Nitrous oxide emission controls and inorganic nitrogen dynamics in fertilized tropical agricultural soils

Use of N fertilizers in tropical regions has recently surpassed use in temperate regions, but understanding of N2O emissions from fertilized soils is based largely on experience from temperate regions. We studied N2O emissions from a sugar cane (Saccharum officinarum L.) plantation on the Hawaiian island of Maul. Young cane fields that were routinely fertilized had > 15 mg NH4+-N and NO3--N kg(-1) soil, whereas mature cane fields not being fertilized had < 2 mg NH4+-N and NO3--N kg(-1) soil. Emissions of N2O were also approximate to 10 times higher in the young cane fields than in the mature cane. The highest nitrification potentials and N2O emissions occurred near buried irrigation lines and lowest values between plant rows. Added (NH4+)-N-15 was nitrified within 48 h in both young and mature cane fields. Hence, microbial populations exist in both young and mature cane fields that can rapidly produce NO3-, and production of N2O is controlled primarily by when and where fertilizer N is applied. In contrast to many temperate agricultural soils where mineralization-immobilization-turnover processes contribute significantly to the supply of crop N, rates of gross N mineralization were low, indicating that the cane crop N came primarily from applied fertilizer. In the mature cane, soil inorganic-N remains low because of high plant and microbial demand, but in the young cane, fertilizer applications exceed the plant and microbial sinks, and N2O emissions are large. Better fertilizer management in this early stage of the cane crop cycle might significantly reduce N2O emissions.