Denitrification-dependent anammox activity in a permanently flooded fallow ravine paddy field

• Denitrification activity in paddy fields was greater than the anammox activity.
• The upper soil layer contained the most anammox microbial activity.
• The contribution of anammox to the total N2 flux changed according to soil depth.
• Anammox activity decreased with inhibition of denitrification.

It is vital to understand the nitrogen dynamics in ravine paddy fields due to their nitrogen removal capacity, providing a potential method for non-point source pollution treatment. This study measured bacterial anaerobic ammonium oxidation (anammox) and denitrification activity using a nitrogen isotope pairing technique (IPT) in fallow ravine paddy soil that is permanently flooded with seepage water from an upland vegetable field. The inhibition experiment explored the correlation between anammox activity and the denitrification processes, using chlorate to inhibit the first step of denitrification.Denitrification activity decreased with increasing paddy soil depth; with anammox activity showing a similar trend, but with even greater declines in activity. In addition, when nitrate reduction to nitrite was inhibited, anammox activity exponentially decreased, likely to be due to a reduction in the intermediate NO2−. Furthermore, the rate of reduction in anammox activity was significantly greater than the reduction in denitrification. This study presents the first direct exploration of the correlation of denitrification with anammox activity in ravine paddy fields receiving nitrate polluted water.