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Despite the known influence of continuous cropping on soil microorganisms, little is known about the associated difference in the effects of continuous cropping on the community compositions of soil bacteria and fungi. Here, we assessed soil physicochemical property, as well as bacterial and fungal compositions across different years (Uncropped control, 1, 6, 11, 16, and 21 years) and in the watermelon system of a gravel mulch field in the Loess Plateau of China. Our results showed that long-term continuous cropping led to substantial shifts in soil bacterial and fungal compositions. The relative abundances of dominant bacterial and fungal genera (average relative abundance > 1.0%) significantly varied among different continuous cropping years (P < 0.05). Structural equation models demonstrated that continuous cropping alter soil bacterial and fungal compositions mainly by causing substantial variations in soil attributes. Variations in soil pH, nutrient, salinity, and moisture content jointly explained 73% and 64% of the variation in soil bacterial and fungal compositions, respectively. Variations in soil moisture content and pH caused by continuous cropping drove the shifts in soil bacterial and fungal compositions, respectively (Mantel R = 0.74 and 0.54, P < 0.01). Furthermore, the variation in soil bacterial and fungal composition showed significant correlation with watermelon yield reduction (P < 0.01). Together, long-term continuous cropping can alter soil microbial composition, and thereby influencing watermelon yield. Our findings are useful for alleviating continuous cropping obstacles and guiding agricultural production. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal (AM) fungi are an important determinant of plant performance. The temporal distribution patterns of AM fungal communities, important groups in forest soil, are affected by various environmental factors. However, the effect of seasonal variation on AM fungal communities in forest soil is still not fully understood. To improve knowledge regarding the seasonal dynamics of AM fungal communities in forest soil, the Juglans mandshurica rhizosphere soil in a 30-year-old natural J. mandshurica and Larix gmelinii mixed forest in northeastern China was sampled and studied during different seasons. AM fungal community composition and structure in the rhizosphere soil were determined by using the Illumina MiSeq sequencing platform. The results showed that forest type significantly affected soil pH and the microbial biomass carbon (C mic), microbial biomass nitrogen (N mic), C, N, P, NH 4 +-N and NO 3 −-N contents. Similarly, soil moisture significantly decreased with seasonal progression in all forest types. Glomus and Diversispora were the dominant genera. The Shannon, Sobs and Chao1 indices of the mixed forest were higher in July than in May and September. The NO 3 −-N and water-soluble phenol (WSPh) contents were the most important factors driving the shift in the soil AM fungal community across the seasonal gradient. Network analysis revealed that AM fungal network structure was significantly different between seasons, and the number of links between species significantly increased with time. We observed a clear tendency for mixed forest management to significantly affect AM fungal community structure and composition but not diversity. Furthermore, the co-occurrence network and keystone taxa among AM fungi can change in response to environmental filtering due to mixed management in different seasons. • Mixed management had a significant effect on AM fungal community structure. • There was obvious seasonal variation in the AM fungal community in the mixed plantation. • Changes in NO 3 −-N and water-soluble phenol altered the AM fungal community. • AM fungal network structure was dynamic throughout the growing season. • Most of the hub OTUs belonged to Glomeromycetes in all seasons. [ABSTRACT FROM AUTHOR]