Applying Compost to Encourage Mycorrhizae Fungi as a Fertilizer Substitute

Mycorrhizae fungi are important symbionts of many plant communities. The symbiosis is considered to be an adaptation of plants to acquire soil nutrients and additional moisture, and of fungi to acquire carbohydrates (Eaton and Ayres). The fungi also fight off parasites such as nematodes and soil pathogens (NIFG). Plants allocate relatively more carbohydrates to roots in nutrient poor soils (Eaton and Ayres). Some of this carbon permeates the rhizosphere, supporting microorganisms, including mycorrhizae fungi (Eaton and Ayres). The metabolic activity of this microbial community enhances the availability of soil nutrients through mineralization (Eaton and Ayres). Mineralization also takes place during microbial decomposition of plant litter. During this process organic nitrogen in the plant litter is converted into inorganic forms of nitrogen that plants can utilize (MIT). This source of nitrogen will probably be reduced in the future though with rising levels of atmospheric CO2. Studies have shown that increased levels of this atmospheric gas will decrease microbial decomposition of plant litter (Shuijin), therefore reducing soil nitrogen from this source. However, as covered in the previous ‘Green Point,’ in most regions of California, nitrogen deposition would more than compensate for the loss of N generated from microbial processes.Mineralization from microbial processes also allows phosphorus, which is persistent in soils but often trapped, to become more available to plants (Lara 56). Adding mycorrhizae soil additives (called transplant inoculants) would enhance this mineralization process and compliment the previously mentioned ‘Green Point’ of letting leaf litter lie, helping to create a sustainable nutrient cycle.

There are several groups of mycorrhizae fungi with more species likely to be discovered. Two of the larger groups are endomycorrhizae and ectomycorrhizae (NIFG). Ectomycorrhizae fruit to form mushrooms, endomycorrhizae do not. The most common of these fungi are the endomycorrhizae, which grow inside root cells and extend threadlike structures called hyphae into the rhizosphere to extract nutrients (NIFG). Endomycorrhizae mainly form associations with herbaceous and broad-leaf woody plants, these relationships are frequently referred to as Vasicular-Arbuscular or VA. Ectomycorrhizae grow between the root cells and outside of the root. They too form extensive networks of hyphae that gather nutrients and water beyond the reach of plant roots (NIFG). Ectomycorrhizae are more commonly associated with woody plants, in particular, coniferous species (Lara 57). Two other mycorrhizae groups are significant to landscape design, arbutoid and ericoid mycorrhizae colonize the Ericaceae plant family (BOTIT). Some of the plants in this family include Arbutus, Arctostaphylos, Erica, Vaccinium, Camellias, Azaleas, and Rhododendrons.

Mycorrhizae are found in native soils, but only in the upper layers of soils. Therefore the landscape and construction practices of grading, trenching, topsoil removal, fumigation and soil compaction, along with erosion, can drastically reduce fungi populations (Lara 57). In light of this, planting holes for landscape material should be shallow (only as deep as the root ball) and broad. Mycorrhizae are annual life forms that have to regenerate (NIFG). Individual mycorrhizae last between 6–16 months. Researchers know that increased nitrogen from pollution reduces the fruiting bodies of ectomycorrhizae, therefore limiting regeneration by their spores, but doesn’t harm the underground structures of the fungi (NIFG). In fact most, but not all, ectomycorrhizal fungi respond to increased mineral N (the form converted by microbes and in N deposition) with increased growth (Eaton and Ayres). On the other hand, increased soil nutrients (including N) can reduce plant allocation of carbohydrates to roots, limiting an essential nutrient for the growth and regeneration of these fungi (Eaton and Ayres). Limiting fertilizer applications benefits this regeneration.

There are commercial blends of mycorrhizae species available as soil inoculants, though they vary in quality. Some tout spore counts in their marketing, and while important, spore counts don’t reveal much about the overall quality of the product (Lara 58). Mycorrhizae also regenerate by fragments of both hyphal and fungi roots, so these too, along with spore counts (collectively known as propagules), should be a part of inoculants mixes (Lara 58). To know the true quality of an inoculant’s mix, a measure of ‘infectivity’ needs to be independently assessed (Lara 58). Of the three methods available to assess the quality of mixes, the Mean Infection Percentage (MIP) assay is the most widely used and recognized. This assay measures the ability of mixes to colonize after a 30 days period (Lara 58). An independent source for testing these products is the International Culture Collection of Arbuscular & Vesicular-Arbuscular Mycorrhizal Fungi (INVAM), (Lara 58).

A limiting, but measurable, factor of how effective mycorrhizal inoculants can be is the level of phosphorus available in the soil of the site to be treated. Phosphorus levels of less than 50 ppm are preferable, with minimum infection occurring at levels above 100 ppm (Lara 60). Therefore, soil test should be performed before an inoculation is applied.

Sources

Eaton, G. K. and Ayres, M. P. 2002. Plasticity and constraint in growth and protein mineralization of ectomycorrhizal fungi under simulated nitrogen deposition. Mycologia, 94(6): 921–932.

NIFG. Ectomycorrhizal Fungi. Northern Ireland Fungus Group Website.

MIT 2002. Mission 2006, Project Amazonia: Characterization-Abiotic-Nutrient Cycles. Massachusetts Institute of Technology Website.

Shuijin, Hu 2001. Nitrogen limitation of microbial decomposition in a grassland under elevated CO2. Nature, January/409. 188–191.

Lara, Joe 2005. Well Rooted for Conservation. Land and Water, May/June.

BOTIT 2002. Ericaceous mycorrhizae. University Wisconsin Botany Department/Botany Instructional Technology Website.