Effect Of Sewage Irrigation Water On Soil Qulitity In Farmland

From the agricultural standpoint, sewage irrigation plays an important role in soil quality and food security. Despite of most fruitful literatures which reported the impacts sewage irrigation at present, some specific problems related to margin fields are urgent to be solved. As to the effect of sewage quality on agricultural soil quality, a lot of papers focused on the influences of inorganic pollutants on soil heavy metals, soil physical, chemical, and biological characteristics, while few literatures concentrated either on the fate of organic pollutants in soil ecological system or on the function to soil quality, resulting in the poor understanding of relationships among irrigation water quality, soil environment, and food security.Recently, China is experiencing serious water shortage in agriculture as well as in rivers in different regions that promote the utility of sewage in future agricultural lands. However, it's difficult to handle the excess of kinds of pollutants in sewage during the process of agricultural modernization, industrialization, and urbanization, because of inferior technology in pollution management and void policies of sewage discharge. Accordingly, effects of pollutants of soil quality are both the challenges of soil and environmental science.Based on long term soil pollutions of the sewage irrigation in Jiaokao, Shaanxi, this study determined the soil physical (soil bulk density, soil compactness, etc), chemical (organic matter, nutrients, calcium carbonate), and biological (urease, catalase, alkaline phosphatase activities) properties to analyze the impact of sewage irrigation on soil quality as compared to that of the control treatments (no irrigations and well irrigation). The main results are listed as follows:1. The sewage irrigation obviously impacted the soil physical properties, which showed soil bulk density and compactness increased as sewage irrigated. The average soil bulk density increased from 1.02 g·cm-3 in 0-10cm to 1.51 g·cm-3 in 20-30cm, and reached the maximum 1.47 g·cm-3 in 30-40cm soil layer despite of small fluctuation, resulting soil compactness in deep soil profile. The soil compactness was less 500KPa in 0-10cm, and then increased considerably in 10-45cm; when the soil moisture content was bigger in deep layer, the soil compactness raised sharply from 415KPa in topsoil layer to 1473KPa in deep layer,about increased by 3.5 times, indicated the problems of soil compactness in subsoil layers. 2. The content of soil calcium carbonate was resulted from the double effects of sewage influx and leaching. The content of carbonate in topsoil for sewage irrigation was higher than that of no irrigation and well irrigation, which illustrated the sewage from Weihe river input carbonate(calcium accumulation) in soil and made soil hard during drying; while the well irrigation lead to carbonate leaching in topsoil. The impact of irrigation on soil layers deeper than 40 cm was very small, and no significant difference existed among treatments.3. Sewage irrigation also had a significant effect on soil chemical characteristics. Under long term sewage irrigation conditions, the organic matter went up evidently, especially in the 0-20cm (20.52g?kg-1), which was extremely significant greater as compared to no irrigation (14.28 g?kg-1) and well irrigation (14.78 g?kg-1). Yet, the organic matter decreased when soil depth increases. The result revealed that the sewage from Weihe River suffered from serious organic pollution increased organic matter accumulation and enhanced carbon sink in agricultural land. Meanwhile, the effect of sewage irrigation on soil organic matter proved that there is great potential of increasing soil carbon in non-sewage irrigated land.Eutrophication appeared in long-term-sewage-irrigated soil: the contents of alkali dispelled nitrogen significantly increased in all layers, especially the content in topsoil (108.26 mg?kg-1); the content of available P increased fast in 0-40cm but slow in deeper layers, and the content was highest in 0-10cm (155.45 mg?kg-1); the available K principally accumulated in 0-40cm, with a content of 353.72 mg?kg-1 in topsoil and few change in deeper layers.4. Soil biological properties were clearly affected by sewage irrigation. As compared to the control, sewage irrigations promoted the activity of urease. The activity of soil urease had a significant positive correlation with organic matter, available P, alkaline dispelled nitrogen, and other nutrients; the sewage irrigation intensified the activity of catalase which was very variable in 0-40cm, and the activity was significantly correlated with organic matter, available P, alkaline dispelled nitrogen, and other nutrients; the activity of soil alkaline phosphatase distinctly increased with the augment of alkaline dispelled nitrogen. Overall, the activity of soil enzymes for sewage irrigation increased compared to those of control, and had significant correlations with primary nutrients. Therefore, the activities of leading soil enzymes can be used to evaluate soil fertility.As described above, long term sewage irrigation significantly affected of soil physical, chemical, and biological characteristics. Although sewage irrigation could increase the content of soil organic matter and nutrients, improve activity of soil microbes, great attention should be paid to the potential harmful effect of soil eutrophication on soil nutrient balance, crop growth, and ground water.