Comparison of the chemical methods for assessing soil N-supplying capacity in calcareous soil

Soil N-supplying capacity is an important parameter to determine the nitrogen application rate.Based on the results from pot experiments treated with leaching soil initial nitrate and native soil,we compared the 8 chemical methods which reflecting soil nitrogen supplying capacity and nitrogen uptake by wheat and maize.25 different types of calcareous soils with big differentia collected from in ploughing layer in the Loess Plateau were used as tested soils.Among the 8 chemical methods,the H2SO4-KMnO4 method was a modified acid KMnO4 method.The results indicated that soil organic matter and total nitrogen can reflect soil nitrogen supplying capacity to a certain extent.However,the sensitivity of these two methods was low.The initial mineral nitrogen,especially soil nitrate,in those calcareous soils,significantly correlated with crop uptake nitrogen with native soil(r=0.884,P0.01),but did not significant with leaching soil initial nitrate(r=0.472,P0.05).The results showed that initial mineral nitrogen could reflect soil nitrogen supplying capacity,but insufficiently reflect soil potential nitrogen supplying capacity.The high content of initial soil nitrate affected the correlations between the chemical methods and crop uptake nitrogen.The acid KMnO4 method reflected both soil potential and total nitrogen supplying capacity.The amount of ammonia-N released by H2SO4-KMnO4 method was similar to that extracted by hot KCl water bath method.The H2SO4-KMnO4 method was similar to acid KMnO4 method in reflecting soil total nitrogen supplying capacity,but it was inferior to acid KMnO4 method in reflecting soil potential nitrogen supplying capacity.Hot KCl water bath method was similar to the acid KMnO4 method in reflecting calcareous soil nitrogen supplying capacity.The boiling water method and NaHCO3 UV method were not suitable for reflecting calcareous soil nitrogen supplying capacity.We could concluded that the acid KMnO4 method was the best one in reflecting calcareous soil nitrogen supplying capacity.The hot KCl water bath method and H2SO4KMnO4 method were also superior to other chemical methods in reflecting soil total nitrogen supplying capacity(with control soil) and soil potential nitrogen supplying capacity(without including soil initial nitrate).