Effects of bird aggregation on the soil properties and microbial community diversity of urban forest fragments

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Forest fragments in urban parks provide important habitat for birds. However, the guano deposited by large aggregations of birds in such fragments can dramatically change soil properties, which in turn, can alter soil microbial community composition, potentially affecting the forests' composition and survival. To investigate the effects of bird aggregations on the soil of fragmented urban forests, we compared the soil properties and microbial communities of two forested islands, one in Liuhuahu park and the other in Wanzuitou park, Guangzhou, where large numbers of birds aggregate yearly to nest. Comparison to sites without bird aggregations suggests that decades of guano deposition appears to have caused soil acidification and an increase in soil nutrients. The relative abundance of the soil bacterial phylum Actinobacteria and the soil fungal phylum Ascomycota were significantly lower in soil under bird aggregations. The aerobic nitrite oxidation, nitrate reduction and cellulolysis bacterial guilds were significantly less abundant under bird aggregations in Liuhuahu park. The wood saprotroph fungi guild was significantly less abundant under the bird aggregation in Liuhuahua park and the pathogenic fungi guild significantly more abundant in Wanzuitou park. Soil properties, including TN, NO3−-N, TOC and pH, explained the variation in Ascomycota and Basidiomycota abundance, and the alpha-diversity of the fungal community. Microbial community variation could potentially slow the rate of decomposition and disease resistance of plant in these forests. We suggest that sufficient contiguous forest should be maintained in urban areas to reduce the density of bird aggregations in isolated forest fragments.