Phosphorus fractions and speciation in rural and urban calcareous soils in the semiarid region of Sulaimani city, Kurdistan, Iraq

The phosphorus (P) status of soils depends, among other influencing factors such as soil properties, on P content and availability. These vary with the P distribution among species of different mobility and chemical behavior. The P distribution, in turn, depends on the parent rock material and pedoclimate, and may differ between land uses. This study assesses the impact of urbanization on P contents, fractions and speciation in rural versus urban soils in the region of Sulaimani city, Iraq. Topsoil samples from calcareous soils experiencing different human impact intensity were taken in and around Sulaimani city, spanning rural soils from near-natural grassland, riverbanks and arable land, and urban soils from city parks, roadsides and industrial areas. Changes in P status and speciation were determined by combining P fractionation through sequential chemical extraction and P K-edge XANES spectroscopy. The sequence of P contents in the different fractions was (percentage of total P extracted): citrate bicarbonate-P (0.9%) < NaOH-P (1.7%) < NaHCO₃-P (2.6%) ≤ citrate bicarbonate dithionate-P (3.8%) < residual P (36.2%) < HCl-P (54.8%). The dominance of stable Ca-phosphates (HCl-P) in all samples is typical for calcareous soils. The P distribution among fractions differed considerably in rural versus urban soils. Poor correlations of P fractions in urban soils with other soil parameters indicated that the P forms and contents were severely altered by human impact. P XANES analyses revealed that P speciation was dominated by P associated with Ca, especially in the urban area, and that irrigated arable land use increased the portions of P associated with pedogenic oxides. Soils of the semiarid regions in and around Sulaimani city exhibit a pattern of P fractions and species typical for calcareous soils. This pattern is clearly altered by human impacts such as agricultural land use and especially urban activities. It is concluded that the P inputs and turnover in urban soils are substantially decoupled from natural processes.