Diversity of arbuscular mycorrhizal fungi in greenhouse soils continuously planted to watermelon in North China

In North China, watermelon is grown in commercial greenhouses in a continuous monoculture and with high application rates of manure or compost. The aim of this study was to determine how the diversity of arbuscular mycorrhizal fungi (AMF) in these soils changed over long periods (0 to 20 years) of monoculture. AMF in control soils (from fields not replanted with watermelon and located near the greenhouses) and in greenhouses (in Daxing, Beijing, and Weifang, Shandong) that had been continuously replanted with watermelon for 5, 10, 15, or 20 years (three greenhouses per year per location) were identified and quantified based on spore morphology and on denaturing gradient gel electrophoresis (DGGE). The total number of AMF species and genera were 13 and 3 in soils replanted for 5-20 years and 19 and 4 in control soils. AMF species richness (SR), the Shannon-Wiener index (H), and spore density declined as the number of years in which watermelon was replanted increased. The available phosphorus, potassium, and nitrogen in the soil increased as the number of years in which watermelon was replanted increased. Values for SR and H were higher when based on DGGE than on spore morphology. The results suggest that current greenhouse practices in North China reduce the AMF diversity in the soil.