Spatial variation and assessment of soil arsenic content in irrigating-agricultural fields of wolfberry in arid area of Qaidam Basin in China

Abstract: Redox condition and pH are the two most important factors affecting the arsenic (As) form. With changes of arsenic forms, desorption and migration of the soil As are likely to occur. In arid regions, the irrigation can result in the low soil gas permeability. It coupled with large amount of organic matter deposited in sediments can facilitate the formation of the reduction environment, in which iron and manganese oxides are easy to dissolve, and As can be converted into the dissolved forms that move with groundwater. The Qaidam Basin is located in the Qinghai-Tibet Plateau with the highest elevation in China. In the basin, there is little rain, the soil pH is 8, and it belongs to a typical arid region with sparse vegetation and high soil pH. The irrigation makes the agricultural cultivation possible here, but also changes the soil redox conditions, creating a good condition for soil As desorption and migration. It is necessary to clarify the influence of irrigation agriculture on soil As distribution and migration. This study aimed to investigate the soil As distribution and its influential factors. A 13.2 hm2 unreclaimed field was selected from the basin for the study. It was reclaimed for wolfberry cultivation. In the field, soil samples (0-20 cm) were collected from 22 points for soil As content detection before the cultivation, and in 1, 2, and 3 years of cultivation. Meanwhile, the factors related with agricultural activities were investigated and the soil As content in these factors were determined. The results showed that the soil As content was 0-26.4 mg/kg with the coefficient of variation (CV) of 0.47 before the cultivation, and about 45.5% of soil samples had soil As content exceeding the Green Food production standard of China (20 mg/kg). After 1-3 years of cultivation, the soil As content was clustered about an average of 15.50-15.88 mg/kg, and the numbers of soil samples points exceeding the standard decreased greatly. The Geographic Information Systems (GIS) spatial distribution map of soil As content revealed that the soil As content before the cultivation was high in the northwest and low in the southeast in the plot, an obvious decreasing trend from the northwest to the southeast. With the increase of farming years, the trend was still maintained but not obvious any more. It indicated that the soil arsenic content at depth of 20 cm was in a dynamic equilibrium with the increase of farming years. By calculating the soil As input from pesticides, fertilizers, and irrigation water and output from wolfberry fruits and stalk, the study showed that the soil As net input per year was 250 g/hm2. However, the measured soil As did not increased after 3 years of cultivation. Therefore, soil As may have the other output ways other than fruits collection and stalk. Downward migration of soil As was likely an output way, which was needed for validation in future.