NITROGEN TRANSFORMATIONS IN FLOODED AGROECOSYSTEMS: A CASE STUDY WITH TARO (COLOCASSIA ESCULENTA)

Wetland agriculture covers an estimated 170 million ha and contributes significantly to global food supply. Nitrogen fertilizers are subject to numerous potential transformation pathways in flooded systems. The present research was focused on improving our understanding of N transformations in a flooded agricultural system by addressing the following two broad objectives: 1) determine whether the presence of anammox bacteria and its activity contribute significantly to N losses, and 2) evaluate the contribution of the coupling of nitrification and denitrification in the rhizosphere to gaseous N losses. Quantitative PCR was performed for anammox 16S rRNA genes. Anammox activity was measured using a slurry-based isotope pairing technique (IPT) and a a whole core 15 NH4 + perfusion technique with porewater equilibrators for the extraction of 14+15 N-NO3 - , NH4 + , and N2 was used to quantify rhizosphere coupling of nitrification and denitrification. Although significant numbers of anammox 16S rRNA genes were detected (~10 5 copies g -1 wet sediment), only negligible anammox activity was measured using the slurry IPT. The whole core 15 NH4 + experiment demonstrated that diurnal O2 transport into the sub-surface stimulated nitrification-denitrification in the extensive root rhizosphere accounting for the majority of NH4 + loss.