ABSTRACT
Microplastics (MPs) are widely present in farmland soil as an emerging contaminant. This paper serves as a comprehensive and systematic review of research progress on the characteristics of distribution, abundance, sources, shape, polymer composition, size, and migration of MPs in farmland soils around the world. Moreover, research prospects were also proposed. MPs have been detected in farmland soils around the world, mainly coming from agricultural plastic films, organic fertilizers, sludge, surface runoff, agricultural irrigation, atmospheric deposition, and tire wear particles. The morphology of MPs in soil mainly includes debris, fibers, and films. MPs polymer forms mainly include polyethylene, polypropylene, and polystyrene. Farmland land use significantly affects soil MPs abundance. Additionally, the abundance of MPs increase with the reduction in size. MPs in soil can migrate to deep soil through tillage, leaching, bioturbation, and gravity. Research on soil MPs detection methods, database establishment, safety thresholds, migration and transformation laws, potential ecological health risk assessment, and the construction of prevention and control technology systems should be strengthened in the future. The paper can provide a reference for the risk control and governance of farmland soil MPs pollution.
ABSTRACT
To study the roles of nitrogen fertilization in regulating the physiological bases of fiber specific strength formation in cotton bolls bloomed at different dates (July 15, July 25, and September 10; expressed as PSB, SB, and AB, respectively), field experiments were conducted in two sites with different ecological conditions, i.e., Nanjing (middle lower reach of Yangtze River Valley) and Xuzhou (Yellow River Valley). Three treatments (0, 240, and 480 kg N x hm(-2)) were installed. The results showed that comparing with applying optimum nitrogen (240 kg N x hm(-2)), no N application decreased the N content and increased the C/N in the leaves subtending cotton bolls significantly, and the effects increased with postponing blooming date, which, to a great extent, led to the decreases of the capabilities of the leaves subtending cotton bolls bloomed at SB and AB in the accumulation and transportation of photosynthetic products, the relative growth rate of fiber in cotton bolls, and the activities of key enzymes (sucrose synthetase and P-1,3-glucanase) during the middle-later period of fiber development, and also, the decline of cellulose speedily accumulating duration in cotton fiber and of the fiber specific strength in the bolls bloomed at SB and AB. On the contrary, applying high nitrogen (480 kg N x hm-2) increased the N content but decreased the C/N in the leaves subtending cotton bolls significantly, and the effects decreased with postponing blooming date, which decreased the distribution proportion of photosynthetic products to fiber, the key enzyme activities during the early-middle period of fiber development, and the momentary rate of cellulose accumulation in the fiber of the bolls blooming at PSB and SB, resulting in a significant decrease of the fiber specific strength. It was suggested that appropriate N application could regulate the 'source-sink' relation of cotton, and benefit the high fiber specific strength formation in cotton bolls bloomed at different dates.
