PRESERVATION OF NITRIFYING CAPACITY AND NITRATE AVAILABILITY IN WATERLOGGED SOILS BY RADIAL OXYGEN LOSS FROM ROOTS OF WETLAND PLANTS

The effects of radial O-2 loss from roots on nitrification and NO3- av ailability were studied. Plants of the flooding-resistant species Rume x palustris and the flooding-sensitive species Rumex thyrsiflorus were grown on drained and waterlogged soils with an initially high nitrify ing capacity. Nitrate reductase activity in the plant leaves was used as an indicator of NO3- availability to the plants. In a separate expe riment these species were shown to have higher levels of nitrate reduc tase activity when NO3- was added to the soils compared to when on NH4 + was provided. In drained soils nitrification was maintained and both plant species showed relatively high nitrate reductase activities in their leaves. In the water-logged series planted with R. thyrsiflorus, nitrification was inhibited, NH4+ accumulated, and the plants grew le ss well compared to those on drained soils. In contrast, waterlogged s oils planted with R. palustris had a redox potential high enough for O -2 to be continuously replenished. Furthermore, the nitrifying capacit y of these latter soils was maintained at a high level. R. palustris g rew well and NO3- must have been available to the plant, since a high level of nitrate reductase activity was observed in the leaves.