ENERGY USE AND CARBON EMISSIONS IN NNSW / SQLD FARMING SYSTEMS - ARE WE AHEAD OR BEHIND?

| Date: 15 Sep 2010

Farming has always essentially been about carbon farming. We estimate that over a 60-year period of cereal cropping under continuous cultivation, soils of NNSW and SQld have lost over 40 t C/ha (146.7 t CO2-e) and 4 t N/ha, valued at $2933 CO2-e/ha ($20/t), and $3200 for N replaced at $800/t, that is, about $100/ha/y for C and N alone.  Assuming 1% emission factor for N2O (see below), almost 40 kg N/ha or 63 kg N2O/ha or 18.7 t CO2-e/ha would have been emitted from soil. Thus, on average 2.75 t CO2-e/ha/y or 750 kg C/ha/y was released into the atmosphere over the 60-year period.

We present management options to increase C sequestration and reduce N2O and CH4 emissions from farming so as to reduce the gap between energy input and energy output, expressed as CO2-e (75 g ≡ 1 MJ).

What do we know about the effectiveness of management options?
From conventional till to reduced till and no-till.  Northern grains region data suggest that in most cropping soils there is similar C input and hence limited or no additional C sequestration (Dalal and Chan 2001; Thomas et al. 2008; Young et al. 2009). However, it appears that in higher rainfall areas (>700 mm) a potential may exist to increase C sequestration in soil under no-till, especially under intensive, response or opportunity cropping (Young et al. 2009, Liverpool Plains; David Lester data from Colonsay trial, SQld).

Ram-1 

Ram-2

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Figure 2: Life Cycle Analysis of CT and NT practices show significant CO2-e contribution to the atmosphere and therefore negative energy balance. Nitrogen fertiliser pre-farm (manufacture and transport) and on-farm (urea hydrolysis and N2O) is the major contributor to CO2-e emissions. Note small benefits from NT practice, mainly from lower amount of diesel use.
 
From continuous cropping to ley pastures. Ley pastures, especially grass-legume pasture increases soil C, around 0.5 t C/ha/year or higher, during the pasture phase, but during the cropping phase soil C decreases again. However, the soil C values may still be higher than from continuous cropping. Moreover, there may also be reduction in total GHG emissions from replacement of synthetic N fertiliser (from manufacture and transport).

Ram-4

Figure 3: Pasture (legume+grass) increases soil carbon during the ley phase but most of this is lost during the cropping phase. Young et al. (2009) data from Liverpool Plains show that NT practice slows soil carbon loss. Increasing the pasture phase and shortening the cropping phase may lead to net carbon gains to the soil.

Table 3: Pasture-crop system (equivalent of 20y pasture -20y crop). Negative value signifies net GHG sequestration and positive value signifies GHG emissions. At $20 CO2-e/t, $2/ha/y is apparent benefit

Ram-5

Assuming 10 t C/ha sequestered under pasture for up to 20 years (0.5 t C/ha/year measured by Hossain et al. 1996 and Dalal et al. 1995 in short-term pasture), this is an optimistic C sequestration since C loss in soil in the cropping phase is not allowed for, assuming NT practice (Young et al. 2009);
BN applications of 100 kg N/ha for grain crops; the N2O emission factor of 1, and GWP of N2O is 298; about 60% CO2-e/ha emitted during manufacture and transport of fertiliser;
CCH4 emissions from grazing animals (0.5 animal (450 kg)/ha/y for 20 years, and 70 kg CH4 emission/animal using IPCC default value, with GWP of CH4 of 25).

From cropping to permanent pasture. We have found that land use change from cropping to permanent pasture increases soil C for up to 35 years and may eventually attain C values similar to the soil under native vegetation or even higher if nutrient limitation is also removed (such as N and/or P, Zn, S).
 

Ram-6

From cropping to afforestation. Increases in soil C have been observed but may not be economic even though timber and tree biomass should additionally provide a useful economic product. Emissions Trading Scheme may provide additional incentives.

From ley pasture to permanent pasture. Usually this transition results in continuous increase in soil C sequestration by eliminating the cropping phase.

Use of manures and sources of waste organic materials. Since manures are high in lignin, some manure C is sequestered in a slow pool of soil C and thus resides in soil longer than the labile crop residue C.

Biochar application to soil. Biochar C persists in soil much longer than the crop residue C, and it may have a place in sandy soils for increasing cation exchange capacity to retain fertiliser nutrients but the net energy use requires further analysis.

Call for action
A large shift in farming systems will be required if most of the carbon sequestration potential is to be realised, that is, a shift from annual to perennial cropping/pasture and afforestation, which may not be economically attractive in the short to medium term.

References
• Dalal RC, Chan KY (2001) Soil organic matter in rain fed cropping systems of the Australian Cereal Belt. Aust. J. of Soil Res. 39, 435-464.
• Farquharson RJ, Schwenke GD, Mullen JD (2003) Should we manage soil organic carbon in Vertosols in the northern grains region of Australia? Aust. J Exp. Ag 43, 261-270.
• Thomas GA, Dalal RC, Standley J (2007) No-till effects on organic matter, pH, cation exchange capacity, and nutrient distribution in a Luvisol in the semi-arid subtropics. Soil and Tillage Res. 94, 295-304.
• Wang W, Dalal RC, Reeves S (2009) Nitrous oxide emissions from cereal cropping systems. Final Report to the Dept. of Climate Change, Comm. of Aust., Oct. 2009.
• Young R R, Wilson BB, Harden S, Bernardi A (2009) Accumulation of soil carbon under zero tillage cropping and perennial vegetation on the Liverpool plains, eastern Australia. Aust. J. of Soil Res. 47, 273-285.

Contact details

Ram Dalal and Weijin Wang
Department of Environment and Resource Management
80 Meiers Road, Indooroopilly, Queensland 4068
Phone: 07 3896 9895
Fax: 07 3896 9591
Email: Ram.Dalal@derm.qld.gov.au  or Weijin.Wang@derm.qld.gov.au