A mesocosm study on biogeochemical role of rice paddy soils in controlling water chemistry and nitrate attenuation during infiltration

Rice paddies typically lie near surface water and play an important role in controlling surface water quality. To evaluate the role of rice paddy soils in determining the fate of nitrate, three different mesocosms were used in this study. Two mesocosms (M1 and M2) were covered with a rice paddy soil and the other mesocosm (M3) with a dry field soil. Rice was harvested in two mesocosms (M2 and M3) during the growing season. An amended groundwater containing high concentrations of NaNO₃ and KBr was repeatedly applied to the mesocosms to infiltrate under saturated conditions. Under these experimental conditions, we monitored time-series variations of pH, Eh, DO, DOC, alkalinity and the concentrations of nitrate, bromide (as a conservative tracer) and dissolved Fe and Mn in the inflow water (flooding water just below the water/soil interface) and outflow water (drained water after infiltration through mesocosm materials) for about two months to examine the fate of nitrate. The Eh and DO data in all mesocosms showed that strong reducing conditions were rapidly and efficiently established in infiltrating waters. In the mesocosms (M1 and M2) with a rice paddy soil, dramatic decreases of nitrate concentrations were observed both in the inflow and outflow waters. Nitrate removal from the inflow waters occurs due to immobilization, while nitrate is removed during infiltration through mesocosm materials by denitrification coupled with organic carbon oxidation, as is indicated by the increases in alkalinity and dissolved Fe and Mn. Interestingly, there was no significant difference in the nitrate removal capacity between M1 (no cultivation) and M2 (rice cropping). In contrast to M1 and M2, there was no large decrease of nitrate concentrations in the inflow waters of M3 (rice cropping with a dry field soil). In addition, nitrate concentrations in the outflow waters of M3 rose steeply again after 30 days, in concert with increasing Eh and decreasing alkalinity. These observations in M3 are attributed to both the low organic carbon content and the generation of large pores by root growth, resulting in the rapid loss of denitrification capacity. This study implies that rice paddy fields play an important role in controlling (attenuate) agricultural nitrate in groundwater discharging to streams; thus, the extent of rice paddies should be carefully considered when evaluating the nitrate loads to streams via groundwater flow.