Deep soil inorganic carbon, an overlooked carbon sink in alkaline croplands

Abstract

Soil carbon sequestration should mitigate climate change, yet the dynamics of deep soil carbon are poorly known. Here, we conducted a 12-year field experiment in a Fluvic Cambisol under a wheat–maize rotation system, comparing straw return with straw removal, and quantified carbon stocks across a 0–200 cm soil profile. We traced carbon sources using both natural ¹³C abundance, and ¹³C-labeled glucose in a 60-day laboratory incubation experiment. Results show that straw return increased soil inorganic carbon stocks by 57.6 t ha⁻¹ in the 100–200 cm layers. Pedogenic carbon increased by 96.6% in the 0–20 cm topsoil and by 97.7% in the 120–140 cm deep soil, whereas lithogenic carbon remained stable, indicating that soil inorganic carbon gains originated primarily from pedogenic carbonate formation. 72.23% of the added ¹³C-glucose was mineralized to CO₂, while 17.27% was transformed into soil inorganic carbon, suggesting that exogenous carbon reaching the 120–140 cm soil layer can enhance pedogenic carbonate formation. Straw return increased microbial biomass carbon by 192% in topsoil and 144% in deep soil, and increased carbonic anhydrase activity by 43% and 73%, respectively. These findings reveal a previously overlooked, microbially-mediated mechanism for deep soil inorganic carbon accumulation, highlighting deep soil inorganic carbon as a stable carbon sink in alkaline croplands.