MODIFIED WATERLOGGED INCUBATION METHOD FOR ASSESSING NITROGEN MINERALIZATION IN SOILS AND SOIL AGGREGATES

A commonly used waterlogged incubation method for measuring mineralizable N was modified to decrease analysis time and increase sample precision by eliminating the sample transfer step. The modified method was tested for speed, ease of use, susceptibility to error, NH4+ recovery, and precision. Modifications included (i) use of a 60-cm3 polypropylene syringe as the incubation vessel rather than a glass test tube, and (ii) KCl extraction of sample NH4+ within the syringe and colorimetric determination of N rather than sample transfer to an extraction flask and subsequent N analysis. The syringe modification reduced sample setup and extraction time by one-third while significantly increasing analysis precision. Net NH4+ production in the syringe compared favorably with that in the standard test tube when a 1-h shake extraction was employed. The potential for error due to NO3- reduction to NH4+ via a dissimilatory NO3- reduction pathway was quantified with N-15. Of the 21 mg N kg-1 of N-15-labeled KNO3 added prior to incubation, 14% was reduced to NH4+, corresponding to a 5% error in the measurement of mineralizable N.