Research advances on the accumulation and degradation of microcapsules derived from polymer-coated controlled-release fertilizers and their effects on soil quality

The controlled-release characteristics of most controlled-release fertilizers (CRFs) rely on polymer-coating materials. The release of coated nutrients into soil simultaneously releases microcapsules. However, it remains unclear whether microcapsule accumulation and their degraded products adversely affect soil quality and safety. This paper reviewed the literature on microcapsules’ accumulation and degradation performance in soil and their possible effects on soil physical, chemical, and biological properties. We found that: 1) The quantity of microcapsules accumulated in fields was almost equivalent to that applied to the fields, as reported in recent studies. The occurrence, distribution, and accumulation of microcapsules in different soil types and depths differ. 2) The degradation process, products, and fate of different microcapsules correlate to the texture of polymer materials, which requires further verification. Moreover, the microcapsule's degradation rate can be improved by modifying the polymer materials. 3) Microcapsule’s impact on crop growth, soil physical, chemical, and biological properties is affected by polymer materials, soil type, and application amounts. The short-term (or low concentration) accumulation of microcapsules showed no significant effects (P>0.05) on soil properties. However, there is a need to investigate the threshold value at which soil properties become vulnerable to the accumulation of microcapsules. Therefore, we propose the following recommendations: 1) Monitoring and evaluating the occurrence, distribution, and accumulation of microcapsules derived from different CRFs applied on farmlands over the long term; 2) Developing and improving the techniques and methods of sampling, visualization, and quantification procedures of microcapsules and their secondary degradation products to establish standardized methods; 3) Exploring microcapsules’ destination and migration process and their secondary degradation products in soil, and their effect on different soil types and crop varieties; 4) Verifying the relationship between the microcapsules’amounts and their effects on soil physical, chemical, and biological properties and the mechanisms, and developing the natural, biological or biodegradable CRF membrane materials, and studying the relationship between membrane degradation and nutrient release, thereby building a new biodegradable CRF membrane degradation and nutrient release model. These can help provide a more comprehensive overview of the application, development, and ecological evaluation of CRFs in the future.