Nitrogen Transformations in Submerged Soils.

Soils with continuous or intermittent submergence by water occur in a range of ecosystems including rice (Oryza sativa L.) fields, wetlands, estuaries, and floodplains. Rice is a staple food for nearly half the world’s population, and about 95% of the global rice production occurs on fields with soil submerged during at least part of the rice-cropping period. The sustained productivity of rice relies heavily on the input and management of N. Nitrate is the dominant form of inorganic N in drained, aerated soils; whereas ammonium is the dominant and stable form of inorganic N that accumulates in submerged soils. Submerged soils as compared with aerated soils are favorable environments for loss of N by nitrification-denitrification and ammonia volatilization and for addition of N via biological dinitrogen fixation. During the past 20 years considerable progress has been made in developing management practices for increasing the efficiency of fertilizer N use for rice. Adjusting the basal and mid-season applications of fertilizer N to optimally match the field-specific needs of the crop for supplemental N can increase yield, increase efficiency of fertilizer N use, and increase profit for farmers, while also reducing losses of reactive N in Asian rice production.