TECHNICAL NOTE
July 2015
Pj. AOTGR1-167
Boosting Soil Carbon, Nth Vic
Rationale for repeat of final sampling
Sequestering Soil Carbon in an Irrigated Landscape turned Dry Ecological Grazing
This Action on the Ground project* is demonstrating on-farm practices that aim to boost
carbon in the soil. These practices are based around the conversion of previous flood irrigated
cropping land to dryland cell grazing on native forage in conjunction with protected
biodiverse vegetation. The project is located near Lake Boga in northern Victoria.
This note examines a technical problem associated with the final project in October 2014, and
the justification for resampling in July 2015.
Compromised samples from the October 2014 event
Initial analysis of the results of the October 2014 sampling indicated that soil carbon densities
(stocks) were on average markedly less than in 2012. Closer inspection of the data showed
that much of the difference could be explained by lower (measured) bulk density (BD) of the
2014 samples. While not recognised as a problem during the sampling, it subsequently
became apparent that the dryness of the soil profile to depth had resulted in substantial
disintegration of soil cores during sampling; and consequently re-sorting and loss of soil bulk
across the sample depth intervals. Moreover, these resorting and losses would likely have also
affected soil C concentrations.
Initial comparison of the 2012 and 2014 data indicated that on average there was a
significant loss of soil carbon stock across the trial paddocks. This is illustrated in Figure 1 for
0-30 cm depth. The apparent losses in some paddocks of up to 10-20 Mg/ha is beyond
reasonable expectation. Scrutiny of the 2014 soil BD revealed values being on average 83%
(0-10 cm) and 91% (10-30 cm) lower than in 2012 (Figure 2). The difference cannot be
attributed to any paddock management effects (i.e. the paddocks are not cultivated).
1.6
0-10 cm
10-30 cm
BD 2014 (Mg/m3)
1.4
1.2
1.0
0.8
0.6
0.6
Figure 1. Comparison of the 2012 and 2014 soil
carbon densities for the 9 sampled paddocks
Boosting Soil Carbon in Northern Victoria
0.8
1.0
1.2
BD 2012 (Mg/m3)
1.4
1.6
Figure 2. Comparison of the 2012 and 2014 soil bulk
densities for the 9 sampled paddocks
Jul 2015
Closer interrogation of the 2014 sampling event revealed that collecting 100% intact
cores was challenging in the very dry conditions at the time of sampling. This was
despite it being only late October. As illustrated in the example in Plate 2 (compared to
Plate 1 from 2012), dryness of the soil profile resulted in substantial disintegration of the
soil core during vibration in the mechanical corer upon penetration into the soil. This has
led to re-sorting and loss of soil from across the sampling depth intervals in a significant
number of the cores.
Plate 1: Example 2012 core. Note the degree of
coherence of the core
Plate 2: Example 2014 core. Note the disintegration
of the core, largely attributable to the dry soil
condition
The case for resampling
The project team subsequently investigated the data in detail to see whether any meaningful
correction, and therefore useful observation, could be made. It was concluded that a full
resampling was required - under appropriate soil moisture condition - to obtain data that can
be used to accurately calculate current soil carbon stocks in all paddocks. However, it is
noted that irrespective of the new results it remains unlikely that any change in soil C stocks
since 2012 will be able to be statistically demonstrated in the short time interval between
samplings (refer Technical Note 1).
Justification for a July 2015 resampling
A resampling in late July 2015 is planned as it balances the needs of project delivery
and budget, as well adequate soil moisture conditions. A July resampling is three
months earlier than a later sampling that would maintain seasonal consistency with the
2012 sampling. However, there is little documented evidence of seasonal variation in
total soil carbon stocks in uncultivated, non-cropped soils.
With the insights gained during the course of this project the proposition held now is that
the 2012 and 2015 samplings will together provide a baseline for sequestration to be
measured over the next 5 or 10 years. This approach reflects the reality of the slow
accrual of soil carbon in semi-arid environments and the time that it will take to
statistically demonstrate change.
Very significantly the current strong El Nino phase of the climate system and its
implication for increased odds for warmer and drier weather across SE Australia means
that there is a real risk of inadequate soil moisture to support a 2015 spring sampling. The
project concludes in February 2016.
Boosting Soil Carbon in Northern Victoria
Jul 2015
For further information:
David Heislers, Kilter Rural
0439 654 066
[email protected]
Supported by:
Project Website: http://www.nccma.vic.gov.au/Land/Dryland/BoostingSoilCarbonKilter/index.aspx
*This project is supported by funding from the Australian Government Department of Agriculture, as part of
its Carbon Farming Futures Action on the Ground program.
Boosting Soil Carbon in Northern Victoria
Jul 2015