Planular-vertical distribution and pollution characteristics of cropland soil Hg and the estimated soil–air exchange fluxes of gaseous Hg over croplands in northern China

Abstract As an important reservoir of mercury (Hg), cropland play an important role in the Hg cycle, but it was poorly understood in northern China. The major objectives of this study are to ascertain the distribution characteristics of soil Hg and then assess its pollution level and potential risk, and further evaluate the role of cropland in northern China in the global soil–air exchange of Hg based on the simulation experiments and regional survey. The average Hg concentration in surface soils of the 30 sites in northern China was 116.1 ± 135.8 ng g−1, which was significantly higher than background values. The surface soils show a significant spatial heterogeneity in Hg concentration, and the Hg levels near provincial capitals were higher than those at corresponding prefecture-level cities, revealing that the soil Hg levels were closely associated with the local industrial and economic development. Profile data shows that topsoil Hg concentration was significantly higher than those in deeper layers at most of sites, indicating the more serious pollution situation in recent years. Generally, the higher the surface soil Hg concentration, the more obvious this top-bottom decreasing trend. The planular-vertical distribution patterns of TOM share similar trends as those of soil Hg concentration, indicating Hg concentration was closely associated with TOM content. Statistical results show that the mean CF, E r i , and Igeo values were 4.0 ± 5.0, 161 ± 198, 0.76 ± 1.34, respectively, and more than two thirds of sampling sites were moderately and considerably polluted. The mean annual accumulative flux of Hg in the northern China was 20.9 ± 43.8 μg m−2 yr−1, and the total net emission fluxes of Hg from the croplands in six provinces were 8.37 ton yr−1. This indicates that although the cropland occasionally acts as a sink, it represents an important natural source of atmospheric Hg as a whole.