Reliability of the in situ incubation methods used to assess nitrogen mineralization: a microbiological perspective

The reliability of in situ incubation methods to measure soil N mineralization was assessed by comparing the initial and final values for several soil parameters inside and outside the isolated soil containers laid in the ground for 2 weeks. We measured soil water content, ammonifying and nitrifying bacteria, soil respiration, and ammonium and nitrate content. Soil water content did not change inside the container, whereas in the surrounding soil significant differences were found, with varying magnitude and sign according to climatic conditions during the incubation period. Ammonifiers number and ammonium content did not change inside the container, whereas nitrifiers number, nitrate content, and soil respiration decreased significantly (49, 22 and 36%, respectively). When comparing container content and the surrounding soil, a similar pattern was found: ammonifiers number and ammonium content did not change, whereas nitrifiers and soil respiration were lower inside the container than in the outside soil (44 and 33%, respectively). At the end of the incubation period, nitrate contents inside the container correlated significantly with both number of nitrifying organisms and respiration rate. It suggests that aerobic processes inside the cylinder were affected, probably because of oxygen depletion due to microbial activity in a closed environment. We conclude that the in situ incubation methods underestimate the N mineralization rate in soils. Shortening the incubation period may minimize this source of error. (C) 2001 Elsevier Science B.V. All rights reserved.