[Characterization of Microplastics and Diazotrophic Community in Greenhouse Soils with Vegetable Planting Ages in Karst Areas].

Abstract

In recent years， microplastics （MPs） contamination in agroecosystems and its adverse effects on soil health and plants has attracted increasing concern. Biological nitrogen fixation is an important source of nitrogen for the agroecosystem， and soil fertility maintenance can be affected greatly by the diazotrophs. However， there are few studies on the evolution characteristics and interrelationships of MPs and diazotrophs in greenhouse vegetable fields with different planting years in karst areas. This study aimed to analyze the changes in MPs and diazotroph community in different planting years （1， 5， 10， and 12） in karst areas. The results showed that the contents of total nitrogen （TN）， ammonium nitrogen （NH₄⁺-N）， and nitrate nitrogen （NO₃^--N） increased with prolongation of growth. Soil urease （URE） and sucrase （SUC） activities first increased and then decreased with prolongation of growth， and the pH value， organic carbon （SOC）， available potassium （AK）， C/N， and catalase activities （CAT） of soils decreased with prolongation of growth. MPs were detected in all greenhouse vegetable field soil samples at concentrations ranging from （286.67±72.23） to（2 454.33±309.73） n·kg^-1， with a mean abundance of （1 518.58±174.03） n·kg^-1. The abundance of MPs increased with increase of planting years. MPs in greenhouse vegetable fields with different planting years had mostly small size （0?1 mm）， accounting for 55.59% of the distribution. The small size （0-0.1 mm） increased with increase of planting year. Fibers， fragments， and films were the main shapes of MPs， occupying 40.81%， 27.34%， and 23.74% of the MPs， respectively. The color was mainly transparent， accounting for 24.78% of MPs， and the most common polymer type was polypropylene， accounting for 20.83%. With increase in planting years in the greenhouse vegetable fields， the nifH gene and diversity index α changed significantly， with the abundances of nifH gene and diversity index α being promoted significantly to year 5 but being significantly inhibited after year 5. The dominant phyla in the soil diazotrophic community included mainly Pseudomonadota （48.60%） and Thermodesulfobacteriota （44.54%）. Desulfuromonas， Bradyrhizobium， Citrifermentans， and Azohydromonas were the dominant genera. After 5 years of planting， the abundances of Pseudomonadota， Desulfuromonas，Bradyrhizobium， and Citrifermentans were significantly increased， but their relative abundances decreased with increase after planting year thereafter. The pH， SOC， NO₃^--N， C/N， AK， NH₄⁺-N， CAT， and MPs values were the main factors affecting the structure of the bacterial community. Partial least squares path modeling （PLS-PM） analysis showed that planting years have a significant inhibitory effect on the diazotrophic community due to enrichment of MPs. Thus， utilizing a reasonable number of planting years and reducing the input of microplastics would improve microbial activity and provide a basis for sustainable utilization and high-quality production in greenhouse vegetable fields.