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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