Mineralization and immobilization of nitrogen in fumigated soil and the measurement of microbial biomass nitrogen

Immobilization of N was measured in a fumigated and in an unfumigated soil by adding (15NH4)2SO4 and following the disappearance of inorganic label from the soil solution and its simultaneous conversion to soil organic N. Calculations based on the measurement of organically-bound 15N gave more consistent values for immobilization than did calculations based on the measurement of the disappearance of label from solution. The fumigated soil immobilized 6.6 μg N g−1 N g−1 soil in 10 days at 25°C, the unfumigated control 4.8 μg. The corresponding gross mineralization rates were 34.9 and 5.6 μg N g−1 soil in 10 days.

Addition of 58 μg N as (15NH4)2SO4 to the fumigated soil increased the quantity of the ynlabelled NH4-N extracted at the end of 10 days from 33.8 to 37.8 μg Ng−1 soil, i.e. there was a positive Added Nitrogen Interaction (ANI). The added labelled N produced this ANI, not by increasing the rate of mineralization of organic N, but by standing proxy for unlabelled N that otherwise would have been immobilized.

A procedure for calculating biomass N from the size of the flush of mineral N caused by fumigation is proposed. Biomass N (BN) is calculated from the relationship BN = F'N/0.68 where F'N is [(N in fumigated soil incubated for 10 days — (N in unfumigated soil incubated for 10 days)].