Carbon-rich organic fertilizers to increase soil biodiversity: Evidence from a meta-analysis of nematode communities

Organic fertilizer applications that boost soil fertility and crop production are expected to enhance soil biodiversity, making ecosystems more resilient to stress. Numerous studies have compared biodiversity in soil receiving organic fertilizer to soil under other fertilizer regimes (inorganic fertilizers, unfertilized), yet the data were not analyzed systematically across studies. We evaluated fertilizer effects on soil nematode communities with a meta-analysis of more than 229 data points from 54 studies around the world that were published between 1996 and 2015. Data were from cropland and considered five fertilizer regimes. These regimes include unfertilized soils and those receiving inorganic fertilizers (2 regimes), as well as organic fertilizers (2 regimes). Species richness and total nematode abundance increased with increasing carbon (C) inputs from fertilizers, whereas greater nitrogen (N) application rates from fertilizers significantly reduced the species richness, Shannon⿿s diversity (H⿲), maturity index (MI) and omnivore-predator nematode abundance. This could indicate that high fertilizer N inputs simplifies the nematode community structure and functions. Species richness, omnivore-predator nematode abundance and structural index (SI) increased with the organically-fertilized regime and declined in inorganically-fertilized regimes, suggesting that organic fertilizers can buffer stresses and sustain soil food web functions. Furthermore, organic fertilizers differed in their impact on soil nematodes, as those with C-rich crop residues supported larger free-living nematode populations and greatly promoted H⿲, SI and enrichment index (EI), whereas N-rich animal manure was more effective in controlling plant-feeding nematodes. Our review suggests that the application of C-rich crop residues is the most effective practice to enhance soil biodiversity in intensively managed agroecosystems, highlighting the importance of regular applications of straw and other C-rich residues to preserve the ecological resilience of cropland.