## 1 Introduction  
The growing human population requires a considerable increase in food production. However, over 50% of traditional fertilizers are transferred to the environment through volatilization and leaching, resulting in serious resource wastage and environmental pollution [1; 2]. Slow-release fertilizers (SRFs) can improve the utilization of fertilizer and provide nutrients for plants over a longer time, thus reducing nutrient loss and alleviating the problem of environmental pollution that is associated with the excessive use of traditional fertilizers [3; 4].  
Slow-release fertilizers are prepared mainly by two methods: (1) coating or encapsulating nutrients with water-insoluble and porous membranes [5; 6], or (2) dispersing nutrients in polymers or inorganic matrices [7; 8]. Slow-release fertilizers with water-retaining properties comprise a new type of multifunctional fertilizer. This fertilizer gradually releases nutrients while absorbing rainwater or irrigation water, thereby enhancing the water-holding capacity of the soil and the drought resistance of the crop. This makes it particularly suitable for arid and semi-arid regions [9; 10]. Palm oil has been used to develop an elastic polyurethane-coated urea fertilizer. The process utilizes acrylonitrile modification to grant the fertilizer its water-retaining capacity [11]. Semi-IPN hydrogels of cassava starch-g-polyacrylic acid/natural rubber/polvinyl alcohol have been developed as membranes for improving slow-released fertilizers with water absorption properties [12].  
Sodium alginate (SA), a natural polysaccharide macromolecule derived from algae, exhibits good biocompatibility and biodegradability. Due to these properties, SA has been widely studied and applied in various fields, such as food, medicine and fertilizer, and is commonly used as a carrier for slow-release fertilizers [13; 14; 15]. SA can be used to prepare microspheres through crosslinking with Ca2+ to form a gel matrix [16; 17]. However, the water absorption capacity of SA microspheres is relatively low. To enhance the water absorption performance of the microspheres, composite materials incorporating SA with materials possessing strong water absorption properties, such as halloysite, bentonite, and nano-cellulose, can be employed [18; 19; 20; 21]. As an important derivative of starch, carboxymethyl starch sodium (CMS) is used in food, pharmacy, fertilizer and other fields. This can rapidly expand in water and form a gel, which makes it suitable as a water-absorbing material [22]. In the previous study, sodium alginate and carboxymethyl starch sodium were crosslinked to form a composite network core, and polydopamine, functioning as a nitrogen metabolism regulator, was adhered to its surface [23]. Thus, a new kind of slow-release fertilizer with water absorption property, called SCPU, was obtained in this process [24].  
In order to reduce production costs and determine optimal reaction conditions, researchers often use the response surface method (RSM) to design experiments, seek optimization and undertake statistical analyses. Multiple quadratic regression equations are used to fit the functional relationship between factors and response values. To solve multivariate problems, the optimal parameters are determined by analyzing regression equations [25; 26]. As RSM can minimize the number of tests and analyze the selected test parameters comprehensively and economically, it has been widely used to optimize various separation and preparation processes [27; 28; 29; 30].  
This study aimed to optimize the process of slow-release fertilizer with water absorption, so as to obtain SCPU with the highest encapsulation efficiency (EE) and water absorption (WA). For prediction and optimization of the conditions, the Box-Behnken design was applied. This work will provide support for the preparation of other slow-release fertilizers with water absorption property.