A presentation discussing spatial patterns of CO2 fluxes across litter amended, non-amended and native soils on cotton farms in southern NSW by Dr Jackie Webb, Deakin University, to Soil Science Australia's Riverina branch workshop on "Parna and Cotton Soils" held at Yanco on 29 April 2021.

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@JackieRWebb
Jackie R. Webb*, Rakesh Awale, Wendy Quayle
Centre for Regional and Rural Futures (CeRRF)
Faculty of Science, Engineering and Built Environment
Deakin University,
Research Station Road, Griffith,
NSW 2680, Australia
*j.webb@deakin.edu.au
Spatial patterns of CO2 fluxes across litter
amended, non-amended, and native soils on
cotton farms in southern NSW

2.  Poultry litter (PL) is becoming a widely adopted soil amendment by southern
cotton growers 28-84%
 PL provides a substantial OM source
 Manure amendments can affect organic C pools and alter decomposition rates
- mixed results on C accumulation
 Australian cotton industry developing sustainability targets to reduce their C
footprint - management strategies for on-farm carbon neutrality.
Australian Cotton sustainability report 2019

3.  Net soil CO2 flux is a measure of plant respiration, microbial respiration,
and abiotic processes.
 Indicator of current soil C mineralisation and organic matter decomposition
 Measure is sensitive to changes in environment e.g. disturbance, plant
cover, climate
Abiotic
(chemical
weathering)
Net CO2
flux
OM
decomposition Root
activity

4. Aim
To capture the spatial variability of soil CO2 flux in southern cotton
soils.
 i) determine the environmental drivers of soil CO2 flux to gain
insights on factors driving organic C decomposition
 ii) assess if cultivation and the practice of PL amendment has had a
legacy effect on soil C and microbial indicators across the region.

5. Method
 Survey of soils across 10 farms, spanning three land management types
 Targeted bare soil to avoid plant respiration
 3 reps, 2 repeated incubations = 180 CO2 flux measurements
Native Poultry litter Conventional

6. Process
LI-COR LI-8100A survey system
Install collars 1 day prior
60s incubations to capture
rate of CO2 ppm change
Collar
Temp &
moisture
probe
Survey chamber
Closed loop
air flow
Portable infrared gas
CO2/H2O analyser

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Range in soil CO2 flux
RESULTS

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RESULTS
LMEM results – drivers of CO2 flux variability
1. Manganese: microbial Mn(II) oxidation which facilitates carbon oxidation and lignin breakdown.
2. DOC/TIN: microbes limited by soluble carbon substrates. High inorganic N limits microbial activity
3. Soil moisture: weakly correlated, no temperature effect.
Soil C pools across most sites were in an advanced state of decay

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 Negative flux (-0.3 µmol m-2 s-1) observed
at a non-amended site
 Significant abiotic contribution to net CO2
flux.
 Usually undetected because microbial
respiration
 High pH (8.55) and inorganic C content
(0.27 %).
 Carbonate weathering
EVIDENCE OF A SOIL CO2 SINK

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 More biological activity in native soils
compared to no-litter
 May change depending on time of
season sampled - moisture
 Less microbial abundance in cultivated
soils – may reflect the low C availability
RESULTS

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 PL improves soil Mn levels back to
natural levels
 Essential micronutrient for cotton
and likely plays a role in OC
stabilisation
 Cultivation has increased soil pH
 mechanical uplifting of alkaline subsoil,
less OM?

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CARBON LIMITATION
 Both mineralisable and total OC
depleted in cultivated soils
 PL sites showed a small improvement
 Farms with a long (>10 years) PL
history maintained natural OC levels
 Higher inorganic N inputs, soil
oxidation over extended fallow, residue
burning.

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 Drivers of CO2 flux indicated that soil carbon cycling was in an advanced
state of decay – cultivated limited in C
 Likely old and mineral associated OM – Mn plays a key role, especially in
PL fields
 Fresh organic carbon likely lost due to prolonged exposure in fallow
fields.
 Found evidence for abiotic CO2 update in agricultural soils – further
study?
 Future research - the combined effect of soil conservation practices with
PL use to improve soil OC.
CONCLUSIONS

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Questions?
Dr Jackie Webb
•0417 634 434
j.webb@deakin.edu.au
Acknowledgements
Cotton growers, IREC, CottonInfo, Deakin University,
Australian Government Department of Agriculture
NLP, and CRDC grant “Optimising the management
of manure in southern NSW cotton production II”

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Jones, M. E., Nico, P. S., Ying, S., Regier, T., Thieme, J., & Keiluweit, M. (2018).
Manganese-Driven Carbon Oxidation at Oxic–Anoxic Interfaces. Environmental Science &
Technology, 52(21), 12349-12357. doi: 10.1021/acs.est.8b03791