The influence of fertiliser type on mycorrhizal colonisation and plant growth in horticultural systems

Conventional synthetic fertilisers are widely used in many horticultural systems and while there is an increasing interest in alternative nutritional management strategies which better promote soil health, there is a gap in studies which objectively compare many of these alternatives. Previous research has highlighted the benefits of alternative nutrient management strategies on soil biota, including mycorrhizal fungi which are important symbionts of many crops. To investigate the difference between conventional fertilisers and a variety of alternative nutrient sources on mycorrhiza and plant growth, research trials were conducted in two systems, sunflower and perennial deciduous tree crops, over three growing seasons. The effect of nutrient source and bio-inoculants on productivity of sunflower was examined in pot trials under glasshouse conditions in three separate experiments. Firstly, inoculation with “effective microbes” (EMs) was examined with an organic humate based soil amendment with and without inorganic fertiliser. Secondly, inoculation with a commercial spore preparation of arbuscular mycorrhizal fungi (AMF, MYCORMAX™) was used in conjunction with a range of fertiliser treatments including a liquid organic fertiliser (LOF) and a liquid inorganic fertiliser (LIF) to test sunflower productivity, mycorrhizal colonisation and nutrient uptake. Thirdly, a range of organic soil amendment treatments (Ferbon®, compost, soluble humate granules (SHG)) were examined in conjunction with the same AMF spore inoculation to further explore the factors described above. In the deciduous tree crops of apples and cherries, the influence of alternative orchard floor management strategies on AMF colonisation, and the availability of nutrients in the soil and leaf nutrient status were examined. Additionally, the response of AMF colonisation to conventional and alternative nutrient regimes, and its correlation with the flavour development of apple and cherry fruit were investigated. Inoculation with EM significantly increased sunflower plant height, number of nodes, stem diameter and early flowering compared to non-inoculated plants but significantly reduced leaf chlorophyll content when nutrient deficiency was observed. Soil amendment with lignite-based humates such as Ferbon® led to an increase in plant height and flower diameter compared to control plants, but did not influence leaf chlorophyll content or number of nodes. The findings demonstrated that, with the availability of sufficient accessible nutrients, both EM and lignite-based humates have the ability to increase plant productivity. AMF inoculation in combination with LIF increased sunflower plant height, stem diameter and leaf chlorophyll content. Sunflower growth attributes were only slightly influenced by AMF with LOF. Node number and flower head diameter also significantly increased with AMF colonisation. Use of LOF as a nutrient source increased concentration of foliar P, K and B and percentage of AMF colonisation. Sunflower productivity was affected positively by AMF inoculation and soil amendment with organic supplements. AMF inoculation with compost greatly increased plant height while AMF and SHG increased both leaf chlorophyll content and stem diameter. Positive relationships were found between AMF inoculation plus organic supplements and nutrient uptake by plants as well as nutrient release in the soil. The inoculation of AMF plus SHG significantly increased leaf nutrient levels of Ca, Mn and Zn in plants grown in an orchard sourced soil compared to those grown in forest sourced soil. In the perennial tree crop studies, nutrient regime had no impact on colonisation of tree roots by resident AMF species. Biochar applied as a soil amendment led to significantly increased AMF colonisation, while planting a combination of clover/grass in the tree row increased the presence of AMF arbuscules and vesicles. In the two cherry sites, mycorrhizal colonisation was not affected by the nutrient regime. An interaction between nutrient regime and EM was observed, with EM application increasing colonisation in ‘Lapin’ cultivar roots when applied to alternative regime plots, but reducing colonisation in conventional plots. However the reverse occurred in the ‘Sweetheart’ cultivar roots. This difference between the two cultivars is most likely influenced by the different soil types in these two orchards. Properties of both apple and cherry fruit were influenced by AMF colonisation with a positive correlation between colonisation and both total soluble solids (TSS) and titratable acidity (TA) but a negative correlation with the ratio of TSS/TA. Soil nutrient levels were adequate in the alternative plots where humates were applied, and levels of some nutrients were higher in these plots than in the conventional plots. Hence it can be concluded that nutrient regimes based on application of humates, particularly in conjunction with biochar or EM inoculation have the ability to improve soil nutrient uptake, leaf nutrient levels and fruit properties in the presence of mycorrhizal activity. The studies reported in this thesis demonstrate that inoculation with EM and/or AMF can have significant effects on plant growth, and that, in the presence of an appropriate level of nutrients, EM and humates have the ability to increase sunflower productivity. These studies have also demonstrated that orchard floor management has a greater impact on AMF colonisation than fertiliser regime. The degree of AMF colonisation and its interaction with both fertiliser applications and orchard floor management are strongly correlated with fruit quality, but not always associated with plant growth.