Soil Ecology and Research Group last update October 29, 2002

Revegetation Methods for the Control of
Dust from Arid/Desert Soil Disturbances in
the Antelope Valley
2001 Annual Report

Introduction

The Antelope Valley, located fifty miles north of Los Angeles in the Mojave Desert, is experiencing air quality problems caused by frequent dust storms. Bordered to the south by the San Gabriel Mountains, to the west by the Coastal Mountain Ranges and to the north by the Tehachapi Mountains, Antelope Valley was intensely farmed up until 10-20 years ago. Increased water costs caused many farmers to abandon farming, leaving vast tracts of bare disturbed land. These abandoned farms, miles of dirt roads, increased construction, summer brushfires and other human disturbances, combined with high winds, has led to an air pollution problem with high levels of PM10 in and around Lancaster and Palmdale. The Dustbusters, a coalition of local farmers, the California Air Resources Board, South Coast Air Quality Management, The United States Department of Agriculture Natural Resource Conservation Service, The City of Los Angeles Department of World Airports and Southern California Edison, invited the Soil Ecology and Restoration Group (SERG) of San Diego State University to join the group in addressing issues of dust mitigation in the Antelope Valley.

SERG was given the task of researching cost efficient methods of revegetating abandoned farmland with native plant species. The goal of this project is to suppress airborne particulate matter and restore the soil to the point that native plant communities can be reestablished. The process of reestablishing such native plant communities in arid and semi-arid areas is hampered by the physical impacts caused by human activities. These activities, including farming, off-road vehicle use and construction impact the soil through compaction, loss of soil microbes and changes in soil nutrient levels. The role of SERG is to determine the specific impacts, develop the most cost-effective methods to mitigate these impacts determine which native species are best suited for restoration on such disturbed lands and establish how best to reestablish a self-sufficient plant community. In June of 1998 two sites were selected for experimental plots. The first site is located on 50th Street East near Palmdale Boulevard and the second site is on 90th Street East just north of Palmdale Boulevard. Soil samples were taken in July 1998 from both sites to determine initial soil nutrient levels and five native plant species were selected to be used in the study. Fourteen hundred seedlings comprised of Atriplex canescens (four-wing saltbush), Atriplex lentiformis (quailbush), Atriplex polycarpa (allscale), Larrea tridentata (creosote bush) and Prosopis glandulosa (mesquite) were grown in the SERG greenhouse at San Diego State University and outplanted in January 1999. This report will give an overview of the project, including site preparation, planting, maintenance and monitoring methods followed by a detailed account of all activities and data collected at the sites from July 2001 through June 2002.

In addition to the review of past work and detailed information on collected data, this report will introduce a new site, located on 85th Street in Lancaster, that was selected to replace the 50th Street site. The 50th Street site (Figure1) is no longer being monitored for data, except survival, as it has been determined not to be representative of recently abandoned farmland. This conclusion was reached after reviewing new information stating that the site had not been farmed for many years but had been regularly flooded with secondary effluent. The secondary effluent dramatically increased the soil nutrient levels over those usually found on farmland and therefore the site is not typical of recently abandoned farmland.

The new site, donated by Phillip Giba, of Giba farms, was farmed as recently as the 2001 season. The experimental design used will be the same as that used on the 50th and 90th Street sites. Initial soil samples were taken in March 2002 and five species of plants, Atriplex canescens (four-wing saltbush), Atriplex lentiformis (quailbush), Atriplex polycarpa (allscale), Larrea tridentata (creosote bush) and Prosopis glandulosa (mesquite) are being prepared for transplanting in the fall/winter of 2002.

Figure 1. 50th Street Shrub Plot 2002

Site Preparation

The Palmdale Regional Airport provided the original two sites for SERG to conduct revegetation experiments. The first site, located on East 50th Street, was thought to be abandoned farmland. We had originally been informed the site had been farmed as recently as three to five years ago, but new information provided by Jim Bort from the City of Los Angeles Airports states the site has lain fallow for the past twenty-five years. The site has, however, been used by the Los Angeles County Sanitation District for the disposal of secondary effluent as recently as five years ago. The furrows, originally believed to be evidence of recent crop activity, were actually used to divert the secondary effluent evenly over the fields. The second site, located on East 90th Street, appears to have been abandoned a minimum of fifteen to twenty years ago.

The 50th Street site was dominated by scattered groups of exotic annual grasses and Salsola tragus (Russian thistle). The soil was only slightly compacted, probably due to the recent furrowing activity. The 90th Street site showed no signs of furrowing and was lightly covered with exotic annual grasses, Salsola tragus (Russian Thistle), and a few native species, including an early seral invader Chrysothamnus nauseosus (rabbitbrush). The soil at 90th Street was also only slightly compacted.

On November 25, 1998 SERG personnel outlined 15m x 30m shrub plots and 30m x 30m windbreaks at both the 50th and 90th Street sites. The Palmdale Regional Airport supplied a John Deer tractor that was used to back blade all the plots, knock down existing vegetation and construct windbreaks. Exotic vegetation knocked down on site was incorporated into the 1m tall by 10m long windbreaks to provide stabilization and organic matter to the soil. The tractor was again used in December 1998, this time using 18" ripping bars to loosen the soil in all plots to a depth of 12" and to incorporate dead exotic plant material into the soil to provide additional organic matter.

In January 1999,SERG personnel installed perforated irrigation pipe into the windbreak mounds at both 50th and 90th Street sites. All plants were outplanted in January and provided with either Tree-pees or Tubex as plant protectors. In addition, each shrub plot was provided with amendments and irrigation treatments as required by the experimental design described in the following section.

Experimental Design

Two types of plot designs were employed, shrub plots (15m x 30m) and wind break plots (30m x 30m). Shrub plots (Figure 2) were established using a 2x3x5 factorial with two types of irrigation methods (surface watering or deep pipe), three types of surface applied soil amendments (wood chips, compost, control) and twenty individuals from each of the five native plant species selected. All treatments were randomly assigned within a plot, with each of the sites having two shrub plots for a total of 640 shrub plot seedlings. Windbreak plots were established with a 2x5 factorial consisting of two irrigation treatments, perforated pipe and surface irrigation, and 16 individual plants per windbreak. Each site has four windbreak plots (Figure 3) and each plot has six windbreaks, three with perforated pipe and three with surface irrigation. Each windbreak has sixteen seedlings for a total of 768 windbreak seedlings.

Figure 2. 90th Street Shrub Plot 2002

Figure3. 90th Street Windbreak 2002

Seed Collecting and Planting

The fourteen hundred seedlings used for this project were grown in the SERG greenhouse at San Diego State University. The seed, obtained from S and S Seeds, had been collected from the Mojave Desert in and around Antelope Valley. Plants were germinated in June 1998 and then transplanted to plant bands (2"x2"x8" and 3"x3"x10"). Seedlings were watered twice each week for the first three months and then on a monthly basis. Seedlings were fertilized for the first three months. In November 1998 the plants were transferred to the University of California, Riverside lathe house to acclimate them to desert conditions.

In January 1999 the sites were prepared for planting. A power auger was used to dig holes and each hole was saturated with water prior to planting. Plant protectors were used to protect the plants from herbivory. All plants were re-watered after planting. Seven hundred plants were put at each site. Of the seven hundred, three hundred and twenty went to shrub plots and three hundred and eighty went to the windbreaks. Surface catchments were built for plants on the windbreaks that did not have perforated pipe installed. The shrub plots were divided equally so that half of the plants had catchments and half were provided with PVC pipe for deep watering. Designated sub-plots within the shrub plots were covered in compost or wood chips.

Maintenance and Monitoring

In March 1999 numerous plant protectors were blown off by high winds at the 90th Street site. Many of the unprotected seedlings had suffered heavy herbivory damage and had to be replaced. Plant protectors were replaced on the remaining unprotected plants and secured with rebar. In March of 2000, 489 plants were replanted at the 90th Street site. Since there were only a limited number of the original species available, other species were used. All replacement plants were grown from Mojave Desert seed at the San Diego State University greenhouse (Table 1).

Table 1.
Species Used to Replant 90th Street site

In July 2000 SERG personnel began removing plant protectors from both sites. Plant protectors were removed from plants when it was determined they were interfering in the natural morphology of the plant. The protectors were removed on a regular basis throughout the year with all removed by June 2001.

In September 2001 and April 2002,soil samples were taken from the 90th Street site and sent to A&L Laboratories, in Modesto CA, to be analyzed for macro and micro-nutrients, organic matter and pH. In April 2002 two samples were also taken from the 85th Street site to provide a baseline for future data. These samples were also sent out for analysis. SERG personnel did europium staining on the 90th Street samples, a process used to measure fungal and bacterial growth, at San Diego State University. This process is used to determine the ratio of bacteria to fungal hyphae in a gram of soil. A shrub dominated habitat, such as those naturally found in the Mohave Desert, typically has a fungal dominated mineralization cycle. Disturbed habitats, dominated by exotic annual species, will have a bacteria dominated mineralization cycle. By comparing the fungal to bacteria ratio it can be determined if the disturbed mineralization cycle is actually being rehabilitated.

June 2002 plant survival was taken for the 50th Street site and survival/biomass was measured and analyzed for the 90th Street site. The 90th Street site was also weeded in June 2002 (Figure 4).

Figure 4. Salsola trasus (Russian Thistle) on a 90th Street Windbreak 2002

Results

Survival Data
Percent survival data was collected for both the 50th and the 90th Street sites in June 2002. Overall survival at 50th Street was 60%, with shrub plots at 59% survival and windbreaks at 61%. The highest shrub plot survival was 60% for the deep pipe/mulch and the deep pipe/surface treatments while the lowest was 55% for the woodchip/surface treatment and the control subplots. Windbreaks with surface irrigation had 58% survival while windbreaks with perforated pipe had 64% survival. No significant difference was found between irrigation or treatment methods at 50th Street. Percent survival by treatment is illustrated in Table 2.

The 90th Street site had an overall survival of 73%, with shrub plots having 64% and windbreaks having 83% survival. Shrub plot survival had a low of 55% in the control subplot and a high of 75% in the deep pipe/mulch treatment. Statistical analysis, using Analysis of Variance (Anova), indicated no significant difference between amendment treatments, however, there was a significant difference between watering methods with deep pipe outperforming surface irrigation (p-value .0098). In the scientific community, any value less than .05 is considered to represent a significant difference. Windbreaks at 90th Street continue to outperform the shrub plots with survival of 82% for surface irrigation and 83% for perforated pipe.

Table 2.
Percent Survival by Treatment

Soils Data
In September 2001 and April 2002 soil samples were collected from 90th Street site and from an undisturbed reference site nearby. Initial soil samples were collected from the new site on 85th Street in Lancaster in April 2002. The samples were then sent to A&L Labs in Modesto, CA to be analyzed. Results (Table 3) of the 90th Street site were then compared to previous samples and to the undisturbed site. Results from the 85th Street site will be used as a baseline for future samples taken from the site. The 90th Street site shows an increase in nitrates and sulfates and a decrease in organic matter. Zinc, copper and manganese remain stable while iron has fallen. The 85th Street site shows numbers typical of irrigated cropland.

Table 3.
Comparison of Soil Analysis

Table 4.
Meters of Hyphae per Gram of Soil

Table 5.
Bacteria per Gram of Soil

Table 6.
Biomass in m3 at Shrub Plots and Windbreaks

When reviewing the results of soil analyses and biomass at the 50th Street site, it came to our attention that the nutrient levels were high even when compared to actively farmed soils. Examples of these high levels include nitrogen levels above 60ppm, iron and sulfate levels above 20ppm, and manganese as high as 39ppm. In addition, the tremendous growth of the plants was beyond our expectations. When considering the source of the growth we suggested it could be attributed to over-spray from the neighboring sod farm, but this did not explain the nutrient levels in the soil. We eventually learned that the land had actually been fallow for the past twenty-five years but had been regularly flooded with secondary effluent by the Los Angeles County Sanitation District. In April 2001 we took samples from a site recently flooded with secondary effluent and found nutrient levels there were comparable in most areas to the 50th Street site soil samples.

At the September 2001 Dustbuster meeting this information was discussed and all members agreed that the 50th Street site was not representative of recently abandoned cropland and would no longer be monitored, except for survival. Discussions with Phil Giba of Giba Farms led to the donation of a new site. Located on 85th Street in Lancaster, California, it has been used for onion crops as recently as 2001. This new site will be prepared and planted in the fall/winter of 2002. The experimental design used will be the same as those used on the 50th and 90th Street sites.

Survival at 50th Street is currently 60%, a drop of 9% from last years 71%. This drop in survival can be attributed to extremely low rainfall and to plant loss due to competition. One of the distinguishing factors seen at 50th Street is the explosive growth, particularly in the Atriplex species. When taking survival data this year it was observed that Mesquite and Creosote plants that were planted between Atriplex had been choked out over the past year. The ones that have survived are located on the edges of the shrub plots and windbreaks with at least one side left clear for growth.

There is no significant difference seen between the different surface amendments and irrigation, although overall survival is higher for all treatments with deep pipe or, in the case of the windbreaks, perforated pipe. Last years data indicated a significant difference in irrigation methods with pipe outperforming surface irrigation. The change this year can be attributed to the fact that there were a number of losses due to competition and lack of rainfall. As time goes on and plant growth begins to stabilize, it is still expected that long term survival for plants watered with pipe will be significantly higher than those that were surface watered.

Overall survival at 90th Street was 73% in June 2002. Despite an extremely dry year these plants are doing better than the 60% survival rate expected in desert restoration projects. The windbreaks, with 83% survival, have a higher survival rate than the shrub plots that have an overall survival of 55%. This may be attributed to the windbreak design, which slopes towards the plant on two sides; therefore these plants receive more water from rain runoff than the plants in the shrub plots. Statistical analysis showed a significant difference between the watering methods used at 90th Street with pipe outperforming surface irrigation. There is still not significant difference seen between the amendments used, which is not unexpected considering the several years of below normal rainfall experienced by the Antelope Valley. In most cases it takes three to four years for any real difference to be seen. Because of the extremely dry years Antelope Valley has been experiencing it may be even longer before the woodchips and mulch begin to decompose at a rate that will effect the overall condition of the soil

When taking survival data it was noted that both shrub plots at 90th Street had new Atriplex seedlings appearing, indicating a successful seed bank is being developed on this site. No seedlings were observed at the 50th Street site, although many of the Atriplex species were producing large quantities of seed despite the low rainfall experienced this past year.
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When reviewing soil analysis data it is important to remember the extreme variability inherent in soil/plant restoration projects. There are many outside factors that can effect the data including extreme weather conditions, animal and insect damage or byproducts and human disturbances. For this reason multiple samples are collected and data sets are analyzed for variability and any number that falls outside the normal range is excluded from analysis. For example, when analyzing the sulfates on 90th Street, all of the data fell between one and thirty six part per million except for one that was sixty parts per million. A number this high indicates an outside influence and is not considered representative of the site.

The results of the April 2002 soil samples taken at 90th Street show a decrease in organic matter and an increase in nitrogen. This indicates that despite the lack of precipitation the OM is being broken down at some level and being made available to the plants in the form of nitrogen. Sulfates at 90th Street have risen from 4.5 in the fall of 2001 to 10.6 in the spring of 2002. It is very common, particularly in arid climates, for there to be a wide fluctuation in sulfate concentrations as environmental conditions play a large role in their mineralization process. During the winter season sulfates usually drop as they are leached out by rainfall. In extremely dry years they begin to build up, but seldom reach levels that are harmful to plants. Sulfate levels in desert soils can exceed 20ppm without any adverse effect on the plants. Although levels may continue to rise, it is expected that they will begin to decrease if the desert experiences normal rainfall this 2002/2003.

Measurements of fungal hyphal lengths and bacteria numbers have shown no significant difference between treatments at this time. The mineralization process, by which organic matter is broken down into its chemical elements by fungi and bacteria, is an extremely slow process in arid habitats such as Antelope Valley. Since water availability is a key factor in mineralization, the limited amounts of precipitation experienced by the Antelope Valley retards the mineralization process, thus preventing a rapid increase in soil microorganisms such as fungi and bacteria.

Bacteria are a primary mineralizer of non-recalcitrant matter, as opposed to fungi that serve as the primary decomposer for the more complex recalcitrant organic matter. Non-recalcitrant matter is made up of simple carbon compounds found in both exotic and native annuals. Recalcitrant matter is made up of more complex carbon compounds such as those found in bark, woodchips and perennial shrubs. With very few exceptions, recalcitrant organic matter must first be broken down from complex to simple carbon compounds by fungi. Once this occurs, bacteria will finish the decomposition process. Because of this, changes in bacterial numbers should develop much faster than changes in fungal activity. Changes in fungal activity would, due to their role in the slower decomposition of more complex organic matter, be much slower to appear. Results from the europium analysis seem to demonstrate this fact with an increase in bacteria numbers appearing over time at the 90th Street site. This increase is most probably the result of the decomposition of the non-recalcitrant annual grasses that were worked into the plots during site preparation. Fungi have demonstrated no such increase. Since desert plant communities are shrub dominated, and therefore provide mostly recalcitrant matter to the soil, desert soils normally have fungi dominated mineralization cycles. It is expected that as the project develops over time, fungal activity will increase while bacteria numbers will stabilize as a self-sufficient plant/soil desert community develops.

The relationship between microorganisms and soil nutrients can be seen when looking at the levels of organic matter, nitrogen, and bacteria. When looked at over time, a trend can be seen between these three parameters. As nitrogen levels go up, so do the bacteria numbers in the soil. As the bacteria break down the non-recalcitrant organic matter the percent of organic matter in the soil drops. Over time this non-recalcitrant organic matter will be slowly released with the more recalcitrant organic matter provided by the native permanent shrubs.
Biomass at 90th Street has increased 221% over the past year with the highest increase seen at the windbreaks with perforated pipe at 334%. The shrub plots, with a 114% increase, had the least amount of growth. Many of the Mesquite and Creosote species had actually lost biomass due to herbivory, but still looked extremely healthy. On the second set of windbreaks a large growth spurt, occurring on the northernmost edge, is contributed to a suspected underground water leak. Between the two northernmost windbreaks there is a water shut off valve that is turned off but is most likely leaking a small amount of water near the root level of the plants. Soil moisture will be measured to determine if this is an accurate assessment of the situation.

Although it is still to early to determine how much more effective deep pipe and perforated pipe is when compared to surface watering, there does appear to be a difference in survival with pipe being superior to surface watering. It also appears, although this years data indicated no significant difference, that over the course of the experiment shrub plots treated with amendments will have higher overall survival then those without. This probably has more to do with water retention than an improvement in soil quality as the lack of significant rainfall has limited the amount of microbial activity seen on the site. All parameters will continue to be monitored at the 90th Street site and survival will be monitored at the 50th Street site for the fourth year of data collection of this project. The 85th Street site will be planted in the early winter of 2002 and data collection will begin in 2003. Our continued goal is to use data collected to determine what restoration methods will be the most cost effective and efficient for controlling dust in the Antelope Valley.

First Annual Report (November 1, 2000)

Second Annual Report (July 23, 2001)

Fourth Annual Report (March 9, 2004)

Fifth Annual Report (December 1, 2004)