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

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 heter ogeneous and are redistributed into islands of fertility with the shif t in vegetation. The research presented here addressed the question of whether soil resources become more temporally heterogeneous as well a s more spatially heterogeneous when grassland undergoes desertificatio n to form shrubland. Within adjacent grassland and creosotebush sites, soil profiles were described at three soil pits, and samples were col lected for description of nutrient resources within the profile. Relat ive abundance of plant cover and bare soil was determined within each site using line transects. Surface samples (0-20 cm depth) of bare soi l and soil beneath the canopies of grasses and creosotebush were colle cted 17 times during 1992-1994. Soil samples were analyzed for moistur e, extractable ammonium and nitrate, nitrogen mineralization potential , microbial biomass carbon, total organic carbon, microbial respiratio n, dehydrogenase activity, the ratio of microbial C to total organic C (C-mic/C-org), and the ratio of microbial respiration to biomass carb on (metabolic quotient). The major differences in the structure of soi ls between sites were the apparent loss of 3-5 cm depth of sandy surfa ce 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 tempor al variation in available soil resources was observed in soils under c reosotebush. When expressed on the basis of area, available soil resou rces were higher in the grassland than in the creosotebush shrubland, primarily due to the difference in plant cover (45% in grassland, 8% i n creosotebush shrubland).