Geostatistical analyses of spatial distribution and origin of soil nutrients in long-term wastewater-irrigated area in Beijing, China

The purpose of this study was to investigate the current status of soil fertility in the long-term wastewater-irrigated agricultural areas of Beiyechang District in the suburb of Beijing. A total of 103 soil samples from the top 20 cm of soil layer were collected and analyzed for macro- and micronutrient elements. The mean values of these elements were: total nitrogen (TN), 0.08 +/- 0.02%; available nitrogen (AN), 60.34 +/- 10.08 mg/kg; available phosphorus (AP), 19.59 +/- 9.75 mg/kg; available potassium (AK), 84.22 +/- 19.66 mg/kg; and availabilities of trace elements (B, Cu, Fe, Mn, Mo, and Zn), 0.29 +/- 0.12, 1.44 +/- 0.34, 8.97 +/- 2.05, 5.44 +/- 0.95, 0.27 +/- 0.16, 1.16 +/- 0.42 mg/kg, respectively. Compared with other similar areas, the overall soil nutrient content in the study area was still at a low level after nearly 30 years of wastewater irrigation. The organic matter content of wastewater-irrigated soils has significant impact on the accumulation of nutrient elements in these soils. Geostatistical analyses suggested that the spatial distribution of TN and AP was on account of agricultural practices, such as wastewater irrigation and fertilizer use. The availability of P, Mo, Cu and Zn may be due to the joint effect of soil parent material and wastewater irrigation. The availability of K, Fe and Mn was attributed to soil parent material. A significant availability of some elements occurred around sewage outfalls in the irrigation area and water canals, where the wastewater could be easily lifted. The knowledge of spatial distribution and sources of soil nutrients in such areas is the basis for undertaking appropriate farming and irrigation activities with rationalized utilization of treated wastewater.