Nitrogen (N) losses from conventional N fertilizers contribute to environmental degradation and low N use efficiency. Highlighting the need for slow-release fertilizers (SRFs) to mitigate these problems, this study aims to develop slow-release N fertilizers using starch-grafted-poly[(acrylic acid)-co-acrylamide] based nanoclay polymer composites (NCPCs) and investigate their efficacy for slow N delivery in soil. Three types of NCPCs, NCPC(A) (poly [(acrylic acid)-co-acrylamide]), NCPC(W) (wheat starch-grafted-poly[(acrylic acid)-co-acrylamide), and NCPC(M) (maize starch-grafted-poly[(acrylic acid)-co-acrylamide) were prepared and characterized using FTIR spectroscopy and X-ray diffraction techniques. N-release behaviour of the products was assessed under two distinct soils, i.e., Assam (Typic Hapludults, pH 4.2) and Delhi (Typic Haplustepts, pH 7.9) soils. Additionally, the effects of varying soil moisture and temperature levels on N release were studied in the Assam soil. The N-release kinetics of the synthesized fertilizers were assessed using zero-order, first-order, Higuchi, and Korsmeyer−Peppas models. Degradability of the NCPCs was evaluated by measuring evolved CO₂–C under various soil conditions as an indicator of microbial degradation. The results indicated that NCPC fertilizers significantly slowed down the release of N compared to urea. According to the R² values obtained, it was evident that the first-order kinetic model most accurately describes the N release from both urea and NCPC-based N fertilizers in the studied soils. Among the formulations, NCPC(A) exhibited the lowest N release (42.94–53.76%), followed by NCPC(M) (51.05–61.70%), NCPC(W) (54.86–67.75%), and urea (74.33–84.27%) after 21 days of incubation. The rate of N release was lower in the Assam soil compared to the Delhi soil, with higher soil moisture and temperature levels accelerating the release. Starch addition improved the biodegradability of the NCPCs, with NCPC(W) showing the highest cumulative CO₂-C evolution (18.18–22.62 mg g⁻¹), followed by NCPC(M) (15.54–20.97 mg g⁻¹) and NCPC(A) (10.89–19.53 mg g⁻¹). In conclusion, NCPC-based slow-release fertilizers demonstrated a more gradual N release compared to conventional urea and the inclusion of starch enhanced their degradability in the soil, which confirms their potential for sustainable agricultural applications. However, soil properties and environmental factors influenced the N release and degradation rates of NCPCs.

This laboratory study was conducted during October, 2023 to December, 2023 at Indian Agricultural Research Institute, New Delhi and focused on the synthesis, characterization, and evaluation of poly (acrylic acid)-co-acrylamide / bentonite based nanoclay-polymer composites (NCPCs) for their potential application as slow-release nitrogen fertilizers. NCPCs were synthesized by co-polymerization reaction between partially neutralized aqueous acrylic acid solution with acrylamide and bentonite clay (in varied amount; 8, 12 and 16% by weight of acrylic acid), in presence of crosslinker and free radical reaction initiator. The composites were characterized using Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) techniques for physical properties. The water absorption and retention studies revealed high swelling capacities of the NCPCs, enabling gradual water and nitrogen release. Nitrogen release studies revealed that NCPCs loaded with urea ammonium nitrate (UAN) in soil showed a slower (40–60% less total N release over 28 days) and more sustained release compared to conventional UAN fertilizer. NCPCs regulated nitrogen availability by releasing ammonium (NH4⁺) and NO₃ at a slower rate during the incubation period, but the effect was more visible upto 15 days. Further N release from UAN-loaded studied using zero-order, first-order and Higuchi’s kinetic reaction model; results highlighted that N release rate followed first-order kinetic reaction evident from higher R² (0.93) over others. In conclusion, NCPCs can sustain N availability from UAN in soil for longer duration and offers its suitability for agricultural application for better nutrient use efficiency over conventional UAN fertilizer.