The following thesis is a two-part study, investigating the influences of biochar (charcoal) on arbuscular mycorrhizal fungi (AMF). The first part of this study is a critical examination and conceptual overview of the literature regarding biochar and AMF available before July 2007. In the second part, I present three experiments all designed to evaluate the influences of biochar applications on AMF abundance in primarily temperate, neutral pH soils. This course of research was selected through an exstensive review of the literature suggesting that biochar presence can strongly affect both soil microbial populations, including mycorrhizal fungi, and biogeochemistry. As both biochar and mycorrhizal associations are subject to management, and because both components are potentially important in various ecosystem services provided by soils (e.g., sustainable plant production) understanding and exploiting interactions between them could be advantageous. After reviewing the experimental evidence for such effects, four mechanisms are proposed by which biochar could influence mycorrhizal abundance and/ or functioning. These mechanisms are: a) alteration of soil physico-chemical properties; b) indirect effects on mycorrhizae through effects on other soil microbes; c) plant-fungus signaling interference and detoxification of allelochemicals; and d) provision of refugia from fungal grazers. Through this overview, a roadmap for research is provided, which is aimed at testing these mechanistic hypotheses. Using this proposed framework as a template, three experiments were designed and implemented, incorporating three different soils, five different biochars, and eight different biochar application rates. Through these experiments, it was illustrated that five different types of biochar are all capable of significantly altering soil orthophosphate availabilities, with four of these biochars not significantly affecting soil pH. Overall, our results indicate that AMF abundances were either unchanged or decreased with biochar amendment across multiple treatments. These results also indicate that biochar, depending on the nature of the feedstock, the temperature attained during pyrolysis and amounts applied can significantly alter soil properties including phosphate availability. These findings may have implications for soil management where the goal is to increase the services provided by AMF.

Inhibition of nitric oxide (NO) signalling may contribute to the pathogenesis of severe malaria. This thesis examines the impact of Plasmodium infection on three key determinants of nitric oxide synthase (NOS) biochemistry: substrate availability, substrate/inhibitor homeostasis and cofactor availability. Arginine, the NOS substrate, is depleted in human patients with severe Plasmodium falciparum malaria and mice infected with P. berghei ANKA. Using heavy isotope tracer infusions to quantify arginine metabolism in infected mice, we found no evidence of increased catabolism by the enzyme arginase, widely assumed to be responsible for arginine depletion. Genetic knock-out of parasite arginase had no effect on arginine depletion in mice. Instead, our findings link arginine depletion to decreased rates of arginine and citrulline appearance in the plasma of infected mice. Asymmetric dimethylarginine (ADMA) competes with arginine for binding to the NOS catalytic site. We observed elevation of the ADMA/arginine ratio in Gambian children with severe malaria, favouring NOS inhibition. In mice infected with P. berghei ANKA, we found evidence of degradation of dimethylarginine dimethylaminohydrolase 1 (DDAH1), the enzyme primarily responsible for ADMA metabolism. We also observed reduced DDAH activity and accumulation of intracellular ADMA in hepatic tissue of infected mice, suggesting that DDAH dysfunction could contribute to disruption of ADMA/arginine homeostasis. Tetrahydrobiopterin (BH4) is an essential NOS cofactor. In P. berghei ANKA-infected mice, BH4 concentrations were decreased in plasma, erythrocytes and brain tissue, which could inhibit NO synthesis and promote NOS-dependent superoxide production. To reverse deficiencies of NOS substrate and cofactor availability, we infused P. berghei ANKA-infected mice with citrulline, an arginine precursor, and sepiapterin, a BH4 precursor. Restoration of systemic arginine and BH4 availability in infected mice improved whole blood nitrite concentrations, a biomarker of NO synthesis, but did not prevent onset of disease symptoms. These studies have identified biochemical disturbances that may contribute to severe malaria pathogenesis by inhibiting NO synthesis.