Preservation of nitrifying capacity and nitrate availability in waterlogged soils by radial oxygen loss from roots of wetland plants

The effects of radial O₂ loss from roots on nitrification and NO _inf3 ^sup- availability were studied. Plants of the flooding-resistant species Rumex palustris and the flooding-sensitive species Rumex thyrsiflorus were grown on drained and waterlogged soils with an initially high nitrifying capacity. Nitrate reductase activity in the plant leaves was used as an indicator of NO _inf3 ^sup- availability to the plants. In a separate experiment these species were shown to have higher levels of nitrate reductase activity when NO _inf3 ^sup- was added to the soils compared to when only NH _inf4 ^sup+ 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, NH _inf4 ^sup+ accumulated, and the plants grew less well compared to those on drained soils. In contrast, waterlogged soils planted with R. palustris had a redox potential high enough for O₂ to be continuously replenished. Furthermore, the nitrifying capacity of these latter soils was maintained at a high level. R. palustris grew well and NO _inf3 ^sup- must have been available to the plant, since a high level of nitrate reductase activity was observed in the leaves.