Sphagnum moss species are key bryophytes in peatland ecosystems, where waterlogged, acidic conditions restrict decomposition and promote peat accumulation. A conservation biologist is investigating factors that limit the growth and primary productivity of these mosses.
Which of the following, A to D, is an abiotic factor that directly limits the growth rate of Sphagnum moss?
the density of mutualistic nitrogen-fixing cyanobacteria inside the hyaline cells
the concentration of nitrogen gas (N2\text{N}_2N2) in the atmosphere
the depth of the water table below the moss surface
the percentage cover of ericaceous shrubs shading the moss carpet
