Effects of Wetting and Drying and N Fertilization on C and N Transformations and Emissions of CO2 in Southeast Texas Rice Soils.

Repeated early season flushing of rice fields greatly affects C and N mineralization, which can result in losses of N and increased greenhouse gas emissions. Therefore, it is important to understand the role of repeated soil drying and rewetting and N fertilization on C and N turnover. This study examined the effects of dry-wet cycling and application of urea on N transformations, dissolved organic matter dynamics, and emissions of CO₂. Topsoil (Ap) and subsoil (A) from two different sites in Southeast Texas was divided into two parts, one of which was amended with urea per annual N recommendations for rice (165 kg N ha^-1 and 180 kg N ha^-1), and another remained unfertilized, resulting in 8 treatments. The soil was incubated in laboratory over 3 consecutive drying and wetting cycles. The concentrations of NH₄-N decreased in the fertilized and unfertilized soil at the end of the first drying cycle. The levels of NO₃-N increased in all treatments during the same period. Short-term decrease in organic-N and a spike in CO₂ emissions during the first drying cycle was likely a result of increase in mineralization and nitrification following initial wetting. The concentrations of total-N in the fertilized treatments were higher in the beginning of the study, but became similar to unfertilized treatments after the first drying cycle. This indicated that the loss of applied N likely occurred during drying and rewetting via ammonia volatilization and N₂O emissions. Our results suggest most of applied N is likely to be lost in the initial wetting, and increased greenhouse gas emissions are likely during that period.