Soil Ecology Restoration Group
last update November 7, 1997
SMER Research
Research Results Disturbances of native communities, whether natural or anthropogenic, often initially increase nitrogen availability which provides a competitive edge to fastgrowing, high nitrogen response exotic invaders. Once established, their rapidly degraded litter provides the labile organic matter that maintains a nitrate-dominated nitrogen cycle beneficial to these exotiv species and, more importantly, may eventually lead to invasion by these exotics into previously undisturbed native habitat. By changing the energy source for belowground microorganisms from labile to recalcitrant, nitrogen should become immobilized in the increased fungal biomass for gradual release as ammonium through the action of fungal grazers. Field plots were established to test the effects of the addition of recalcitrant organic matter to disturbed soils in both coastal sage and creosote scrub habitats. Parameters monitored included fungal hyphae, bacteria counts, nitrate, ammonium, total nitrogen, soil moisture and soil organic matter. Native seedlings were transplanted into test plots to measure above ground effects of the addition of organic ammendment.
Test plots were established with bark amendment, straw amendment and no amendment treatments. Control plots were expected to reflect the annual exotic life cycle of available nitrogen buildup during hte summer drought months and decrease during the active growing winter and spring months. Amended plots were expected to show less available nitrogen during both summer and winter months through an increase in nitrogen immobilization caused by an increase in microbial biomass from the addition of the organic amendment.
Data on available nitrogen, both nitrate and ammonium, confirmed the above expectations. Nitrate and ammonium accumulated in the soil during the summer drought months and decreased during the wet winter months when exotic annual growth occured. Lower values in amended plots coincided with increased fungal hyphal lengths and bacteria counts found in amended plots. This apparent increase in immobilization removed nitrogen from the available soil nitrogen pool by tying it up in microbial tissue. Nitrogen thus immobilized would be released slowly, providing a much smaller amount of available nitrogen at any one time. This condition is more beneficial to slow growing native perennials than faster growing exotic annuals.
The reaction of native perennial seedlings to the addition of organic amendment also gave an indication that changes beneficial to native perennials had occurred. Artemisia californica seedling growth data showed bark amended plot seedlings had tremendous growth compared to control plot seedlings and, to a lesser degree, even straw amended plot seedlings. This increased gwoth could be connected to the increase in fungal hyphal length, bacteria counts and nitrogen immobilization seen in the amended plots.
It appears the addition of organic amendments, such as bark or straw, affects the nitrogen cycle found in disturbed soils by increasing the immobilization of nitrogen through increasing the microbial biomass. This decrease in available nitrogen, necessary to support the quick nutrient turnaround required by exotic annuals, could be a significant factor in providing a competitive edge to native perennials over invading exotic annuals.
