Vertical distribution of soluble organic nitrogen,nitrogen mineralization,nitrification, and amidohydrolase activities in a manure‐treated soil

Recent studies indicate that soil soluble organic nitrogen (SON) plays an important regulatory role in the soil–plant N cycle. The aims of this study were to identify the vertical distribution of SON and its correlation with N mineralization, nitrification, and amidohydrolase activities, in a soil repeatedly amended with cow manure or chemical fertilizer. For this purpose, soil samples were collected from 0–20, 20–40, 40–60, 60–80, and 80–100 cm depths of a calcareous soil, which has been annually amended for 5 y with cow manure (CM) at two rates of 50 (CM₅₀) and 100 (CM₁₀₀) Mg CM ha^–1 y^–1. Treatments with chemical fertilizer (CF) and a control (CT) were also included. Soluble organic N, N mineralization, nitrification rates, as well as L‐glutaminase and L‐asparaginase activities were determined. Both CM₅₀ and CM₁₀₀ enhanced SON content throughout the soil profile. Nitrogen‐mineralization rate (N_m) was increased at the 0–20 cm depth of the CM₁₀₀ treatment and remained unaffected at the deeper depths. Nitrification rate (N_n) was significantly higher at the 0–60 cm depth of CM₁₀₀ compared to CF and CT. L‐glutaminase and L‐asparaginase activities were significantly increased at the 0–40 cm depth in both CM₅₀ and CM₁₀₀ compared to CF and CT. The amidohydrolase activities could not be detected below 40 cm, regardless of the fertilizer treatments. Our results suggest that SON makes a minor contribution to N mineralization in deep soil layers. It was also concluded that changes in the SON throughout the soil profile were not associated with changes in the N‐transformation rates (N_m and N_n) and amidohydrolase activities. While we conclude that SON is a major N pool in the whole profile of the manure applied soil further investigation is required to characterize SON and to investigate the bioavailability of SON for microbial activity in different soil depths.