Influence of initial nitrate-nitrogen on methods of measuring soil nitrogen supplying capacity in calcareous soils

Seventeen calcareous soils representing different organic matter,total N and NO3--N contents were sampled from Chengcheng ,Yongshou and Yangling area of Shannxi Province, and the soils with and without leaching of NO3--N were used for pot experiments using wheat as indicator to discuss the relationship between initial NO3--N and mineralizable N determined by various methods with wheat uptake N. The results showed that when initial NO3--N were not leached, soil initial NO3--N extracted by KCl directly and KCl boiling method, and leached by CaCl2 before aerobic incubation were closely related with wheat N uptake, the correlation coefficients were 0.934, 0.856, 0.862, respectively, all reached at 1% significant level. On the contrary, it was no good relationship between mineralizable N extracted by aerobic incubation, water logging method, boiling water method, alkaline permanganate extraction, acid permanganate extraction and NaOH hydrolyzation diffusion method with wheat uptake N. When initial NO3--N were leached, the correlation coefficient between initial NO3--N extracted by KCl directly and KCl boiling method, and leached by CaCl2 before aerobic incubation with wheat uptake N were decreased greatly, and could not reach 5% significant level. However, the correlation coefficients between mineralizable N extracted by aerobic incubation, water logging method, boiling water method, alkaline permanganate extraction, acid permanganate extraction and NaOH hydrolyzation diffusion method with wheat uptake N were raised obviously, and all reached 5% or 1% significant level. Of which, the NH4+-N mineralized by one week waterlogging and NO3--N leached by two weeks formal aerobic incubation, as well as NH4+-N extracted by Conway method varied greatly. The correlation coefficients between mineralizable N and uptake N of wheat above ground raised from 0.443, 0.119, 0.259 to 0.866, 0.767, 0.763, respectively. It showed that the poor effects of mineralizable N on reflecting soil N capacity were influenced and disturbed by initial NO3--N, warning researcher to pay more attention to correctly evaluate the function of mineralizable N when soil initial NO3--N is higher .