In chapter 23, we briefly discussed how humans have more than doubled the quantity of fixed nitrogen cycling through the biosphere. In chapter 15, we reviewed studies by Nancy Johnson (1993) on the effects of fertilization on the mutualistic relationship between mycorrhizal fungi and grasses. The increases in fixed nitrogen cycling through the biosphere, particularly that portion deposited by rain, are analogous to a global-scale fertilization experiment. Reasoning from the results of Johnson's study, how should increased fixed nitrogen supplies affect the relationship between mycorrhizal fungi and their plant partners? How would you test your ideas?