John Tiszler and David Bainbridge
last update July 16, 1996
The formation of soil organic matter pools and re-establishment of normal nutrient cycling is essential for restoration of a plant community to its condition prior to disturbance. Establishment of adequate soil organic nitrogen pools is particularly important as nitrogen is the element most likely to limit growth in desert systems (Crawford and Gosz 1982). The great majority of soil nitrogen is held in the organic fraction and the plant available (inorganic) nitrogen pool is constantly replenished from this reservoir by microbial decomposition. If the accumulated soilorganic nitrogen is inadequate, nitrogen cycling will be impeded(Munshower 1994). Insufficient nitrogen supplies can result in reductionsin plant growth and ultimately shifts in species composition, as plant species capable of successfully exploitingunnaturally low soil nitrogen concentrations predominate.
The re-establishment of predisturbance shrub species at arevegetation site leads directly to the accumulation of soil organicnitrogen and the rehabilitation of the nitrogen cycle. Soil nutrientpools beneath shrub canopies are enriched primarily through translocation of nutrients from surrounding soils by root uptake andthe subsequent deposition and retention of litter beneath the canopy(Charley and West 1975, Garner and Steinberger 1988). In addition, shrubsmoderate the subcanopy soil environment and produce carbon-rich root exudates, promoting the growth ofinvertebrate and microbial populations necessary for organic matterdecomposition and inorganic nitrogen release.
The nutrient accumulation process is self-augmenting and leads to the formation of the "islands or fertility" commonly found beneath plant canopies in desert shrub ecosystems. However, nutrient accumulation is largely limited by the amount of nitrogen retained in the disturbed soils. At excavated materials sites, where the relatively nutrient-rich topsoils have been removed, other processes such as trapping of wind borne organic matter, clay particles, and microorganisms by shrub canopies (Allen 1988) and symbiotic atmospheric nitrogen fixation by legumes (Virginia 1986) may be important in soil development and nutrient replenishment.
In this study we assessed the recovery of soil organic nitrogen at a Cal Trans materials site five years after it was both planted and seeded with native species. The site is located near the intersection of 84th Ave and Buchanan St., west of Highway 86 near the San Diego - Riverside County line. In April 1990, the borrow pit was ripped and then planted with honey mesquite (Prosopis glandulosa var. torreyana) seedlings and seeded with a mix of species common to the area. Prior to planting, the site was bare of vegetation due to excavation. The materials site shows good vegetation recovery, with the successful establishment of both the planted mesquite and numerous brittlebush (Encelia farinosa) and creosotebush (Larrea tridentata) from seed.
Soils were collected in June 1995 from beneath six of the honey mesquite planted in April 1990 and three creosotebush which germinated from seed in spring, 1991. After clearing away leaf litter, paired soil samples were taken from 0-2.5 cm and 2.5-10 cm depths beneath shrub canopies and in open, unvegetated soils 50 cm from the edge of each shrub canopy. Samples were analyzed for total organic nitrogen by a micro-Kjeldahl method (Bremner and Mulvaney 1982) and compared statistically for differences in nitrogen concentration among locations and between depths.
Results and Discussion
Organic nitrogen concentrations in the surface soils (0-2.5 cm) beneath shrub canopies were over two times greater than in unvegetated soils adjacent to plant canopies (Table 1). The unvegetated soil nitrogen concentrations were not significantly different from soils collected immediately prior to planting in 1990. Subsurface soils (2.5-10 cm) beneath mesquite and in unvegetated areas had organic nitrogen concentrations similar to the 1990 samples, while subsurface soils beneath creosote bush showed a significantly greater average nitrogen concentration than other subsurface soils. Organic nitrogen concentrations in surface soils beneath shrub canopies were significantly higher than in subsurface soils while soils outside shrub canopies did not change with depth.
The accumulation of organic nitrogen in the upper soil layers beneath the shrub canopies indicates that the materials site is returning from its uniform post-disturbance nutrient distribution to the heterogeneous "fertile island" nutrient distribution characteristic of typical desert ecosystems (Crawford and Gosz 1982). However, comparison of creosote bush soils with soils collected in nearby undisturbed bajada and wash sites at an earlier date indicates that recovery of soil nitrogen pools at the materials site is incomplete (Table 2). At both the bajada and the wash, organic nitrogen concentrations beneath shrubs were over three times greater than those currently found at the materials site. Soil nitrogen concentrations outside the canopy were comparable in the wash, but almost two times greater on the bajada.
It was anticipated that mesquite soils would benefit from nitrogenfixation and it is surprising that the average surface and subsurface soilorganic nitrogen concentrations beneath non-fixing creosote bush were 50percent greater than beneath mesquite (Table 1). Although the mesquite at the materials site were inoculated with Rhizobium, it is not known if the plants were actually fixing nitrogen. In any event, our findings indicate that the most rapid recovery of nitrogen pools is not necessarily associated with legumes.
We plan to do a more extensive examination of both the speciescomposition and nutrient distribution at the revegetated materials site. Soils will be sampled from additional mesquite and creosote bush and fromall other species common at the site. In addition to organic nitrogen, concentrations of organic carbon,plant-available (NaHCO3-extractable) phosphorus, sodium, magnesium, andcalcium will be determined. Texture, pH, and salinity (electricalconductivity) will also be determined. Organic nitrogen andplant-available phosphorus will be directly comparable to samples taken atthe site prior to planting in 1990. All plant soil properties will becompared to unvegetated areas within the materials site and to soilsassociated with the same plants found in nearby undisturbed areas. Nutrient pools will beassessed with respect to plant species and size and the relative rate ofsoil nutrient recovery associated with individual plant species will bedetermined.
Allen, M. F. 1982. Below ground structure: A key to reconstructing aproductive arid ecosystem. Pages 113-135 in E. B. Allen, editor. TheReconstruction of Disturbed Arid Lands. Westview Press, Boulder, CO. 267p.
Bremner, J. M., and C. S. Mulvaney. 1982. Nitrogen-Total. Pages595-624 in A. L. Page, R. H. Miller, and D. R. Keeny, editors. Methods ofsoil analysis. Part 2. Chemical and microbiological properties, 2ndedition. Number 9 in the series Agronomy. American Society of Agronomy and Soil Science Society of America, Inc.,Madison, Wisconsin.
Crawford, C. S., and J. R. Gosz. 1982. Desertecosystems: Their resources in space and time. Environmental Conservation9:181-195.
Charley, J. L., and N. E. West. 1975. Plant-induced soilchemical patterns in some shrub-dominated semi-desert ecosystems of Utah.Journal of Ecology 63:945-964.
Garner, W., and Y. Steinberger. 1989. A proposed mechanism for the formation of "Fertile Islands" in the desertecosystem. Journal of Arid Environments 16:257-262.
Virginia, R. A. 1986. Soil development under Legume tree canopies. Forest Ecology and Management 16:69-79.