EVALUATION OF THE ABILITY OF 3 LABORATORY METHODS TO ESTIMATE NET NITROGEN MINERALIZATION DURING THE GROWING-SEASON

Three laboratory methods for estimating net N mineralization in arable soils were evaluated on the basis of two 3-year field experiments wit h spring barley in each of the countries Denmark, Finland, Norway and Sweden. In these, net N mineralization and crop uptake of soil N durin g the growing season were recorded. Soil samples were taken at the har vest of the preceding crop and in the forgoing late autumn and early s pring. The laboratory methods were: (1) aerobic incubation at 30 degre es C for 14 days, (2) UV light absorption at 205 and 260 nm in water e xtracts and (3) electro-ultrafiltration (EUF). Net N mineralization un der field conditions and crop uptake of soil N generally were not rela ted to nitrogen release during incubation but were significantly assoc iated with mineral nitrogen in soil prior to incubation of air-dried s oil samples (''initial'' mineral N), and generally also with light abs orption at 205 nm and EUF-NO3-N. Mostly, the best relationships were d etected for samples taken in late autumn. Light absorbance at 205 nm a nd EUF-NO3-N were closely related to nitrate N in the samples, extract ed with 2 M KCl. This indicates that light absorption at 205 nm and EU F-NO3-N reflected ''initial'' mineral N. During sample handling and pr eparation, nitrogen was mineralized, thus affecting the amounts of ''i nitial'' mineral N. It was supposed that this released N could have co rresponded to part of the nitrogen mineralized in the held trials duri ng the subsequent growing season. Tests of the best relationships for light absorption at 205 nm and EUF-NO3-N yielded rather encouraging re sults as regards their ability to predict N mineralization under held conditions. It was judged, however, that predictions of N mineralizati on for advisory purposes must be more reliable if they are based on di rect field measurements of plant-available soil N.