Abstract: Nitrogen loss with farmland drainage is a major cause for water quality degradation in aquatic environment. Denitrification bioreactors installed at field edges have been found effective in reducing nitrogen loss from agricultural fields; they have been widely investigated in recent years as an emerging technology to reduce agricultural nonpoint source pollution. Filled with carbon media, such as woodchip, rice husk and wheat straw etc., denitrification bioreactors may remove nitrate-nitrogen through denitrification process from drainage water, which was either partially or fully introduced into the device. This paper reviewed existing studies on the denitrification bioreactors that were designed to purify farmland drainage water; different factors that affect the denitrification processes were synthesized, and the potential application of such bioreactors in the humid regions of southern China was analyzed. The current research findings from both laboratory and field studies showed that denitrification bioreactors with variable sizes and designs can effectively improve drainage water quality; the reported annual NO3--N load reductions ranged from 23% to 98% in existing literature. Compared with other water treatment devices (e.g. wetlands) in agricultural landscape, denitrification bioreactors occupy only a small portion of land area but they exhibit high efficiency in reducing nitrogen load to receiving waters; they have been advocated as economical and effective water quality purification devices. The denitrification effect of bioreactors is closely related to the carbon media characteristics, drainage inflow and outflow conditions, and some environmental factors. Internal temperature control and carbon media selection are important factors for sustainable operation of field bioreactors. How to select carbon media to maintain their hydraulic conductivity and performance life in the bioreactors needs further study. Determining the appropriate drainage inflow and its hydraulic retention time based on the local drainage pattern is crucial for achieving certain nitrogen reduction goal. Nitrate reduction rate normally increases with the hydraulic retention time, but very high retention time is related to over reduction in the bioreactors. Installing a control structure at the inlet of bioreactor may help adjust hydraulic retention time of drainage water to prevent undesirable reaction processes. To size a bioreactor according to drainage system design remains a challenge due to variable weather, soil and cropping conditions; applicability of the existing methods proposed in different studies needs further examination. Field monitoring of bioreactor performances is needed to ensure proper system function; the advances of monitoring technologies may help provide better insights on internal processes in bioreactors. In the warm southern plain area of China, agricultural drainage processes are relatively concentrated in the summer growing season. Such environmental conditions are favorable for adopting denitrification bioreactors to treat drainage water discharge. Based on a regression analysis using reported data of nitrate reduction and the hydraulic retention time, the calculated land requirement for bioreactor installation in a case study area in southern China was only 0.117% of the drainage area to achieve 50% nitrogen reduction in drainage water. Findings from this research may help guide denitrification bioreactor research and their applications to reduce agricultural non-point source pollution from drainage water discharge.