‘WITH environmental issues one needs a soft heart and a hard head.’
Few people would challenge this opinion, expressed recently by Virginia
Bottomley, junior minister at Britain’s Department of the Environment. Her
department has published a paper entitled The Nitrate Issue.* This ‘study
of the economic and other consequences of various local options for limiting
nitrate concentrations in drinking water’ could form the backbone of government-enforced
measures for controlling nitrate in water supplies. The options include
‘water options’ such as blending water from various sources and removing
of nitrate by chemical or biological means. There are also ‘agricultural
options’, restricting the way farmers use land and apply nitrogen fertilisers.
Pressure to control nitrate in drinking water comes mainly from the
EEC rather than from medical research. But although it is prudent to minimise
the concentration, this must be on a scientific basis. In some aquifers,
control measures may need decades to become effective, so they must be right
first time.
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They also need to be right because of their impact on the lives of the
farming community and on food prices. These measures must, in particular,
take proper account of the complex web of physical, chemical and biological
processes that contribute to the leakage of nitrate from soil. The ‘agricultural
options’ set out in the paper from the Department of the Environment (DoE)
do not do so, and appear to be based on a misunderstanding of the problem.
The Nitrate Issue evaluates the agricultural options in a series of
‘desk studies’ that use a computer model developed by the Water Research
Centre (WRC). This is a good model, but as with all models we need to consider
its purpose, the concepts underlying it, and how well its simulations match
reality. The purpose of the model, according to the WRC, is ‘to simulate
nitrate movement from the soil zone down to the water table’. This means
the movement of nitrate from the soil down to the water stored in the aquifer.
The model does not explicitly simulate the movement of nitrate in the soil.
The agricultural options, however, are very much concerned with the progress
of various forms of nitrogen through the soil to the point at which nitrate
is washed beyond the reach of the roots of crops.
The concepts underlying the model are also relevant to the way in which
the DoE has used it. Within an aquifer, the model simulates the downward
movement of peaks of nitrate concentration using a sophisticated set of
mathematical flow equations. It does this well, suggesting that the equations
are soundly based. But the model is much less sophisticated in its treatment
of soil than of aquifers. For the soil, the model relies on a set of empirical
rules obtained by relating the peaks of nitrate concentration measured at
various depths in the aquifer to ‘snapshots’ of the way in which the land
had been used.
The first of the model’s rules for the movement of nitrate in the soil
allows for a leakage of 280 kilograms of nitrogen per hectare when grassland
is ploughed up. (In fact, the loss depends on the age of the grassland.)
The second rule concerns leakage when winter cereals are grown, and this
rule must be got right because more land in Britain grows winter cereals
than any other crop.
Here the model assumes that nitrogen equivalent to 40 per cent of the
amount applied as fertiliser leaks as nitrate. This is not the same as saying
that 40 per cent of the fertiliser nitrogen itself leaks. But it does imply
that a cut in fertiliser nitrogen automatically cuts the leakage of nitrate
by the same proportion. This assumption clearly has a profound influence
on the evaluation of the agricultural options, but it has little scientific
backing.
We know of no direct evidence for the ’40 per cent’ rule. It fails one
very simple test. Forty per cent of nothing is nothing, so the rule implies
that when farmers do not add fertiliser, no nitrate leaks. Even 100 years
ago, researchers knew from observations at Rothamsted that soil carrying
an unfertilised crop could, in a year, leak about 20 kilograms of nitrogen
per hectare as nitrate. The WRC modellers counter this objection by saying
that the model should not be used outside the range of fertiliser applications
against which it was developed, a range that did not include no fertiliser.
But even within this range, there is no evidence for the ’40 per cent’ rule.
Michael Goss and his team at Rothamsted have for several years measured
the amounts of nitrate in the water draining from plots growing crops under
various conditions. Their results show no relation between the amount of
nitrogen applied as fertiliser in spring and the amount of nitrate in drainage.
When farmers grow spring barley, they leave the soil bare during winter,
and more nitrate leaks from it than when they grow winter wheat. Rodney
Dowdell and his colleagues at the Letcombe Laboratory (now closed by government
cuts) measured nitrate leaking from the soil in which they grew spring barley
over four consecutive years. The researchers applied 80 or 120 kilograms
per hectare of nitrogen as fertiliser each year. In both cases, a mere 7
per cent of the nitrate that leaked from the soil came from the fertiliser.
The total quantity of nitrate lost was no greater when they added 120 kilograms
than when they added no fertiliser at all.
The ’40 per cent’ rule diverts attention from the real root of the problem,
fuelling the popular misconception that nitrate pollution originates from
excess fertiliser washed out of farmland. There is, however, little excess
to be washed out. This was shown by researchers at Rothamsted when they
‘labelled’ nitrogen fertiliser with nitrogen-15, a heavy isotope of nitrogen,
to find out where it went. Winter wheat fertilised with nitrogen, at rates
similar to or exceeding the national average, absorbed most of the fertiliser
nitrogen. This left only between 1 and 5 kilograms per hectare of fertiliser
in the soil as nitrate at harvest.
There was much more nitrate in the soil from other sources, and the
amount washed out during autumn and winter was far greater than that left
from fertiliser. Most of the extra nitrate would have been liberated from
the soil’s vast reserves of organically bound nitrogen, about 5000 kilograms
per hectare, by the large and anarchic population of microbes living in
the soil.
These microbes function when the conditions suit them, rather than when
the crop needs nitrate. The warmth and increasing moistness of the soil
in autumn stimulate them to produce nitrate vigorously – just when the rain
is beginning to exceed evaporation so that water flows down through the
soil. This flow continues through winter and carries with it the nitrate
that the microbes have produced. This is the cause of the nitrate problem,
not fertiliser applied to growing crops in spring.
Although there is no evidence that fertiliser given to winter wheat
directly affects the subsequent leakage of nitrate, could there be indirect
effects? The most likely is that the fertiliser increases the amount of
organic nitrogen in soil that microbes can readily break down. The amount
of readily decomposable material is minute compared with the main bulk of
largely inert organic nitrogen, but it could have a disproportionately large
effect on the losses of nitrate. There is little evidence to show whether
such an effect occurs.
What we do have, though, are measurements on the soil of the Broadbalk
experiment at Rothamsted. For more than a century, winter wheat growing
on this soil has received 0, 48, 96 and 144 kilograms of nitrogen as fertiliser.
Another plot has received 192 kilograms but for a much shorter time. Research
led by David Jenkinson showed that microbes produce more nitrate as the
concentration of fertiliser rises (see Table). We do not know how long it
took for these differences to build up, but they imply that fertilisers
could indirectly affect the leakage of nitrate by increasing the amount
of the readily decomposable organic material.
Whether this really happens depends on the other effects of nitrogen
fertiliser. Our own ‘desk study’, using a well-tested model developed at
Rothamsted, simulates the microbial production of nitrate, its uptake by
the crop (winter wheat) and its loss when rain washes it out. The results
suggest that the potential indirect effect does not seem to have been important
in practice (see Table and Figure). This is because where more fertiliser
has been given, more is absorbed by the crop – up to a point. If more than
192 kilograms had been applied, the concentration would probably have increased
again. Even so, however, there is still no support for the ’40 per cent’
rule. If the soil had been left bare during winter, the leakage of nitrate
would have increased over the full range of nitrogen applications. This
would have supported the rule – but the crop would not have been winter
wheat.
These studies empha-sise that nitrogen fertiliser is not the major cause
of leakage by nitrate. It is arable farming itself that is the key. However
careful the farmer is there is always a ‘window’ for nitrate to leak into
the aquifer when the soil has been ploughed but the next crop is not yet
established. The implication is that we need to take land out of arable
cropping and put it down to low-productivity grass. This is one option for
the ‘protection zones’ envisaged in the DoE’s paper, and it certainly would
cut leakage. But if the land were ploughed and returned to arable cropping,
nitrate would flood out.
We cannot turn all our arable land over to grass, because we would need
to import wheat and other arable crops; our balance of payments is bad enough
as it is. Neither can we live off the ‘grain mountain’: the wheat in intervention
in Britain would last us about 10 days. We need arable crops, but we have
to minimise the leakage of nitrate.
The DoE’s paper mentions three ways of doing so: cutting down on fertiliser,
modifying cropping systems and ‘light control measures’. Such measures include:
applying nitrogen fertiliser strictly according to professional advice;
not applying it in autumn; leaving the soil covered over winter, perhaps
by growing a ‘catch crop’ to mop up the nitrate; sowing winter crops early;
ploughing up grassland as rarely as possible; taking great care with manures.
The report dismisses the ‘light control measures’ as ‘not having as
much effect on nitrate leaching as the other options’, suggesting that they
are less effective than cuts in the use of fertiliser. Yet the past 10 years
of agricultural research have shown that improving farm practice does cut
losses of nitrate. This is supported by recent reductions in the nitrate
concentrations in two aquifers, which seem to be related to earlier sowing
of winter wheat and less use of nitrogen fertiliser in the autumn. The WRC
recently modified its model to take account of these changes.
Cutting the use of fertiliser has a less certain effect on the leakage
of nitrates. The WRC’s updated model suggests that a cut of 20 per cent
in the 190 kilograms of nitrogen now typically applied to each hectare of
winter wheat would cut the leakage by no less than 42 per cent. Our results
show that a cut of 20 per cent would have no perceptible effect on the leakage.
Someone must be wrong, and we think that the problem lies in the ’40 per
cent rule’.
Fortunately, the DoE’s paper presents its conclusions as ‘tentative’,
which is a good thing; implementing these proposals without further thought
might well cause unnecessary hardship to rural communities and waste taxpayers’
money. The paper also suggests that further changes to the WRC’s model may
be necessary.
The real problem, however, is the way in which the DoE used the model.
The model was designed to simulate the movement of nitrate in an aquifer
after it had escaped from the soil above. It has little to say about the
soil and nothing about the complex web of processes that go to make up the
nitrogen cycle of the soil. Yet it is exactly these processes that determine
how much nitrate escapes. A proper understanding of these processes will
help farmers to control losses of nitrogen from their arable land. We cannot
ignore what goes on in the soil. The future of agriculture and the environment,
not to mention ourselves, depend on it. Mrs Bottomley was right; you do
need a hard head when dealing with environmental issues.
—————————————————————– Fertiliser
N applied (kg/ha/yr) 0 48 96 144 192
—————————————————————– Inputs
Mineral nitrogen in 28 42 50 50 48 metre top
kg/ha Rate of nitrate-N production kg/ha/day at 20oC 0.50 0.55
0.68 0.80 0.88 —————————————————————–
Outputs N leached as nitrate kg/ha 23 34 41
41 40 Nitrate concentration in drainage mg/l 34 50
60 61 59 —————————————————————–
Dr Tom Addiscott and Dr David Powlson are soil scientists working at Rothamsted
Experimental Station, Harpenden. Rothamsted is part of the AFRC Institute
of Arable Crops Research.
* Department of Environment. The Nitrate Issue. December 1988. HMSO.