Hunan Research Academy of Environmental Sciences

The angle of repose is a fundamental parameter for assessing the stability of coral reefs. However, predictive models for this angle are currently lacking. In this study, a series of laboratory experiments were undertaken to investigate the angle of repose by varying moisture content, particle shape, and particle size. Based on our experimental data, variation in the angle of repose with moisture content is classified into five distinct zones. It is demonstrated that the range of moisture content for each zone varies with particle size. Coral sands of dendrite, flake, rod, and block particles have a descending order of angle of repose, as demonstrated for a sieve size of 4.5 mm. The angle of repose for dry, submerged, and steady coral sands exhibits a correlation with the nominal diameter of particle size. Finally, extended models are proposed for predicting the angle of repose of coral sands (R2 = 0.8, Dn50 = 0.317−5.470). To facilitate use of these models, a linear relationship between sieve particle size diameter, nominal particle size diameter, and Corey shape factor, allowing for conversion among these parameters, is established. This study thereby helps to enhance our understanding of how moisture content affects angle of repose and improve our ability to predict the angle for coral grains with intricate geometries.

Gastrodia dayaoshanensis, a new species from Dayaoshan National Nature Reserve, Guangxi, China is described and illustrated. The new species is morphologically similar to G. peichatieniana, G. longistyla and G. menghaiensis, but can be distinguished from G. peichatieniana mainly by its brown flowers, sepals and petals entire or with a few teeth at margin, united portion of lip with 2 ridges, and from G. longistyla mainly by its wider free portions of sepals, united portion of lip with 2 ridges, stigma without appendage, and from G. menghaiensis mainly by its brown and straight perianth tube, sepals and petals entire or with a few teeth at margin, united portion of lip without calli. A detailed taxonomical description along with photoplates and a comparison with the allied species have been provided to aid in easy identification.

ABSTRACT Irrigation is essential for agricultural production, serving not only to transport water but also to facilitate the migration of pollutants such as microplastics (MPs). Although the presence of MPs in agricultural soils is well documented, the effects of irrigation hydrodynamics on their distribution have rarely been explored. This study examines the occurrence and sources of MPs in various sections of paddy soils from the Quyuan Irrigation District (QID) in China. MP analysis was conducted using density separation, microscopy, and laser direct infrared spectroscopy (LDIR) methods. Results showed that MP abundance ranged from 73.30 to 3880.00 items/kg. Statistical analyses revealed that agricultural plastic residues were the primary source of MPs. As a result of irrigation activities, the abundance of < 0.5 mm MPs in paddy soils near branch canals was 2.17 times higher than that in soils near the main canal, indicating more severe MP pollution near the branch canals. In addition, the MP abundance in soil at the drainage outlet (DO) was 2560 items/kg, significantly higher than the 1440 items/kg found at the irrigation inlet (IN) ( p < 0.05). This was due to an increase in water velocity and flux, which generated stronger water impact forces that flushed most MPs into the DO soil. The irrigation hydraulic conditions and MP characteristics influenced the horizontal migration of MPs in surface paddy soils. Overall, this study elucidated the relation between MP pollution and irrigation hydrodynamics, providing valuable insights for precise monitoring and control of MP pollution in farmland.