Organic amendments, soil microbial diversity and extracellular enzyme activity: a critical review with emphasis on tropical and Nigerian soils

Organic amendments are central to restoring degraded agricultural soils, yet their effects on soil microbial communities and the enzymes that drive nutrient cycling are often summarized too simply. This critical review synthesizes evidence from global meta-analyses and from field studies in sub-Saharan Africa, with emphasis on Nigeria, on how compost, animal manures, crop residues, green manures and biochar affect microbial biomass, diversity, community composition and extracellular enzyme activity. Across meta-analyses, organic amendments increased microbial biomass carbon by 36–63% relative to mineral fertilization or unfertilized controls, and increased C-, N- and P-acquiring enzyme activities by about 22–48%. Effects on α-diversity were smaller and less consistent than effects on biomass and community structure. Compositional shifts commonly favoured Bacteroidetes and Proteobacteria over Acidobacteria, but amendment-induced pH change was a major driver of these shifts. Biochar differed from labile amendments: it increased microbial biomass and some N- and P-cycling enzymes while sometimes suppressing C-cycling enzymes, mainly through pH, habitat and sorption effects that can also bias enzyme assays. Responses were moderated by amendment quality and rate, duration, soil pH, texture and initial organic carbon. African field studies, including long-term trials in Burkina Faso and Kenya, broadly support these patterns, but Nigerian evidence remains scarce, short-term and often culture-based. Priorities include long-term trials on Nigerian Alfisols and Ultisols, standardized enzyme assays, sequencing-based community analysis, explicit tests of diversity–function links, and assessment of trade-offs such as antibiotic-resistance genes, metals and nitrous oxide emissions.