Plant communities across large portions of the southwestern United States have shifted from grassland to desert shrubland. Studies have demonstrated that soil nutrient resources become spatially more heterogeneous and are redistributed into islands of fertility with this shift in vegetation. This research addressed the additional question of whether soil resources become more temporally heterogeneous along a grassland-shrubland ecotome. Within adjacent grassland and creosotebush sites, soil profiles were described at 3 pits and samples collected for description of nutrient resources within the profile. Relative cover of plant species and bare soil were determined within each site by line transects. The top 20-cm of bare soil or soil beneath the canopy of grasses/creosotebush were collected 17 times during 1992-1994. Soil samples were analyzed for soil moisture, extractable ammonium and nitrate, nitrogen mineralization potential, microbial biomass carbon, total organic carbon, microbial respiration, dehydrogenase activity, ratio of microbial C to total C (C[mic]-to-C[org]), and microbial respiration to biomass carbon (metabolic quotient).
The major differences in the structure of soils between sites were the apparent loss of a 3 to 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 shown in soils under creosotebush. When expressed on an area basis, greater temporal variation in the total amount of available soil resources was shown in the grassland site, primarily due to greater plant cover (45% in grassland vs. 8% in creosote).
Study Area - The study area was located on the Sevilleta National Wildlife Refuge (NWR) in Central New Mexico, a 93,000 ha wildlife refuge managed by the U.S. Fish and Wildlife Service established in 1973. Prior to establishment of the refuge, the area was grazed by livestock, but domesticated livestock are now totally excluded. An area within the Sevilleta NWR locally known as "5-Points" has been the focus of intense study as part of the Sevilleta Long-Term Ecological Research (LTER) project. Our study area is located approximately 2.5 km west of 5-Points, 13 km east of the Rio Grande, and 1 km north of Palo Duro Canyon, an ephemeral drainage. The study area was selected in 1989 for intensive study because it appeared "typical" of the grassland-creosote bush ecotone in the northern Chihuahuan Desert, with creosotebush extending from the area closest to the rim of Palo Duro Canyon into the grasslands to the north.Sampling Webs - Within the study area, experimental plots used to quantify arthropod and rodent populations were established in 1989. These circular plots, or "webs" (Anderson et al. 1983), have a 100-m radius. Five webs were in a site dominated by creosotebush and five other webs were in a site within the grassland to the northeast. Each web was at least 200 m from other webs. Each web comprised a sample unit for this study. Meteorological collections - Meteorological conditions were measured at a permanent station (Deep Well) approximately 3 km to the northeast of the study area. This station continuously measures meteorological conditions including precipitation, air temperature, relative humidity, mean wind speed, mean vectored wind speed, mean vectored wind direction, maximum and minimum wind speeds, precipitation, 1- and 10-cm depth soil temperature, 10- and 30-cm depth soil moisture, and solar flux. All variables are recorded on an hourly basis, except for precipitation which is recorded on a 1-minute basis during periods of precipitation. The data are downloaded at the Sevilleta Field Research Station every 8 hours through radio links.Soil characterization - Soil pits were excavated to characterize the soil's morphological properties and to collect representative samples from individual horizons for laboratory analyses of physical and chemical properties. Three pits were dug in each vegetative community near the grassland and creosotebush webs. Genetic horizons were identified and their mean depth and thickness, color, and root score were determined. Samples were analyzed at New Mexico State University's Soil, Water and Air Testing Laboratory for the following: organic matter; exchangeable Ca, Mg, Na, and K; pH; electrical conductivity; extractable phosphorus; total phosphorus; KCl-extractable ammonium and nitrate; and % CaCO3 equivalents. Water content was measured gravimetrically after 24 h desiccation at 105 degrees C. Soil pH was measured using a pH meter on a 1:1 wt/wt slurry of soil and 0.01 M CaCl2. Soil texture was measured by the hydrometer method (Day 1965).Vegetative community structure - Aboveground cover of individual plant species, as well as non-vegetative ground cover by categories (bare soil, litter, gravel and rock), were measured in the grassland and creosotebush communities using the Community Structure Analysis (CSA) technique of Pase (1981) (Wolters et al. 1996).Soil sampling for C and N dynamics - For each soil collection, a randomly located point on the circumference of each web was selected. A different point was used for each of the 10 webs, and this point was removed from the pool of possible points for future collections. Soil samples consisted of 4-cm diameter by 20-cm long cores collected from the perimeters of the webs (5 webs for each site). Samples were collected from beneath the canopies of the two closest individual plants (within the grass clumps or a single core at 1/3 the canopy radius from the base of the closest two creosotebushes in their respective webs) and two from open soils approximately mid-point between the closest plants. Comparable samples from diametrically opposite points on the circle were collected and pooled in the field, resulting in one pooled sample consisting of 4 cores each of canopy soil and open soil from each of the ten sample webs.
Sample webs were collected 17 times during 1992-1994, beginning in April 1992 and ending in August 1994. Samples were placed in an ice chest and transported on ice to the Sevilleta Field Station or the University of New Mexico, where they were sieved (2 mm), mixed, split into two portions and stored at 5 degrees C. Total carbon was measured on all soils by a wet oxidation technique (Nelson and Sommers 1982). Microbial biomass and respiration analyses - Microbial measurements were made within 3 weeks of sample collection. Basal respiration was measured as the change in CO2 concentration in the headspace gas in 60-ml serum vials containing soil samples as they were incubated (Anderson 1982). CO2 was measured using gas chromatography as described by Kieft et al. (1991). Soil samples (10 g wet wt.) were placed into serum vials, moistened to approximately field capacity with deionized water, and the vials were sealed with rubber septa. Respiration was measured in triplicate subsamples of each sample during 24-h incubation at 22oC, beginning 24 h after the vials were sealed. Microbial biomass carbon was measured using the substrate induced respiration method (Anderson and Domsch 1978) as modified by West and Sparling (1987). Soil samples (10 g wet wt.) were placed into 60 ml serum vials along with 5 mg glucose and were then wetted to field capacity. Vials were sealed with rubber septa and incubated at 22oC. Headspace gas was sampled at intervals during a period beginning 0.5 h after sealing the vials and extending for 2 h. Respiration rates were converted to Cmic by the equation of Anderson and Domsch (1978): y = 40.04x + 0.37; where y = Cmic (mg 100 g-1 dry wt. soil) and x = respiration rate (ml CO2 100 g-1 dry wt. sediment h-1). Four replicates of each sample were tested. Dehydrogenase activity - Soil dehydrogenase activity was measured using the substrate iodonitrotetrazolium (INT). The method was a slight modification of Griffiths' (1989) method. Tetrazolium salts are colorless in solution and are reduced by cellular dehydrogenases to colored tetrazolium formazans. Soil (1.0 g wet wt) was added to a screw-cap test tube, and then 2.0 ml of a 0.5% (W/V) INT and 1.5 ml deionized water (W/V) were added. The soil suspensions were vortex-mixed and the tubes incubated in the dark at 40oC. After 2 h incubation, enzymatic activity was halted and INT-formazan was extracted by adding a 10.0 ml of a 1:1 mixture of dimethylformamide and ethanol. Extraction was carried out in the dark for 1 h with vortex mixing every 20 min. The soil was then removed by centrifugation. INT- formazan concentrations of the supernatants were determined spectrophotometrically at 460 nm using the extracting solution as a blank. Triplicate tubes were set up for each soil sample along with an autoclaved control of each sample. Absorbance in the control extract was subtracted from the average of the extracts in the live soils. N mineralization potentials- After determining water-holding capacity (WHC)(White and McDonnell 1988), a portion of each sample was adjusted to 500f determined WHC and up to 11 subsamples were apportioned into plastic cups. Each cup contained approximately 30 g dry-weight (DW) mineral soil. One subsample of each sample was immediately extracted with 100 ml 2 N KCl for NO3--N and NH4+-N analyses. The remainder of the cups were covered with plastic wrap, sealed with a rubber band, and incubated in the dark at 20oC. The plastic wrap minimized water loss during incubation, yet exchange of CO2 and O2 was sufficient to keep the subsamples aerobic during incubation. Moisture content was monitored by mass loss and replenished as needed. At weekly intervals, one subsample of each sample was removed and extracted with KCl for 18-24 h. The clarified KCl was filtered through a Kimwipe and analyzed for NH4+-N and NO3--N+NO2--N on a Technicon AutoAnalyzer as described in White (1986). LITERATURE CITEDAnderson, D. R., K. P. Burnham, G. C. White, and D. L. Otis. 1983. Density estimation of small-mammal populations using a trapping web and distance sampling methods. Ecology 64:674-680.Anderson, J. P. E. 1982. Soil respiration. Pages 831-871 in A. L. Page, editor. Methods of soil analysis, Part 2, Chemical and microbiological properties. American Society of Agronomy, Madison, Wisconsin.Anderson, J. P. E., and K. H. Domsch. 1978. A physiological method for the quantitative measurement of microbial biomass in soils. Soil Biology and Biochemistry 14:273-279.Day, P. R. 1965. Particle fractionation and particle-size analysis. Pages 562-566 in C. A. Black, editor. Methods of soil analysis Part I: Physical and mineralogical properties including statistics of measurement and sampling. American Society of Agronomy, Madison, Wisconsin.Griffiths, B. S. 1989. Improved extraction of iodonitrotrazolium-formazan from soil with dimethylformamide. Soil Biology and Biochemistry 21:179-180.Nelson, D. W., and L. E. Sommers. 1982. Total carbon, organic carbon, and organic matter. Pages 539-579 in A. L. Page, editor. Methods of soil analysis, Part 2, Chemical and microbiological properties. American Society of Agronomy, Madison, Wisconsin. Pace, C. P. 1981. Community structure analysis - a rapid, effective range condition estimator for semi-arid lands. Pages 425-430 in H. G. Lund, et al. (tech. coord.). Arid land resources inventories: developing cost-efficient methods. USDA Forest Service General Technical Report WO-28, Washington, D.C.West, A. W., and G. P. Sparling. 1986. Modification to the substrate induced respiration method to permit measurement of microbial biomass in soils of differing water contents. Journal of Microbiological Methods 5:177-189.White, C. S. 1986. Effects of prescribed fire on rates of decomposition and nitrogen mineralization in a Ponderosa pine ecosystem. Biology and Fertility of Soils 2:87-95.White, C. S., and M. McDonnell. 1988. Nitrogen cycling processes and soil characteristics in an urban versus rural forest. Biogeochemistry 5:243-262.Wolters, G. L., S. R. Loftin, and R. Aguilar. 1996. Changes in plant species composition along a Chihuahuan desertscrub/desert grassland transition. Pages 614-615 in N. West (ed.) Fifth International Rangeland Congress Proceedings. July 23-38, 1995. Salt Lake City, Utah.
10-09-96 added keywords, methods, and persons involved with the data (JAC)10-10-96 reformatted document to fit an 80 column screen (JAC)10-10-96 added abstract (JAC)10-11-96 appended formated data and began variable descriptions (JAC)12-09-96 appended water data (JAC)12-10-96 completed variable descriptions (JAC)02-24-99 linked /db/archive/nutrient/soils/FPCN-data.dbf to the web site. GM