Plant communities of large portions of the southwestern United States have changed from grassland to desert shrubland. Previous studies have demonstrated that soil nutrient resources become spatially more heterogeneous and are redistributed into islands of fertility with the shift in vegetation. The research presented here addressed the question of whether soil resources become more temporally heterogeneous as well as more spatially heterogeneous when grassland undergoes desertification to form shrubland. Within adjacent grassland and creosotebush sites, soil profiles were described at three soil pits, and samples were collected for description of nutrient resources within the profile. Relative abundance of plant cover and bare soil was determined within each site using line transects. Surface samples (0–20 cm depth) of bare soil and soil beneath the canopies of grasses and creosotebush were collected 17 times during 1992–1994. Soil samples were analyzed for moisture, extractable ammonium and nitrate, nitrogen mineralization potential, microbial biomass carbon, total organic carbon, microbial respiration, dehydrogenase activity, the ratio of microbial C to total organic C (Cmic/Corg), and the ratio of microbial respiration to biomass carbon (metabolic quotient). The major differences in the structure of soils between sites were the apparent loss of 3–5 cm depth of sandy surface soil at the creosotebush site and an associated increase in calcium carbonate content at a more shallow depth. Soils under plants at both sites had greater total and available nutrient resources, with higher concentrations under creosotebush than under grasses. Greatest temporal variation in available soil resources was observed in soils under creosotebush. When expressed on the basis of area, available soil resources were higher in the grassland than in the creosotebush shrubland, primarily due to the difference in plant cover (45% in grassland, 8% in creosotebush shrubland).

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract.Plant communities of large portions of the southwestern United States have changed from grassland to desert shrubland. Previous studies have demonstrated that soil nutrient resources become spatially more heterogeneous and are redistributed into islands of fertility with the shift in vegetation. The research presented here addressed the question of whether soil resources become more temporally heterogeneous as well as more spatially heterogeneous when grassland undergoes desertification to form shrubland. Within adjacent grassland and creosotebush sites, soil profiles were described at three soil pits, and samples were collected for description of nutrient resources within the profile. Relative abundance of plant cover and bare soil was determined within each site using line transects. Surface samples (0-20 cm depth) of bare soil and soil beneath the canopies of grasses and creosotebush were collected 17 times during 1992-1994. Soil samples were analyzed for moisture, extractable ammonium and nitrate, nitrogen mineralization potential, microbial biomass carbon, total organic carbon, microbial respiration, dehydrogenase activity, the ratio of microbial C to total organic C (Cmic/Corg), and the ratio of microbial respiration to biomass carbon (metabolic quotient). The major differences in the structure of soils between sites were the apparent loss of 3-5 cm depth of sandy surface soil at the creosotebush site and an associated increase in calcium carbonate content at a more shallow depth. Soils under plants at both sites had greater total and available nutrient resources, with higher concentrations under creosotebush than under grasses. Greatest temporal variation in available soil resources was observed in soils under creosotebush. When expressed on the basis of area, available soil resources were higher in the grassland than in the creosotebush shrubland, primarily due to the difference in plant cover (45% in grassland, 8% in creosotebush shrubland).

Rangeland restoration through surface application of biosolids (municipal sewage sludge) is an increasing practice. Biosolids supply both organic matter and available nutrients that are lacking in degraded rangeland soils. This study investigated the effects of biosolids applications of 0, 22.5, 45, and 90 Mg ha−1 to a degraded rangeland in 1985 on the soil chemical properties and N mineralization potentials 8 and 9 yr after the application, respectively. Following an initial increase in proportion to the application rate, levels of water‐extractable Ca, Mg, Na, nitate (NO3‐N), and electrical conductivity declined to that near the control soils after 8 yr. Nearly all diethylenetriaminepentaacetic acid (DTPA)‐extractable metals (Fe, Zn, Cu, Cd, and Pb) were highest in the collection 4 yr after application, but were near concentrations in the control soil after 8 yr. Nitrogen mineralization potentials were significantly higher (P < 0.05) in the 45 and 90 Mg ha−1 applications after 9 yr, indicating that site fertility remained higher even though most soil chemical properties were returning to untreated levels.

Woody perennials have invaded semiarid grasslands throughout the Southwestern United States. This invasion was coupled with decreased grass cover and increased runoff and soil erosion. Fire, which was a natural force that shaped and maintained the grasslands, is a management tool that may aid in restoring and maintaining grass cover. However, fire also poses the risk of increasing erosion and further soil degradation because protection afforded by vegetation is reduced immediately after the fire. Using a randomized block study design, this study measured vegetation cover, soil potentially mineralizable N, and erosion associated with the first application of prescribed fire on 2 semiarid grasslands. The potential for adverse effects from these fires was great because the fires occurred at the beginning of a drought period. However, the effects of the burn were minor relative to the effects of the drought, which caused the greatest change. Grass cover on the burn plots was nearly equal to grass cover on the controls 1 year after the fire. After 2 growing seasons, grass cover was equal on the control and burn plots. Potentially mineralizable soil N and sediment transport were similar on the control and burned plots during the 2 years following the fire. Thus, prescribed fire for reducing the cover of woody perennials may not increase the risk of site degradation over that caused by drought and weather fluctuations.