Importance of sub-surface rhizosphere-mediated coupled nitrification–denitrification in a flooded agroecosystem in Hawaii

The fate of fertilizer nitrogen (N) in flooded agroecosystems is difficult to predict given the multitude of potential N transformation pathways. In particular, rhizosphere effects are known to play a significant role in N cycling, but are especially difficult to quantify in large emergent macrophytes. To address these issues, we utilized a whole core 15NH4+ perfusion technique with porewater equilibrators for the extraction of 14+15N–NO3−, NH4+, and N2. Sub-surface denitrification was found to be an important N loss pathway in wetland sediments vegetated with aerenchymatous taro (Colocasia esculenta) versus bare sediments. Driven by hypothesized thermo-osmotic mechanisms linked to photosynthesis, diurnal O2 transport into the sub-surface stimulated nitrification–denitrification in the extensive root rhizosphere. Porewater denitrification rates were also positively influenced by airflow across leaf surfaces. Depth-integrated porewater denitrification rates in this system were very high, ranging from 23 to 845 μmol N2 m−2 h−1. The N cycling functional genes nosZ and amoA were found at high abundances throughout the sub-surface with nirS dominating nitrite reduction in these sediments. Overall we were able to account for >82% of added 15NH4+ in the vegetated cores over a ten-day incubation through both plant incorporation and surface/sub-surface coupled nitrification–denitrification. In summary, these results suggested (1) that oxygen flux through the taro stem and root system into the flooded sediment may be an important driver of nitrification and coupled denitrification in these systems, and (2) that oxygen flux is mediated by air movement (wind) and the diurnal light-cycle related to photosynthesis.