Based on the text, which statement best explains why ECM-dominated forest stands maintain greater long-term soil organi…

According to the passage, AM stands exhibit rapid decomposition and lower carbon storage because saprotrophic microbes 'encounter minimal competition from mycorrhizal fungi for organic substrates.' By contrast, ECM fungi possess specialized enzymes that directly access complex organic nitrogen, meaning they compete with saprotrophs for organic substrates in ECM-dominated stands. This competition limits the saprotrophs' decomposition rate, allowing more organic carbon to be preserved over the long term.

Choice A accurately synthesizes these points.

Choice B is incorrect because the passage states that AM fungi 'lack these nitrogen-mining enzymes' and rely on inorganic nitrogen liberated by saprotrophs, rather than consuming organic nitrogen directly.

Choice C is incorrect because the text specifies that ECM fungi—not the trees themselves—produce enzymes, and that these enzymes degrade organic nitrogen rather than synthesizing carbon compounds from inorganic nitrogen.

Choice D is incorrect because the text indicates that AM-dominated soils have active saprotrophic microbes that drive high decomposition rates, rather than lacking them.