Identifying the influence of urbanization on soil organic matter content and pH from soil magnetic characteristics

Soil magnetic characteristics are correlated with soil pH and organic matter content. Analyzing soil magnetic characteristics, organic matter content and pH can indirectly evaluate soil pollution caused by human activities. This study analyzed the soil magnetic characteristics, organic matter content and pH in surface soil samples from different land use types in Shihezi city, a newly and rapidly developing oasis city in Xinjiang of China. The aims of this study were to explore the possible relationships among the soil magnetic parameters and thereby improve the understanding of influence of urbanization on soil properties. Eighty surface soil samples at the depth of 0–10 cm were collected from 29 July to 4 August 2013. The results showed that the magnetic minerals in surface soil were dominated by ferromagnetic minerals. Spatially, the magnetic susceptibility (χ LF), anhysteretic remanent magnetization susceptibility (χ ARM), saturation isothermal remanent magnetization (SIRM) and “soft” isothermal remanent magnetization (SOFT) were found to be most dominant in the new northern urban area B (N-B), followed by built-up areas (U), suburban agricultural land (F), and then the new northern urban area A (N-A). The values of χ LF, χ ARM, SIRM and SOFT were higher in the areas with high intensities of human activities and around the main roads. Meanwhile, the property “hard” isothermal remanent magnetization (HIRM) followed the order of U>N-B>F>N-A. Built-up areas had an average pH value of 7.93, which was much higher than that in the new northern urban areas as well as in suburban agricultural land, due to the increased urban pollutant emissions. The average value of soil organic matter content in the whole study area was 34.69 g/kg, and the values in the new northern urban areas were much higher than those in the suburban agricultural land and built-up areas. For suburban agricultural land, soil organic matter content was significantly negatively correlated with χ LF, and had no correlation with other magnetic parameters, since the soil was frequently ploughed. In the new northern urban areas (N-A and N-B), there were significant positive correlations of soil organic matter contents with χ ARM, SIRM, SOFT and HIRM, because natural grasslands were not frequently turned over. For the built-up areas, soil organic matter contents were significantly positively correlated with χ LF, χ ARM, SIRM and SOFT, but not significantly correlated with frequency-dependent susceptibility (χ FD, expressed as a percentage) and HIRM, because the soil was not frequently turned over or influenced by human activities. The results showed that soil magnetic characteristics are related to the soil turnover time.