The Silent Sink: A Comprehensive Review of Microplastic Accumulation in Agricultural Soils and its Impact on Crop Physiology

The abundance of microplastics (MP) in agricultural soils has launched a "silent sink" of pollution that endangers global food security. Originating from plastics from plasticulture, sewage sludge and irrigation water, MPs are accumulating in arable lands at alarming rates, leading to a fundamental change in the soil biophysical environment. This review performs a critical synthesis of the mechanisms of disruption of the soil-plant continuum by MPs. We highlight that MPs contribute to increasing the porosity and impair aggregate stability of soils, worsening erosion and drought stress. Chemically, they interrupt the cycle of nutrients in a way that the biodegradable plastics have a paradoxical effect of "carbon catabolite repression" that starves plants of nitrogen. Physiologically, below microscopic level value particles enter root tissues and cause oxidation stress, genotoxicity and hormonal imbalances which stunt root growth and reduce the rate of photosynthesis. Furthermore, MPs are vectors for heavy metals, antibiotics and pathogens, which help to transfer them into the food chain. We also point out a "biodegradable paradox", through which environmentally benign alternatives are potentially more phytotoxic than conventional plastics. Finally, we assess new approaches for remediation such as the potential of biochar amendment and microbial bioaugmentation approaches to restoration of soil health. This review highlights the importance of urgent standardization of monitoring and global governance for the reduction of the increasing threat of 'white pollution' in agroecosystems.