With the decline of oil resources, the development and utilization of biopolymer materials based on renewable resources has received wide attention from researchers [1]. Black mulch film is widely used in agriculture due to its excellent thermal insulation and moisture retention capabilities. Typically, black mulch films raise soil temperatures steadily and gently, reducing damage to plant roots and reducing light penetration to inhibit weed growth, thus reducing nutrient competition between weeds and crops in the field [2]. At present, black commercial mulch films are made of low-density polyethylene (LDPE) mixed with black masterbatches or humus or carbon black [3]. The main drawbacks of using black commercial LDPE mulch films are nondegradability and poor mechanical performance [4]. When discarded or buried in the soil, commercial mulch breaks down into “microplastics”, which would be accumulated in the human body through food chain and cause serious harm to human health [5]. Therefore, the development of biodegradable black mulch films to replace traditional black commercial LDPE mulch films is popular in the field of building “green” agriculture [6]. Polyvinyl alcohol (PVA), a sort of semicrystalline polymer, has been demonstrated as “green” materials in constructing composite films because of their biocompatibility, biodegradability, non-toxicity, excellent chemical resistance and odorless nature [7,8]. In addition, PVA also has well gas barrier, well water solubility and easy processing [9]. However, PVA is expensive, not tough enough, and has poor water resistance, making it difficult to apply them on a large-scale [10].

Sodium carboxymethyl cellulose (CMC) is one of the most important derivatives of cellulose, which is rich in carboxyl (-COOH) and hydroxyl (-OH) groups [11]. CMC is a water-soluble, non-toxic, low-cost, biodegradable materials with excellent film-forming ability [12]. To obtain black CMC films, black materials such as carbon black, humic acid, and other substances are added to CMC [13]. However, these methods only increase the darkening of the film and have little effect on the overall performance of the film [14]. Due to the rigid molecular chain segments, the elongation at the break of CMC film is poor, making it difficult to apply them on a large-scale [15].

Sodium lignosulfonate (SLS) is a tannin-like polyphenolic polymer compound with high affinity [16]. As a phenolic ligand, the hydroxyl groups on SLS provide abundant chelating sites for metal ions [17,18]. The catechol-like group on SLS helps drive the ligand crosslinking, thus forming a stable three-dimensional metal polyphenol network structure [19,20]. This three-dimensional stabilized metal polyphenol network structure is beneficial for improving the mechanical properties of composite films [21,22]. To promote the growth of plants and development of “green” agriculture, an ideal black composite mulch film should own good soil moisture retention as well as controlled nutrient release properties [23]. Urea is commonly used as a cost-effective nitrogen fertilizer with high nitrogen content. However, biopolymer-based mulch films often experience rapid urea release when exposed to water or during rainy periods, which fails to meet the crop's nitrogen requirements during later growth stages [24]. Therefore, there is a growing demand for biodegradable mulch films that possess adequate mechanical strength and controlled urea release [25]. The utilization of darkened lignin as the nanofiller for the design of black, biodegradable, urea slow-release and structurally robust mulch film was less investigated.

Herein, we proposed a novel eco-friendly and mechanically robust sodium mulch film fabricated by coordination between polymers and Fe ions and redox reactions between SL and Fe ions. The basic physicochemical characteristics such as tensile mechanical performance, gas barrier, hydrophobicity, controlled urea release and the underlying mechanisms were further analyzed. The practical application of manufactured materials as black mulch films for cabbage seeds growth were carried out. Our finding not only proposed a novel strategy for designing eco-friendly black mulch films, but also contributes to “green” economic development.