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Excess Na+ and high pH result in poor structures in Saline-Sodic soils, which reduces extracellular enzyme activity (EEA) and causes nutrient limitations. The application of manure improved the Physical-Chemical properties of soil and balanced the soil nutrient supply, which was reflected in the soil EEAs and stoichiometry. Five experimental treatments were designed according to the manure application duration as follows: manure application for 11 years (11a), 16 years (16a), 22 years (22a), and 27 years (27a) and a control treatment with no manure application (CK). The results of the redundancy analysis (RDA) showed that physical properties (mean weight diameter (MWD)) and EEA (β–glucosidase (BG)) significantly increased and bulk density (ρb) significantly decreased when the nutrient content increased. Additionally, soil pH, electrical conductivity (EC), exchangeable sodium percentage (ESP) and sodium adsorption ratio (SAR) significantly decreased after manure application. Based on stepwise multiple linear regression models (SMLR), total nitrogen (TN) was the dominant variable that significantly increased EEA, and the Mantel test showed that soil C:N significantly influenced enzyme stoichiometry. Furthermore, RDA showed that pH, soil C:N and TN were the main factors influencing EEAs and enzyme stoichiometry. Soil EEAs significantly increased with TN and decreased with pH and soil C:N, which affected enzyme stoichiometry. The enzyme stoichiometry increased from 1:2.1:1.2 and 1:2.7:1.5 to 1:1.7:1.2, and the vector angle (vector A) increased, which showed that the N limitation was relieved after the application of manure. The vector length (vector L) showed no significant difference in the C limitation at depths of 0–20 cm and significantly increased at depths of 20–40 cm. In conclusion, soil EEAs and stoichiometry improved with changes in TN and soil C:N, and pH decreased with changes in the soil structure after the application of manure, which accelerated the soil nutrient cycle and balanced the soil nutrient supply.

Poor soil physical properties, serious salinization and low soil nutrients are the limiting factors for crop yield in saline-sodic soil. Long-term cattle manure application is an important measure that can affect the physicochemical properties and increase the maize yield of saline-sodic soil. This experiment included five treatments according to the history of cattle manure application: a control treatment with no cattle manure (CK) and treatments with cattle manure application for 14 years (14 a), 17 years (17 a), 20 years (20 a), and 25 years (25 a). The results indicated that compared with the CK treatment, longterm cattle manure application to saline-sodic soil resulted in significant increases in soil organic matter (SOM), soil total nitrogen (TN) and available nutrients at the 0-20 cm and 20-40 cm depths (p<0.05). The soil physical properties improved significantly, and cattle manure application significantly decreased the soil bulk density (ρb) and soil density (ρd) and increased the soil total porosity (ft) and water-holding capacity (WHC). With the number of years of cattle manure application, the soil pH, electrical conductivity (EC), exchangeable sodium percentage (ESP) and sodium adsorption ratio (SAR1:5) decreased significantly, and the maize yield gradually increased over time from 8690 kg/hm² in the CK treatment to 14 690 kg/hm² in the 25a treatment. There were significant differences among all treatments (p<0.05). The results showed that long-term cattle manure application decreased the soil ρb and saline-alkaline properties, which was the main factor that affected the maize yield in the saline-sodic soil, especially for soil ρb. [ABSTRACT FROM AUTHOR]