Several long-term studies at the Sevilleta LTER measure net primary production (NPP) across ecosystems and treatments. Net primary production is a fundamental ecological variable that quantifies rates of carbon consumption and fixation. Estimates of NPP are important in understanding energy flow at a community level as well as spatial and temporal responses to a range of ecological processes. The NPP weight data (SEV 157) is obtained by harvesting a series of covers for species observed during plot sampling. These species are always harvested from habitat comparable to the plots in which they were recorded. This data is then used to make volumetric measurements of species and build regressions correlating biomass and volume. From these calculations, seasonal biomass and seasonal and annual NPP are determined. These sampled are then vouchered for use to do analyses of inorganic and organic components such as carbon, nitrogen, and phosphorous as well as and other macro and micro nutrients and organic components such as cellulose and lignin.
After all aboveground net primary production (ANPP) quadrat measurements are complete, plants of similar size classes are harvested outside the permanent quadrats. These samples are sorted, dried, and weighed and the resulting data (weight dataset- SEV157) is used to create regressions that estimate aboveground biomass. Then the harvest samples of all size classes, for each species, are then combined to make a voucher sample. A subsample of that combined sample is then ground up mechanically and stored in a sealed glass vial. These samples are available for quantitative chemical analysis of their inorganic and organic composition. Seasonal as well as inter-annual compositions of the various species on the Sevilleta can be derived from this material. The samples are stored at the Sevilleta Field Station. Please contact Stephanie Baker for sample access.
This study examined concentrations of organic and inorganic phosphorus in surface soils of a Bouteloua gracilis-Bouteloua eriopoda grassland and a Larrea tridentata shrubland in the northern Chihuahuan Desert, New Mexico, USA. In this desert, where grassland vegetation has a uniform spatial distribution and individual shrubs have a patchy distribution, vegetation strongly influences the locations and concentrations of soil nutrients. Most studies of soil phosphorus (P) fractions in desert soils have focused on inorganic P fractions and have demonstrated the importance of geochemical controls on soil P cycling. This study addressed whether organic phosphorus, determined by the presence of different vegetation types, also contributes to soil P cycling. Within soils of similar age, topography, parent material, and climatic regime, samples were collected under and between vegetation and analyzed for P fractions following a modified sequential fractionation scheme.
Field Sample Collection: We established four 10 m X 10 m plots in grassland and shrubland habitats near the Five Points area of the Sevilleta Long Term Ecological Research Site (LTER) in the northern Chihuahuan Desert, New Mexico, USA. In the grassland, paired plots (two plots separated by 50 m) were located at sites where Bouteloua eriopoda (Torr.) Torr. (Black grama) and B. gracilis (H.B.K.) Lag. ex Steud. (Blue grama) dominated. In the shrubland, paired plots were located in areas dominated by Larrea tridentata D.C. (Cov.) (creosotebush).
In 1989, we collected a total of 40 soil samples to 10 cm depth in grassland and shrubland plots following a random stratified design and noted whether samples were taken under or between vegetation. All soil samples were air-dried and sieved through a 2 mm sieve prior to analysis. We used unground soils for the fractionation.
Laboratory Analysis: Phosphorus was extracted using a modified sequential Hedley fractionation (Tiessen et al. 1984; Tiessen and 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) (Abrams and Jarrell 1993; Cooperbrand and Logan 1994).
Samples were shaken end-over-end for 16 hours 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 1M HCl for 4 hours (resin-extractable P). Subsequently, the remaining soil sample was extracted with 30 ml of 0.5M NaHCO3 (pH 8.5) in the 50 ml centrifuge tube (bicarbonate-extractable P). This process was repeated with increasingly stronger reagents to remove the more tightly bound P using NaOH (hydroxide-extractable P), HCl (HClextractable P), cHCl (concentrated HCl P), and H2SO4-H2O2 (residual P).
The NaHCO3, NaOH, and cHCl extracts were divided and half of the sample digested with H2SO4-H2O2 to determine total P. For these extracts, organic P was calculated by subtraction. All extracts were analyzed for orthophosphate with the Total Phosphorus procedure for the TRAACS 800 Autoanalyzer (Murphy and Riley 1962; Technicon Bran-Luebbe 1987).
Recovery of organic P from standards mixed with fructose-6-phosphate was (78 95%) (r2 = 0.997). Total P values calculated from the sum of the individual fractions following Tiessen and Moir (1993) averaged 4.5% less than total P values for a subset of samples digested for only total P.
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.
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.
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 ofTransect 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.
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