Cropland has higher soil carbon residence time than grassland in the subsurface layer on the Loess Plateau, China

- Soil C mineralization rates were higher in grassland than those in cropland.
- The active C pool was higher in grassland than that in cropland.
- Conversion of cropland to grassland accelerated soil C turnover rate in subsoil.

Carbon (C) residence time is one of the key factors that determine the capacity of C storage and the potential C loss in soil, but it has not been well quantified. Assessing C residence time is crucial for an improved understanding of terrestrial C dynamics. To investigate the responses of C residence time to land use, soil samples were collected from millet cropland (MC) and enclosed grassland (EG) on the Loess Platea, China. The soil samples were incubated at 25 °C for 182 days. A Bayesian inverse analysis was applied to evaluate the residence times of different C fractions based on the information contained in the time-series data from laboratory incubation. Our results showed that soil organic carbon (SOC) mineralization rates and the amount of SOC mineralized significantly was increased after the conversion of cropland into grassland due to the increase in labile C fraction. At the end of the incubation study, 2.1%-6.8% of the initial SOC was released. Our modeling analysis indicated that the sizes and pool of active C in EG were larger than those in MC. The residence times of slow C pool at the 10-30 cm layers were longer in MC (11.8-15.7 years) than those in EG (14.7-21.9 years). Our inverse analysis also directly reflected that the active C pool dominated C mineralization in the first 92 days, while the slow C pool was the largest contributor after 92 days. Conversion of cropland to grassland increases SOC decomposition, probably through increased active C pool resulted from higher litter input and no ploughing activities. The declined SOC in subsoil may be attributed to the short carbon residence times in grassland.