Nutrient Characteristics and Redox Potential Changes as Affected by Water Management in Rice Paddy Weighing Lysimeter

Abstract

This study was designed to assess the nutrients characteristics and redox potential changes as affected by water management condition in rice paddy weighing lysimeter. Rice was grown from June to October 2015, and the water management was consisted of two methods: continuously flooding (CF) and alternate wetting and drying (AWD). Under the CF condition, the redox potential (Eh) of 5 cm topsoil began to decrease rapidly and maintained around -260 to -200 mV. The AWD showed the oxidation condition. The changes of soil pH in CF and AWD ranged from 6.0 to 7.0 and 5.7 to 7.1, respectively. Soil pH in the AWD condition indicated up and down patterns by irrigation. In changes in nutrient contents of the soil solutions, nitrate nitrogen (NO3-N) was initially 12.2 to 26.6 mg L-1 at the highest concentration, and the concentration of ammonia nitrogen (NH4-N) and phosphate (PO43-) appeared to be lower than 1 mg L-1 during rice cultivation. The grain yield of rice was significantly higher in the CF than in the AWD. Nitrogen and magnesium content in both rice grain and straw were not significantly different between the water management conditions. Phosphorous, potassium, and calcium content in the rice grain were not statistically different between CF and AWD conditions. However, the nutrient contents in rice straw were higher in the CF, compared with that in the AWD. Although the iron content in rice straw was higher in the CF than in the AWD, the manganese content showed an opposite tendency. Interestingly, soil maximum temperature at the daytime was significantly higher in the AWD than in the CF, whereas the soil minimum temperature at the night time was 0.5-0.9℃ lower in the AWD, compared with that of the CF. Therefore, these results suggest that the decrease in rice yield in AWD condition can be linked to the inhibition of phosphate absorption into rice plants under oxidized-soil condition and/or high temperature difference between day and night in the soil. Changes in the redox potential (Eh) of (A) 5, (B) 10, and (C) 15 cm soil depth under water management conditions of the continuously flooding (CF) and alternate wetting and drying (AWD).