Nitrogen mineralisation, immobilisation and loss, and their role in determining differences in net nitrogen production during waterlogged and aerobic incubation of soils

Twenty air-dried soil samples were incubated for 14 days at 30 degreesC to determine net N production (np) under aerobic and waterlogged conditions. The results showed that np was not always higher under waterlogged than under aerobic conditions, which differed from some previous reports. Five analytical equations were compared and used to estimate gross rates of N mineralisation, immobilisation and loss. It was elucidated that the equations based on changes in the AT and AL pools gave estimates of gross mineralization and consumption rates, with the values obtained with the equation of Shen et al. (Shen, S.M., Pruden, G., Jenkinson, D.S., 1984. Mineralization and immobilization of nitrogen in fumigated soil and the measurement of microbial biomass nitrogen. Soil Biology & Biochemistry 16, 437-444) greater than or equal to the equation of Kirkham and Bartholomew (Kirkham, D., Bartholomew, W.V., 1954. Equations for following nutrient transformations in soil, utilizing tracer data. Soil Science Society of America Proceedings 18, 33-34) > the equation of Tiedje et al. (Tiedje, J.M., Sorensen, J., Chang, Y.-Y.L., 1981. Assimilatory and dissimilatory nitrate reduction: Perspectives and methodology for simultaneous measurement of several nitrogen cycle processes. Ecological Bulletin 33, 331-342). The equations based on AT, AL and OL pools estimated gross immobilisation rates, of which the equation of Shen et al. (1984) gave higher values than the equation of Guiraud et al. (Guiraud, G., Marol, C., Thibaud, M.C., 1989. Mineralization of nitrogen in the presence of a nitrification inhibitor. Soil Biology & Biochemistry 21, 29-34). The difference between gross consumption and immobilisation rates represented the rate of N loss from the exchangeable NH4+, NO3- and organic N pools. Gross N mineralization was not always higher under aerobic than under waterlogged conditions during the 14 days incubation of air-dried soils. Immobilisation was greater under aerobic conditions than under waterlogged conditions. Significant amounts of N were lost from some soils during the 2 weeks of incubation. Soils that lost N during aerobic incubation also lost substantial amounts of NH4+-N under waterlogged conditions. However, soil that lost NH4+-N during waterlogged incubation did not necessarily lose N when incubated aerobically. Mechanisms causing the difference in net N production between waterlogged and aerobic conditions are soil-dependent. Although the rate of gross mineralisation predominantly determines the amount of mineral N that may accumulate in soils, immobilisation and loss have the potential to significantly affect the quantity of mineral N accumulation. Crown Copyright (C) 2001 Published by Elsevier Science Ltd. All rights reserved.