Effects of long-term drainage of stony open deep-narrow drainage ditches on groundwater level, soil chemical characteristics and rice grain quality in cold-waterlogged paddy fields

Cold-waterlogged paddy fields across Jiangnan rice-growing regions belong to low-yield paddy fields. The main characteristics of these paddy fields include low soil temperature, high reducing agents and bad soil structure. However, the fields have the potential to significantly increase paddy rice yields. A 30-year stony open deep-narrow drainage ditch system in Shunchang County, Fujian Province, was investigated for its effects on groundwater level, soil chemical characteristics and rice grain quality in cold-waterlogged paddy fields using a consecutive 3-year monitoring. The results showed that soil conditions in paddy fields 75 m, 25 m, 15 m and 5 m from the ditches were deep-foot mud, shallow-foot mud, blue mud and blue-bottom mud, respectively. The groundwater levels in paddy fields 75 m, 25 m, 15 m and 5 m from the drainage ditches were 5.0 cm, 8.3 cm, 5.4 cm and 16.7 cm, respectively. The highest variation in groundwater level (from 62 cm to 13 cm) was in paddy field 5 m from the drainage ditches. The closer the distance to the drainage ditches, the lower the reducing agents and the higher the contents of available N, P and K. Compared with the field 75 m from the drainage ditches (CK), soil reducing agents content at the field 5 m from the drainage ditches decreased by 62.6% while available N, P and K contents increased respectively by 40.7%, 38.8% and 184.5%. At closer distances to the drainage ditches (e.g., 5 m, 15 m and 25 m sites), amino acids content of rice grain were significantly higher than that of CK. Amino acids content of rice grain was significantly negatively correlated with soil reducing agents content, but positively correlated with soil available N and P contents. Starch content of rice grain was also significantly negatively correlated with soil reducing agents content. Soil physiochemical properties and rice amino acids content were improved by long-term drainage of stony open deep-narrow drainage ditches. Soil improving effect was obvious in paddy fields within 15 m distance from stony open deep-narrow drainage ditches.