The influence of fertiliser nitrogen on soil nitrogen and on the herbage of a grazed kikuyu pasture in Natal.
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Date
1994
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Abstract
The work reported in this thesis was designed to develop a better
understanding of the fate of fertiliser nitrogen applied to a
tropical pasture under field conditions, with the eventual
objective of improving the economy of livestock production off
such pastures. This involved an examination of the
concentrations of soil total nitrogen, ammonium nitrogen and
nitrate nitrogen at different depths within the soil profile
following the application of different levels of fertiliser
nitrogen to a grazed kikuyu (Pennisetum clandestinum) pasture,
as well as the influence of such applications on pasture yield
and some elements of pasture quality. The trial was conducted
over a two year period at Broadacres in the Natal Mistbelt.
A labelled [15]NH[4]N0[3] fertiliser experiment was also conducted
to ascertain how the labelled ammonium ion moved through the
soil, roots and herbage after being applied in spring onto a
kikuyu pasture.
In the absence of fertiliser N, a total of 15.45 t/ha of
soil N was recorded at an average concentration of 0.15%. More
than 30% of the soil total N was, however, situated within the
top 10cm of soil. organic matter (OM) content in the top 0-10cm
of the profile was high (4.75%), reflecting an accumulation of
organic matter in this zone. However, as organic C (and thus c: N
ratios) declined with depth, so too did soil total N
concentration.
Not surprisingly, fertiliser
measurably increase soil total N,
N applications did not
but indirectly may have affected soil N dynamics by increasing net mineralisation (due
to its "priming" effect) thereby stimulating plant growth and
thus increasing the size of the organic N pool through greater
plant decay.
Total soil N concentration did not change significantly from
the first to the second season. This could be attributed to the
fact that N gains and losses on the pastures, being over 15 years
old, were probably in equilibrium. Generally similar trends in
soil total N down the profile over both seasons was further
confirmation of this.
Before the application of any fertiliser, 331.9 kg NH[4]-N was
measured in the soil to a depth of 1m, on average, over both
seasons. This amount represented only 2.1% of the soil total N
in the profile. The concentration of NH[4]-N followed a quadratic
trend down the soil profile, irrespective of the amount of
fertiliser N applied, with the largest concentrations
accumulating, on average, in the 0-10cm and 75-100cm depth
classes and lowest concentrations in the 50-75cm depth class.
Laboratory wetting/drying experiments on soil samples collected
from a depth of 75-100cm showed that NH[4]-N concentrations
declined only marginally from their original concentrations. A
high organic C content of 1.44% at this depth was also probable
evidence of nitrification inhibition. Analysis of a similar
Inanda soil form under a maize crop did not exhibit the
properties eluded to above, suggesting that annual turn-over of
the soil was causing mineralisation-immobilisation reactions to
proceed normally.
Addition of fertiliser N to the pasture significantly increased the amount of NH[4]-N over that of the control camps.
Furthermore, the higher the application rate, the greater the
increase in NH[4]-N accumulation within the soil profile. As N
application rates increased, so the NH[4]-N:N0[3]-N ratio narrowed in
the soil complex. This was probably due to NH[4]-N being applied
ln excess of plant requirements at the high N application rates.
On average, 66.7 kg more NH[4]-N was present in the soil in
the first season than in the second after fertilisation.
Although this amount did not differ significantly from spring
through to autumn, during early spring and late summer/autumn
concentrations were higher than in mid-summer. Observed soil
NH4-N trends were also very similar to the soil total N trends
within both seasons, suggesting that soil total N concentrations
might well play an important role in determining soil NH4-N
concentrations.
Before fertilisation, only 45.6 kg N0[3]-N, representing 0.29%
of the soil total N, was on average, found in the profile to a
depth of 1m. The highest concentration of N0[3]-N was lodged in
the top 10cm of the soil. Nitrate-N declined, on average, with
depth down the profile. However, during the second season, even
though the concentration of N03-N declined down the profile, it
increased with depth during relative to that of the first season,
suggesting the movement of N0[3]-N down the profile during this
period.
Fertilisation significantly increased the concentration of
N0[3]-N above that of the control camps. Concentrations increased
as fertiliser application rates increased, as did N0[3]-N
concentrations with depth. This has important implications regarding potential leaching of N03-N into the groundwater,
suggesting that once applications reach levels of 300 kg
N/ha/season or more, applications should become smaller and more
frequent over the season in order to remove this pollution
potential.
On average, 94.3 kg N0[3]-N/ha was present down to a depth of
1m over both seasons. However, significantly more N0[3]-N was
present in the second season than in the first. This result is
in contrast to that of the NH[4]-N, wherein lower concentrations
were found in the second season than in the first.
No specific trends in N0[3]-N concentration were observed
within each season. Rather, N0[3]-N concentrations tended to vary
inconsistently at each sampling period. Nitrate N and ammonium
N concentrations within each month followed a near mirror image.
A DM yield of 12.7 t/ha, averaged over all treatments, was
measured over the two seasons. A progressive increase in DM
yield was obtained with successive increments of N fertiliser.
The response of the kikuyu to the N applied did, however, decline
as N applications increased.
A higher yield of 1.8 t DM/ha in the first season over that
of the second was difficult to explain since rainfall amount and
distribution was similar over both seasons.
On average, 2.84% protein N was measured in the herbage over
both seasons. In general, protein N concentrations increased as
N application rates increased.
On average, higher concentrations of protein-N were measured
within the upper (>5cm) than in the lower <5cm) herbage stratum,
irrespective of the amount of N applied. Similar bi-modal trends over time in protein-N concentration
were measured for all N treatments and within both herbage strata
over both seasons, with concentrations tending to be highest
during early summer (Dec), and in early autumn (Feb), and lowest
during spring (Oct), mid-summer (Jan) and autumn (March). spring
and autumn peaks seemed to correspond with periods of slower
growth, whilst low mid-summer concentrations coincided with
periods of high DM yields and TNC concentrations.
The range of N0[3]-N observed in the DM on the Broadacres
trial was 0.12% to 0.43%. As applications of fertiliser N to the
pasture increased, N0[3]-N concentrations within the herbage
increased in a near-linear fashion.
On average, higher concentrations of N0[3]-N, irrespective of
the amount of fertiliser N applied, were measured wi thin the
upper (>5cm) than the lower <5cm) herbage stratum.
A similar bi-modal trend to that measured with protein-N
concentrations was observed in both seasons for N0[3]-N in the
herbage. High concentrations of N0[3]-N were measured during
spring (Nov) and autumn (Feb), and lower concentrations in midsummer
(Dec & Jan), very early spring (Oct) and early autumn
(March). During summer, declining N0[3]-N concentrations were
associated with a corresponding increase in herbage DM yields.
A lack of any distinctive trend emerged on these trials in
the response of TNC to increased fertilisation with N suggests
that, in kikuyu, applied N alone would not materially alter TNC
concentrations.
Higher concentrations of TNC were determined in the lower
<5cm) height stratum, on average, than in the corresponding upper (>5cm) stratum. This may be ascribed to the fact that TNCs
tend to be found in higher concentrations where plant protein-N
and N0[3]-N concentrations are low.
A P concentration of 0.248% before N fertilisation, is such
that it should preclude any necessity for P supplementation, at
least to beef animals. Herbage P concentrations did, however,
drop as N fertiliser application rates were increased on the
pasture, but were still high enough to preclude supplementation.
Even though no significant difference in P concentration was
measured between the two herbage strata, a higher P content
prevailed within the lower <5cm) herbage stratum.
On average, 2.96% K was present within the herbage material
in this trial. The norm for pastures ranges between 0.7 and
4.0%.
On these trials, applications of fertiliser N to the camps
did not significantly affect K concentrations within the herbage.
The lower <5cm) herbage stratum, comprising most of the
older herbage fraction, was found to contain the highest K
concentration in the pasture.
The presence of significantly (although probably
biologically non-significantly) less K within the herbage in the
second season than in the first may be linked to depletion of
reserves of ยท this element in the soil by the plant and/ or
elemental interactions between K and other macro-nutrients.
An average Ca content of 0.35% within the herbage falls
within the range of 0.14 to 1.5% specified by the NRC (1976) as
being adequate for all except high-producing dairy animals.
Increasing N application rates to the pasture increased the Ca content within the herbage .
No significant differences in Ca concentration were found
between the upper (>5cm) and lower <5cm) herbage strata over
both seasons, even though the lower stratum had a slightly higher
Ca concentration, on average, than the upper stratum.
Calcium concentrations did not vary between seasons,
probably because concentrations tend rather to vary according to
stage of plant maturity, season or soil condition. However,
higher concentrations of the element were measured in the second
season than in the first. The reason for this is unknown.
On average, 0.377% Mg was present within the herbage over
both seasons. This compares favourably with published data
wherein Mg concentrations varied from 0 . 04 to 0.9% in the DM,
with a mean of 0.36%.
All camps with N applied to them contained significantly
more Mg in their herbage than did the material of the control
camps.
On these trials, the Ca :Mg ratio is 0.92: 1, which 1S
considered to be near the optimum for livestock and thus the
potential for tetany to arise is minimal.
Magnesium concentrations remained essentially similar within
both herbage strata, regardless of the rate of fertiliser N
applied.
As in the case of Ca, Mg concentrations within the herbage
were significantly higher in the second season than in the first.
Calcium:phosphate ratios increased, on average in the
herbage, as N application rates increased. This ratio was high
in spring, dropped off in summer and increased again into autumn, suggesting that the two ions were following the growth pattern
of the kikuyu over the season.
The K/Mg+Ca ratios were nearly double that of the norm,
suggesting that the pasture was experiencing luxury K uptake
which may be conducive to tetany in animals grazing the pasture.
This ratio narrowed as N application rates were increased,
probably as a result of ion dilution as the herbage yields
increased in response to these N applications. The ratio was low
in spring (October), but increased to a peak in December, before
declining again to a low in March.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1994.
Keywords
Plants, Effect of nitrogen on., Kikuyu grass., Pastures--Fertilizers., Soils--Nitrogen content., Theses--Grassland science.