Why grain yield of transplanted rice on permanent raised beds declines with time

摘要

Permanent raised beds are being promoted as a resource conservation technology for rice-wheat systems in Indo-Gangetic plains (IGP) to improve the water productivity of rice and wheat in addition to other benefits, as furrow irrigation can be more efficient than flood irrigation. However, several studies carried out in the NW IGP have shown rice grain yields to decrease with the increasing age of the beds. The present study was conducted on a deep alluvial loam (Ustochrept) in a farmer's field at Phillaur, Punjab, India, to identify possible reasons for the declining grain yield of rice on the permanent raised beds (37.5cm wide alternating with 30cm wide furrows 15cm deep) in comparison to fresh raised beds. The beds were formed with a bed planter drawn by a 35-hp 4-wheel tractor, which was also used to direct drill wheat on the permanent beds each year, and to reshape the beds prior to each rice crop. This paper reports a study of rice root distribution and mass at the end of the vegetative stage, and soil bulk density after harvest, for transplanted rice on permanent beds (4th rice crop, 8th crop) in comparison with transplanted rice on fresh beds (1st crop). Rice grain yield declined linearly with increasing age of the permanent beds. It decreased by 19% in 2004, 45% in 2005 and 59% in 2006 from 4.64thap# in 2003. In situ exposure of root profiles on permanent and fresh beds revealed that the horizontal spread of roots on permanent beds (6cm from the base of the plant at 18cm depth and 12cm at 27cm depth) was much less than on fresh beds (12cm at 18cm depth and 18cm at 27cm depth). The root mass density in at 0-15cm the middle of the fresh beds was 59% higher than on the permanent beds. Bulk density was significantly higher under the shoulder and side of the permanent beds to the depth of sampling (0-15cm) than under the fresh beds at the same positions across the furrow. The decline in performance of rice on beds as the beds aged was at least partly due to compaction of the permanent beds by the tractor tyres, which had width similar to that of the top of the furrow. This hindered the spread of the roots particularly towards the beds.