Unexpected increases in soil carbon eventually fell in low rainfall farming systems

Area of Science:

• Agricultural Science
• Soil Science
• Environmental Science

Background:

• Understanding soil organic carbon (SOC) dynamics is crucial for effective carbon trading schemes.
• Predicting SOC changes requires accurate data on temporal variations and influencing factors.

Purpose of the Study:

• To investigate temporal changes in SOC and its fractions over 16 years across four contrasting farming systems in a low rainfall environment.
• To assess the accuracy of the APSIM model in simulating SOC dynamics under these conditions.
• To provide insights for land managers participating in climate change mitigation schemes.

Main Methods:

• Monitoring total SOC, total nitrogen (N), and carbon fractions (POC, ROC, HOC) over 16 years (1999-2015).
• Comparing four farming systems: conventional tillage (CT), reduced tillage (RT), continuous cropping (CC), and perennial pasture (PP).
• Utilizing APSIM C and N modules to model SOC dynamics.

Main Results:

• SOC increased for all systems over the first 12 years, primarily in the particulate organic carbon (POC) fraction, but decreased to baseline levels by 2015.
• Perennial pasture (PP) consistently showed higher total organic carbon (TOC), POC, humus organic carbon (HOC), and SOC stocks to 30 cm depth.
• Total nitrogen (TN) decreased over time in all systems except PP; average C:N ratio increased then decreased.
• The AusFarm/APSIM model did not accurately represent the observed temporal SOC dynamics.

Conclusions:

• Observed SOC changes over 12 years do not guarantee the long-term sequestration permanence required for carbon trading, due to POC's susceptibility to degradation.
• Establishing reference land uses (control plots) is essential for accurately determining net carbon sequestration and climate change mitigation value.
• Current APSIM/AusFarm models do not fully capture SOC temporal dynamics in low rainfall environments, necessitating model refinement or alternative approaches.