The Effects of Arbuscular Mycorrhizas on Phosphorus Cycling and Leaching In Soils

Abstract

The leaching of phosphorus (P) from soils is of major concern worldwide. In agricultural production systems P leaching represents an inefficient use of a limited resource, as well as potentially leading to contamination of aquatic systems. Arbuscular mycorrhizal fungi (AMF) establish a symbiotic with the majority of terrestrial plants. When the fungi form these associations with plants, they can take up P from the soil and deliver it to the plant. In doing so, they not only improve plant P nutrition, but can also reduce the risk of soil P loss via leaching. While effects of arbuscular mycorrhizas (AM) on plant P acquisition are very well understood, their impacts on soil P leaching are only just starting to be explored. The aim of the work presented in this thesis was to study the effects of AM on soil P leaching, with an emphasis on P dynamics in the soil-plant-leachate system. A series of greenhouse and field experiments were conducted to explore this issue, including: two microcosm studies to investigate mycorrhizal and soil effects on plant biomass, plant P nutrition, soil P after leaching events, leachate volume, and the amount and chemical composition of P in leachates (Chapter 2 and 3); a field-based study to investigate mycorrhizal effects on fruit yield, soil moisture, soil (16S) bacterial community composition, and soil P loss under realistic field conditions (Chapter 4); and a final set of experiments using nuclear magnetic resonance (NMR) spectroscopy, to investigate the storage of P in the external hyphae of AMF (Chapter 5). All leaching experiments (field and glasshouse) made use of a mycorrhiza defective tomato mutant and its mycorrhizal wild-type progenitor, to study AM effects in the field. This approach avoids the potentially confounding effects of soil sterilisation, which is commonly used to establish non-mycorrhizal control treatments. The outcomes of the work presented in this thesis confirm the positive impacts of AM on plant P uptake, plant biomass and tomato fruit yield and nutrients (Chapters 2, 3, 4 and 5). The presence of roots, regardless of mycorrhizal colonization, had a significant impact on soil P, total P and the chemical composition of P in leachate (Chapter 2 and 3). An important finding of this work was that roots increased the concentration of dissolved organic carbon (DOC) in leachate, and this increase in DOC concentration coincided with an increase in the concentration of P leached (Chapter 2 and 3). In the field experiment, AM had no significant effect on soil moisture, leachate volume or soil (16S) bacterial communities (Chapter 4). Importantly, soil texture affected mycorrhizal colonization, plant P and the amount of P and DOC leached, highlighting the need for results to be carefully considered in context (Chapter 3). This may be especially important in the context of nutrient leaching given the importance of soil texture in water movement through soil. While not a major theme of the work presented here, the 31P NMR spectra identified a polyphosphate (PolyP) peak in mycorrhizal external hyphae, confirming the importance of PolyP in hyphal P storage (Chapter 5). Taken together, this study provides new insights into the impacts of the below-ground plant systems and AMF on soil P leaching in soils.