Since at least 1980, conservation and restoration biologists have expressed concern that high concentrations of soil NO3- and NH4+ may favor introduced annual weeds (e.g., annual ryegrass, wild oat) over California native plants, including herbaceous species, shrubs and trees. Lignin-rich organic mulches may promote survival and growth of newly planted trees and shrubs, in part by suppressing some species of weeds. A better understanding of these soil-plant dynamics may help determine best practices for establishing hedgerows and diversifying field edges in agricultural settings.

At the Santa Margarita Ecological Reserve near Temecula, San Diego County, Calif., researchers Zink and Allen used a randomized complete block design with plot size 1.0 x 0.5 m to evaluate effects of organic mulches on survival and growth of seedling California sagebrush (Artemisia californica). The three treatments were: 1) pine bark; 2) oat straw; 3) no mulch (control). Response variables were assessed on eight occasions from February 1993 to June 1995, including seedling survival and estimated above-ground volume of California sagebrush plants, soil total N, soil N03-, several indices of soil microbial activity, and soil organic matter. Growth of potentially competing annual vegetation (e.g. wild oat [Avena fatua]) was not measured. Separate analyses of variance (ANOVA) were conducted for data from each date, with Fisher’s protected least significant difference test used for detected differences among pairs of means.

A summary of the data shows that:

• Pine-bark mulch plots showed the greatest sagebrush survival rates over the course of the experiment (66% survival as contrasted with 42% for oat-straw and control plots).
• California sagebrush growth was also greatest in pine-bark mulch plots.
• Both mulches had significantly lower soil NO3- concentrations on six of eight sampling dates.
• From January 1994, mulch-amended plots had greater active fungal biomass than control plots.
• There were no strong nor consistent differences in bacterial biomass among the three mulch levels.
• Soil organic matter content did not differ among the three treatments for the first six months; from January 1994 through the end of the research, organic matter content increased significantly under the bark-amended plots compared to control plots. No change in organic matter was detected under the oat-straw treatment.

The authors attributed increased survival and growth of California sagebrush to reduced NO3- availability in the mulched plots and suggested that this mediated competition by wild oat and other introduced annual weeds for other nutrients and water. The authors also mentioned the role of mulches in conserving soil moisture as a possible factor influencing plant growth. As no measurements were made of potentially competing vegetation nor of soil moisture, the mechanisms for the improved survival and growth of California sagebrush remain speculative.

For more information: Thomas Zink, Department of Biology, San Diego State University, San Diego, CA 92182.

DEC. 603 Contributed by Robert L. Bugg