By Peter Darbishire
Denitrification is a good thing.
By Peter Darbishire
McGill University researchers at the Bruce Centre of Water Resources Management,
Macdonald Campus in Quebec, have found that total nitrate-nitrogen losses, in
terms of loads (kg/ha), could be 60 percent to 88 percent less with subirrigation.
"There are two principal reasons for this reduction in nitrate loads with
subirrigation: denitrification, and the dilution of the nitrates in the saturated
zone between the water table and the tile lines due to the introduction of the
subirrigation water," says professor Chandra Madramootoo.
Denitrification is a process of the nitrogen cycle, whereby nitrates are converted
to two gaseous products: nitrogen gas (N2) and nitrous
oxide (N2O). It only occurs under saturated soil conditions.
N2O is a greenhouse gas and contributes to global warming.
Does subirrigation contribute to the production of greenhouse gases? The answer
is 'yes', but in very small amounts.
Further studies conducted by McGill researchers show the ratios of N2O
to N2O+N2 at three different soil
depths are lower in subirrigated plots compared to plots with conventional subsurface
drainage (see Table 1).
|Table 1. Ratio of N2O to N2O
|Soil depth||Free drainage||Sub-irrigation||Free drainage||Sub-irrigation|
|Zero to 15cm||0.33||0.22||0.25||0.22|
|15cm to 30cm||0.39||0.25||0.56||0.29|
|30cm to 45cm||0.58||0.39||0.92||0.70|
The reason for the lower ratios in the subirrigated plots is that there is
a more complete conversion of N2O to N2
with denitrification. Therefore, less N2O and more N2
are being produced under subirrigation. This is good news, since N2
is not as environmentally harmful as N2O.
"The overall significance of these findings is that subirrigation can
be viewed as a best management practice for crop producers because it increases
crop yields while protecting water and air quality," he adds.
The Bottom Line
In the nitrogen cycle all the processes are happening at once, some
more than others at different times of the crop's growth. This article talks
about nitrogen loss (N2O) at a very small portion of
the nitrogen cycle and in a very small period of time. The fact is that subirrigation
will help grow a good crop. Any good growing crop will take up nitrogen… a
lot of nitrogen. And that means less nitrogen to be lost. Andy
van Niekerk, CCA, farms near Stayner, Ontario.