Nitrogen Fertiliser Response on New Zealand Hill Country
Introduction
Steep non-arable hills below 1000 m occupy 40 % of New Zealand’s land surface. These
lowland steeplands are commonly known as New Zealand hill country (Blaschke et al 1992).
Until approximately 150 years ago these areas were predominantly forested, comprising
evergreen, mixed coniferous and broadleaved species. Deforestation of hill country began with
European contact and settlement, with the rate of deforestation peaking in the decades around
the beginning of the 20th century. Hand felling of forest trees followed by burning and sowing of
grass for extensive pastoral use was the primary method used.
Approximately 60 % of farmland in New Zealand is used for sheep and beef farming (Statistics
NZ 2003) and the majority of this area is hill country. A characteristic of New Zealand hill country
is that it is very diverse. Diversity in climate, soil type, soil fertility, slope class, and slope aspect,
plant species, and farming system used to harvest herbage grown occur on a regional, farm,
paddock and even on a within paddock scale (Ball et al 1982). Hill country on the east coast of
both the North and South Islands is subject to hot dry summers, whilst much of the hill country in
the lower North Island is subject to a warmer and wetter climate. North facing slopes are
generally drier and warmer than south facing slopes. Decreasing slope steepness is generally
associated with increasing soil fertility due in part to increased nutrient transfer by grazing
animals as well as increased soil moisture status due to greater water retention ability/reduced
incidence of rainfall runoff on easier slopes (Ball et al, 1982).
Over the past 25 years sheep and beef numbers have dropped, however national production
from sheep and beef farms has increased. This has been attributed to a combination of a 25 %
increase in national lambing percentage and an increase in carcass weights of 25, 18 and 13 %
for lamb, mutton and beef respectively during this time period (Meat and Wool Economic Service
2004). The increases in lambing percentage and in sheep and beef carcass weights are
attributed to both improved genetics and better feeding of these animals.
In 2000 McKinsey & Co stated that in order to retain economic viability and international
competitiveness, the sheep and beef industry must grow at approximately 4 % per annum. This
places increasing pressure on sheep and beef farmers to intensify their operation in order for
their business to remain viable and profitable.
Increasing fertiliser input is one of the major tools that farmers use to intensify their operation.
The benefits of maintaining adequate soil phosphorus (P) and sulphur (S) levels on hill country
using fertiliser are well known (Lambert et al, 2000; O’Connor et al, 1990; Roach et al 1996).
Nitrogen (N) fertiliser use by hill country farmers has until recently been negligible, but has
begun to increase in the last decade (Parliamentary Commissioner for the Environment, 2004).
Even if the major and trace element requirements of hill pastures are met, these pastures will
always remain chronically nitrogen deficient. This means that there is potential to achieve
increased pasture production from the use of nitrogen fertiliser in this environment. An excellent
review of pasture response to fertilizer nitrogen on hill country up to 1982 was presented by Ball
et al (1982) and the reader is encouraged to examine this concise summary. Evidence from a
range of trials conducted up to 1982 showed that nitrogen response efficiencies (kg of extra dry
matter grown/kg of nitrogen fertiliser applied) ranged from 7 to 33 kg (Ball et al, 1982 -Table
9.4). At that time the authors concluded that there was “scope for nitrogen fertiliser use in hill
country – as an adjunct to pasture improvement on low fertility soils and – to relieve seasonal
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nitrogen stress in better pastures”. The authors were confident that “if cost-price relationships
remain favourable then there will be a considerably greater use of nitrogen fertiliser on hill
pastures”.
In 1978 nitrogen fertiliser use by the pastoral sector (including sheep, beef, dairy and deer)
amounted to 7,500 tonnes per year (Menzies, 1982). The author estimated that by 1995 that
figure would increase to 25,000 tonnes (Menzies, 1982). In retrospect this was a conservative
estimate since the figure for 1996 was 86,000 tonnes. In 2002 that figure had climbed to
474,000 tonnes and it continues to increase to date. The average amount of nitrogen (N) applied
to hill country has risen from 0.7 kg N/ha in 1996 to 5.7 kg N/ha in 2002 (Parliamentary
Commissioner for the Environment, 2004). Whilst this figure is not great in absolute terms, the
area over which this nitrogen is applied is large, and it would be prudent to look carefully at the
consequences of such a magnitude increase in nitrogen use in hill country.
This report will focus on published work relating to nitrogen fertiliser use in hill country since
1982, and will consider only grazing trial evaluations. This is because the subsequent cutting
trial information does not add substantially to the information contained in the Ball et al (1982)
review. In addition, cutting trials may underestimate responses that are realised under grazing
since there is an absence of excretal (and therefore nutrient) return to the area. Up to 1982 most
of the work on nitrogen fertiliser responses in hill country had been done in moist environments,
particularly those in the lower North Island. It is ironic to note that this was still the case in 1998
(Gillingham et al, 1998), and remains largely unchanged to date.
In the 24 years since the 1982 review, there have been five grazing trials evaluating the use of
fertiliser N in hill country. Two in the southern North Island (Lambert and Clark, 1986 and
Lambert et al 2003), one in the Waikato (Ledgard et al 1983), and two at Waipawa in the eastern
North Island (Gillingham et al 1998 and 2004a,b).
Moist Hill Country - North Island
Large N fertiliser response rates (17-34 kg extra DM/kg N applied) were obtained by Lambert
and Clark (1986) when urea (up to 50 kg N/ha) was applied in late autumn, and pasture
production response extended through winter into spring. Much lower response efficiencies were
found by Ledgard et al (1983) to urea application (up to 90 kg N/ha) in autumn (less than 8 kg
extra DM/kg N applied) and in early spring (less than 20 kg extra DM/kg N applied).
Ledgard et al (1983) concluded that in setting guidelines for obtaining reliable and maximum
pasture response to N fertiliser in hill country “priority must be given to areas of dense, ryegrass
dominant, easy slopes in late winter/early spring (3-4 weeks before the additional pasture is
required in early spring)”.
Lambert et al (2003) applied high rates of N fertiliser (400 kg N/ha annually as eight split
dressings of 50kg N/ha) to historically low, medium and high fertility (P status) areas of hill
country. Similar N fertiliser response rates of 18-22 kg extra DM/kg N applied were observed
across fertility sites. They suggested that higher application rates of fertiliser N would be an
economic option for hill country farmers.
Pasture composition responses to N fertiliser in the above trials was variable and inconsistent,
and depended on year, P status of the soil, and on slope and aspect. In general, legume content
tended to decrease with added N, but sometimes this was transient, not always significant and
dependant on whether extra pasture grown was eaten. More intensive sampling of herbage, as
well as increased replication of treatments are often needed to pick up any measurable effects
of N fertiliser on long term changes in botanic and nutrient composition of pastures (Ledgard et
al, 1983).
Dry Hill Country – North Island
Two trials were conducted over a period of 7 years on dry, steep hill country in the Hawkes Bay
(Gillingham et al 1998; Gillingham et al 2004 a, b). A very high N response efficiency of 43 kg
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extra DM/kg N applied in mid to late winter was measured on steep north facing slopes of low
phosphorus status (Gillingham et al 1998), with an average DM response efficiency of 24 kg
DM/kg N across all slopes and aspects of the low P status soils. With higher P status soils the
average DM response to applied N was only 16 kg DM/kg N applied.
In summarising the findings from nitrogen fertiliser work at Waipawa, Gillingham (2004a)
concluded that three important factors drive profitability in a dry hill country environment. The
first is to maximise pasture growth during the reliable wet (winter) period, especially when slopes
dry quickly once rain ceases. This is best achieved using nitrogen fertiliser. The second factor is
that pasture must be fully utilised during the period of active pasture growth if it is to be
economically worthwhile. The third important factor is to use the most profitable stock class to
make use of that growth. Gillingham concluded that with increased nitrogen fertiliser use and
more strategic use of P fertilisers in dry hill country, and more intensive management of stock,
economic returns would increase. However, the best combination of practices would vary from
property to property.
In all of the above trials investigating the response to nitrogen fertiliser on hill country, between
year variation in pasture production response to nitrogen fertiliser was often large. This
highlights the importance of running multiple year grazing trials when attempting to give reliable
estimates of response efficiencies to fertilizer nitrogen even for a given region. It is difficult and
cost prohibitive to generate information from trial work that is directly applicable to risk
management at the farm level. The above studies have all added to the general knowledge of
hill country response to added nitrogen fertiliser. How this knowledge can be utilised to help
decision making at the individual farm level across all regions is more difficult to determine.
Perfect knowledge is rare in decision making in agricultural systems. Risk and uncertainty exist
simply because agricultural systems are based on biological processes. Biological systems are
inherently variable and often unpredictable given our still limited knowledge of the same. At the
farm level, decision makers are faced with both uncertainties due to the nature of biological
systems, as well as the vagaries of the global economy.
Parker et al (1994) categorised sources of risk and uncertainty associated with nitrogen fertiliser
decisions for pastoral livestock systems into three areas, namely production, price (or market),
and financial. Of these three areas, production risk was deemed to be the major source of
uncertainty associated with nitrogen fertiliser use. Production risk arises from the variability
inherent in the biological processes of pasture growth and the conversion of this into animal
products. Specifically these were identified as weather (rainfall and soil temperature), season
and soil condition at the time of fertiliser application, and grazing management imposed post
application. Issues surrounding price and financial were deemed to be of much lower importance
when assessing risk and uncertainty associated with nitrogen fertiliser use.
Through surveying and extensive questioning of experts in the field Parker et al (1994)
concluded that once the most important sources of risk were identified a decision tree framework
to systematically evaluate management alternatives could be utilised. They recommended
simple steps that could be taken to minimise risk i.e. monitoring of soil, pasture and weather
conditions. Regardless of region, the best outcomes from using nitrogen fertiliser would be
when:
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fertiliser is applied immediately prior to a light rain, and avoiding heavy rain (> 50mm)
events 3-4 days post application
10 cm soil temperatures must exceed 6° and remain above this for at least 4 weeks post
application
soil conditions are not excessively wet and soil is not pugged
herbage mass is at least 1500 kg DM/h
an “optimum” level of mass is reached before stocking and that this level is maintained to
maximise dry matter production
the greatest responses will be obtained in spring, followed by autumn, and then winter
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The increased use of fertiliser nitrogen in hill country has resulted in more intensive
management of that land. The loss of nutrients from increasingly intensively managed farmland
(including hill country), is of great concern to land owners and managers, and regional councils.
The New Zealand public is also becoming increasingly aware of issues surrounding the
degradation of our natural capital.
There is very little published information available on nitrogen losses from hill country. There is
even less information on the effects of nitrogen fertiliser per se on losses of nitrogen from hill
country. Sources of nitrogen loss include leaching of soluble nitrogen (nitrate, ammonium and
dissolved organic nitrogen) below the root zone of pasture plants and in overland flow during
heavy rainfall events, volatilization of nitrogen in the form of ammonia gas, and the emission of
nitrous oxide as a result of denitrification occurring in the soil. Apart from the environmental
effects of nitrogen loss from hill country, it is an agriculturally important process, since it can lead
to losses of valuable nitrogen from the system.
Nitrogen Loss through Leaching
Hill country soils have a high soil carbon to nitrogen ratio relative to more intensively managed
soils. Thus, it has been thought that any available nitrogen not being utilised by plants would be
rapidly immobilised and would be unlikely to contribute to significant nitrogen leaching (Lambert
et al, 2003). This hypothesis has yet to be to be rigorously tested (e.g. Sakadevan et al, 1993).
One method of measuring nitrogen leaching from hill country has been the monitoring of water
quality and flow rate from streams draining specific catchments (e.g. McColl et al 1977; Bargh,
1978; Lambert et al 1985; Quinn and Stroud 2002), and relating land use of those catchments to
nitrogen levels and export in the stream water. Estimates of nitrogen leaching obtained in this
way have generally been lower than that when compared to that of more intensively managed
fertile lowland soils (e.g. 68-80 kg N/ha /year on sheep grazed pastures (Field et al 1985)).
McColl et al (1977) reported nitrogen export from sheep grazed hillsides to be 1.4 kg nitrateN/ha/year vs. 0.01 and 0.04 kg nitrate-N/ha/year for native and exotic forests respectively.
Lambert et al (1985) estimated that losses from cattle grazed hillsides were 12.1 kg total N/ha
per year compared to sheep grazed hillsides which yielded 8.7 kg total N/ha per year. Stroud
and Quinn (2002) reported exports of 10.0 and 23.2 kg total N/ha/year for two grazed hill
catchments and compared this to mixed (grazed plus forest) and native forest catchments which
exported 6.8 and 2.1 kg total N/ha year respectively.
Monitoring water quality and flow rates from streams draining hill country catchments is able to
only partly assess the impact of intensification on nitrogen leaching loss from these areas. It may
take much longer than anticipated for management effects to be exhibited in stream water
quality, as leached N may take the order of years to move from below the pasture root zone into
a waterway. It must also be noted that not all water and thus nutrients draining from hill
catchments finds its way into an immediate stream. Deep percolation of water also occurs,
which results in nutrients being exported into water bodies sometimes spatially quite distant from
the area in question. This must be taken into account when attempting to describe losses of
nitrogen from hill country.
It is of interest to note that nearly 30 years ago, Bargh (1978) pointed out that “deterioration in
the quality of natural waters is a problem of increasing concern in New Zealand and overseas”.
Bargh hoped that his study of a small catchment in the Tararua ranges “would provide new
information and point to directions for future research in New Zealand”. Very little work on
nutrient leaching (in the widest sense) from hill country has been done since. This is of concern
since modelling of nutrient leaching from managed farmland has concentrated mainly on more
intensively managed flatter areas. Extrapolation of results from these areas to that of hill country
in its many diverse forms may not be a realistic model of nutrient export in hill country. It may
however be the best option until further work is done.
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Nitrous Oxide Emissions
Nitrous oxide is a very potent greenhouse gas; absorbing longwave radiation emitted from the
earth’s surface at a rate 250 times that of carbon dioxide, unit for unit. It is also an important
factor in altering the concentration of ozone in the stratosphere, which in turn affects the
penetration of ultra violet radiation to the Earth’s surface.
Approximately 5% of the total global atmospheric greenhouse gas effect is attributed to nitrous
oxide (Mosier 2001). Nitrous oxide is a gaseous product of denitrification in soils, and may also
be produced during the oxidation of ammonium and nitrite ions in soils (Bolan et al, 2004).
Denitrification is promoted when soils are anaerobic (saturated or water logged), have a high
concentration of nitrate, a readily available source of carbohydrate, and is positively related to
soil pH and temperature (Bolan et al, 2004). It is estimated that 70 % of the annual global
anthropogenic (originating from human activity) emissions of nitrous oxide come from animal
and crop production (Mosier, 2001).
Whilst a number of studies of nitrous oxide emissions from dairy and sheep pastures under
relatively fertile conditions have been done in New Zealand (Ruz-Jerez et al, 1994; Saggar et al
2004, Carran et al 1995; de Klein et al 2001), few measurements of nitrous oxide emission from
New Zealand hill country have been done (Carran et al, 1995). Despite high spatial and
temporal variability in emission rates within measurement sites in that study, emission rates
were low both at low fertility and high fertility sites relative to that from more intensively grazed
and managed flatter land.
Until more nitrous oxide measurements are made it is difficult to estimate losses of nitrogen from
this source in hill country and in particular, the effect of increased nitrogen fertiliser use on
emissions of this gas from hill country. If one could extrapolate from more intensively managed
flat land systems, then an increase in nitrogen fertiliser use would be expected to result in an
increase in emissions of nitrous oxide from hill country. There would be both direct and indirect
effects. Fertiliser nitrogen would directly increase the amount of nitrate in the soil, and an
increase in stocking rate as a result of increased dry matter grown would increase the amount of
dung and urine return to that area. Both of these factors have been shown to increase nitrous
oxide emissions (Bolan et al, 2004; Luo et al 1999 a and b; Luo et al 2000; de Klein et al 2001).
Increased stock numbers per unit area with nitrogen fertiliser application would also increase
areas in the soil experiencing anaerobic condition at a given soil moisture content, due to
increased incidence of treading damage (Betteridge et al, 1999; Carran et al 1995). Increased
treading damage has been shown to increase nitrous oxide emissions in dairy pastures (de
Klein et al, 2001).
Ammonia Volatilisation
In grazed pastures, biological degradation of dung and urine, and hydrolysis of fertiliser nitrogen
leads to the continuous formation of ammonia gas in the soil, which then volatilises into the
atmosphere. These losses are greatest from recent dung and urine patches, and in the first few
days after nitrogen fertiliser application. Factors that affect the rate and amount of ammonia loss
include soil temperature, wind speed, and moisture conditions of the soil at the time (Bolan et al.
2004). At high soil and ambient air temperatures, dry soil conditions at the soil surface, and high
winds, ammonia losses are high. While few measurements of ammonia losses from hill country
have been made, it would be expected that increased nitrogen fertiliser use would increase the
amount of ammonia volatilisation from these areas. The loss of ammonia directly from fertiliser
nitrogen would be expected to be small, since the majority of fertiliser nitrogen is applied when
conditions for ammonia loss are minimal. Increased stocking density to make use of added
pasture grown on nitrogen fertilised areas however, would increase the number of dung and
urine patches and hence the opportunity of ammonia loss to occur.
Conclusion
Issues surrounding fertiliser nitrogen use and response on hill country are multi-faceted. Of
immediate importance from a farmer’s perspective is the likely response efficiency (kg extra
DM/kg applied nitrogen) to be had given a specific set of climate, soil and pasture conditions at
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the time of application. Broad guidelines for these are available, but realised responses will vary
for each individual farm. Farmers who monitor and keep good records of climate, soil, and
pasture data on their farms will be in a better position to make good decisions regarding nitrogen
fertiliser use on their hill country. Good utilisation of the extra pasture grown is vital to
maximising profit from the nitrogen applied, irrespective of the response efficiency obtained. In
addition, utilising the best class of stock possible to eat that extra feed will improve profit from
any nitrogen applied. The best class of stock will depend on the farm system and on current
stock values.
Increased awareness by all sectors of the community of the environmental consequences of
intensification of land use has resulted in the need for a much greater understanding of nutrient
export from hill country. It is unlikely that the application of fertiliser nitrogen per se to hill country
using best practise guidelines as they stand to date will adversely affect nutrient export from that
area. The resultant increase in stock numbers necessary to utilise the extra herbage grown,
however, increases the amount of urine and dung returned to that area, and this in turn will most
likely impact on nitrogen losses from that system.
If the economic, environmental and social sustainability of hill country farming is to be realised,
then those parties responsible for investigating the impact of nitrogen fertiliser use on hill country
must look carefully, remain open minded, and consider all of the implications put before them.
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