PICCC PhD graduate: Rachelle Meyer
Rachelle recently completed her PhD at UM, supervised by Richard Eckard and Brendan Cullen. Rachelle's thesis explored the use of a whole-farm system modelling approach to quantify the agro-ecosystem benefits and mitigation implications of soil carbon in grazing systems of western Victoria in both recent and future climates.
Rachelle has published two papers on this topic, and provided the following summary:
We investigated both the on-farm benefits and the net greenhouse gas balances of maintaining high levels of soil organic matter (SOM) using a whole-farm system modelling. We looked at two pasture scenarios: high SOM, where initial carbon amount was similar to long-term pasture; and low SOM, where initial carbon condition was similar to long-term cropping with annual cultivation.
The analysis compared mineralisation, plant-available water-holding capacity (PAWHC), associated productivity and net greenhouse gas balances between the high- and low-SOM soils with the same management, each in two climatic zones and with two soil types.
Some of the key findings included:
- On high-rainfall sites, annual pasture production in high-SOM soil was 590–900 kg DM/ha greater than for low-SOM soil, mainly due to increased nitrogen mineralisation overcoming a nitrogen limitation in spring.
- On low-rainfall sites, a reduction in annual pasture production of 290–810 kg DM/ha on low-SOM soils compared to high-SOM soils was due to reduced PAWHC.
- The increased pasture production associated with higher SOM was valued between $26 and $95/ha, across the soils and sites.
- The value of maintaining high SOM on low-rainfall sites ($26-$85) was attributable to differences in PAWHC, while on high-rainfall sites, increased pasture production was attributed to nitrogen mineralisation valued from $85–$105/ha.
- Because of greater pasture productivity, and consequent higher sheep stocking rates, high-rainfall sites were associated with greater livestock-related greenhouse gas emissions that could not be offset by carbon sequestration.
- On these high-rainfall sites, the higher rate of soil organic carbon (SOC) increase on sites modelled with low initial SOC offset an average of 45% of the livestock emissions on the modelled chromosol and 32% on the modelled vertosol, compared to only 2–4% of emissions on high-SOC soils.
- On low-rainfall sites, carbon sequestration in low-SOC soils more than offset livestock emissions, whereas the modelled high-SOC soils offset 75–86% of emissions.
- Greater net emissions on high-SOC soils were due primarily to reduced sequestration potential and greater nitrous oxide emissions from nitrogen mineralisation and livestock urine.
These results indicate that soil carbon sequestration, through increased SOM, can provide substantial on-farm benefits that contribute to future productivity. However, as SOM increases there is a trade-off between diminishing greenhouse gas offsets and increasing ecosystem services, including mineralisation and productivity benefits.
Read more about Rachelle’s research in her published papers: