TEMPORAL DYNAMICS IN SOIL CARBON AND NITROGEN RESOURCES AT A GRASSLAND–SHRUBLAND ECOTONE

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 creo- sotebush were collected 17 times during 1992-1994. Soil samples were analyzed for mois- ture, extractable ammonium and nitrate, nitrogen mineralization potential, microbial bio- mass 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 concen- trations 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 shrub- land, primarily due to the difference in plant cover (45% in grassland, 8% in creosotebush shrubland).