Biofertilisation with anaerobic digestates: A field study of effects on soil microbial abundance and diversity

There is much interest in understanding the effects of repeated applications of anaerobic digestates on soil microbial communities, as well as the biofertiliser value of the microbial community in the digestates. The objective of this study was to evaluate the effects of repeated applications of different types of digestates on soil microbial abundance and diversity in a grassland. Microbial communities in the biofertiliser and soil were quantified by gene copy numbers (GCN) (16S/18S quantitative polymerase chain reaction (qPCR)), deoxyribonucleic acid (DNA) sequencing (Illumina) and colony forming units (CFU). The fertilisation trial was conducted over two years in a ryegrass-dominated grassland. Fertiliser treatments included four different types of digestate, undigested cattle slurry, a nitrogen control with calcium ammonium nitrate (CAN) 27 % N, and a no-fertilisation control. Treatments were randomised in blocks with three replicates. Bacteria had the highest GCN in the anaerobic digestates, followed by archaea; fungi had the lowest. Genes from microorganisms with agronomic/environmental importance were detected in the digestates, including N-fixing bacteria, plant-growth promoting bacteria (PGPB), nitrifying and denitrifying bacteria, arbuscular mycorrhizal fungi (AMF), cellulolytic microbes, methanogens and saprotrophic organisms; however, most of them were found in very low abundances. AMF (Acaulospora) and methanogens were found in considerably higher abundances than other microbes with recognised soil-plant effects or functions. Soil bacterial, fungal and archaeal GCN were not significantly influenced by the type of fertiliser (p > 0.05), and only temporary effects of the application of digestates were noted on the soil bacterial and fungal CFU populations (p < 0.05). The application of digestates had no detectable impact on the soil microbial diversity. Microbial DNA sequences found in abundance in the digestates were not found or only found in low abundance in the soil, an indication that dominant microorganisms present in the biofertiliser failed to establish in soil and/or replace the native microbial populations there, possibly due to niche incompatibilities and competitiveness of indigenous soil microbes.