The importance of soil sampling depth for accurate account of soil organic carbon sequestration, storage, retention and loss

• When a soil is sampled to different depths the SOC stock change varies.
• When only the 0–5 cm layer was sampled SOC sequestration did occur.
• When root zone sampled the losses were greater than the 0–5 cm gains.
• No SOC sequestration occurred for a 75 cm root zone.
• NT treatment lost more SOC stock in 5 to 30 cm layer than MP.

Soil organic carbon distribution within soil profile is highly influenced by management practices, especially tillage systems where soil environment is altered. Such changes in soil environment will affect soil carbon retention or accumulation in different layers of the soil profile. However, much published research in the area of soil organic carbon (SOC) sequestration focuses on shallow sampling depths within the 0–30 cm tillage zone when determining SOC stocks and sequestration. The objectives of this study are to quantify the SOC stock differences with depth between tillage treatments after 20 years and to determine the appropriate sampling depth when assessing SOC stocks as influenced by management practices. A 20-year moldboard plow (MP), chisel plow (CP) and no-tillage (NT) study was established with a maize–soybean rotation. The 75-cm root zone was sampled in 5-cm intervals to measure SOC stocks. The SOC sequestration, storage, retention and loss were determined for the 0–5 cm, 0–15 cm, 15–75 cm and 0–75 cm layers. The NT treatment did retain more SOC stock than the MP treatment to a 20 cm depth but the SOC stock of the 20–35 cm layer NT system was lower than the MP system. It is recommended that the depth of soil sampling has to include the entire root zone to accurately report SOC stock and the effect of tillage system on change in SOC.