Arbuscular mycorrhizal fungi (AMF) can form a mutualistic symbiosis with over two-thirds of all land plants, providing phosphorus and/or nitrogen in exchange for carbon. They can have a significant effect on the surrounding soil, altering pH, water content, structure, and drainage. Important greenhouse gases (GHG) including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) can be influenced by these factors, yet to date the interactions between AMF and soil GHG fluxes are surprisingly understudied.
A microcosm system was developed to study GHG fluxes in the presence and absence of AMF hyphae. A central compartment contained an AMF host plant (Zea mays L.), with two outer compartments, that either allowed (AMA) or prevented (NAMA) AMF hyphal access. Organic matter patches of dried, milled, Z. mays leaves mixed with soil were added to the outer compartments to encourage proliferation of AMF hyphae and GHG production. Soil-atmosphere fluxes of N2O, CO2 and CH4 from the outer compartments were quantified, and gas probes were developed to measure N2O concentrations within the organic matter patches.
Data from a series of microcosm experiments provide evidence for AMF interactions with soil fluxes of N2O and CO2, but not CH4. Soil CO2 fluxes were found to be a useful non-invasive method for determining the presence of AMF in hyphal compartments. The N2O concentrations in organic patches decreased in AMA treatments, and a subsequent experiment demonstrated that N2O production by nitrifiers may be limited in the presence of AMF hyphae. In contrast, following harvesting, N2O fluxes from organic matter patches were higher in the AMA treatment; possibly because carbon release from severed AMF hyphae fuelled denitrification. These interactions have important implications for N cycling and sustainable agriculture. The evidence presented in this thesis suggests that AMF may play a previously unappreciated role in reducing soil-atmosphere losses of N2O.
