Abstract: Based on pot experiments which including leaching soil initial nitrate and without leaching, we studied the correlations between the 9 biological methods which reflecting soil nitrogen supplying capacity and nitrogen uptake by wheat and maize. Test soils were taken from 25 different calcareous surface soil on Loess Plateau. Dry-wet alternation aerobic incubation method was a modification of the aerobic incubation (for 2 weeks) method according to the actual farmland moisture. The results showed that the nitrogen mineralization potential (N0) was significantly correlated with crop uptake nitrogen with leaching soil initial nitrate (r=0.790, p<0.01). The mineralized nitrogen obtained by aerobic incubation for 4weeks, dry-wet alternation aerobic incubation method and long-term alternate leaching aerobic incubation method had remarkably positive correlations with crop uptake nitrogen with leaching soil initial nitrate (Correlation coefficient were 0.777, 0.768 and 0.764, respectively, p<0.01). The correlation coefficient was low between the water logged incubation method and crop uptake nitrogen with leaching soil initial nitrate (r=0.530, p<0.01). If mineralized nitrogen obtained by any methods included the soil initial nitrate, the correlation between the obtained nitrogen and crop uptake nitrogen without leaching soil initial nitrate was significantly increased. The correlation coefficient were highest for short-term leaching aerobic incubation method and the aerobic incubation (for 2 weeks) method with crop uptake nitrogen (r=0.963，p<0.01). The nitrogen obtained by aerobic incubation for 4weeks, dry-wet alternation aerobic incubation (for 2 weeks) method, long-term alternate leaching aerobic incubation method and N0 had remarkably positive correlations with crop uptake nitrogen without leaching soil initial nitrate (Correlation coefficient were 0.962, 0.959, 0.825 and 0.812, respectively, p<0.01). It could be seen that high contents of soil initial mineral nitrogen, especially nitrate affected the correlations between the biological methods and crop uptake nitrogen. The correlation coefficient between the microbial biomass carbon (BC), microbial biomass nitrogen (BN) with crop uptake nitrogen with leaching initial nitrate were respectively 0.555 (p<0.01) and 0.465 (p<0.05). The correlation between BC, BN with crop uptake nitrogen without leaching initial nitrate were not significantly. Thus, due to long-term alternate leaching aerobic incubation method and N0 needed long term incubating, they were not suitable for general analysis in laboratory. So, we choose the dry-wet alternation aerobic incubation method as the best index for reflecting calcareous soil nitrogen supplying capacity, which were accord with the actual farmland character.

Key words: soil nitrogen supplying index, biological methods, mineralized N