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
Associated with a project that was based upon the assumption that nitrogen may limit net primary plant production in desert grasslands, this project began measuring available inorganic soil N and potentially mineralizable N of soils at two desert grassland locations. Both available N and potentially mineralizable N were greatest following a drought period in 1989, declined during wetter periods that followed and remained relatively stable until another extended drought period. After drought in 1995-6, both forms of soil N increased, indicating the potential for greater NPP following drought and lower potential NPP during periods of normal precipitation.
This project began as part of a study to identify the interactions between soil inorganic nitrogen supply and El Nino-La Nina oscillations that bring wetter-than-average or drier-than-average (respectively) winter-spring precipitation to the central New Mexico region (El Nino-La Nina Fertilization Experiment). This project was known locally as "The Fertilizer Study" and was initiated by Dr. Sandra Turner. To test whether soil inorganic N was limiting aboveground net primary production, fertilizer was added to designated plots within a block of study plots. At first, the soil sampling was designed to confirm or establish the actual fertilizer contribution to each site. The amount added was the amount of inorganic N in soils from the fertilized plots minus that in the control plots. The amount in the controls plots was interpreted as being the amount of inorganic N that had not been utilized by the plant/microbial community and was "available" at that time for utilization in the future. Potentially mineralizable N measurements were performed to determine how the amount of readily mineralizable N was changed by the fertilizer addition; termed a priming effect when the addition of inorganic N results in even greater increases in mineralizable N. Early results from this study were reported at the 1990 Ecological Society of America annual meetings (White et al. 1990).
Following the first year's work, measurements of soil available N and potentially mineralizable N were continued. Except in 1990 when a computer hard-drive that contained most of that year's soil N data crashed and the data were not able to be recovered, the period from 1989 until the spring- summer of 1999 was covered by at least one collection per year.
Each 30m by 30m plot was given a number at the beginning of the study. Before each year's fertilizer application or collection, plots were chosen by selection of random numbers from a table or by using a phone book and using the last 2 digits of the phone numbers. Plots that were used in prior years were excluded from future use. The outer 2.5 m of each plot were left as a buffer strip, so the inside 25m by 25m plot constituted the actual sampling unit.It is not known if the fertilizer was applied to the 30 x 30 m plot or only to the 25 x 25 m plot excluding the buffer. Thus, values given below are based upon application to the 30 x 30 m plot and should be used cautiously.
In 1989, fertilizer treatments were applied as NH4NO3 in 2 treatment levels with controls (low, high, and no fertilizer). Three plots received 61.8 kg N/ha, three plots received 30.9 kg N/ha, and 3 plots were not fertilized (controls). All of these plots were selected in a random manner. It turned out that plot 7 on the west side was too disturbed to be used, so plot 51 was substituted. On the east side, plot 66 was mistakenly fertilized instead of plot 56, which had been selected, so that plot 66 became the 3rd heavily fertilized plot. Hence the plots used in their respective treatments were as follows:
Treatments East side West side
---------- ---------- ----------
Control 16, 33, 42 16, 33, 42
Low 7, 10, 25 19, 25, 51
High 1, 26, 66 1, 26, 56
The fertilizer was applied in April on the east and west sides. Fertilizer was applied with a cyclone spreader by a person walking 5 equally spaced paths across each plot while cranking the spreader. The light treatment was applied with passes in only one direction, while passes were made in both directions across the plot for the heavy treatments.
Soil samples were collected on April 18, 1989, to verify the nitrogen additions and to measure net N mineralization and nitrification potentials. Soil cores (4.2 mm in diameter) were taken to a depth of 20 cm at 4 m intervals along a diagonal from the NE to the SW corner of the plot (starting at the edge of the 2.5 m buffer zone). These cores (15 in all) were placed in the same bag and comprised one sample. The 3 plots represented the sample units (n=3). Plot 66 was not collected until July 4th, when the mistake was identified because plot 56 had no elevated levels of nitrate or ammonium.
In 1990, it was decided that the number of treatment levels would be reduced to one fertilizer application rate and that the number of replicate treatment plots be increased to 4. Fertilizer was applied in two applications, instead of the single application in 1989, in attempts to reduce "burning" from the fertilizer. The first application was on March 1, 1990, and the second was April 4, 1990. Over an inch of rain occurred on both the east and west sides between applications. For each application, fertilizer was applied with a cyclone spreader by a person walking 3 evenly spaced passes across each plot in one direction, and then repeating the process in the perpendicular direction. The setting on the cyclone spreader was 3 1/4. The plots used were:
Treatments East side West side
---------- -------------- --------------
Control 24, 34, 43, 48 24, 34, 43, 48
High 3, 22, 30, 60 3, 22, 30, 60
Within each 30m by 30m plot, 3 ea. half-meter-square (1/2 or 0.5 m^2) quadrats were randomly selected for soil sampling within the northwest quarter of the plot (the 12.5 m by 12.5 m portion excluding the 2.5 m buffer). To identify the location of the 0.5 m^2 quadrats, two tape measures were run parallel to each other, one along the top of the plot and the second along the mid-line of the NW quarter. A third tape was run between the two tapes on the edges to locate the long side of the randomly selected 0.5 m^2 quadrats. Along each diagonal, 4 soil cores were taken at equal spacing starting with the corner of the quadrat and ending in the diagonal corner. Soil cores (4.2 mm diameter) were taken to a depth of 20 cm. The 4 cores from the quadrat were placed in the same bag and comprised one sample. Thus, each plot had 3 soil samples, one from each quadrat, with each treatment having 4 plots.
Protocols for 1991 through 1994
Starting in 1991 and continuing through 1994, 4 plots were sampled with 3 quadrats in each plot; however, only three soil cores were collected (2 from the opposing corners and one from the middle of the quadrat) per quadrat. As in the other years (except 1989 where only one sample was taken per plot), soils from each quadrat were analyzed separately and later averaged to get the value for the plot (3 replicate soil analyses, one per quadrat, and these values were averaged to get a plot value for each of 4 plots; n=4).
Protocols for 1995 to present: Soil Bridges and associated soil collections
In 1995, 5 soil bridges were installed about 30 m east of the Fertilizer Plots. The primary function of the soil bridge is to measure small changes in soil microtopography; erosion or deposition. At the time of soil bridge measurements, two soil cores (same dimensions, 4.2 mm diameter to a depth of 20 cm) for initial available N and potentially mineralizable N were collected from vegetated (mostly grass, 2 cores each) and unvegetated (2 cores each) areas that appeared comparable to those areas beneath the soil bridge. The two sample-types (under plants or open soils) were analyzed separately and the "average" for the bridge was determined by weighting the soil- types by their respective coverages determined beneath the bridge. For example, if vegetation cover was 35% (0.35 of area beneath the bridge had vegetation) and bare soil 65%, the N values for the soil beneath the vegetation were multiplied by 0.35 and summed with the N values for the bare soils multiplied by 0.65. This procedure gave 5 average values for soil N at each collection.+
In the summer of 1998, a study was performed to make direct comparison of the two different collection techniques (collection of soils from fertilizer plots or soils associated with the bridges) and the potential differences the collection method could have on soil nitrogen mineralization potentials.
Methods used for comparative study
Collection on fertilizer plots (fp): Four plots were randomly selected as per previous methods; 4 each 30 X 30 m plots within a 300 X 300 m study site. Within each plot, three randomly selected 0.5 m^2 quadrats within the 15 X 15 m NW quarter of the plot were selected. Within each 0.5 m^2 quadrat, a soil core (4 cm diameter by 20 cm long) was taken from the NW and SE corners of the plot. Both cores were composited to make one sample from each quadrat. This sampling method gave 3 soil samples for each of four plots, for a total of 12 samples.
Collection from the bridge sites: The 5 bridges were equally spaced along the 300 m east border of the fp study area. At each bridge, 2 soil sample cores (4 cm diameter by 20 cm long) were taken from under vegetation cover (under) and 2 from non-vegetative soil surfaces (open). The two cores were composited to give a single soil sample from under and open at each bridge.
Analyses of N mineralization potentials: After determining the water holding capacity (WHC; White and McDonnell 1988), a portion of each sample was adjusted to 50% of determined WHC and 5 subsamples were apportioned into plastic cups. Each cup contained approximately 30 g (+/- 0.05 g) dry-weight mineral soil. One subsample of each sample was immediately extracted with 100 ml 2 N KCl for NH4+-N and NO3--N analyses. The remainder of the cups were covered with plastic wrap, sealed with a rubber band and incubated in the dark at 20 degrees C. Moisture content was monitored by mass loss and replenished as needed. At days 14, 28, 35, and 42, one subsample of each sample was removed and extracted with KCl for 18 - 24 hours. The clarified KCl was filtered through a Kimwipe and analyzed on a Technicon AutoAnalyzer.
Fp: Extractable N at each time interval was averaged for the three soil samples from the 0.5 m^2 quadrats per plot to yield a mean value for each plot. The mean values then were used for statistical analysis (n=4).
Bridges: The relative cover for under and open was obtained from the most recent bridge measurements (made within 2 weeks of soil collections). Extractable N in each soil sample was multiplied by the respective relative cover, and N conc.-times -cover values for soils collected at each bridge were summed to give a cover-weighted value for each bridge (n=5).
The extractable N values for the four fp's were compared to the five bridges over the entire extraction period to determine differences between collection techniques (Factor 1), the effect of extraction time (Factor 2), and their interaction (Factor 1 X 2) at each time interval by analysis of variance. In addition, the variance associated with each collection method was determined and analyzed by analysis of variance to determine if the techniques significantly affected the variance associated with each technique.
The two collection techniques were not significantly different for N mineralization (P = 0.47). There was no significant incubation time x collection technique interaction (P = 0.87). The bridge method did increase variance slightly, but not significantly (P = 0.114).
Net N Mineralization Potentials by different Collection Techniques:Factor d.f. S.S. M.S. F-test P-value------ ---- ---- ---- ------ -------Collection 1 0.316 0.316 0.531 0.47TechniqueExtraction/ 4 330 87.5 147 0.001Incubation timeColl. X time 4 0.719 0.18 0.302 0.87error 35 70.8 0.596 Net N. Mineralization Potentials for Soil Collected by Different Techniques (sum ammonium and nitrate; mg N / kg soil).Incubation Bridge Collections Fertilizer Plots CollectionsTime (days) Mean S.E. Mean S.E.----------- ------------------ ---------------------------- 0 1.96 0.43 2.02 0.24 14 5.55 0.804 5.28 0.45 28 7.33 0.77 7.6 0.17 35 8.29 0.74 8.62 0.63 43 9.59 1.34 10.04 1.12Soil Collection MethodsAll soil samples were taken with a 4.2 mm diameter soil corer to a depth of 20 cm and placed into an ice chest and transported on ice to the University of New Mexico, where they were sieved (2 mm), obvious live roots removed, and stored at 5 degrees C.Soil Analysis MethodsSoil Moisture and Organic MatterFor each collection, soil moisture content of each fresh sample was determined by mass loss upon heating at 105 degrees C for 24 hours. Organic matter was determined by loss-upon-ignition from oven-dried samples placed in a muffle furnace and brought to 500 degrees C for 2 hours. Soil Texture and Total N and P.At select times, texture (percent of sand, silt, and clay) was determined by the hydrometer method (Day, 1965). Total nitrogen and phosphorus were determined by Kjeldahl digestion with copper sulfate as catalyst (Schuman et al. 1973) followed by analysis of ammonium by an automated phenolate method (Technicon AutoAnalyzer Industrial Method #19-69W) and orthophosphate by an ascorbic acid method (Technicon AutoAnalyzer Industrial Method #94-70W).Water-holding capacityWater-holding capacity (WHC) was determine by saturating about a 50 g portion of the sieved soil contained in a funnel with DI water, allowing to saturate for 30 min., and then to drain by gravity for 30 min. The drained soil was transferred to pre-weighed soil tins and dried in an oven at 105 degrees C for 24 hours. The water lost upon drying was the water-holding capacity of the soil sample.Field available N and N mineralization/nitrification potentialsAfter determining water-holding capacity (WHC), a portion of each sample was adjusted to 500f determined WHC and subsamples were apportioned into plastic cups. Each cup contained approximately 30 g dry-weight mineral soil. One subsample of each sample was immediately extracted with 100 ml 2 N KCl (with 5 ppm phenyl mercuric acetate as a preservative) for NH4+-N and NO3--N analyses. The remainder of the cups were covered with plastic wrap, sealed with a rubber band, and incubated in the dark at 20 degrees C. 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 approximately weekly intervals, one subsample of each sample was removed, 100 ml KCl was added and shaken with the plastic wrap coveringthe top, and allowed to settle 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 (Technicon, Terrytown, NY) as described in White (1986). This procedure gave up to three different measurements for inorganic ammonium and nitrate: The initial extractions (time 0, wetted but not incubated) indicated field available concentrations (their sum gave total field available N); The amount of ammonium and nitrate extracted after 35-day incubation; and The maximum sum of ammonium and nitrate in any extraction up to the 35-day incubation (some samples peaked prior to day 35).
File assembled 1/19/2001 JAC. File updated with data through 2005, 10/25/2005 Carl White.
File updated with data through 2008, 5/20/2009 Carl White.
Additional Information on the Data Collection Period
Data collected four times per year (usually January, April, July and October). Sampled one week per season, four seasons per year.
Site DescriptionThe fertilizer study was initiated at two locations on the Sevilleta NWR; one on the east side and one on the west side of the refuge. The east side study site was located within a 300m by 300m block on MacKenzie Flats. The vegetation was dominated by black grama and was thought to be a "typical" black grama desert-grassland. Within that area were 100 ea. 30m by 30m plots (10 plots along the side). The outer 2.5 m of the plot were left as a buffer strip, so the inside 25m by 25m plot constituted the actual sampling unit. When the fertilizer study was no longer active, the plots were sampled to continue the long-term soil record. In 1995, 5 locations along the east 300 m border of the fertilizer study area were established for future soil monitoring. Each of these new sites had soil erosion bridges installed to measure changes in soil microtopography (erosion/deposition). In the methods section are the results of a study that verified that sampling at the 5 new locations gave results comparable to those from the 4 random plots within the fertilizer study area. In 2003, the 5 new locations of 1995 were burned by prescribed fire on 19 June 2003. Additional bridge and soil sites were established within the original fertilizer study area (established in control or untreated plots) and sampled before the prescribed fire to establish similarities to continue the control or untreated soil collections at that area. These new bridge sites are numbered 7196, 7197, 7198, 7199, and 7200.The original study also had a grassland site on the west side of the Sevilleta NWR. This site was west of the Field Station, accessed by entrance at the north gate near Bernardo and following the roadbed of the old highway alignment. This site was dominated by C-3 grasses with C-4 grasses a minor component. Shrubs were primarily 4-wing saltbush. The west side only had room for 61 each 30m by 30m plots. When the fertilizer study was ceased, no further soil collections were made on the west side for long-term study.
Method ReferencesDay, Paul R. 1965. Particle Fractionation and Particle-size Analysis. IN: C.A. Black (ed). Methods of Soil Analysis. Part 1. American Society of Agronomy, Inc., Madison, USA.
Schuman, G.E., Stanley, M.A., and Knudsen, D. 1973. Automated total nitrogen analysis of soil and plant samples. Soil Sci. Soc. Am. Proc. 37:480-481.