Disturbing land to develop energy producing technologies in Mojave Desert areas usually destroys existing shrub vegetation that cannot be repaired by natural restoration processes within the time frame requried by environmental protection legislation. Such is the case, especially, for central receiver solar thermal power systems which perturb large areas of land. Once the natural vegetation is destroyed, the denuded land is more vulnerable to water and wind erosion, and the subsequent dust-prone environment generally becomes a nuisance to construction engineers and installation operators. The primary effort of the vegetation management project during FY 1980 was directed toward the development of cost effective methods of restoring native vegetation on disturbed Mojave Desert land. Emphasis was placed upon improving techniques for producing transplanting stocks of native shrub species, investigating different ways to protect and encourage regrowth from crown sprouting and new seedling germination, developing improved transplanting methods, and searching for more practical means of conserving seasonal soil moisture necessary for the survival of new plant seedlings and transplanted specimens. An overall project goal is to develop methods of restoring native vegetation on disturbed Mojave Desert land within a two or three year time frame in order to provide engineers with a usefulmore » tool to combat disturbed site erosion and fugitive dust problems.« less

Fifty samples of Atriplex hymenelytra (Torr.) S. Wats, were collected from several different locations in southern Nevada and California to test variability in mineral composition. Only Na, V, P. Ca, Mg, Mn, and Sr in the samples appeared to represent a uniform population resulting in normal curves for frequency distribution. Even so, about 40 percent of the variance for these elements was due to location. All elements differed enough with location so that no element really represented a uniform population. The coefficient of variation for most elements was over 40 percent and one was over 100 percent. The proportion of variance due to analytical variation averaged 16.2 ± 13.1 percent (standard deviation), that due to location was 43.0 ± 13.4 percent, and that due to variation of plants within location was 40.7 ± 13.0 percent.

Above-ground and below-ground biomass, percent dead shrubs by species, and percent of dead stems of living species were determined for a site in the northern Mojave Desert.

Fourteen species of native shrubs were transplanted to bare areas of the northern Mojave Desert in 1972 and 1973. By 1978 plants surrounded by small fences were larger (0.26 vs 0.11 m/sup 3/ overall average for several species) and survived better (42 percent versus 23 percent) than unfenced plants. These effects are primarily due to reduced grazing of shoots. Loss of shrubs to pocket gophers or other burrowing rodents was not prevented by fencing.

The revegetation of disturbed, arid lands is one of the great challenges of a desert. An attempt to encourage it is not an impossible task, however, if the natural and the man-made resources available are utilized and managed. Where rainfall and temperature conditions approach or exceed those of the Great Basin desert, restoration of disturbed land will occur through natural revegetation processes within a reasonable period of time. This is not generally the case in the more arid Mojave Desert areas where the moisture and temperature conditions are less favorable for germination and seedling survival. Restoration of vegetation by natural reseeding can, however, occur within local sites where moisture has concentrated as the result of terrain features forming catchment basins. Otherwise, the natural revegetation processes in the Mojave Desert areas require much longer periods of time (possibly decades or centuries) than are practical for meeting environmental protection standards imposed by current legislation. Through better understanding of the processes governing revegetation and the ability to control them, it is possible for man to more rapidly restore disturbed desert lands. Terrain manipulation to form moisture catchment basins, selection of seed from pioneering shrub species, preservation of existing shrub clump fertile islands' in the soil, supplemental fertilization, irrigation, organic amendments, and transplanting vigorous shrub species are some of the important things that can be done to help restore disturbed desert land.

Residual effects of sprinkle irrigation from 1968-1970 on populations of Mojave Desert shrub communities were observed in late 1974. The sprinkle-irrigated plots showed a residual increase in density of four species, but other species either failed to reproduce in significant numbers or lost all gains made during the years following treatment. The seven-year change for the irrigated plots was equivalent to a gain of 1178 perennial plants per ha, but the nonirrigated plots lost an average of 1050 plants per ha equivalent during the same period. The biomass gain after seven years was equivalent to 1000 kg/ha for irrigated plots and 310 for nonirrigated plots.

New Artemisia seedlings are not established each year. Many that are established fail to survive because of unfavorable rainfall in succeeding years. A total of 184 young plants was examined for the number of annual growth rings to ascertain the year of establishment after all vegetation had been killed near the time of a nuclear test event in 1965. The three most important recent years for establishment and survival of new seedlings (as of 1976 and based on a sample of 184 plants) were 1966 (9 percent), 1969 (29 percent), and 1973 (36 percent). A total of 2 percent was established in the other years from 1965 to 1976. These three years were also the years with high rainfall input during preceding winter and spring months. If old plants are killed, seeds germinate with mud, lower input of precipitation. Many seedlings germinated in 1968 at a site where old ones had been burned off even though the rainfall was not favorable. Plants of a given age varied greatly in size according to their competition. Seedlings germinating in old stands grew little in comparison with those germinating in areas where old plants had been killed. One exception was an area where intense competition occurred due to large numbers of new plants, resulting in growth restriction on all plants.

Populations of pocket gophers and rabbits regulate or control the perennial vegetation on relatively large sites in the northern Mojave Desert. Aboveground shoots are pruned and whole plants are killed by complete cutting of main roots.

At a site near Rock Valley, Nevada, dominated by volcanic rocks, both Larrea tridentata (Sesse and Moc. ex DC.) Cov. And Lycium andersonii A. Gray were restricted in distribution. Larrea tridentata did not grow in the many small washes in the area, but L. andersonii grew only in the washes. Ambrosia dumosa (A. Gray) Payne was more dense and more dominant in wash areas than in nonwash areas.

Six Larrea tridentata (Sesse and Moc. ex DC) Cov. plants were exposed to /sup 14/CO/sub 2/ in a field experiment for 2 h. Three of the plants had been irrigated regularly in the preceding year. Ten small twigs from each plant were removed and counted for /sup 14/C activity at the end of 2 h. The stem portion of the twigs was of equal dry weight for the two sets of plants, but those irrigated had a greater weight of leaves per twig. The activity of /sup 14/C in leaves was equal for the two groups, but was higher in stems for watered plants than for unwatered plants. The results were best expressed as ratios. Dry weight of leaves divided by dry weight of stems was high for watered plants; cpm/g dry weight of leaves divided by cpm/g dry weight of stems was higher for unwatered plants. In another experiment in which leaves were removed before exposing stem portions of twigs to /sup 14/CO/sub 2/, small green stems accounted for about 1/8 the total photosynthesis for a plant; the coefficient of variation was around 100%.

Multivariate analysis of soil and plant data from the northern Mojave Desert was used to investigate aspects of the mineral nutrition of Larrea tridentata (Sesse and Moc. ex DC.) Cov. Larrea tridentata biomass was significantly correlated with soil NO/sub 3//sup -/ and pH and leaf Fe content. Leaf cation accumulation was negatively correlated with leaf Fe concentration.

Effects of trickle irrigation and 25 or 100 kg N/ha applied as NH,NO, were studied in Mercury Valley, adjacent to Rock Valley. During 1975, shrub growth continued at a more rapid pace in irrigated plots than in nonirrigated plots, b_ut vegetative production shifted more from new stem growth toward proportionally greater production of deciduous structures. Fewer new shrub seedlings germinated and survived in 1975 than in 1974. Both irrigation and nitrogen treatments increased the numbers and biomass of winter annuals (primarily the grasses Bromus rube1is and Festuca octoflora). Biomass on irrigated and fertilized plots was 342 kg/ha vs. 141 kg/ha on the controls. Summer annuals were nearly absent from each of the plots in 1975. Seed production was studied in the major shrub species. Irrigation increased fruit production per plant in all species; nitrogen increased it in some and decreased it in others. Ambrosia dumosa fruit production was reduced almost 50% by 100 kg N/ha, and Larrea tridentata fruit production was little affected by either irrigation or nitrogen. INTRODUCTION A study is in progress to determine the effects of irrigation and nitrogen fertilization on a Mohave Desert shrub community. Earlier investigations (Romney et al. 1974), and our results from treatments applied in 1974 (Hunter et al. 1975), indicated that the marked response to supplemental irrigation generally masked any beneficial effects of nitrogen fertilizer on shrub growth. We did, however, observe an increase in the nitrogen content of some shrub and annual plant species in plots where nitrogen treatments were applied. We have not yet seen widespread evidence of nitrogen deficiency where shr"ubs have shown marked growth response to supplemental moisture applied over a period of three years. There have been, however, some cases where some individual shrubs and a number of annual species showed marked growth response to nitrogen applied to irrigated plots, We believe these observations are related to the reserve status of the available nitrogen pool underneath shrub clumps, and that insufficient treatment respollse time has elapsed to allow nitrogen deficiency to develop as the result of higher productivity during several successive years of optimal soil moisture. The basic premise of our continued work is to determine the extent to which the demand for nitrogen will limit plant growth, as available water is increased, and hence make N fertilization more effective in increasing primary productivity. This is the third in our series of progress reports.

This progress report summarizes additional investigations concerning the nitrogen cycle in the northern Mohave Desert. A point transect method was used to estimate lichen crust cover. Lichen crust covers only a very small part of the northern Mohave Desert and could account for much less than 1 kg/ha of fixed nitrogen per year. Free-living organisms may be more important. Acetylene reduction studies were continued. Findings of other years in relationship to specific plant species were not always reproducible. The semisymbiotic and rhizosphere nitrogen fixation appeared to be very irregularly distributed. Nitrogen applied one year previously was not found as mineral nitrogen in the soil surface in the subsequent analysis. Soil nitrate analyses for five different shrub clumps and adjacent bare areas were determined periodically (about three-week intervals) for a year at three different soil depths (0-7.5, 7.5-15, 15-22.5 cm). The values consistently decreased with depth and generally were highest in the March-April period, although the December-February period was almost as high. These results indicate that sufficient nitrogen is mineralized to meet the needs of plant growth in the spring. Ambrosia dumosa was used as a test plant to determine yearly changes in nitrogen concentrations. The rate of turnover of nitrogen in litter measured by plant uptake of various kinds was studied when N-containing plant materials had been incorporated in soil. Considerable differences were observed for different kinds of plant material. Field plots were established to study nitrogen transfer rates and movement using N as a tracer. Isotope ratio techniques were used to estimate soil pools of nitrogen and the root space for a given plant. All studies reported are being continued. INTRODUCTION Studies in the northern Mohave Desert have suggested that there are ample supplies of nitrogen for normal growth (Wallace and Romney 1972; Romney et al. 1974; Wallace et al. 1974; Hunter et al. 1975; R. B. Hunter et al., unpublished data). There are also indications of N inputs through lichen-algal crust and symbiotic plant-bacterial fixation (Hunter et al., unpublished data; Wallace and Romney 1972). Estimates of rates of utilization of N are on the order of 4-10, and sometimes more, kilograms per hectare per year for above-ground plant productivity (Hunter et al., unpublished data; Wallace and Romney 1972). It appears that there is a store of nitrate sufficient for at least two years' growth, but soil NO 3 concentrations are very variable and NO 3 is present in marked concentrations at some points in the soil. OBJECTIVES 1. To determine rates of biological N fixation in the desert systems studied via: a) symbiotic relationships with higher plants; b) symbiotic relationships with algal crusts and lichens; c) free-living nonsymbiotic forms including semisymbiotic forms. 2. To determine losses from the ecosystem via: a) volatilization of NH t; b) leaching; c) runoff of litter, surface leaching or wind removal of litter; d) denitrification; e) erosion of soil organic matter. 3. To determine rates of transfer of nitrogen between various soil-plant compartments as influenced by: a) soil moisture; b) soil and air temperature; c) salinity; and d) soil pH; and to determine relationships among various compartment sizes and to other factors affecting nitrogen cycling under desert systems, 4. To determine rates of uptake of different forms of nitrogen by some desert plants. 5. To characterize and develop some reasons for variations in the C:N ratio of soils in the northern Mohave Desert. METHODS ESTIMATION OF LICHEN COVER A point-transect technique was used to estimate lichen-crust cover. A 50-m steel tape marked every 0.5 m was stretched out and the soil surface where each mark touched the edge was categorized and recorded. A point on the north edge of the IBP validation site was selected and ten 500-m transects run in random directions, with a new one starting where the previous transect ended. The area varied from sandy wash to mature desert pavement. ACETYLENE REDUCTION Ass A y Techniques were reported last year. A modification made during 1975 was incubation of serum bottles for a period of hours to days in the soil at the site of root harvest in the field. Activity was stopped with ethanol or CuSO4-H 2SO,. mixtures when the bottles were unearthed. They were then sent to UCLA for analysis. Data are stored under DSCODE A3UWSI2. PERSISTENCE OF NH(NO3 FERTILIZER IN SOIL Samples of soil from plots which received NH,NO, (0, 25 and 100 kg N /ha in March 1974) were taken June 6, 1975. Depths sampled were 0-7.5, 7.5-15 and 15-22.5 cm. Analysis for NO 3 was as previously reported. Analysis for NH4 was by two techniques, the first as reported in 1974 and the second a similar assay performed on a lOX dilution of soil with 2 N KC! (Bremner 1965). The soil was incubated in 10 N KCl for 0.5 hr and stirred every 15 min, then centrifuged and NH4 analyzed on the supernatant with an Orion NH4 electrode. Data are stored under A3UWS09. SEASONAL NO3 CHANGES Five shrub clumps were selected on a control plot established in 1967 near Mercury, Nevada, sewage ponds. The top 22.5 cm of soil was periodically sampled (three cores), both in the clumps and in the adjacent bare areas throughout 1975. Nitrate analysis was by an Orion nitrate electrode on a dilution of 20 g soil with 40 ml of 0.1 M Na citrate and 1 ppm NO3-N as NaNO3. Analysis was performed on sieved soil (2 mm) the same day as samples were taken (A3UWS09). CHANGES IN Ambrosia N PooLs In January 1975, a group of 30 Ambrosia dumosa plants was selected and divided into those with no visible buds, partially developed buds, and visible leaflets. Those with no buds were harvested in random groups of three at biweekly intervals until exhausted, followed by partially developed and well-developed plants. This was an attempt to provide equivalent physiological leafing-out stages. From June through December other plants were selected at random from the same area from which the 30 original plants were taken. The plants were harvested, oven-dried (68 C for > 3 days) and separated into component parts. Root crowns were harvested, but not the entire root system (root:shoot ratios were reported for Ambrosia by Wallace et al. 1974). Soil samples were taken from below the plant at 0->7 .5 cm, 7.5-22.5 cm and the "root zone" (0.5-30 cm in most cases). Soil NO 3 was determined on those soil samples the day of harvest using the technique recorded above. LITTER DECOMPOSITION RATES Ground plant material tagged with N by hydroponic culture of four desert species (Lycium andersonnii, Larrea tridentata, Atriplex hymenelytra and Ambrosia dumosa) was added to pots of a sieved mixture of several Rock Valley soils. Uptake of N and 15 N into plants grown from January 1975 to the present on these soils in the greenhouse was measured by optical emission spectroscopy. A-values for available soil N were calculated as descibed by Smith and Legg (1971). Pots were watered with deionized water containing less than 0.1 ppm NH4 and NO 3 • UPTAKE OF FIELD-APPLIED 15N Nitrate salts enriched in N were applied to two shrub clumps on January 29, 1975, at rates of 1.76 and 4 kg N/ha to a 5-m 2 area centered on the clump. Soil and plant samples were taken prior to addition of NO3 and on March 27 and June 23, and plant samples on October 1, 1975. On March 21, two similar plots were treated with NH4 salts with and without N-Serve (2-chloro-6 [trichloromethyl] pyridine). Litter traps were constructed around each plot. Samples were separated into tissues and analyzed by optical emission spectroscopy. Analyses of soil samples and the October plant samples have been recently completed, but the data are not y~t analyzed.