Read, Zoe Joan
Description
The flux of carbon (C) between the terrestrial biosphere and the
atmosphere occurs naturally as part of the carbon cycle (CC).
However, anthropogenic activity can disrupt the natural
equilibrium of the CC. Although fossil fuel burning is the
largest cause of human induced C emissions to the atmosphere,
land use and land use change (LULUC) also results in emissions.
Of particular concern is soil disturbance and erosion caused by
LULUC activities including...[Show more] deforestation, tillage and
overgrazing. These activities can lead to a net loss of C from
the terrestrial biosphere leading to increased levels of
greenhouse gas in the atmosphere. This outcome is a major factor
widely understood to adversely affect the rate of global warming,
climate change and extreme weather events.
There has been a great deal of research undertaken to show that
LULUC activities can also lead to the sequestration and long term
storage of C in soil. LULUC practices that result in increases in
soil organic matter (SOM), humification and aggregation improve
soil quality, water quality, enhance food security and increase
biodiversity. The activities also result in the sequestration of
atmospheric CO2 as soil organic carbon (SOC) and therefore may
contribute to the reduction or stabilisation of atmospheric CO2
levels and mitigate the adverse effects of climate change. This
thesis describes the effects of two different LULUC activities on
the C sequestration potential of soil.
In this research two case studies have been used to explore three
principal aims. Firstly to identify the C sequestration potential
of agricultural soil following LUC; then to determine whether
there is a steady rate of SOC sequestration over time following
LUC, and finally to identify biogeochemical factors that
contribute to the C sequestration potential of soil after LUC.
The first case study was undertaken on the Southern Tablelands of
New South Wales (NSW), a temperate region with approximately 670
mm mean average rainfall (MAR). The study involved comparing the
soil C stock (SCS) to 30 cm depth, in agricultural land (AL)
principally used for grazing, with the SCS from 20 biodiverse
environmental plantings (BEP) established between one and 19
years prior to sampling. BEPs, otherwise referred to as mixed
species environmental plantings (MSEP), windbreaks or
shelterbelts, can be established using either a direct seeding
method or from tube-stock. The sites included in this study were
all directly seeded in either linear or block configurations
using a mix of native eucalypt and acacia trees and understory
shrubs. At each site three paired 10 m transects were established
on a tree line (TL), the inter-row (IR) and in the adjacent AL.
The AGB was determined for each TL transect. Three soil cores to
30 cm depth were taken from each transect and separated into 0-5,
5-10, 10-20 and 20-30 cm increments. The soil samples were
analysed for SOC, total nitrogen (TN), total phosphorus (TP), and
by using mid infrared spectroscopy (MIRS) analysis soil fraction
changes were also determined. The measured AGB and SCS results
were compared with FullCAM modelled data. The overall aim of this
case study was to determine whether BEPs sown into AL led to SOC
sequestration, and whether the rate of increase could be
predicted based on tree age and biomass.
The results of the BEP case study show that the average soil CS
in the AL based on an equivalent soil mass (ESM) was 41.1 Mg C
ha-1 (41.1 tonnes C ha-1). BEPs aged 11-15 years had the highest
SCS of 49.3 Mg C ha-1. The oldest BEP sites aged between 16 and
19 years had a lower SCS of 45.7 Mg C ha-1. Overall, the
average SCS rate of increase was 0.5 Mg C ha-1 yr-1. However, the
results show significant variation between sites and no trend
suggesting there is a temporal increase in SCS. The average AGB
for all sites was found to be 31.4 Mg C ha-1 with the range
between 0.2 Mg C ha-1 in a two year old site, and 97.0 Mg C ha-1
in a site aged 16 years. The average annual rate of AGB change
was found to be 2.4 Mg C ha-1 yr-1. The results show there is no
relationship between AGB and SCS. Therefore the prediction of
changes to SCS based on AGB increase alone is not possible. The
results also show that site factors and nutrient availability
have an influence on the SOC sequestration potential of the BEPs
and the sustainability of the SCS.
The second case study was undertaken in the semi-arid rangelands
of the Central West of NSW. The MAR of the region is
approximately 440 mm. The study examined whether waterponding, an
effective environmental restoration activity used to rehabilitate
scalded soil, would result in increases in SCS. The research
involved comparing the SCS of three scalded sites with 12
waterpond sites established between 1 and 27 years prior to
sampling. Nine soil cores to 30 cm depth were taken at each scald
site. Three waterponds were sampled at each of the 12 waterpond
sites. Nine soil cores to 30 cm depth were taken from three
positions (wall, mid and top) within in each waterpond. Each soil
core was separated into 0-5, 5-10, 10-20 and 20-30 cm increments.
Every sample was analysed for pH, electrical conductivity (EC),
SOC, TN, and total C. The results show that waterponds have a
significantly lower EC than scalded soil. This change occurs
because soluble salts are leached from the saline scald soil
profile after waterponding. The results also show that the
scalded soil has an average SCS to 30cm depth of 18.7 Mg C ha-1.
Waterponds aged five years had a SCS of 26.1 Mg C ha-1. This
represents a rate of increase of 1.5 Mg C ha-1yr-1. The 10 year
old and 25-27 year old waterponds have a slightly lower SCS of
25.3 and 24.9 Mg C ha-1. The probable reason for the apparent
decline in SCS in the older water ponds is associated with the
lower bulk density found in these older waterponds and the
increased presence of SOM.
A number of edaphic factors were found to influence the potential
of the scalds to sequester SOC. To make such assessments within
research resource constraints three different subsets of the soil
samples were compiled. Initially, a sub-set of 144 samples
including one scald, and waterponds aged 1 year, 5 years and 25
years since establishment were analysed for aggregate stability,
TP, total sulfur (TS), and available P (AP). The results from
this subset were used to identify temporal changes to soil
stability and to ascertain whether these soil nutrient
concentrations were affected by waterponding or influenced the
SOC sequestration potential of the soils. The aggregate stability
results show there is a temporal change in the dispersability of
the soils following waterponding. Scalded soils are stable
because of the high concentration of soluble salts. After the
salts are leached from the profile the soils become more
susceptible to dispersion. The results also indicate that total
N, P and S is not limiting, but AP is limiting ongoing
sequestration of C in the waterponds.
A smaller subset of samples made up of one scald core and a
paired 27 year old waterpond core were analysed for cation
exchange capacity, x-ray diffraction and x-ray fluorescence to
identify any mineralogical differences that may be attributable
to waterponding. Calcite was found to be leached from the scald
profile, hematite increased due to iron oxide formation, gypsum,
kaolinite, illite and vermiculite was higher in the waterponded
soil than the scalded soil whereas smectite was lower in the
waterponds. These differences are most likely due to weathering
processes and aeolian deposition.
Across the scalded soil surface there are occasional hummocks of
vegetation. Cesium-137 analysis was undertaken to determine
whether the hummocks were recent deposits or relics of the
original soil profile. For this analysis composite samples were
made up from the three scald sites, two hummock sites and
waterponds aged between 25 and 27 years since establishment. The
results from this small study indicate the hummocks are most
likely to be recent deposits of aeolian material.
The experimental results from this project show that LULUC can
lead to an increase in SCS. The higher SCS found in the BEPs may
not be sustainable because the oldest sites show a lower SCS than
the younger sites. The results suggest reasons for the decline
are most likely due to site factors and lower levels of nutrient
availability. The SCS sequestered in the scalded soil
rehabilitated by waterponding however, is likely to have been
sequestered permanently so long as the rehabilitated landscape is
managed sustainably.
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