Is land condition a useful indicator of soil organic carbon stock in Australia’s northern grazing land?

Abstract

The grazing lands of northern Australia contain a substantial soil organic carbon (SOC) stock due to the large land area. Manipulating SOC stocks through grazing management has been presented as an option to offset national greenhouse gas emissions from agriculture and other industries. However, research into the response of SOC stocks to a range of management activities has variously shown positive, negative or negligible change. This uncertainty in predicting change in SOC stocks represents high project risk for government and industry in relation to SOC sequestration programs. In this paper, we seek to address the uncertainty in SOC stock prediction by assessing relationships between SOC stocks and grazing land condition indicators. We reviewed the literature to identify land condition indicators for analysis and tested relationships between identified land condition indicators and SOC stock using data from a paired-site sampling experiment (10 sites). We subsequently collated SOC stock datasets at two scales (quadrat and paddock) from across northern Australia (329 sites) to compare with the findings of the paired-site sampling experiment with the aim of identifying the land condition indicators that had the strongest relationship with SOC stock. The land condition indicators most closely correlated with SOC stocks across datasets and analysis scales were tree basal area, tree canopy cover, ground cover, pasture biomass and the density of perennial grass tussocks. In combination with soil type, these indicators accounted for up to 42% of the variation in the residuals after climate effects were removed. However, we found that responses often interacted with soil type, adding complexity and increasing the uncertainty associated with predicting SOC stock change at any particular location. We recommend that caution be exercised when considering SOC offset projects in northern Australian grazing lands due to the risk of incorrectly predicting changes in SOC stocks with change in land condition indicators and management activities for a particular paddock or property. Despite the uncertainty for generating SOC sequestration income, undertaking management activities to improve land condition is likely to have desirable complementary benefits such as improving productivity and profitability as well as reducing adverse environmental impact.

Sustainability of beef production from brigalow lands after cultivation and mining. 3. Pasture rundown, climate and grazing pressure effects

“…The difficulty of adopting a grazing management regime in a variable climate that achieves productive returns while avoiding loss of productivity through overgrazing is well documented (McKeon et al 2004;McIvor 2010). Loss of pasture condition (and thus, productivity) is a frequent occurrence in the Darling Downs region and, more generally, in northern Australia (Tothill and Gillies 1992;Bortolussi et al 2005b;Bray et al 2016). Examples in the present study are the pastures in C and D condition at the BMK 10 and 11 sites and the Roundview and Mirrabooka paddocks (Table 4).…”

“…It is also assumed that improvements in productivity do not occur. Such improvement may occur by rebuilding soil fertility through the contribution of pastures to soil organic matter (Partridge et al 2009;Sanderman et al 2010;Clewett 2015;Bray et al 2016) and, particularly, through use of both summer-and winteractive legumes (Peck et al 2011;Paton and Clewett 2016;Whish 2017). Further development of GRASP to more adequately represent legume-based pastures, soil N availability and changes in pasture quality would be useful.…”

Incentivising fire management in Pindan (Acacia shrubland): A proposed fuel type for Australia's Savanna burning greenhouse gas emissions abatement methodology

“…) and possibly maintaining top‐soil pools (Bray et al . ). The technical feasibility of similar fire‐mediated abatement and sequestration accounting methods in arid rangelands has been demonstrated but requires further development (Burrows ).…”

“…Savanna burning projects offer economic diversification opportunities for pastoral enterprises, Indigenous land managers and the formal conservation estate (National Parks, Indigenous Protected Areas) (Walton et al 2014), as well as a variety of significant environmental, social and cultural benefits (Russell-Smith et al 2013, 2015bRobinson et al 2016). In future, it is likely that additional Savanna burning methodologies will become available, based on carbon sequestration through longer-term accumulation of coarse woody debris , increasing living tree biomass (Murphy et al 2010) and possibly maintaining top-soil pools (Bray et al 2016). The technical feasibility of similar fire-mediated abatement and sequestration accounting methods in arid rangelands has been demonstrated but requires further development (Burrows 2014).…”

Accounting for liabilities related to ecosystem degradation

“…The ABCD land condition classifications for northern rangelands are designed in relation to flows of provisioning services for livestock (i.e., forage), and only provide an indication of regulating services such as soil retention services (to limit soil erosion) and habitat services (to support conservation of biodiversity) (Parsons et al 2017). The condition measures are not designed to indicate the capacity for carbon storage (Bray et al 2016) or cultural services. Nonetheless it would be expected that land in A and B condition under this classification system can be regarded as providing more regulating and cultural services than land in C or D condition.…”