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The hypothesis of an anammox hotspot in river riparian zones was put forward based on our investigation on freshwater ecotones for over 25 years and previous anammox research. Here we used a complementary array of methods including isotope-pairing technique, quantitative PCR assays, and 16S rRNA and hydrazine synthase gene (hzsB) clone libraries to document the spatiotemporal evidence for a high abundance zone of anammox bacteria in river riparian sediment with observed abundance of 1.3-12 x 106 (summer) and 1.4-20 x 108 (winter) hydrazine synthase gene copies g-1, which is the highest abundance in natural environments recorded so far. Meanwhile high anammox bacterial biodiversity were detected with 'Brocadia' and 'Kuenenia' dominating. However, the high anammox bacterial abundances were not related with high activities and contributions for nitrogen gas generation. The anammox activities ranged from 0.07 to 0.15 nmol N cm-3 h-1 (summer) to 1.0-2.6 nmol N cm-3 h-1 (winter) with high temporal heterogeneity. The retrieval of archaeal and bacterial amoA sequences indicated that nitrifying microbes might be the major source of nitrite for anammox bacteria in winter, while in summer the anaerobic nitrate reduction is more likely the main source. On the basis of 15N tracing technology, it was estimated that a total loss of 0.67-9.62 g N m-2 yr-1 is linked to anammox in the riparian zone while denitrification contributed 96.2-170.3 g N m-2 yr-1 in Pearl River riparian sediments. [ABSTRACT FROM AUTHOR]