Radiocarbon in soil organic carbon and soil pore space CO2 in sub-humid to semi-arid regions of western India: Implications to tropical soil carbon dynamics

CO<inf>2</inf> emitted from soil to the atmosphere is one of the largest components of terrestrial carbon exchange. Therefore, it is important to understand soil organic carbon (SOC) cycling and factors governing it. Radiocarbon (¹⁴C) in SOC and soil pore space CO<inf>2</inf> (soil CO<inf>2</inf>) is a powerful tool to determine SOC residence time and identify the pool of organic matter contributing to CO<inf>2</inf> release. A combined analysis of ¹⁴C in SOC and soil CO<inf>2</inf> offers a comprehensive understanding of the influence of climate and soil properties on the SOC cycling. We measured ¹⁴C in SOC and soil CO<inf>2</inf> from surface to a depth of ∼ 120 cm in agricultural and forest lands in sub-humid to semi-arid regions of Gujarat, western India. The SOC content in the top layers (0–10 cm) ranges from 0.001 to 0.5 % in agricultural lands and from 0.9 to 3.3 % in forest sites, which decreases to 0.001–0.02 % and 0.05–0.5 % at a depth of ∼ 100 cm for the respective soil types. The lower SOC content in agricultural lands is attributed to carbon loss due to tillage practices. We estimated that ∼ 90 % of the SOC has been depleted from the agricultural lands. The mean residence time (MRT) of SOC is about a century at the surface (0–10 cm) and increases with depth, reaching 1300 to 10000 years at a depth of ∼ 100 cm. MRT is lower in forest soils than in agricultural lands, indicating faster cycling of SOC in forests. MRT decreases with decreasing rainfall and its dependence on air temperature in agricultural lands is not obvious. Soil CO<inf>2</inf> throughout the profile is mostly modern, primarily originating from root respiration and fresh organic matter decomposition, with less contribution from older SOC present in different soil horizons.