Humans are creating significant global environmental change, including shifts in climate, increased nitrogen (N) deposition, and the facilitation of species invasions. A multi-factorial field experiment is being performed in an arid grassland within the Sevilleta National Wildlife Refuge (NWR) to simulate increased nighttime temperature, higher N deposition, and heightened El Niño frequency (which increases winter precipitation by an average of 50%). The purpose of the experiment is to better understand the potential effects of environmental drivers on grassland community composition, aboveground net primary production and soil respiration. The focus is on the response of two dominant grasses (Bouteloua gracilis and B eriopoda), in an ecotone near their range margins and thus these species may be particularly susceptible to global environmental change.
It is hypothesized that warmer summer temperatures and increased evaporation will favor growth of black grama (Bouteloua eriopoda), a desert grass, but that increased winter precipitation and/or available nitrogen will favor the growth of blue grama (Bouteloua gracilis), a shortgrass prairie species. Treatment effects on limiting resources (soil moisture, nitrogen availability, species abundance, and net primary production (NPP) are all being measured to determine the interactive effects of key global change drivers on arid grassland plant community dynamics and ecosystem processes.
On 4 August 2009 lightning ignited a ~3300 ha wildfire that burned through the experiment and its surroundings. Because desert grassland fires are patchy, not all of the replicate plots burned in the wildfire. Therefore, seven days after the wildfire was extinguished, the Sevilleta NWR Fire Crew thoroughly burned the remaining plots allowing us to assess experimentally the effects of interactions among multiple global change presses and a pulse disturbance on post-fire grassland dynamics.
This data set provides soil N availability in each plot of the warming experiment for the monsoon season (also see SEV176).
Our experimental design consists of three fully crossed factors (warming, increased winter precipitation, and N addition) in a completely randomized design, for a total of eight treatment combinations, with five replicates of each treatment combination, for a total of 40 plots. Each plot is 3 x 3.5 m. All plots contain B. eriopoda, B. gracilis and G. sarothrae. Our nighttime warming treatment is imposed using lightweight aluminum fabric shelters (mounted on rollers similar to a window shade) that are drawn across the warming plots each night to trap outgoing longwave radiation. The dataloggers controlling shelter movements are programmed to retract the shelters on nights when wind speeds exceed a threshold value (to prevent damage to shelters) and when rain is detected by a rain gauge or snow is detected by a leaf wetness sensor (to prevent an unintended rainout effect).
Each winter we impose an El Nino-like rainfall regime (50% increase over long-term average for non-El Nino years) using an irrigation system and RO water. El Nino rains are added in 6 experimental storm events that mimic actual El Nino winter-storm event size and frequency. From January-March, there are 4x5mm applications, 1x10mm application and 1x20mm application. For N deposition, we add 2.0 g m-2 y-1 of N in the form of NH4NO3 because NH4 and NO3 contribute approximately equally to N deposition at SNWR (57% NH4 and 43% NO3; Báez et al., 2007, J Arid Environments). The NH4NO3 is dissolved in 12 liters of deionized water, equivalent to a 1 mm rainfall event, and applied with a backpack sprayer prior to the summer monsoon. Control plots receive the same amount of deionized water.
Soil N is measured using Plant Root Simulator Probes (PRS® Probes, Western Ag Innovations, Saskatoon, Saskatchewan, Canada https://www.westernag.ca/innov).
Probes are installed in late June or early July prior to the monsoon season and removed in October each year.
Study Area Name: Warming site
Study Area Location: Within the Sevilleta, the site is located just Northeast of Deep Well meteorological station. The site can be reached by parking on the main road next to the signs for deep well and the minirhiztron study. Note that the road to Deep Well met station does not permit vehicles. Travel on foot towards deep well and look for a well-trod path off to the right shortly before the met station.
Vegetation: The vegetation is Chihuahuan Desert Grassland, dominated by black grama (Bouteloua eriopoda & B. gracilis).
Vegetation throughout the southwestern United States has changed from perennial grassland to woody shrubland over the past century. Previous studies on the development of 'islands of fertility' focused primarily on only the most limiting, plant-essential element, soil nitrogen (N). The research presented here addressed the question of whether other plant-essential elements, namely phosphorus (P) and potassium (K), showed similar concentration gradients under the desert shrub Larrea tridentata (creosotebush). It also examined whether the spatial distribution of N, P, and K differed from that of essential, but non-limiting nutrients, namely calcium (Ca), magnesium (Mg), and sulfur (S), and non-essential elements, namely sodium (Na), chloride (Cl), and fluoride (F). Within adjacent grassland and shrubland plots, surface soils were collected under and between vegetation and analyzed for a suite of soil nutrients. Soil nutrient distribution followed a uniform pattern that mirrored the spatial homogeneity of bunchgrasses in the grassland, but followed a patchy distribution that mirrored the spatial heterogeneity of individual shrubs in the shrubland. The main differences were that in the grassland, all elements were uniformly distributed, but in the shrubland the plant-essential elements, nitrogen, phosphorus, and potassium, were concentrated under the shrub canopy, and the non-limiting and non-essential elements were either concentrated in the intershrub spaces or were equally concentrated under shrubs and in the interspaces. Our results show how vegetation shifts from grassland to shrubland contribute to long-term, widespread change in the structure and function of desert ecosystems.
The research presented here addressed the question of whether other plant-essential elements, namely phosphorus (P) and potassium (K), showed similar concentration gradients under the desert shrub Larrea tridentata (creosotebush). It also examined whether the spatial distribution of N, P, and K differed from that of essential, but non-limiting nutrients, namely calcium (Ca), magnesium (Mg), and sulfur (S), and non-essential elements, namely sodium (Na), chloride (Cl), and fluoride (F).
Four 10 X 10-m plots were established in grassland and shrubland sites in 1989. In the grassland, two plots were located where B. eriopoda dominates and two plots were located where B. gracilis dominates. In the shrubland, paired plots were located in two shrubland areas dominated by L. tridentata.
Aboveground plant biomass was estimated for each vegetation type at the height of the summer growing season in July 1989. Estimates of grassland biomass were based on clippings of aboveground plant material from a composite of three separate 1 m2 quadrats adjacent to each grassland plot. Estimates of shrub biomass were calculated from shrub volume measures – one height and two canopy diameter measurements – taken on all L. tridentata shrubs in each shrubland plot following Ludwig et al. (1975). Volume measurements were not taken on the few desiccated or dead individuals of the sub-shrub G. sarothrae in the plots.
Ludwig, J. A., Reynolds, J. F. & Whitson, P. D. 1975. Size-biomass relationships of several Chihuahuan Desert shrubs. Am. Midl. Nat. 94:451-461.
To characterize overall soil nutrient composition and soil properties, we collected 25 soil samples from 0–10 cm in depth using a stratified-random sampling design in each of the eight 10 X 10-m plots, noting whether the sample was taken from beneath vegetation or in the bare space between plants. This resulted in a total of 100 soil samples from the grassland and 100 samples from the shrubland habitats. Soil samples were taken at the height of summer drought and although they appeared dry to the touch, all samples were air dried and sieved through a standard 2 mm mesh sieve prior to analysis.
Soil Nutrient Analysis
We analyzed all soil samples for NO3-N, total N, K, total organic C, Ca, Mg, SO4-S, F, Cl, Na, and P. Ground soil samples were analyzed for total organic carbon and total nitrogen using a Carlo-Erba CHN Analyzer. Anions, except for phosphorus, were extracted by shaking a 6-g sample in 30 ml of deionized H2O for 30 min. The extract was filtered through a 0.45 um millipore filter, and analyzed with a Dionex 2010i ion chromatograph. Cations were extracted by shaking a 10-g soil subsample with 50 ml of NH4C2H2O2 (ammonium acetate) at pH 7.0. The extract was filtered gravimetrically through a #40 Whatman filter and analyzer with a Perkin Elmer 3100 Atomic Absorption Spectrophotometer. Phosphorus was extracted using a modified sequential Hedley fractionation (Tiessen et al. 1984; Tiessen &Moir 1993). A 2-g soil sample was placed in a 50 ml plastic centrifuge tube with 30 ml of deionized water and a 2.5 cm2 anion exchange membrane (AR- 204UZR-412 Ionics, Watertown, MA). Samples were shaken end-over-end for 16 h at 25 degrees C. The anion-exchange membrane was removed and phosphorus retained on the membrane was eluted by shaking the strip with 30 ml of 1 M HCl for 4 h (resin-extractable P). Subsequently,the remaining soil sample was extracted with 30 ml of 0.5 M NaHCO3 (pH 8.5) in the 50-ml centrifuge tube (bicarbonate-extractable P). This process was repeated with increasingly stronger reagents that remove more tightly bound, less plant-available, fractions using NaOH, HCl, and H2SO4-H2O2. Each sample was also sonicated and resuspended in NaOH to remove P that is otherwise encapsulated in Al and Fe minerals. All extracts were analyzed for orthophosphate with the Total Phosphorus procedure for the TRAACS 800 Autoanalyzer.
Tiessen, H., Stewart, J. W. B. & Cole, C. V. 1984. Pathways of phosphate transformations in soils of differing pedogenesis. Soil Sci. Soc. Am. J. 48: 853-858.
Tiessen, H. & Moir, J. O. 1993. Characterization of available P by sequential fractionation. Pp. 75-86. In: Carter, M. R. (ed.), Soil sampling and methods of analysis. Lewis Publishers, Boca Raton.
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