Potential for biological denitrification of fertilizer nitrogen in sugarcane soils

Abstract

Nitrogen (N) fertilizer is being lost from sugarcane soils following application to the crop. This study was conducted to estimate the quantity of N being lost from the soil through biological denitrification and to determine the proportion of gaseous N being emitted either as N2O or as N2. Field studies were conducted on four different soils (humic gley, alluvial massive earth, red earth and gleyed podzolic), and on different crop management systems, by installing plastic (PVC) cylinders (23.5 cm diam., 25 cm long) in the soil to a depth of 20 cm beside the plant row in a ratoon sugarcane crop. 15N-labelled KNO3 was applied as a band across each cylinder to a depth of 2.5 cm at a rate of 160 kg N/ha. After rainfall or irrigation, the cylinders were capped for 3 h intervals and gas in the headspace sampled in the morning and afternoon, for up to 4 days. Denitrification losses from the humic gley ranged from 247 g N/ha.day for cultivated plots to 1673 g N/ha.day for no-till plots. Over the sampling period, this was equivalent to 3.2% and 19.7% of the N applied, respectively. Nitrous oxide accounted for 46% to 78% of the total N lost. For the alluvial, massive earth and the red earth and gleyed podzolic, losses over the sampling period ranged from 25 to 117 g N/h.day and represented <1% of the N applied. Recovery of 15N in the soil ranged from 67% at the first sampling on the red earth soil to 4.9% at the third sampling on the alluvial, massive earth soil. In a glasshouse study, intact soil cores (23.5 cm diam., 20 cm long), taken from the humic gley and the alluvial, massive earth, were waterlogged after band application of 15N-labelled KNO3 at a rate of 160 kg N/ha. Gas samples from the headspace were taken after 3 h, and then morning and afternoon for the next 14 days. Denitrification losses ranged from 13.2 to 38.6% of N applied with the majority of gaseous N loss occurring as N2. Total recoveries after 14 days, including the evolved gases, ranged from 68.7 to 88.2%. We conclude that denitrification is a major cause of fertilizer N loss from fine-textured soils, with nitrous oxide the major gaseous N product when soil nitrate concentrations are high.