Investigation of Plant Growth and Associated Soil Microbial Stimulation by Digestate Fertilisers

Abstract: The main aim of this PhD thesis was to investigate how different types of liquid anaerobic digestates fertilisers affect plant growth responses, and if these plant growth responses can be associated with a microbial stimulation of the soil due to repeated applications of these biofertilisers. Recent field and laboratory trials have indicated that anaerobic digestates may stimulate the soil/plant interaction in a different way to other fertilisers, with growth enhancement effects sometimes being higher than expected for the amount of nutrients supplied, especially for grass species. The mechanism for this stimulation of plant growth is not fully clear, but it is thought that digestate may stimulate complex interactions between the plant, soil, and soil microorganisms. The thesis is subdivided into five research chapters, where, in two of them described the characterisation of the physical-chemical and microbial properties of different types of anaerobic digestates. The second part is divided into three chapters, based on the results of a one-season fertilisation trial in a glasshouse using different types of plants combination with perennial ryegrass (Lolium perenne L.) and white-clover (Trifolium repens L.), and a two-season fertilisation trial performed in field conditions using a ryegrass sward. Effects of the digestates on plant growth and soil physical-chemical and microbial properties were investigated. Different types of liquid anaerobic digestates exhibited significant differences for most of the physical and chemical traits evaluated, with higher variability found for dry matter (DM) and K (CV= 17.2 and 16.8 respectively), and lower variation for pH and P (CV= 1.78 and 3.55 respectively). Anaerobic digestates exhibited varied quantities and fertiliser potential in terms of plant macro and micronutrients. Most of the anaerobic digestates met the recommendations of Irish standards on the quantity of pathogen indicators and potentially toxic elements. Bacterial and fungal colony-forming units (CFU) ranged widely in liquid anaerobic digestates (105 to 1010; 0 to 105 g-1 DW, respectively). Bacterial, archaeal and fungal gene copies numbers (GCN) showed narrower ranges v than CFU (108 to 1010; 107 to 109; 104 to 106 g-1 dry weight (DW), respectively) between different commercial anaerobic digestates. Microorganisms with agronomic importance were detected in all anaerobic 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. Digestates with different chemical composition, when equally balanced in terms of dry matter, drove comparable forage yield responses in ryegrass and mixed ryegrass/white clover pots. In the glasshouse trial, the soil bacterial (16S) GCN responded to the interaction between fertiliser/vegetation (p<0.05), while archaeal (16S) and fungal (18S) GCN only to the type of vegetation (p