Kl. Weier et al., POTENTIAL FOR BIOLOGICAL DENITRIFICATION OF FERTILIZER NITROGEN IN SUGARCANE SOILS, Australian Journal of Agricultural Research, 47(1), 1996, pp. 67-79
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.