A pH-responsive/sustained release nitrogen fertilizer hydrogel based on aminated cellulose nanofiber/cationic copolymer for application in irrigated neutral soils

Abstract

Stimuli-responsive release nitrogen (N)-fertilizer hydrogels are new materials recently introduced to improve control and sustained release N-fertilizer hydrogels efficacy by adding additional functionality for greater N use efficiency. However, the stimuli-responsive behavior and release mechanisms of these fertilizer hydrogels in response to irrigated-soil stimulus fluctuations during different plant growth stages are still facing limitations and have not been considered. Herein, a novel N-fertilizer nanocomposite hydrogel with the function of N pH-responsive/sustained release (pHRSRNFH) was prepared based on wheat straw aminated-cellulose nanofibers (A-CNFs) and cationic poly(acrylamide-co-2-aminoethyl methacrylate hydrochloride) (PAM-PAEM) by direct ammonium nitrate (AN) fertilizer encapsulation. The fertilizer hydrogel structure, properties, and effects on N use efficiency and N metabolism process were characterized by FTIR, zeta potential, SEM, porosity, swelling-release behaviors, TEM, flowcytometric analysis, and N use efficiency indicators. The hydrogel nanocomposite with 3 wt% A-CNFs and 4 wt% AEM (pHRSRNFH4) exhibited desirable enhancement of characterizations, pH-responsive/sustained behaviors, and biodegradability. The soil and buffers release kinetic studies in-vitro revealed the best pH-dependent/sustained AN release of 3.00 and 2.69 mg.day−1 at pH 5.5 and 0.92 and 0.55 mg.day−1 at pH 7.4 from 1g pHRSRNFH4 for 58 and 65 days, respectively, desirably synchronizing with the pH fluctuations of irrigated-neutral soil. The whole lifetime of AN release data revealed that the three sustained delivery stages were governed by the First-order mechanism, followed by Zeroth-order, and finally via the Higuchi mechanism, covering the N crop requirements during different plant growth phases. Application of pHRSRNFH4 at 1 g kg−1 irrigated-neutral soil successfully improved water-holding capacity and N recovery efficiency by 15.7% and 38.6%, respectively, compared with free AN treated soil with over 90-days sustained AN release. Furthermore, positive impacts on rice growth indicators, N-nutrition status, and cell progression were achieved with a feasible-compatible application, demonstrating its further significant potential to be a viable alternative fertilizer formulation for N/water management of cropping systems in the irrigated-neutral soils.