Determination of soil nitrogen availability in alfalfa pasture by chemical and bioassay test

Abstract: The supply of soil available nitrogen in the 5year and 9year alfalfa pasture soils in a rainfed region was determined by three chemical extractants (0.01 mol/L CaCl2, hot water and 1 mol/L KCl) in combination with biological tests on ryegrass.This study showed that the CaCl2 extracted more NO3-N than hot water and KCl did. The extraction capacity of the three extractants for soil ammonium was hot water 1 mol/L KCl 0.01 mol/L CaCl2. N uptake by ryegrass in 5year and 9year alfalfa pasture soils was 2 100 mg/m2 and 3 100 mg/m2, respectively, and N uptake by ryegrass was a significantly positive correlation with the amount of NO3-N extracted by hot water, KCl and CaCl2, implying that the amounts of NO3-N extracted by hot water, KCl and CaCl2 was used to indicate the nitrogen supply of soil. However, the relationship between the amounts of NH4+N extracted by hot water, KCl and CaCl2 and nitrogen uptake by plants is not certain, indicating that and the amounts of NH4+N was not good indicator to describe the nitrogen supply of soil.