Biochar-based slow-release nitrogen fertilizers (BSRFs) have been arousing a great deal of interest from researchers and academics, but many of these fertilizers do not possess water-retention capacity. In this work, a polymer matrix has been introduced into NH4 +-loaded biochar (N-BC), using microwave (MW) irradiation, for the purpose of acting as a superabsorbent polymer to improve soil water-retention capacity. The polymer matrix is composed of cotton stalks (CSs), acrylic acid (AA), 2-acrylamide-2-methylpropanesulfonic acid (AMPS), and bentonite (bent.). N-BC was prepared via the adsorption of NH4 + onto BC, and its adsorption behavior was investigated. The structure and properties of the resulting samples were characterized using various characterization methods. Results suggest that BSRFs could significantly improve the water-holding and water-retention capacity of soil. BSRFs effectively reduced the nitrogen-release rate (69.8% of nitrogen was released after 30 days), and possessed low nitrogen-leaching-loss amounts (10.3%), low nitrogen migrate-to-surface-loss amounts (7.4%), and high nitrogen-use efficiency (64.27%), as compared to NH4Cl and N-BC, consequently effectively promoting cotton plant growth. The soil burial degradation test of BSRFs indicated that BSRFs possessed good degradability. BSRFs may therefore have promising applications in modern, sustainable agriculture, while MW irradiation is an important strategy with which to produce BSRFs.

A semi-interpenetrating polymer network (semi-IPN) slow-release nitrogen fertilizer with water absorbency was synthesized via microwave irradiation. Urea was incorporated as the nitrogen source into a cotton stalk-g-poly(acrylic acid)/ bentonite (CS-g-PAA/bent) network and linear polyvinylpyrrolidone (PVP). A reaction mechanism for synthesis of CS-g-PAA/ bent/PVP/urea was proposed. Variables influencing water absorbency, such as microwave power, irradiation time, and contents of CS, PVP, and bent were evaluated and optimized. The sample prepared under optimized conditions had a water absorbency of 1018.4 g/g in distilled water and 71.3 g/g in 0.9 wt % NaCl solution. The structure and properties of samples were characterized through various analysis technologies. Swelling measurements and water-retention study indicated that the sample possessed excellent water absorbency in different environments and enhanced the water-retention capacity of soil by introduction of bent and formation of a semi-IPN structure. Investigation of leaching behavior indicated that the sample has the potential to carry the necessary N. The results indicated that about 60% of nutrients were released after 30 days based on anomalous transport, and the sample improved the seed germination rate and promoted cotton growth. Thus, microwave irradiation is a possible method to produce slow-release fertilizers that could be widely applicable in agriculture and horticulture.

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