Review of the high bed - low ditch system as an alternative to lowland paddy in tropical and subtropical Asia Provisionally Accepted

Fei Zhao^{1, 2} Shiming Luo³ Jiaen Zhang^3*

• ¹Research Base of Guangzhou Agricultural Heritage Systems, South China Agricultural University, China
• ²South China Agricultural University, China
• ³College of Natural Resources and Environment, South China Agricultural University, China

Many forms of traditional raised bed systems could be found around the world. Several of them have been identified as Globally Important Agricultural Heritage Systems (GIAHS) sites by Food and Agriculture Organization of the United Nations. Unlike traditional raised bed systems with similar structures in the Americas, the high bed - low ditch (HBLD) system in tropical and subtropical Asia, which originated and developed from rice production, has been playing an important role in enhancing food security and maintaining farmer livelihood. Moreover, products provided by HBLD system are not only important for the livelihood of the local farmers, but also important for people living in the nearby towns and cities especially for vegetable and fruit supply. In this system, the ditches or sunken beds can be used to lower the groundwater table, retain nutrients and soil particles washed from the bed, grow rice or aquatic vegetables, and raise fish or shrimp. The HBLD system can also help to reduce salinity in coastal lowlands. The raised beds can be used to grow various upland crops. Compared with rice monocropping, the adoption of HBLD system significantly improves the cropping intensity, productivity, employment, and income of farmers. Famers’ long-term practices fully demonstrate that these systems are a type of sustainable agriculture with strong adaptability to the changes of natural environment. However, it should also be noted that the large-scale development of HBLD systems is not simply dependent on natural conditions, but is also determined by specific socioeconomic factors, such as excellent external transportation facilities, well developed commercial environment, and a sufficient supply of labor. As a model of equilibrium between food production and high levels of biodiversity maintenance, the HBLD system is an important agricultural heritage system with global significance, and it should be well preserved and utilized in new ways to realize its important multiple functions under conditions of rapid urbanization in lowland regions.