Impacts of sediment dewatering and rehydration on sediment nitrogen concentration and macrophyte growth

River sediments were experimentally dewatered via exposure to air from an initial moisture content of 75% to 61% (~20% dewatered), 32% (~60% dewatered), and 4% (~95% dewatered). Sediments dewatered by 20% and 60% and rehydrated exhibited a lower exchangeable ammonium-N (NH4-N) concentration and a lower rate of nitrate/nitrite-N and NH4-N release from sediments compared with controls. In contrast, sediments dewatered by 95% and rehydrated exhibited a marked increase in exchangeable sediment NH4-N and a much higher rate of nitrate/nitrite-N (under oxic conditions) and NH4-N (under oxic and anoxic conditions) release from sediments. Sediment dewatered by 60% and 95% and rehydrated lost total N (18%), suggesting denitrification. Growth of Potamogeton pectinatus responded to changes in the exchangeable sediment NH4-N concentration; it was lower on partially dewatered and rehydrated sediments that exhibited lower exchangeable sediment NH4-N and elevated on sediment that was dewatered by 95%. Our results suggest that sediment N may be altered via sediment dewatering and rehydration to affect macrophyte growth.