I have noticed that Magnolia leaves seem to decompose more slowly than other leaves…can you tell me why that might be the case?
There is a lot of anecdotal evidence regarding your observation. Finding information to specifically confirm it is not easy, however. But some facts about plants physiology help, especially in combination with considerations about the conditions required for decay. I referred to Introduction to Plant Physiology (William G. Hopkins, 1995) for most of the information below.
Lignin is a compound that is an integral part of the cell walls of plants. (It is the second most abundant organic compound on earth after cellulose.) Lignin fills the spaces in the cell walls of various plant tissues, providing mechanical strength to the cell wall and thus to the entire plant.
Lots of lignin in a leaf would result in a slow process of decomposition because it is difficult to degrade. That is, it is not easy for bacteria and water (necessary for decomposition) to penetrate the chemical structure of lignin.
Suberin is a waxy substance that is highly hydrophobic (repels water); its main function is to prevent water from penetrating plant tissue. Suberin is found in the outermost layer of the bark (in the dead corky tissue). The cells in this layer are dead and abundant in suberin, preventing water loss from the tissues below.
Suberin can also be found in various other plant structures, including leaves, where it also prevents the movement of water.
So, the combination of a structural function of lignin and the water-repelling characteristics of suberin – in leaves, in this case – is quite helpful in explaining why magnolia leaves decay at a slower rate than other leaves.
An article about composting from University of Florida Extension mentions that magnolia leaves would need to be shredded in order to be usable in compost (or as mulch).