Background Saline-alkali soil is widely distributed in our country, while desulfurized gypsum, citric acid slag and fly ash are produced and accumulated in large quantities, which cause great pressure on national economy and ecological environment. But all of them can be used to improve saline-alkali soil. Exploring the improvement of saline-alkali soil by these calcium-containing wastes and the effect on plant growth is conducive to reducing environmental pollution and eventually provides scientific basis for the recycling and reuse of calcium-containing wastes and the improvement of saline-alkali soils.

Methods Conditional control method was used in the research. Three kinds of calcium-containing waste materials (desulfurized gypsum, citric acid slag and fly ash)were applied to pot-planted Lespedeza bicolor Turcz. with saline-alkali soil soils respectively. Indexes of plant biochemical and soil composition such as net photosynthetic rate P_n, stomatal conductance G_s, transpiration rate T_r, intercellular CO₂ concentration C_i, stomatal limitation L_s, water use efficiency (WUE), chlorophyll, content of main ion, exchangeable sodium percentage and sodium adsorption ratio were measured accordingly.

Results 1) The main soil ion content after improved such as Na⁺, CO₃^2-, and HCO₃^- reduced significantly, while the content of Ca²⁺, Mg²⁺, SO₄^2- and CEC increased. 2) Na⁺ and alkaline ions were replaced, and the content of exchangeable Na⁺ even decreased by 84.3% in desulfurized gypsum treatment. 3) The alkali-reducing degree of each treatment soil exceeded 50%, and the soil was transferred from moderate saline to mild saline-alkali, the degree of salinity in each treatment also shifted from moderate to mild, all of the pH after the experiment decreased, and especially the citric acid residue treatment even reached to 7.1, which was basically neutral. 4) P_n, G_s, T_r and WUE of L. bicolor Turcz. were all higher, while C_i and L_s were lower.5) The contents of chlorophyll a, chlorophyll b and total chlorophyll were all over 2.4 mg/g, 1.1 mg/g and 3.5 mg/g in each treatment, that in fly ash treatment was the highest, mainly because the salt resistance was coupled, meanwhile it showed that by changing salt stress, calcium-containing materials changed the external environmental factors, and then affected the synthesis of plant chlorophyll.

Conclusions Calcium-containing wastes such as desulfurized gypsum, citric acid slag and fly ash may improve the physicochemical properties of the saline-alkaline soil by changing the composition and contents of salt-damaging ions in saline-alkali soil. Normal physiological functions are maintained by adjusting the feedback regulation mechanism of L. bicolor Turcz. under salt stress, and reducing the effect of salt stress on the growth of L. bicolor Turcz. This study may provide scientific basis and reference for the recycling and reuse of calcium-containing wastes and the improvement of saline-alkali soil.