Spatial distribution of inorganic nitrogen contents of marsh soils in a river floodplain with different flood frequencies from soil-defrozen period

Contents of inorganic nitrogen ( NH4+ - N and NO3- - N) in soil profiles were measured in five typical zones ( including permanently flooded flood-plain ( B), 1-year floodplain (O), 5-year floodplain ( F), 10-year floodplain ( T), and 100-year floodplain (H)) from Huolin River floodplain in Erbaifangzi, Jilin Province of China, in the soil-defrozen period ( May of 1999). Contour maps and profile maps were constructed to describe the spatial distributions of NH4+ - N and NO3- - N in order to identify the influences of flood frequencies on them. Results showed that NH4+ - N generally increased with depth in soil profiles from the five areas, but NH4+ - N contents in T or H areas significantly differed from those in other areas. For NO3- - N, with the exception that there was a significant cumulative peak ( 6.77 +/- 0.08 mg kg(-1)) at 15-cm depth ( 10-20 cm) in B area, no significant difference was observed between NO3- - N contents in soil profiles from the other four areas. The horizontal distributions of NH4+ - N and NO3- - N in top soils ( 0-10 cm) were different in the five areas, which were greatly influenced by flood frequencies. The highest content of NH4+ - N or NO3- - N did not appear in B area but in the floodplain with certain flood frequency. For example, NH4+ - N content (16.81 mg kg(-1)) in 5-year floodplain wetland was highest, and the highest content of NO3- - N ( 1.69 mg kg-1) appeared in 1- year floodplain wetland. In addition, NH4+ - N contents were significantly correlated with soil pH, and NO3- - N contents had significant correlation with inorganic carbon, but there were no significant correlations between inorganic nitrogen and other selected soil properties.