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 ( 15 NH 4 ) 2 SO 4 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 15 N 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 ( 15 NH 4 ) 2 SO 4 to the fumigated soil increased the quantity of the ynlabelled NH 4 -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 (B N ) is calculated from the relationship B N = F' N /0.68 where F' N is [(N in fumigated soil incubated for 10 days — (N in unfumigated soil incubated for 10 days)].