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John Lloyd recently completed his Ph.D. degree
in entomology at The Ohio State University,
Ohio Agricultural Research and Development Center,
in Wooster, and is currently an assistant professor
in the Department of Plant, Soil, and
Entomological Sciences at the University of Idaho in
Moscow, Idaho;
Dan Herms, Ben Stinner, Harry Hoitink, |
As with plants, soil microorganisms (fungi, bacteria) require energy and essential nutrients to grow and reproduce. While plants derive their energy from carbon acquired from the atmosphere by means of photosynthesis, the carbon in decomposing organic matter provides soil microbes with their energy supply.
However, both plants and soil microbes utilize the same pool of essential soil nutrients. Since nitrogen is the nutrient that most often limits plant growth, the effects of mulch on soil fertility generally will be determined by how mulch impacts the outcome of competition between plants and microbes for this key nutrient.
Nitrogen and other nutrients are cycled as organic matter is decomposed by soil microbes (Figure 1). Rate of decomposition of organic matter is affected by many factors including soil moisture, temperature, and oxygen levels but is highly dependent on the total biomass of microbes in the soil (Wardle, 1992). Since microbes are generally limited by the supply of available carbon, microbial biomass can increase quickly when a biodegradable source of organic matter is applied to the soil surface.
Figure 1. A conceptual model of nitrogen cycling in ornamental landscapes. Decomposing organic matter releases organic nitrogen that is mineralized into forms available for plant and microbial uptake. Fertilization supplements the available nitrogen pool. Nitrogen acquired by microbial biomass is immobilized and thus is unavailable for plant uptake. As microbes die and are themselves decomposed, nitrogen is returned to the available pool.
As microbes decompose mulch on the soil surface, they acquire nutrients from the soil below in several ways. Fungal hyphae forage for nutrients in the soil much like plant roots (Frey etal., 2000). Nutrients can also be carried toward the surface in soil water by diffusion as well as mass flow driven by evapotranspiration. Furthermore, the soil is worked continuously by earthworms, insects, and natural weathering processes, which act to stir the nutrient pool and incorporate decomposing organic matter.
In a process known as nitrogen mineralization, inorganic forms of nitrogen (ammonium, nitrite, and nitrate) are released from organic matter as it is decomposed. Once in mineral form, nitrogen can be taken up and used by plants. Recent studies indicate that plants can also utilize dissolved organic nitrogen released from decaying organic matter (Nasholm etal., 1998, 2000).
Plants and microbes compete for available nitrogen, and the process of nitrogen uptake by microbes is referred to as nitrogen immobilization, since any nitrogen acquired by microbes is not available to plants. Microbial turnover occurs as microbes die and are themselves decomposed, which releases nutrients that can then be acquired by living microbes or by plants.