Monsoon Rainfall Manipulation Experiment (MRME): Soil Nitrogen Data from the Sevilleta National Wildlife Refuge, New Mexico (2007 - present)


The Monsoon Rainfall Manipulation Experiment (MRME) is designed to understand changes in ecosystem structure and function of a semiarid grassland caused by increased precipitation variability, by altering rainfall pulses, and thus soil moisture, that drive primary productivity, community composition, and ecosystem functioning. The overarching hypothesis being tested is that changes in event size and frequency will alter grassland productivity, ecosystem processes, and plant community dynamics. Treatments include (1) a monthly addition of 20 mm of rain in addition to ambient, and a weekly addition of 5 mm of rain in addition to ambient during the months of July, August and September. We predict that soil N availability with interact with rainfall event size to alter net primary productivity during the summer monsoon. Specifically, productivity will be higher on fertilized relative to control plots, and productivity will be highest on N addition plots in treatments with a small number of large events because these events infiltrate deeper and soil moisture is available longer following large compared to small events.

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Experimental Design

MRME contains three ambient precipitation plots and five replicates of the following treatments:
1) ambient plus a weekly addition of 5 mm rainfall, 2) ambient plus a monthly addition of 20 mm rainfall.

Rainfall is added during the monsoon season (July-Sept) by an overhead (7 m) system fitted with sprinkler heads that deliver rainfall quality droplets. At the end of the summer, each treatment has received the same total amount of added precipitation, delivered in different sized events.

Each plot (9x14 m) includes subplots (2x2 m) that receive 50 kg N ha-1 y-1. Each year we measure: (1) seasonal (July, August, September, and October) soil N, (2) plant species composition and ANPP, (3) annual belowground production in permanently located root ingrowth cores, and (4) soil temperature, moisture and CO2 fluxes (using in situ solid state CO2 sensors).

Soil Measurements: We use plant root simulator probes (PRS® Probes, Western Ag Innovations, Saskatoon, Saskatchewan, Canada

Instrumentation: Plant root simulator probes

On August 4th, 2009, lightning ignited a ~3300 ha wildfire that burned through the entire experiment and its surroundings allowing us to assess experimentally the effects of interactions among rainfall pulse dynamics and wildfire on post-fire grassland dynamics and ecosystem processes.

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Data was not collected in 2011.

Additional Study Area Information

Study Area Name: Monsoon site
Study Area Location: Monsoon site is located just North of the grassland Drought plots
Vegetation: dominated by black grama (Bouteloua eriopoda), and other highly prevalent grasses include Sporabolus contractus, S.cryptandrus, S. lexuosus, Muhlenbergia aernicola and Bouteloua gracilis.

North Coordinate:34.20143
South Coordinate:34.20143
East Coordinate:-106.41489
West Coordinate:-106.41489

Rio Grande Water Chemistry Data from Bernalillo County, New Mexico (2006-2007)


Human populations in Colorado, New Mexico and Texas depend on the Rio Grande for municipal water, agricultural irrigation, and recreation. The Rio Grande and its riparian corridor also support thousands of species of plants, invertebrates and vertebrates, some of which include over 300 species of migratory birds and the endangered Rio Grande silvery minnow and southwestern willow flycatcher. Eutrophication and salinization are the two most important types of water quality degradation which negatively impact the human and nonhuman biological communities in this water poor region. In spite of their significance, few published studies have investigated anthropogenic and natural sources of nutrients and dissolved solids to the Rio Grande. This study investigated the patterns and trends of nutrients and dissolved solids in the Middle Rio Grande (MRG) on a monthly basis from September 2005 – January 2008. During all months, wastewater treatment plants were the major source of nutrients to the MRG. Under high flow conditions, nutrient levels remained elevated for 260 river kilometers below the wastewater inputs. During months when significant portions of the river flow were diverted for irrigation, nitrate and phosphate were removed from the MRG and concentrations at the downstream end of the reach were returned to levels comparable to the un-impacted northern reach of river. Dissolved solids were added to the river by both wastewater and saline tributary inputs. Both anthropogenic and natural inputs of dissolved solids were found to affect water quality in the MRG. Continuous real-time measurements of temperature, pH, turbidity, dissolved oxygen, and conductivity also were initiated at four sites above and through the urban reach of the City of Albuquerque. Preliminary results show increasing turbidity and dissolved oxygen depletions associated with storm runoff from urban areas. 

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The objectives of this study were to: 1) conduct a detailed assessment of the temporal and spatial trends in water quality of the MRG, 2) determine sources of eutrophication and salinization along the MRG, 3) estimate instream nutrient processing and retention, 4) calculate the effects of urbanization on dissolved oxygen and stream metabolism values in the MRG, and 5) provide baseline data for future water-quality monitoring and assessment in the MRG. 


Experimental Design:  

Samples were collected along the lenth of the MRG over a two to three day period, approximately monthly. Single grab samples were collected at each site. During 'Monthly' collections samples were taken from just the mainstem of the MRG. During 'Synoptic' collections samples were taken from both the mainstem sites and all of the major tributaries to the MRG. Mainstem sites were located ~ 5 km downstream of each major tributary to  the MRG to allow complete mixing of the tributary and mainstem water bodies and tributaries were sampled just prior to their convergence with the mainstem of the Rio Grande. Samples were collected during periods of stable flow (samples were not collected during storm pulses). 

Field methods: 

Surface-water samples were collected for measurement of temperature, pH, and conductivity, and analysis of major dissolved inorganic nutrients (nitrate, phosphate, and ammonium), major cations (sodium, potassium, magnesium and calcium), major anions (sulfate, bromide and chloride), dissolved organic and inorganic carbon (DOC, DIC), specific ultraviolet absorbance (SUVA), and chlorophyll a at each site. Sampling began in September 2005 and continued through February 2008. All samples were collected as close to the stream thalweg as flows permitted. Water samples for analysis of nutrients, cations, and anions were collected as grab samples in 130 ml syringes and immediately filtered in the field through ashed 0.7 um pore size glass fiber filters. Unfiltered water samples for chlorophyll-a analysis were collected in acid washed or unused HDPE bottles. All samples were placed on ice and transported to the laboratory for analysis.

Laboratory Procedures:  

Ammonium samples were analyzed using the phenyl hypochlorite method and a 10 cm flow path modified from Hansen and Koroleff (Hansen and Koroleff 1983). Bromide, chloride, nitrate, phosphate and sulphate were analyzed by ion chromatography (Dionex, Standard Method EPA 300.1, 2).  Organic and inorganic carbon were analyzed using a Shimadzu TOC-5050A carbon analyzer using Standard Method 5310 B (Clesceri et al. 1998). Sodium, potassium, magnesium, and calcium were analyzed using a Perkin Elmer Optima 5300 DV ICP using Standard Method 3120 B (EPA 200.7) (Clesceri et al. 1998). Clesceri, L. S., A. E. Greenberg, and A. D. Eaton, editors. 1998. Standard Methods for the Examination of Water and Wastewater. 20 edition. American Public Health Association, American Water Works Association, Water Environment Federation, Baltimore. Hansen, H. P. and F. Koroleff. 1983. Determination of Nutrients. Pages 159-226 in K. Grasshoff, K. Kremling, and M. Ehrhardt, editors. Methods of Seawater Analysis. Weinheim: Verlag Chemie.

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* Instrument Name: Carbon Analyzer

* Manufacturer: Shimadzu

* Model Number: TOC-5050A

* Instrument Name: Ion Chromatograph

* Manufacturer: Dionex

* Model Number: 

* Instrument Name: Inductively Coupled Plasma Optical Emission Spectrometer

* Manufacturer: Perkin Elmer

* Model Number: Optima 5300 DV ICP 

Temporal Dynamics of Soil Carbon and Nitrogen Resources Within a Grassland-Creosote Ecotone at the Sevilleta National Wildlife Refuge, New Mexico (1992-1994)


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).

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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).


Anderson, 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

Ecological Effects of Prescribed Fire on Soils in a Chihuahuan Desert Grassland at the Sevilleta National Wildlife Refuge, New Mexico (2003)


Fire resulting from natural ignition has become a more common event on the Sevilleta National Wildlife Refuge (NWR) since the exclusion of domesticated livestock. Efforts to return fire to the native landscape has resulted in the use of prescribed fire during periods that meet burn prescriptions. A prescribed fire was performed on the Sevilleta NWR in June 2003. Among the measured site and burn characteristics that were measure, this project sampled soils before and after the fire from 5 previously-sampled locations that were burned in June 2003 and from 5 newly established locations that served as controls. The controls were within an area that was sampled between 1989 and 1996 for similar properties measured in this study and had previously been tested to be similar to the locations burned in 2003. The soil properties that are repeatedly measured at the burn and control locations include: field water content; water-holding capacity; organic matter; field extractable nitrate and ammonium; and potentially mineralizable nitrogen.

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The removable bridge is placed upon the end rebar and the middle pin is secured in the depression on the nail beneath the middle hole (#16). The bridge is then leveled and individual pins are inserted to the soil surface. If the surface is firm enough, the pins are left unsecured. If the surface is too soft, the pins are secured with the tip at the soil surface by attaching a clothspin above the bridge. The heights of each pin above the bridge are recorded, and cover is recorded if the pin struck vegetation when being inserted and basal cover is recorded if the pin rested upon the basal portion of a plant at the ground surface. The standard soil bridge developed for the Sevilleta was used. The bridge contains 31 holes at 5 cm intervals with the middle hole used to orientate the bridge above a nail left at the ground surface, and which provides a reference to secure the line and the bridge height. Also referenced at (

Soil Collection

For inorganic N extractions and potentially mineralizable N measurements, a soil core of 4-cm diameter was taken to 20-cm depth beneath two nearby grass clumps (the two cores were compostited; termed under) and from two bare soil patches (two cores were composited; termed open) within 5 m of the stake identifying each bridge or from the bridge stake with the identification tag (new control bridges). All soil samples were placed into an ice chest and transported on ice directly to the University of New Mexico UNM, where they were sieved (< 2 mm), mixed, and stored at 5 degrees C. All soil N measurements were performed at UNM.

Soils Analysis

After determining fresh water content and water-holding capacity (WHC)(White and McDonnell 1988), fresh portions of each sample were adjusted to 50% of determined WHC and subsamples of 20 g dry-weight were apportioned into plastic cups. One subsample of each sample was immediately extracted with 100-ml 2 N KCl for NH4+-N and NO3-N analyses to determine field-available N. Two additional 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. After contact and settling 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). After incubation for 6 weeks, a subsample of each soil was extracted with KCl and analyzed for NH4+-N and NO3--N+NO2--N. Potentially mineralizable N was determined to be the amount of extractable N in the 6-week extraction.

Organic Matter

Organic matter was determined by loss-upon-ignition in tin cups following heating at 500C for two hours.

Soil Physical/Chemical Properties

Soil cores were taken beneath grass clumps in which the temperature pellets were placed both before and after the fire. At least 300 g of soil were taken to a depth of 10 cm (NOTE:different depth than nitrogen cycling)

Fire Temperature

Fire temperature was determined with pellets supplied by Tempil (2901 Hamilton Blvd., South Plainfield, NJ 07080; A set of 15 foil-wrapped tablets, with melting temperatures ranging from 85 C to 1533 C, were strungon wire and suspended about 1 inch above the ground. The fire temperature was assumed to be greater than the temperature at which the corresponding pellet showed signs of melting and less than the temperature of the next highest undamaged pellet. The pellets were suspended within two clumps of dominant grasses at the site (black grama).

Pre-existing briges (1.1 through 1.5) were selected to be included within a prescribed burn area. Data collected from the bridges were consistent with existing data collection: (http// ) and included soil surface elevation, plant aboveground cover and basal cover. Soils from beneath a nearby grass clump and from bare interspaces were collected for analysis of soil properties. Soil temperature pellets were placed within grass clumps from beneath which soils were collected. Pre-fire on control and expected burn plots, and post-fire on burn plots only for soil elevation, aboveground plant cover and basal cover, N mineralization potentials, field moisture, water holding capacity, and loss upon ignition for organic matter. Pre and post burn soil samples were collected beneath grass clumps at the existing bridges for analysis of soil properties (sent ot Jane Belnap). Fire temperature was measured with temperature tablets placed about 1 cm above the ground within the grass clump that was sampled for soil properties and in an adjacent grass clump of similar appearance.

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Changes to the data: Data were updated to include 2007 data on 5/15/2008 by Carl White.

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Additional Study Area Information

Study Area Name: Bridge 1.1

Study Area Location: north end of five bridges; black grama dominated grassland; MacKensie Flats; Site is 5 m area around bridge; Bridges setup in 1994 to monitor changes in soil surface elevations to understand the dynamics of soil particles and associatednutrients.

North Coordinate: 34.3358
South Coordinate: 34.3358
East Coordinate: -106.6954
West Coordinate: -106.6954

Snakeweed (Gutierrezia sarothrae) Habitat Soils Data from the Sevilleta National Wildlife Refuge, New Mexico (1984)


In 1984, a research project was initiated on a relatively small disturbance patch just south of Deep Well. This disturbance was thought to be the result of an old praire dog town, probably dating back to when a nearby ranch was active, and a lot of old mammal mounds remained in the disturbed area. One of the things that made the disturbance patch particularily noticeable was the lush growth of snakeweed (Gutierrezia sarothrae) within the patch. This prompted the designation of the disturbance patch as the "snakeweed patch" or "Gutierrezia patch." In addition, there was an obvious increase in bare ground and a shift in vegetation composition across the patch boundary. The dominant vegetation was not consistent around the boundary, with a marked dominance of black grama on the west side of the plot and a blue/black grama mix on the other three sides. To obtain information on the cause and/or effect of this disturbance, a survey of the soil and vegetation was performed.

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Sample collection - The soil samples were collected using a hammer-driven soil corer. The barrel of the corer was fitted with a plastic sleeve that allowed extraction of the soil core generally intact. The  soil corer was driven to a depth of 50 cm and soils split ito 10 cm fractions. This data set contains data for only the top 30 cm.

Samples were taken along six 100 m transects. Four of these transects crossed the patch boundary on the four cardinal points. On these four transects the 0m sample was taken starting 50 m outside the boundary, the 50 m sample was taken at the patch boundary and the 100 m sample was taken 50 m into the patch. The other two transects formed a cross near the center of the patch.

Twenty-one cores were collected along each transect, with increased sampling intensity near the boundary. However, this data set contains data from only the 10 m intervals for a total of 11 samples.

Sample processing - Soil samples were kept in a refrigerator prior to analysis. Each sample was weighed and samples were well-mixed before analysis. Samples were sieved through 2mm screens to remove pebbles and roots. A sample of 25 g was added to a preweighed soil can. Samples were dried for 24 hours at 105 degrees C then cooled and then reweighed. This dry/wet moisture correction was used to calibrate weights for other samples. A 1 g sample was taken from the oven-dried samples and ashed at 500 degrees C for 2 hours and re-weighed after cooling. This provided a measure of organic content. A 12 g sample was weighed into a 125 ml plastic bottle and 100 ml of 2 N KCL added before the bottles were well-shaken. After standing for 24 hours, the KCL was decanted and the samples analyzed for NO3-N and NH4-N on a Technicon Autoanalyzer. Another 5 g sample was weighed into a centrifuge tube and extracted repeatedly with pH 7 ammonium acetate. These samples were brought up to 250 ml and analyzed for Ca, Mg and K using atomic absorption. Fifty g samples of soil were mixed and texture determined using the hydrometer method. Samples were mixed 2:1 with 0.001N CaCl2 and pH measured. From the oven-dried samples 1 g samples were digested using sulfuric acid using the Kjeldahl method. Samples were then brought up to 250 ml and analyzed on a Technicon Autoanalyzer for total nitrogen and phosphorous.

Coordinates (NAD27): 

End of

Transect Transect Latitude Longitude

North 0 34 21' 1.2" 106 41' 8.3"W

100 34 20' 57.9"N 106 41' 8.6"W

East 0 34 20' 47.0"N 106 41' 1.6"W

100 34 20' 46.5"N 106 41' 5.4"W

West 0 34 20' 53.7"N 106 41' 16.3"W

100 34 20' 53.7"N 106 41' 12.4"W

GCSA 0 34 20' 49.1"N 106 41' 9.2"W

100 34 20' 45.6"N 106 41' 9.2"W

GCSB 0 34 20' 47.1"N 106 41' 8.9"W

100 34 20' 47.4"N 106 41' 5.1"W


12/10/00 (DM) File created.2/10/2009. (DM) Metadata was updated and compiled.

Long-term Dynamics in Soil Field Available Nitrogen and Potentially Mineralizable Nitrogen in a Chihuahuan Desert Grassland at the Sevilleta National Wildlife Refuge, New Mexico (1989-2014)


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.

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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.

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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.

1989 Protocols

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.

1990 Protocols

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. 

Statistical Analysis

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     0.47

Extraction/         4        330     87.5      147       0.001
Incubation time

Coll. X time       4       0.719    0.18    0.302     0.87

error                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 Collections
Time (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           1.12

Soil Collection Methods

All 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 Methods

Soil Moisture and Organic Matter

For 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 capacity

Water-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 potentials

After 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: 

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 Description

The 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 References

Day, 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.

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