SOIL DENITRIFICATION RATES AT WASTE-WATER IRRIGATION SITES RECEIVING PRIMARY-TREATED AND ANAEROBICALLY TREATED MEAT-PROCESSING EFFLUENT

Nitrous oxide emission rates and corresponding N2O:(N2O+N2) ratios were measured at three sites receiving meat-processing effluent at an annual loading rate of approximately 1000 kg N ha-1. Two of the sites (both a Horotiu sandy loam) were in pasture. One received primary-treated effluent and the other, anaerobically treated effluent. The third site (a highly modified Maori gravelly sand) was planted in trees and received primary-treated effluent. At all sites N2O emission rates increased (peaked) immediately following an irrigation event. Emission of N2O then returned to baseline levels within 24 h. Peak rates at the pasture sites were higher with primary-treated effluent (1-137 g N2O-N ha-1 h-1) than with anaerobic effluent (1-62 g N2O-N ha-1 h-1). This was attributed to the higher organic carbon concentration in primary-treated effluent and possibly to the higher soil pH (5.9 compared to 5.0 at the anaerobic effluent site). At the forest site, peak rates were 12-240 g N2O-N ha-1 h-1. Peak nitrous oxide emission rates increased with increasing surface soil temperature. It is concluded that at soil temperatures below 12-degrees-C denitrification is not an important nitrogen-removal mechanism.