Department of Biological Sciences, Fort Hays State University Instructor: Mark Eberle                  Course Homepage

Plants and Nutrients

Nitrogen (as N2 gas) is the most abundant element in the air (about 78% of the atmosphere), and nitrogen is an essential component of proteins and other organic molecules. Some bacteria in the soil (or water) can convert N2 into ammonia (NH3). This process is referred to as nitrogen fixation. The ammonia can be converted into ammonium ion (NH4⁺), which is picked up from the soil by plants, especially trees and grasses, and by the symbiotic fungi (mycorrhizal fungi) associated with the roots of plants. Other bacteria can convert ammonia into nitrite (NO2^-) and then nitrate (NO3^-), which is used by most flowering plants. This process is referred to as nitrification. In addition to nitrogen fixation and nitrification, nitrogen becomes available to plants through decomposition of organic materials. Fungi and bacteria are the dominant decomposers, breaking down complex organic molecules, including proteins, into smaller molecules, such as amino acids and ammonia.

To assist them in obtaining nitrogen, legumes, such as the many species of lupine (Lupinus; photograph below), and red alder (Alnus rubra; a tree) have globular nodules on their roots occupied by symbiotic bacteria that can fix nitrogen, some of which is used by the plant. The plant provides high-energy molecules and other necessities to the bacteria.

Bog soil presents a challenge to most plants because the soil is often saturated with water, but it also is low in nitrogen (nitrate is less soluble in acidic soils, such as those in bogs). Rather than assistance from symbiotic bacteria in their roots, cobra lilies (Darlingtonia californica; photograph below) obtain their nitrogen from insects attracted to their modified, tubular leaves that produce aromatic nectar. The insect moves inside the tube through an opening on the underside of the "cobra head". Long, downward-pointing hairs help to prevent the escape of the insect, and an enzymatic fluid inside the tube helps to decompose it, which provides nitrogen needed by the cobra lily.

Some flowering plants lack chlorophyll (achlorophyllous) and the metabolic (chemical) pathways for photosynthesis. Thus, they rely on other photosynthetic plants directly or indirectly for their organic molecules. The roots of pinedrops (Pterospora andromedea) and Indianpipe (Monotropa uniflora; photographs below) are associated with mycorrhizal fungi that are connected with the roots of a conifer, forming what is called a common mycelial (or mycorrhizal) network (CMN). Through this connection, the achlorophyllous plants indirectly obtain their nutrients from the photosynthetic plant by way of the mycorrhizal fungus. In this CMN, the conifer provides organic molecules derived from photosynthesis to the fungus, and the fungus provides nutrients to the conifer. Whether the achlorophyllous plant provides anything to the fungus from which it derives its nutrients is unknown, although the plant might produce substances that stimulate the growth of mycorrhizal fungi (summary by Trudell, 2002).

Another feature of plant competition for nutrients in temperate rain forests is the nurse log. These fallen trees open the canopy, allowing more sunlight to reach the forest floor, and they serve as a home and nutrient source for a wide variety of organisms. Initially, wood-boring arthropods (e.g., beetles, ants, termites, mites) tunnel into the wood to feed or nest. Into these tunnels come the fungi and bacteria that will help to decompose the wood. Leaves and other forest litter, as well as mosses, accumulate on top of the log and provide a place for seeds to germinate. Western hemlock (Tsuga heterophylla), Sitka spruce (Picea sitchensis), and some shrubs are among the plants that germinate on the log (photograph below, left). As the log continues to decompose, the tree roots, with the help of their mycorrhizal fungi, continue to penetrate deeper into the moist microhabitat of the decaying log toward the soil. Water produced by the respiration of the decomposers helps to keep the interior of the decaying log moist during the regular summer "drought", and this improves the chances of success for the young trees, with their active mycorrhizae, compared to those growing on the soil, whose mycorrhizal fungi can become dormant during the summer (summary by Schultz, 1990:268-279). As the process progresses, some of the trees do not survive, but those that do eventually form a colonnade along the line of the largely decomposed nurse log (photograph below, right).

• Trudell, S. 2002. Mycorrhizas (5): fall mushrooms, ghostly fungus-robbers, and a definition revisited. Mushroom: the Journal of Wild Mushrooming, Issue 77, Fall. On-line version accessed 21 June 2006 at http://www.mykoweb.com/articles/Mycorrhizas_5.html.