POTENTIAL FOR BIOLOGICAL DENITRIFICATION OF FERTILIZER NITROGEN IN SUGARCANE SOILS

Nitrogen (N) fertilizer is being lost from sugarcane soils following a pplication to the crop. This study was conducted to estimate the quant ity of N being lost from the soil through biological denitrification a nd to determine the proportion of gaseous N being emitted either as N2 O or as N-2. Field studies were conducted on four different soils (hum ic gley, alluvial massive earth, red earth and gleyed podzolic), and o n different crop management systems, by installing plastic (PVC) cylin ders (23.5 cm diam., 25 cm long) in the soil to a depth of 20 cm besid e the plant row in a ratoon sugarcane crop. N-15-labelled KNO3 was app lied 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 f or 3 h intervals and gas in the headspace sampled in the morning and a fternoon, 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 . da y 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 a nd the red earth and gleyed podzolic, losses over the sampling period ranged from 25 to 117 g N/h . day and represented less than or equal t o 1% of the N applied. Recovery of N-15 in the soil ranged from 67% at the first sampling on the red earth soil to 4.9% at the third samplin g on the alluvial, massive earth soil. In a glasshouse study, intact s oil cores (23.5 cm diam., 20 cm long), taken from the humic gley and t he alluvial, massive earth, were waterlogged after band application of N-15-labelled KNO3 at a rate of 160 kg N/ha. Gas samples from the hea dspace were taken after 3 h, and then morning and afternoon for the ne xt 14 days. Denitrification losses ranged from 13.2 to 38.6% of N appl ied with the majority of gaseous N loss occurring as N-2. Total N-15 r ecoveries after 14 days, including the evolved gases, ranged from 68.7 to 88.2%. We conclude that denitrification is a major cause of fertil izer N loss from fine-textured soils, with nitrous oxide the major gas eous N product when soil nitrate concentrations are high.