Science behind build-up

Sound science behind soil carbon build-up


On Farm
Dr Susan Orgill, of NSW Department of Primary Industries, found the better a pasture is the higher organic carbon is likely to be in a recent study.

Dr Susan Orgill, of NSW Department of Primary Industries, found the better a pasture is the higher organic carbon is likely to be in a recent study.

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Bob Freebairn says soil organic carbon (a major part of organic matter) is largely influenced by pasture productivity. The better the pasture the higher organic carbon is likely to be.

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Soil organic carbon (a major part of organic matter) is largely influenced by pasture productivity.

The better the pasture the higher organic carbon (OC) is likely to be.

This seems to me to be the essence of the paper “Parent material and climate affect soil organic carbon fractions under pastures in south-eastern Australia” published in the CSIRO Soil Research journal.

It notes how soil type and climate are large determinates of soil OC, largely because of their influence on production.

Fertiliser use (soil fertility) is far more critical than management issues like grazing methodology. 

Dr Susan Orgill NSW Department of Primary Industries, Wagga Wagga Agricultural Institute and Graham Centre, Charles Sturt University, was senior author of the study. Other researchers involved were Jason Condon (CSU Wagga) Mark Conyers (now retired DPI research agronomist) Stephen Morris (DPI), Brian Murphy (NSW Office of Environment and Heritage, Cowra) and Richard. Greene (Australian National University, Canberra).

There were 72 sites sampled to assess the effect of climate (Monaro, Boorowa and Coleambally regions) and parent material (Monaro region; basalt and granite) on OC under perennial pastures.

In the higher-rainfall zone (Monaro and Boorowa; over 500mm mean annual rainfall), OC stocks under introduced and native perennials were compared.

In the lower-rainfall zone (Coleambally; under 500mm annual rainfall) OC stocks under crops and pastures were compared.

Carbon fractions assessed in the study were total OC (TOC), particulate OC (POC), resistant OC (ROC) and humic OC (HUM). 

Higher OC stocks were associated with higher spring and summer rainfall and lower annual temperatures.

Within a climatic zone, parent material affected the level of OC in the 0–30cm soil layer.

Within a climatic zone, when grouped by parent material, there was no difference in OC stock with vegetation type. TOC was also positively correlated with fertility, especially in the higher rainfall areas, including available sulphur and nitrogen, indicating importance of elements like them to maximise production. 

There were good correlations between soil factors related to parent material and OC concentration.

Positive correlations existed between cation exchange capacity. Basalt derived soil had a greater stock of OC for the 0–10 and 0–30cm soil layers compared with all other parent material. Deep granite-derived soil (Monaro) had greater levels of OC in the 0–10 and 0–30cm soil layers compared with shallow granite. 

When grouped by parent material, there was no difference in OC with native versus introduced pastures in the Monaro and Boorowa regions or pastures versus crops in the Coleambally region.

Total OC decreased with increased temperature. Large differences in OC occurred when annual rainfall was compared, higher OC associated with higher rainfall.

The paper notes that over the past 20 years research has focused on management options to increase OC permanence stocks.

In particular, pastures in the Australian south-eastern agricultural zone offer a considerable opportunity for OC accumulation in agricultural soil. 

Other results from these studies highlight the variable and often little response of OC to pasture management; notably, there was little difference in OC under annual versus perennial, introduced versus native and rotationally versus continuously grazed pasture.

An example of management increasing OC in soil was where phosphorus applied to introduced perennial pastures increased OC stocks by 0.26 - 0.72t/C/ha/year. 

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