Effects of biochar on the photosynthetic and antioxidant characteristics of ryegrass and alfalfa under saline-alkali stress

Plants generally behave in different response mechanisms under stress. This study aims to reveal the effects of biochar on photosynthesis and antioxidant system of ryegrass and alfalfa at the early stage of salt-alkali stress. Four treatments were conducted in the pot experiment: saline-alkali stress treatment (C0, 150 mmol/L equal mol NaCl, Na2CO3, NaHCO3 mixed saline solution), saline-alkali stress +1% biochar (C1), saline-alkali stress +3% biochar (C2), saline-alkali stress +5% biochar (C3), and set blank control (CK) without additives. Conventional cultivation was also carried out before the experiment, and then a stress test was conducted 40 d later. The stress treatment lasted for a total of 14 d, where 100 mL mixed saline solution was added to C0, C1, C2, and C3 treatments at one time, and 100 mL deionized water was added to CK treatment. Subsequently, the response of ryegrass and alfalfa was determined on the 14th day of the experiment, including growth indices, photosynthetic characteristics, malondialdehyde content, and antioxidant enzyme activities under various levels of biochar addition. The results showed as follows: 1) 14 d salt-alkali treatment dominated the growth of ryegrass and alfalfa. Specifically, there was a significant decrease in biomass accumulation, net photosynthetic rate, stomatal conductance, transpiration rate, and intercellular CO2 concentration. But there was no significant effect on the chlorophyll content and root length. Furthermore, the content of malondialdehyde increased significantly, indicating the antioxidant system responded positively. Similarly, the activities of superoxide dismutase, peroxidase, and catalase increased to alleviate the saline-alkali stress on plants. 2) The addition of biochar effectively improved the stress resistance of perennial ryegrass and alfalfa, thereby inducing a positive response in the antioxidant system. As such, the salinity-alkalinity stress was effectively relieved, particularly from the osmotic stress. The malondialdehyde content decreased obviously, whereas the alfalfa superoxide dismutase increased significantly, indicating that the perennial ryegrass antioxidant system was given a priority with the decomposition of hydrogen peroxide enzyme. The biomass, plant height, and root length of ryegrass and alfalfa increased by 48.50%-82.34%, 31.19%-44.16%, and 17.15%-48.09%, respectively, under 3% biochar treatment, compared with that under the salt-alkali stress. Additionally, the stomatal conductance and transpiration rate increased by 118.69%-358.99%, and 98.66%-526.53%, and the chlorophyll content and net photosynthetic rate increased by 7.97% and 519.09%, respectively. 3) The antioxidant enzyme activities and malondialdehyde content of ryegrass and alfalfa remained stable or decreased with the increase of biochar amount under the short-term salt-alkali stress. A trend was also found that the promoting at a low supplemental level and inhibiting at a high supplemental level for the growth indexes and photosynthetic characteristics. Consequently, there was a significant effect of salinity-alkalinity stress on the growth of perennial ryegrass and alfalfa, where a positive response was found at the beginning of stress. Correspondingly, an optimal addition of biochar can be expected to improve the stress resistance of perennial ryegrass and alfalfa, while the positive reaction of the oxidation system can effectively relieve the short-term salinity-alkalinity stress. The best effect was also achieved in the 3% biochar application. The findings can provide a sound reference for the scientific application of biochar to promote crop growth and yield. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.