Effects of land use and cultivation time on soil organic and inorganic carbon storage in deep soils

The vertical distribution and exchange mechanisms of soil organic and inorganic carbon (SOC, SIC) play an important role in assessing carbon (C) cycling and budgets. However, the impact of land use through time for deep soil C (below 100 cm) is not well known. To investigate deep C storage under different land uses and evaluate how it changes with time, we collected soil samples to a depth of 500 cm in a soil profile in the Gutun watershed on the Chinese Loess Plateau (CLP); and determined SOC, SIC, and bulk density. The magnitude of SOC stocks in the 0–500 cm depth range fell into the following ranking: shrubland (17.2 kg m−2) > grassland (16.3 kg m−2) > forestland (15.2 kg m−2) > cropland (14.1 kg m−2) > gully land (6.4 kg m−2). The ranking for SIC stocks were: grassland (104.1 kg m−2) > forestland (96.2 kg m−2) > shrubland (90.6 kg m−2) > cropland (82.4 kg m−2) > gully land (50.3 kg m−2). Respective SOC and SIC stocks were at least 1.6- and 2.1-fold higher within the 100–500 cm depth range, as compared to the 0–100 cm depth range. Overall SOC and SIC stocks decreased significantly from the 5th to the 15th year of cultivation in croplands, and generally increased up to the 70th year. Both SOC and SIC stocks showed a turning point at 15 years cultivation, which should be considered when evaluating soil C sequestration. Estimates of C stocks greatly depends on soil sampling depth, and understanding the influences of land use and time will improve soil productivity and conservation in regions with deep soils.