Carbon dynamics in topsoil and subsoil along a cultivated toposequence

• Topsoil temperature sensitivity of SOC turnover was independent of slope position.
• Subsoil SOC stocks and temperature sensitivity of turnover depend on slope position.
• Soil surface CO2 effluxes were similar at shoulderslope and footslope positions.

Topography-induced spatial heterogeneity influences soil organic carbon (SOC) stocks and microbial degradation (respiration) both in topsoil and subsoil compartments. However, the interaction between topographic position and soil horizons has rarely been assessed. This study aimed to investigate SOC dynamics in topsoil (5 cm) and subsoil horizons (40 and 80 cm) at shoulderslope and footslope positions in a toposequence in a Danish winter wheat field. In addition, SOC was quantified for 20-cm depth intervals to 100 cm depths. Over a 1 year period, gas samples for carbon dioxide (CO2) and oxygen (O2) analyses were collected from seven different soil depths (5 to 80 cm) at the shoulder- and footslope positions. Soil surface CO2 fluxes were measured over a shorter period (January to June 2012). Soil samples from 5 and 40 cm depths were incubated at 5 to 34 °C to determine the temperature sensitivity (Q10) of soil respiration. Results showed that SOC stocks to a soil depth of 1 m were larger at footslope (202 Mg C ha− 1) compared to shoulderslope (44 Mg C ha− 1) positions. Mean annual soil CO2 concentrations were higher at footslope positions, and increased with depth at both shoulder- and footslope positions. Temperature sensitivity of C turnover was similar in topsoil at shoulderslope (Q10 = 2.5) and footslope (Q10 = 2.6) positions; in subsoil (40 cm), however, Q10 was lower at shoulderslope (Q10 = 2.0) than footslope (Q10 = 3.2) positions. Further, shoulderslope subsoil had less non-complexed organic C than footslope subsoil, suggesting that the shoulderslope subsoil was not C saturated and had higher potential for C stabilization. Despite the dissimilar subsoil characteristics at shoulder- and footslope positions, soil surface CO2 effluxes were similar, suggesting low contribution of subsoil C to short-term surface CO2 fluxes at footslope positions.