Investigating the effects of organic amendments on soil microbial composition and its linkage to soil organic carbon: A global meta-analysis

Understanding the dynamics of soil microbial communities responsible for soil element cycling is vital to understanding organic amendments' mechanisms in agricultural soil. However, several studies show inconsistencies in whether and how organic amendments affect the taxonomic composition of soil microbial communities compared to the application of sole chemical fertilizers. This first global meta-analysis demonstrated that organic amendments increased the bacterial diversity indices (Shannon and Chao1) but had no significant effect on fungal diversity indices. When considering both bulk and rhizosphere soils, only copiotrophic strategies such as Proteobacteria, Bacteroidetes, and Zygomycota phylum demonstrated a significant increase in response to organic amendments, mainly because the environment with a significant increase in nutrients content preferentially supports the growth of copiotrophic species after the use of organic amendments. Additionally, the factors influencing the response of different dominant microbial phyla to organic amendments varied. Besides soil pH, the effect of organic amendments on different microbial phyla was significantly influenced by soil texture, organic fertilizer type, crop type, and climate type, providing insights into the diverse responses of microbial communities to organic amendments under varying conditions. Organic amendments significantly increased soil organic carbon (SOC) content and enzyme activities related to nitrogen (N) and phosphorus (P) decomposition but had no significant effect on enzymes related to carbon (C) decomposition. Notably, the effect of organic amendments on the relative abundance of three dominant phyla (Mortierellomycota, Nitrospirae, and Firmicutes) was related to the effect on SOC, where the increase in the relative abundance of Firmicutes was significantly positively associated with the increase in SOC. This result has implications for understanding the relationship between the dynamics of microbial community composition and C turnover in agroecosystems.