Thesis (PhD(For))--Stellenbosch University, 2012. / The subtropical ecosystem of the Zululand coastal plain is prized by the South African
commercial plantation forestry industry for its rapid clonal Eucalyptus growth, short rotations (6
to 7 years) and high yields. This region is typified by sandy soils that are low in clay and organic
matter, have small nutrient reserves and are poorly buffered against nutrient loss. The subtropical
climate induces rapid decomposition of residues and tree litter resulting in small litter nutrient
pools and rapid nutrient release into the soil, particularly after clearfelling. A combination of
large nutrient demands through rapid growth, rapid nutrient turnover and small soil nutrient
reserves implies that sites in this region are sensitive and may be at risk of nutrient decline under
intensive management. The work in this study set out to determine the risk of nutrient depletion
through harvesting and residue management on a site within the Zululand region, to assess
nutritional sustainability and the risk of yield decline in successive rotations. Some bulk
biogeochemical cycling processes of macro-nutrients nitrogen (N), phosphorus (P), potassium
(K), calcium (Ca) and magnesium (Mg) were assessed, and assessments also included sodium
(Na). An existing Eucalyptus stand was clearfelled and treatments were imposed on the residues after
broadcasting to simulate various levels of nutrient loss through levels of harvesting intensity and
residue management. These included residue burning (Burn), residue retention (No-Burn),
fertilisation (stem wood nutrient replacement), whole tree harvesting and residue doubling. Outer
blocks of the stand were not felled, but included as replicates of an undisturbed standing crop
treatment. Biogeochemical nutrient cycling processes were assessed primarily in the standing
crop, Burn and No-Burn treatments, in the assumption that these represented the furthest
extremes of nutrient loss. Data collection commenced a year prior to clearfelling and continued
to two years and six months after planting with key data collection over a 20.1 month period
from clearfelling to canopy closure (one year after planting). Water related nutrient pools and
fluxes were assessed as atmospheric deposition (bulk rainfall, throughfall and stemflow) and
gravitational leaching to 1m soil depth. Drainage fluxes were predicted using the Hydrus model
and real-time soil moisture data. Zero tension lysimeters collected soil solution for chemical
analysis. Sequential coring in the 0 to 30cm soil layer was used to determine in situ soil N
mineralisation. Soil chemical and physical properties were assessed over the first meter of soil at clearfelling and new crop canopy closure to determine soil nutrient pools sizes. Biomass nutrient fluxes were assessed from litterfall, residue and litter decomposition, and above ground accretion
into the tree biomass. Leaching and N mineralisation were monitored in the No-Burn, Burn and
standing crop treatments only. Atmospheric deposition, while variable, was shown to be responsible for large quantities of
nutrients added to the Eucalyptus stand. Nitrogen and K additions were relatively high, but
within ranges reported in previous studies. Rapid tree canopy expansion and subsequent soil
water utilisation in the standing crop permitted little water to drain beyond 1m resulting in small
leaching losses despite a sandy well drained soil. Further leaching beyond this depth was
unlikely under the conditions during the study period. Mineralisation and immobilisation of N
also remained low with net immobilisation occurring. The standing crop was shown to be a
relatively stable system that, outside of extreme climatic events, had a relatively balanced or
positive nutrient budget (i.e. nutrient inputs minus outputs).
Large quantities of nutrients were removed with stem-wood-only harvesting in the No-Burn
treatment leaving substantial amounts on the soil surface in the harvest residues. Whole tree
removal increased losses of all nutrients resulting in the largest losses of P and base cations
compared to all other treatments. This was mostly due to high nutrient concentrations in the
removed bark. Loss of N in the Burn treatment exceeded whole tree N losses through
combustion of N held in the harvest residues and litter layer. The majority of K leached from the
residues prior to burning and a relatively small fraction of the base cations were lost from the
partially decomposed residues during burning. Ash containing substantial amounts of Ca and
relatively large amounts of N and Mg remained after burning. Surface soil Ca and Mg was
significantly increased by the ash which moved into the soil with rainfall directly after burning. Rapid soil moisture recharge occurred within a few months after clearfelling, increasing leaching
from the upper 50cm of soil. Clearfelling increased net N mineralisation rates, increasing mobile NO3-N ions in the soil surface layers. Nitrate concentration peaked and K concentration dipped
in the upper soil layers of the Burn treatment directly after burning. Deep drainage and leaching
(beyond 1m depth) over the 20.1 month period was, however, not significantly different between
the Burn and No-Burn treatments. Rapid soil moisture depletion and nutrient uptake with new
crop growth reduced leaching fluxes to levels similar to the standing crop by six months after
planting. Taking the full rotation into account, clearfelling induced a short-lived spike in N and
cation leaching compared with the low leaching losses in the undisturbed standing crop. Soil N
mineralisation over the 20.1 month period in the burnt treatment was half that of the No-Burn
treatment.
Growth and nutrient accumulation was significantly higher in the fertilised treatment than in
other treatments up to 2.5 years of age. Growth in the Burn treatment was greatest compared to other treatments during the first few months, but slowed thereafter. No significant growth
differences were found between all other treatments from a year to 2.5 years after planting. Early
growth was therefore apparently not limited by N supply despite large differences in N
mineralisation between Burn and No-Burn. Foliar vector analysis indicated that fertilisation
improved growth initially through increased foliar N and P at six months after planting followed
by Mg and Ca at one year. The Burn treatment was not nutrient limited. These growth results
contrasted with similar international research on sandy tropical sites where growth was reduced
after residue removal and increased after residue doubling. The combined nutrients released from
pools in the litter layer or ash and soil in addition to atmospheric inputs were sufficient to
provide most nutrients required to maintain similar growth rates across all treatments. This
demonstrated the importance of residue derived nutrients to early growth nutrient supply.
Reduced N mineralisation through a lack of substrate may limit N supply later in the rotation
where residue had been removed. Construction of a nutrient budget for the system revealed that high levels of atmospheric inputs
have the potential to partially replenish a large proportion N, K, and Ca lost during clearfelling,
provided losses are constrained to stemwood removal only. However, loss of Mg that occurred
primarily through leaching may not be replaced under the low Mg inputs recorded in this study.
Larger nutrient removals (i.e. stemwood plus other plant parts) placed a heavier reliance on the
small soil nutrient pools at this site which can limit future productivity. More intense harvesting
and residue management practices dramatically increased the risk of nutrient depletion. Losses of
specific nutrients depended on a combination of clearfelling biomass removal, residue burning
and subsequent leaching. Nitrogen losses due to harvesting and burning were more substantial
than those due to leaching. Mg and K losses depended most strongly on the time after
clearfelling before re-establishment of the new crop and rainfall patterns, while Ca and P losses
depended directly on the amount of biomass removed. Depletion risk was the greatest for Mg
and K through rapid leaching, even after stem wood only removal. Deep root uptake and deep
drainage with associated cation loss needs to be investigated further to quantify ecosystem losses
and recovery of cations displaced beyond 1m. Atmospheric deposition is one of major factors countering nutrient losses. However,
atmospheric inputs may not be reliable as these may lessen in future through pollution control
legislation and climate change. Changes in growth rate under poor nutrient management
practices are small and difficult to detect relative to the large impacts of changing weather
patterns (drought), wildfire and pest and disease. This makes it difficult to prove nutrient related
growth decline. It may be possible that improvements in genetics, silvicultural technologies and atmospheric inputs may also be masking site decline (in general) and in part explain the lack of
evidence of a growth reduction in the region.
As the poorly buffered sandy soils on the Zululand Coast are at risk of nutrient depletion under
the short rotation, high productivity stands, it may be necessary to stipulate more conservative
harvesting and residue management practices. A more conservative stem-wood only harvesting
regime is recommended, retaining all residues on site. Residue burning should be avoided if N
losses become a concern. The length of the inter-rotation period must be kept short to reduce
cation leaching losses. Site nutrient pools need to be monitored and cations may eventually need
to be replenished through application of fertilisers or ash residues from pulp mills. Management
practices therefore need to be chosen based on the specific high risk nutrients in order to
maintain a sustainable nutrient supply to current and future plantation grown Eucalyptus.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/71720 |
| Date | 12 1900 |
| Creators | Dovey, Steven Bryan |
| Contributors | Du Toit, Ben, De Clercq, Willem, Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 218 p. : ill., maps |
| Rights | Stellenbosch University |
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