Effects of Cover Crop Species and Incorporation Ratios on Soil Fertility and Economic Returns in Salinized Cotton Fields

Aiming at the soil degradation problems (including low organic matter content, nutrient imbalance, and secondary salinization) caused by long-term continuous cropping of cotton fields in northern Shandong, this study aimed to identify the suitable catch crop species and optimal incorporation ratio for salinized cotton fields in this region, so as to provide the technical support for ecological restoration and green production of cotton fields.

The experiment was conducted at the experimental demonstration base in Wucheng County, Dezhou City, Shandong Province from 2022 to 2023. A split-plot design was adopted, with the main plots being three catch crop species: Orychophragmus violaceus, Vicia villosa Roth, and Lolium perenne L., and the subplots being four incorporation ratios: 100%, 70%, 30%, and 0. Winter fallow bare land was used as the control (CK). The biomass and nutrient accumulation of catch crops were systematically determined before incorporation, and soil physicochemical properties (such as available phosphorus, available potassium, alkali-hydrolyzable nitrogen, organic matter, pH, and electrical conductivity), microbial biomass carbon and nitrogen, and activities of key soil enzymes (such as urease, sucrase, and catalase) were dynamically monitored at the seedling, flowering, and harvest stages of cotton. The comprehensive soil fertility index (IFI) was calculated using the fuzzy mathematics membership function combined with principal component analysis, and a comprehensive evaluation was conducted by integrating cotton yield and annual economic benefits.

(1) There were significant differences in soil improvement functions among different catch crops (P<0.05): Vicia villosa Roth had the highest total nitrogen accumulation (70.4 kg·hm^-2), showing the best performance in improving soil nitrogen supply and comprehensive fertility; Lolium perenne L. had the highest fresh weight (31951.4 kg·hm^-2), dry weight (6058.0 kg·hm^-2), and total carbon accumulation (3129.9 kg·hm^-2), with significant advantages in carbon input and biomass accumulation; Orychophragmus violaceus had stable nutrient release and balanced overall performance. (2) The incorporation ratio significantly regulated soil indicators (P<0.05), and the 100% incorporation treatment performed the best. Compared with CK, it reduced soil electrical conductivity by 32.7% on average, increased organic matter by 15.9%, and enhanced the activities of urease, sucrase, and catalase by 142.3%, 46.4%, and 54.6%, respectively. (3) There were growth stage differences between soil physicochemical properties and biological activities: organic matter was the core for driving factor at the cotton seedling stage (significantly positively correlated with microbial biomass carbon and nitrogen, and enzyme activities, P<0.05); the inhibitory effect of electrical conductivity was significantly enhanced at the harvest stage (correlation coefficients -0.5--0.9). (4) The IFI under 100% incorporation treatment of Vicia villosa Roth was the highest (0.76), with a cotton yield of 5263.6 kg·hm^-2 and a net income of 21477.0 yuan/hm², an increase of 50.0% compared with CK; although 100% incorporation of Lolium perenne L. increased soil carbon storage, it significantly inhibited cotton growth, and the net income decreased by 24.0% compared with CK.

100% incorporation of Vicia villosa Roth was the optimal improvement model for slightly salinized cotton fields in northern Shandong, which could achieve the coordinated improvement of ecological and economic benefits. The matching mechanism between catch crop functional characteristics and incorporation ratio under this model provided a reference for the improvement of similar salinized farmland.