Explanation

In submerged (flooded) soil conditions, which are typical for rice paddies, the oxygen levels are low, leading to anaerobic conditions. Under these conditions, ammonium (NH4+) is more stable and available for plant uptake compared to nitrate (NO3-) and nitrite (NO2-), which are less stable and can be lost through denitrification processes. Rice plants are well-adapted to take up nitrogen in the form of ammonium, making NH4 the preferable form of nitrogen in submerged soils.