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.