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AbstractBiodegradable polymer-coated controlled-release fertilizers (PC-CRFs) are essential means to reduce cost, improve marketability, conserve land fertility, achieve high crop yields and combat climate challenges. It is known that about 15–30% of any fertilizer packed in a PC-CRF does not get released due to the concentration gradient difference across the polymer coatings. To release the trapped fertilizer(s), it is desired that polymer-based coatings should biodegrade after the fertilizer is completely released into the soil. This review has aimed to provide a comprehensive account for various biodegradable polymers/blends derived either from natural or synthetic sources which are cited in the literature for PC-CRFs. In addition, this review covers the discussion on their classification criteria, trends in the processes of fertilizer coatings, methodological issues for their biodegradation assessment, coating attributes that affect the biodegradability and an outlook into their biodegradation kinetic models that involve enzymes and microbial processes. It also concludes that experimental as well as modeling data are insufficient to assess the biodegradation contribution of the overall nutrient release in PC-CRFs.

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Field geomorphology and remote sensing data, supported by Optical Stimulated Luminescence (OSL) dating from the Mandakini river valley of the Garhwal Himalaya enabled identification of four major glacial events; Rambara Glacial Stage (RGS) (13 ± 2 ka), Ghindurpani Glacial Stage (GhGS) (9 ± 1 ka), Garuriya Glacial Stage (GGS) (7 ± 1 ka) and Kedarnath Glacial Stage (KGS) (5 ± 1 ka). RGS was the most extensive glaciation extending for ∼6 km down the valley from the present day snout and lowered to an altitude of 2800 m asl at Rambara covering around ∼31 km 2 area of the Mandakini river valley. Compared to this, the other three glaciations (viz., GhGS, GGS and KGS) were of lower magnitudes terminating around ∼3000, ∼3300 and ∼3500 m asl, respectively. It was also observed that the mean equilibrium line altitude (ELA) during RGS was lowered to 4747 m asl compared to the present level of 5120 m asl. This implies an ELA depression of ∼373 m during the RGS which would correspond to a lowering of ∼2 • C summer temperature during the RGS. The results are comparable to that of the adjacent western and central Himalaya implying a common forcing factor that we attribute to the insolation-driven monsoon precipitation in the western and central Himalaya.

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The urea-crosslinked starch (UcS) film has a major drawback of very rapid biodegradability when applied as slow release fertilizer in soil. Lignin reinforcement of the UcS was used to prepare composite films, aimed to reduce the starch biodegradability and slow the release of nitrogen in aerobic soil condition. Study results revealed that mineralization of the composite films was delayed from 6.40 to 13.58% more than UcS film. Inhibition of composite films mixing with soil, the Michaelis-Menten reaction rates for α-amylase were inhibited ~1.72–2.03 times whereas the Michaelis-Menten reaction rates for manganese peroxidase were increased ~1.07–1.41 times compared to UcS film. Saccharides–glucose, maltose and maltotriose demonstrated that their rates of formation (zero-order reaction) and depletion (first-order reaction); both were slowed more in aerobic soil which received the composite films. Increasing of lignin in composite films, the acid to aldehyde ratios of vanillyl and syringyl phenols of the lignin declined from 1.18 to 1.17 (~0.76%) and 1.59–1.56 (~1.78%), respectively. The diffusivity of nitrogen was effectively slowed 0.66–0.94 times by the lignin in composite films and showed a “Fickian diffusion” mechanism (release exponent n=0.095–0.143).

High biodegradability of starch is a major limitation for its commercial usage in developing urea‐crosslinked starch (UcS) film as slow‐release fertilizer. For solving this problem, UcS films were reinforced with 5–20% kraft lignin. Implication of lignin as a macromolecule was tested for slowing the biodegradability of UcS films. These films were biodegraded and characterized in an aerobic soil burial test up to the 60th day. The results were drawn for biodegraded lignin‐reinforced films through comparison made with biodegraded control film, which received 0% lignin. The Fourier transform infrared spectroscopy peaks at 1625 and 1665 cm−1 corresponded to UcS and were found to be more conspicuous in biodegraded lignin‐reinforced films. Thermogravimetric analysis of biodegraded lignin‐reinforced films showed higher thermal stability. This was inferred from the decrease of ∼85.45°C in the thermal decomposition temperature at 5% weight loss (onset temperature), the increase of ∼31.69°C in the thermal decomposition temperature at maximum weight loss, and the increase of ∼12.90% in char. The molecular weight distribution of the biodegraded lignin‐reinforced films reduced not more than ∼1% and the polydispersity index was conserved to 1.4. Light microscopy of the biodegraded lignin—reinforced films reduced not more than ∼1% and polydispersity index was conserved to 1.4. Light microscopy of biodegraded lignin‐reinforced films showed the shape of starch particles was oblong, less disrupted, and wrinkled. Field‐emission electron microscopy showed lignin addition favored more the fungal growth beside formation of cavities. Atomic force microscopy showed the average surface roughness increased 2.00–7.32 times more as a result of residual lignin's accumulation in biodegraded lignin‐reinforced films. Based on the understanding of biodegradability in UcS films, a theoretical framework has also been proposed for biodegradability‐driven urea‐nitrogen release in soil.