in a sandy loam (3.2-fold reduction over the 5-month period at 16 dS/m `EC_w` salinity). Overall, it is concluded that higher reliance on organic osmolytes (hence, associated carbon costs) for osmotic adjustment, poor `K^+` retention, a lack of efficient vacuolar `Na^+` sequestration, and higher reduction in stomata aperture in M. arborea are the main factors that explain poor performance of this species under long-term (5 months) salinity treatments in high salinity conditions (`EC_w` 16 dS/m). On the contrary, A. lentiformis had superior performance in each of the above components and showed no time-dependent changes in any of the measured characteristics. The above conclusions obtained in glasshouse experiments were further supported by a field trial. Atriplex halimus and A. lentiformis were shown to benefit from the application of saline water (up to 16 dS/m) at 800 mm/year irrigation rate in addition to natural rainfall (on average 500 mm/year), without any detrimental impact on plant growth and biomass production. On the other hand M. arborea could only be irrigated with up to 8 dS/m in the same conditions. The strongest reduction in M. arborea yield was observed in clay dominated soil under high irrigation regimes and was most likely associated with a transient waterlogging. Glasshouse data demonstrated that soil salinity `(EC_(se))` increased with increasing salinity of irrigation water (`EC_w`). Regardless of irrigation regime, `EC_(se)` was higher in clay dominated soil (clay and texture-contrast soil) compared with sandy loam soil. For the 16 dS/m `EC_w` treatment, clay soil salinity did not exceed the salinity level of irrigation water when irrigated at a high irrigation rate, and it was about 1.5 fold higher compared with low irrigation treatment. In the sandy loam soil, `EC_(se)` values were only half the value of `EC_w`. On the other hand, field experiment data showed a sharp increase of `EC_(se)` in the soil profile (including the top sandy soil) after 2.5 years of application of saline water (16 dS/m) but the value was less than the `EC_w`. In the high salinity condition, topsoil (0 - 10 cm) total organic carbon (TOC) was significantly reduced. Whilst, in the upper 50 cm depth, both the exchangeable `Na^+` and its ratio to total base cations (ESP) were significantly increased but the individual exchangeable base cations (`Ca^(2+)`, `Mg^(2+)`, `K^+`) were significantly decreased. They also show that the `Na^+` in the saline waters causes differential leaching of base cations from the rooting zone to deeper in the soil. In the last part of this project saline water irrigated A. halimus, A, lentiformis and M. arborea forage samples were collected after 3 years of field growth and analysed by wet chemistry methods for selected nutritional values. The measured nutritional parameters varied between species with M. arborea having a higher calcium (1.6%) but others minerals were higher in Atriplex species. In Atriplex species calcium content increased with increasing `EC_w` but was reduced in M. arborea. Regardless of irrigation levels and plant species, ash percentage increased from 16.8 to 17.8 % and `Na^`+ content from 3.7 to 4.3 % when salinity of irrigation water increased from 0.8 to 16 dS/m but `K^+` content decreased from 1.7 to 1.5 % under the same condition. Collectively, nutrient detergent fibre (NDF) values of plants were reduced from 35 to 33 % DM when the `EC_w` increased from 0.8 to 16 dS/m. In summary, this work has shown a possibility of sustainable production of M. arborea and Atriplex species using up to 8 and 16 dS/m saline water, respectively, with an irrigation rate of 800 mm/year on a Brown Sodosol in a 500 mm annual rainfall region. As the topsoil salinity increased with irrigation water salinity and some `Ca^(2+)`, `Mg^(2+)` and `K^+` were leached down to the subsoil from the active root-zone area in the soil profile. Such a practice may be not recommended to be used on prime agricultural land. However, for already salt-contaminated land not suitable for conventional farming the usage of high irrigation saline water regimes is likely to be beneficial, as at no treatment conditions did soil `EC_(se)` exceeded that of `EC` of the irrigation water. This study has also provided further insights into halophyte physiology demonstrating (in long-term experiments) that the better ability of halophyte species to cope with salinity is related to their much higher reliance on inorganic osmolytes for osmotic adjustment. This strategy allows these plants to avoid the yield penalties experienced by glycophytes due to a need to redirect a major pool of photosynthates for de novo synthesis of organic osmolytes. Finally, this study has also showed that the nutritional value of these plant species will not be affected by the saline water irrigation with the exception of increased shoot `Na^+` ion content.