Storing and removing nitrogen in drainage from paddy field by using aquatic crops wetland

Aquatic crop (Zizania latifolia Stapf–Ipomoea aquatica Forsk) wetlands were constructed to remove nitrogen (N) in drainage from paddy rice fields, with three different storage water depths (namely 4H, 7H and 10H, where H is the routine paddy drainage water depth). Concentrations of ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3−-N) and total nitrogen (TN) during the first two drainage events were reduced within 8–14d of storing by 82.3–92.8%, 84.5–94.3% and 74.9–92.4%, respectively. Generally, the higher N concentration in inlet water, the higher removal efficiency of nitrogen. The removal rate in wetland with 4H water storage was observed higher as compared to those at 7H and 10H. Overall, the aquatic crop wetlands at these storage water depths could intercept more than 93.2% of N output from paddy fields, and the removal efficiency was slightly better in wetland at 4H. For a routine drainage event (about 30–50 mm) occurred following a routine rainfall (about 100 mm at 5–10 years return period) with local water-level management practice in rice field, wetlands with 4H and 7H storage could effectively remove N in drainage by reusing it with aquatic crops. Nevertheless, the larger the proportion (low water depth in wetland for one drainage) of wetland to paddy field, the higher the production efficiency, because the value of aquatic vegetable is much higher than rice. The current results suggested that aquatic crop wetlands constructed in low-lying part within paddy fields could intercept a large amount of N output from paddy field, and with high economic return. Designing wetlands and paddy fields in a proper area ratio is vital important for balancing removal effect of nutrients in paddy drainage and the demand of food security.