A meta-analysis of the effect of organic and mineral fertilizers on soil microbial diversity

Organic agriculture, employing manures or composts, has been proposed as a way of mitigating undesirable impacts of mineral fertilizer use. Of particular interest is the effect of fertilizer regime on soil microbes, which are key to nutrient cycling, plant health and soil structure. However, the effect of fertilizers on soil microbial diversity remains poorly understood. Since biological diversity is an important determinant of ecosystem function and a fundamental metric in community ecology, the effects of fertilizer regimes on soil microbial diversity are of theoretical and applied interest. Here, we conduct a meta-analysis of 37 studies reporting microbial diversity metrics in mineral fertilized (NPK), organically fertilized (ORG) and unfertilized control (CON) soils. Of these studies, 32 reported taxonomic diversity derived from sequencing, gradient gel electrophoresis, or RFLP. Functional diversity, derived from Biolog Ecoplate™ measures of carbon substrate metabolism, was reported in 8 studies, with 3 studies reporting both diversity metrics. Bacterial and archaeal diversity was reported in 28 taxonomic studies, and fungal diversity in 8 taxonomic studies. We found that functional diversity was 2.8% greater in NPK compared with CON, 7.0% greater in ORG vs. CON, and 3.8% greater in ORG vs. NPK. Bacterial and archaeal taxonomic diversity was not significantly different between NPK and CON, but on average 2.9% greater in ORG vs. CON, and 2.4% greater in ORG vs. NPK. Fungal taxonomic diversity was not significantly different between any treatment pairs. There was very high residual heterogeneity in all meta-analyses of soil diversity, suggesting that a large amount of further research is required to fully understand the influence of fertilizer regimes on microbial diversity and ecosystem function.