By Liz Poon
An eight-year study on the effects of grazing and
tree cover on soil fertility in north Queensland has shown that
while grazing tends to produce a decline in landscape condition,
the presence of trees has a greater positive impact on landscape
condition and soil fertility.
The study was carried out in heavy and lightly stocked paddocks
within the Wambiana grazing trial and a nearby ungrazed paddock, 70
km south of Charters Towers, in the Upper Burdekin river catchment,
North Queensland. The ungrazed paddock had never carried domestic
livestock due to the presence of heartleaf, a plant toxic to
cattle. Although the ungrazed paddock had somewhat different soils
to the grazed paddocks, it presented a rare opportunity to observe
a long-ungrazed area.
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Relationship between the nutrient cycling index
and soil biological activity at Wambiana, across grazing
intensities, landform elements and canopy/intercanopy zones. The
graph above shows that the zones under trees have higher soil
biological activity, therefore more nutrient cycling occurs there.
(Also greater water infiltration as reported by Dawes-Gromadzki et
al, Savanna Links Issue No 32). Soil is more stable there than in
the intercanopy zones, irrespective of grazing intensity or
landscape element.
Bar graphs above and below show differences between nutrient
cycling index, landform element and canopy/intercanopy zones in
ungrazed and grazed paddocks. In general, the nutrient cycling
index was higher in the ungrazed paddock and highest in the canopy
zones in all paddocks.
Total N (%) was greater in the canopy zones than
intercanopy zones in all paddocks. In general levels of N were
greater in the ungrazed paddock than the grazed paddocks.
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Soil fertility was estimated by measuring soil nitrogen (N)
which accounts for 80% of nutrients taken up by plants. Most of the
nitrogen in soil is in organic form and to be taken up and used by
plants, it has to be converted to inorganic forms by soil
micro-organisms. The two major forms of inorganic ‘plant
available’ N are ammonium (NH 4 + ) and
nitrate (NO 3 - ). Nitrate, which is highly
soluble in water and thus more mobile, was the more dominant form.
The highest levels of plant available N were detected during the
early and mid-wet season, lowest levels occurring in the mid-dry
season — reflecting the importance of soil moisture in
the mobility and availability of these nutrients. (As we became
aware of the ungrazed paddock later in the project, this paddock
during the early wet season only, when levels were high.)
Nutrient, trees and landscape position
Distribution of different plant-available pools of N were also
influenced by grazing, tree cover and position in the landscape. In
the wet season, concentrations of inorganic N were generally
greatest under trees. Ammonium concentrations were higher in grazed
paddocks, particularly under the trees in heavily stocked paddocks,
compared to the ungrazed paddock. Since the distribution of
ammonium in the ungrazed and lightly stocked paddocks was
comparatively more even, this may indicate that heavy stocking
rates create greater patchiness in nutrient distribution and a
greater potential for nutrient loss.
Concentrations of nitrate, which is water soluble and more
likely to leach away, were higher in the grazed paddocks than the
ungrazed paddock and in the drainage lines of the grazed paddocks.
However, in the ungrazed paddock, most nitrate was on the crest and
the least was in the drainage line. The higher vegetation cover in
the ungrazed paddock may have aided the retention of the nitrate in
the crest zone. Overall variability in the data was great and was
indicative of natural variation in soil moisture and vegetation
cover in the field.
Landscape condition
Landscape condition was assessed by measuring vegetation and
soil crust cover, soil micro-topography and the size and spatial
distribution of vegetated and bare soil patches (Tongway &
Hindley 2004); these influence the capture and distribution of
resources such as water and nutrients.
Results suggested that areas under tree canopies were in better
condition than areas outside the canopy in terms of soil stability,
infiltration rate, nutrient cycling capacity and soil biological
activity. These differences were greater between ungrazed and
grazed paddocks than between heavy and light stocking rates.
Analysis of nitrogen isotopes in the soil and leaves in the grazed
paddocks indicated that more nitrogen ‘leaks’ out of
the system in grazed paddocks, possibly due to erosion or grazing
and that trees in the grazed paddocks were not as efficient in
using nitrogen as those in the ungrazed paddock. Overall, these
results emphasise the importance of trees as zones of fertility and
highlights the benefits of having a healthy landscape where
topsoil, organic matter, water and nutrients are retained and
recycled in the system.
References
Tongway, D. J. & Hindley, N. L. 2004, Landscape Function
Analysis: Procedures for monitoring and assessing landscapes, with
special reference to minesites and rangelands, CSIRO SE,
Canberra.
Liz Poon is a PhD student with University of Queensland, Dr John
Ludwig (CSIRO SE) and Dr Susanne Schmidt (UQ) are Liz’s
advisers.
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