Denitrification Losses In Cropped Soils With Subsurface Drip Irrigation.

Abstract

Denitrification is a microbial process of anaerobic respiration in which nitrate (NO₃⁻) is chemically reduced to gaseous nitrous oxide (N₂0) and molecular N2. Fertilizer N can be lost to the atmosphere through this process. Subsurface drip irrigation may create favorable conditions for denitrification, such as high moisture and NO₃⁻ content. The objectives of this research were to: 1) determine the denitrification rate in drip-. irrigated cauliflower and sweet corn crops; 2) evaluate the effect of soil water tension on the denitrification rate, and; 3) estimate an N balance under subsurface drip irrigation, including denitrification losses. Two field experiments with subsurface drip-irrigated cauliflower were conducted during the 1996-98 winter growing seasons at the Maricopa Agricultural Center, in Maricopa, AZ. An additional study with subsurface drip-irrigated sweet corn was conducted at the Campus Agricultural Center in Tucson, AZ. All the experiments were complete factorial designs with two soil water tension levels (low, high), two levels of N fertilizer (zero, adequate), and three replications. The denitrification rates evaluated at ambient temperature were <12 g N ha⁻¹ d⁻¹ during the cauliflower winter seasons. When soil cores taken during the 1997-98 winter season were incubated at room temperature (24 ±2°C), denitrification rates were five to 50 times higher than the rates evaluated at ambient temperature. The denitrification rate measured at room temperature in the cauliflower winter season was similar to the rate observed in the sweet corn during summer. Soil cores from the cauliflower 1997-98 season that received 100 kg N ha⁻¹ had denitrification rates from 10 to 45 g N ha⁻¹ d⁻¹ ; when these cores were amended with additional soluble carbon, the denitrification rate increased to 800 to 3500 g N ha⁻¹ d⁻¹. All of the three experiments showed higher denitrification rates at the end of the season. This trend coincided with increases in denitrifying enzyme activity and soluble organic carbon. The denitrification loss of fertilizer N was <1% in cauliflower and almost 2% in summer sweet corn, when irrigated at the higher soil water tension. Lower soil water tension did not increase the denitrification rate in the winter, but in the summer the loss of N due to denitrification increased to almost 6% of the applied N.