Plant nitrate reductase activity as an indicator of availability of nitrate in forest soils

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Abstract
The nitrate reductase activity of Deschampsia flexuosa (L.) Trin., an evergreen grass species common in northern coniferous forests, was tested as an indicator of nitrate availability in forest soils. A positive linear correlation (correlation coefficient significantly different from zero, p < 0.01) was found between nitrate supply and nitrate reductase activity in shoots of seedlings in sand cultures. Throughout a vegetation period the nitrate reductase activity of D. flexuosa was constantly 2-16 times higher in a clear-cut areas as compared with an adjacent forest. High enzyme activity, however, indicated significant amounts of nitrate also in the forest during spring and autumn. On plots heavily fertilized with urea the nitrate reductase activity was up to three times higher than on control plots, which suggests considerable nitrate formation on the former. The plant nitrate reductase activity method seemed to be an inexpensive, rapid, nondisturbing, semiquantitative indicator of nitrate availability in soils, particularly for time-course studies and in comparisons between experimental plots. Tests of maximal induction of nitrate reductase activity can reveal to what extent factors other that than the supply of nitrate are limiting the activity.