- Improved grazing practices have the potential to sequester carbon and can have significant benefits at local, national and international level.
- The mechanisms of soil carbon sequestration under improved grazing are well established
- Associated N2O emissions and loss of soil carbon due to priming do occur but there is still significant net sequestration.
Soil Carbon and Grazing Lands
Estimates of globally terrestrial carbon stocks (carbon stored in soils) is typically reported to be in the order of 1500–1600 Pg C which is 3 times the size of carbon stored in vegetation (~560 Pg C) (Johnston & Groffman, 2004) and 2 times the size of the carbon in the atmosphere (~770 Pg C). These estimates are for the first 300 or 1000 mm of soil depth which are generally assumed to contain the bulk of the carbon. Estimates of deeper soil carbon add an additional 33% in the 1-2 m zone and a further 23% to 2-3m zone taking the total estimated storage in soil to 3 m to 2344 Pg C (Jobbágy & Jackson, 2000) which is more than 3 time the carbon in the atmosphere.
Grazing lands  occupy around 3.5 billion hectares which is 26% of global land area and 70% of global agricultural land (Follett & Reed, 2010) (Conant, 2010). It is estimated that soil in grazing lands sequester around 0.7 Pg CO2y-1 (191 Tg Cy-1) or about 20-25% of the total carbon sequestered in all soils (Lal, 2011) (Follett & Reed, 2010)(Conant, 2010).
Historic land use and soil degradation, particularly conversion of native vegetation to agriculture, has resulted in the loss of 25 to 75% of original soil carbon in many areas of Australia, estimated to be equivalent to around 286 Tg CO2 ( reported as 78 Gt of C).
Within Australia grazing lands occupy approximately 444 million hectares or 57% of the total land area and more than 90% of the agricultural land (Sanderman, Farquharson, & Baldock, 2010). Dean (Dean, Wardell-Johnson, & Harper, 2012) using the land use definition of “rangeland” estimate 661Mha of rangeland of which 369 is occupied by commercial livestock properties.
Restorative agricultural practices across all agricultural land ( cropping, grazing and rangelands) is estimated to have the potential to sequester 1.8-4.5 Tg Cy-1( reported as 1.2–3.1 billion tons Cy-1) (Lal, 2011). Estimates of the scale of the net benefit of improved management of agricultural land range from 5 to 14% of total emission.
Estimates of potential sequestration in grazing land vary depending on assumptions:
- 27 Tg Cy-1in grazing lands (Cosier, Flannery, Harding, & Karoly, 2009)
- The Garnaut Review (Garnaut, 2011) quotes CSIRO estimates that rehabilitating 200 million hectares of overgrazed rangelands could have a technical potential to sequester 100 Tg CO2ey-1 (27 Tg Cy-1) between 2010 and 2050.
- The Garnote Report estimates 77 Tg C y-1 (286 MtCO2y-1) over 358 million hectares of degraded land for 20 to 50 years at 0.2 C ha-1 y-1 although some of the assumption in this estimates are challenged by other analysis
- Globally rehabilitation of overgrazed grasslands can sequester approximately 45 Tg C y-1, most of which can be achieved simply by cessation of overgrazing and implementation of moderate grazing intensity. Within this estimate 4.5 Tg Cy-1is in the Asia pacific region (Conant & Paustian, 2002).
There remains considerable uncertainty in soil carbon stock estimates and sequestration potential and there are limitations in the extrapolations inherent in meta-analysis studies of global sequestration potential. However, while the change in soil carbon per unit area may be small and uncertain the total area is so large such that even conservative estimates show the potential of soil carbon sequestration as a significant contribution to national and global carbon budgets and efforts to address climate change.
There is a paucity of long terms paired studies of potential soil carbon improvements under grazing systems and very limited data on well managed Cell Grazing Systems. This is compounded by the heterogeneous nature of grazing land and grazing management practices. However, a significant number of studies do provide strong evidence that properly managed grazing can positively affect soil carbon stocks.
- Conant ( 2002) estimates rates of sequestration across Australia to be between -5 to 1 Mg C ha-1 y-1 with rates of increase of 0.5 to 1 Mg C ha-1 y-1 in favourable sites covering much of the northern parts of Australia.
- Unpublished paired samples undertaken by CarbonLink of cell and continuous grazing properties indicate an average annual sequestration of 0.7 Mg C ha-1 y-1 in the top 15cm.
- The Chicago Climate Exchange use 0.3 to 1 Mg C ha-1 y-1 as well as providing correlation to sequestration rates with annual precipitation and differentiated risk and insurance factors in various regions.
- Studies of improved pasture and changes to grazing management for a number of studies as reported by Sanderman et al ( 2010) suggest a sequestration rate in the range of 0.1 to 3 Mg C ha-1 y-1 though generally <1 Mg C ha-1 y-1 and averaging 0.5 Mg C ha-1 y-1. The estimates are qualified by comments on a paucity of specific field trials and confounding impacts of soil, climate and management practices.
- Lal (2011) reports sequestration rates of between 0.05 to 1.5 Mg Cha-1 y-1r for various types of recommended agricultural management practices.
- NSW DPI summarise available data to give a range of sequestration rates of -0.25 to 1.3 Mg C ha-1 y-1 (Chan, Cowie, Kelly, Singh, & Slavich, 2008).
- Estimates of conversion of crop land to grassland 1.44 Mg C ha-1 y-1 in Europe and 0.64 Mg C ha-1 y-1 (144 g C/m2/y) in the USA (Abberton, Conant, & Caterina, 2010).
- In a study to support the development of sequestration factors for IPCC Good Practice Guide which reviewed available data in a number of areas around the world Ogle (Ogle, Conant, & Paustian, 2004) derived sequestration rates for conservative grazing to be in the range 0.1 to 0.9 Mg C ha-1 y-1 covering low, medium and high levels of intervention.
- Conant identifies improved grazing management as a major opportunity to sequester carbon and estimates an average sequestration rate across all potential improved grazing practices of 0.35 Mg C ha-1 y-1(Conant, 2010)
Base on the studies listed above a conservative estimate of the expected rates of sequestration applicable to Australia appears to be around 0.5 to 1 Mg C ha-1 y-1. However, the listed studies provide no guarantee and it is recognised that sequestration may be higher or lower and will depend on local soil type, climate and management practices.
Duration and saturation
The time period for which sequestration can be expected to continue, the total change in carbon stocks and the final saturation or stabilisation carbon level are all dependant on climate, soil and management practice (West & Six, 2006). Total changes in soil carbon range from a few percent of initial carbon in cold temperate moist environments to over 100% in tropical locations with expected sequestration durations of between 25 and 45 years (West & Six, 2006). Significantly longer sequestration periods have been observed in other landscapes and management systems (Batjes, 1999).
The concept of saturation of carbon in soil – a maximum level of soil carbon achievable – is discussed by several authors(Stewart, Paustian, Conant, Plante, & Six, 2007)(West & Six, 2006)(Sanderman et al., 2010). That there is a limit to the amount of carbon that soil can contain appears logical but specifying such a limit is more problematic. It is not clear whether equilibrium, steady state or saturation are appropriate to describe soil carbon levels over time (West & Six, 2006). The quantity of carbon in soil is based on the dynamics between inputs and losses. Changes in soil, climate, management or other environmental factors can affect soil carbon levels. These impacts can occur at multiple time scales and in a three dimensional environment which may also gain or lose mass and volume over time. While a particular soil stratum may achieve equilibrium there could be ongoing changes in other stratum. Accordingly it is difficult to quantify a saturation level across a larger parcel of land. However, within in the context of a carbon accounting framework it is likely that a point will be reached where the rate of sequestration will decline to the point that it is no longer cost effective to continue further sampling in order to quantify ongoing changes. The expected time period for this to occur is 25 to 40 years.
How grazing effects soil carbon stocks.
The term grazing covers a wide variety of practices from nomadic herds moving through native vegetation and rangelands to intensively managed pastures and livestock. Grazing is one of the most widely practiced agricultural activities occurring in over half of the global lands and over one third of the above and below ground carbon stores ( from (Han et al., 2008) citing Allen Dais 1996).
A large portion of the literature discussing the impacts of grazing provides relatively simplistic classifications of grazing intensity ( eg heavy, medium, light) and does not quantify important factors such as the stage of growth of various pasture species, precent of biomass removed, the intensity or number of grazers per unit area and the length of rest between grazing events if any, all of which can affect pasture growth and productivity. There is however ample evidence that excessive grazing pressure results in a decrease in productivity. Less commonly recognised is that there is a level of grazing that is both sustainable and may in fact have higher Net Primary Production then ungrazed pasture (McNaughton, 1979).
There is a considerable body of research on the impacts of grazing on soil carbon. These include overviews of potential carbon sequestration in agricultural systems (for example Follett & Reed, 2010; Lal, 2011; Sanderman et al., 2010; Slavich, 2008), and numerous studies of specific mechanisms (Conant & Paustian, 2002). Pasture species have several characteristics that support, and even require, grazing. Perhaps not surprisingly, there is evidence of stepwise reciprocal adaptations in grass and grazers ((Herrera, 1982) citing (Janzen, 1980)) suggesting co evolution. Recognising this McNaughton (McNaughton, 1979) coined the term “obligate grazophils” meaning there are species of grasses that have evolved to be more productive when grazed.
The broad conclusions of the literature on grazing impacts are that:
- Grazing can have negative, neutral or positive impacts on soil carbon.
- The impacts are determined by the management system and the biophysical constraints of the area being grazed.
- The key mechanisms that result in soil carbon increases include:
- Increases in the net primary production that increases the carbon inputs into the soil.
- Changes in the partitioning of above ground and below ground biomass with an emphasis on benefits of deep rooted perennial pasture species.
- Increased photosynthetic rates in residual tissue.
- Reallocation of substrates elsewhere in the plant.
- Removal of older tissue that is functioning at below maximum photosynthetic levels.
- Consequent increase of light intensity on potentially more active underlying tissue.
- Reduction in leaf senescence thus prolonging the active photosynthetic period of residual tissue.
- Hormonal redistribution promoting cell division and elongation and activation of remaining meristems, thus resulting in more rapid leaf growth and promotion of tillering.
- Enhanced conservation of soil moisture by reduction in the transpiration surface and reduction of mesophyll resistance relative to stomatal resistance.
- Nutrient recycling through dung and urine.
- Direct effects of growth promoting substrates in ruminant saliva.
- Reduction in conditions that result in carbon loss in particular bare ground and over grazing.
- Structural changes, both physical and chemical at multiple scales and depths that that result in longer residence times of carbon in the soil profile.
 Including both rangelands and pasture, scrubland and cropland sown with pasture
 Referenced in (Chan et al., 2008)
 This is based on Chicago Climate Exchange sequestration rate.
 negative means carbon emissions