Spatial heterogeneity and short-term oxygen dynamics in the rhizosphere of Vallisneria spiralis: Implications for nutrient cycling

Aquatic macrophytes modify the sediment biogeochemistry via radial oxygen loss (ROL) from their roots. However, the variation in ROL and its implication for nutrient availability remains poorly explored. Here, we use planar O-2 optodes to investigate the spatial heterogeneity of oxic niches within the rhizosphere of Vallisneria spiralis and their alteration following variable light and ambient O-2 levels. The effect of ROL on NH4+ and PO43- distribution in the rhizosphere was evaluated by a combination of N-15 isotopic techniques, 2D sampling, and electron microscopy. A single specimen of V. spiralis could maintain an oxidised sediment volume of 41-47 cm(3) and 10-27 cm(3) in the rhizosphere at 100% and 38% dissolved oxygen saturation in the overlying water, respectively. Whatever the environmental conditions, the ROL was, however, very heterogeneous and dependent on root age and architecture of the root system. ROL stimulated the coupling between denitrification and nitrification in the sediment both under dark (+25 mu mol N-N-2 m(-2) hr(-1)) and light (+70 mu mol N-N-2 m(-2) hr(-1)) conditions. This, in combination with plant uptake, contributed to intense removal of NH4+ from the pore water. Similarly, PO43- was highly depleted in the rhizosphere. The detection of Fe-P plaques on the roots surface indicated substantial entrapment of P as a consequence of ROL. The extensive spatio-temporal heterogeneity of oxic and anoxic conditions ensured that aerobic and anaerobic processes co-occurred in the rhizosphere and this presumably reduced potential nutrient limitation while maximising plant fitness in an otherwise hostile reduced environment.