Rainfall and Seasonality Drive Pyrogenic Carbon Stocks in Coarse-Textured Mineral Soils

Pyrogenic carbon (PyC) from biomass burning is a large, but poorly quantified, slow-cycling component of the soil organic carbon pool. Modeling of soil carbon dynamics can be improved by including the processes governing the input and cycling of PyC in the soil. The carbon isotope composition of PyC (δ¹³C_PyC) provides a tracer for the partitioning of PyC into the soil from biomass. We report the stocks and δ¹³C values for PyC and organic carbon (OC) for 41 regions dominated by savannas and seasonally wet to arid regions of Australia and Africa. Stocks of PyC in the 0–5 cm interval ranged from 0 to 1.17 MgC ha⁻¹ (mean 0.43 ± 0.25 MgC ha⁻¹) and in the 0–30 cm interval ranged from 0.25 to 3.89 MgC ha⁻¹ (mean 1.65 ± 0.77 MgC ha⁻¹). PyC stocks averaged 8% (but were up to 25%) of total organic carbon (TOC) stocks. Stocks tended to highest in relatively wet, but seasonally dry, regions such as tropical savannas. PyC abundance could be predicted (r = 0.8 to 0.95) from environmental variables only. δ¹³C_PyC values varied widely between regions, but with no systematic differences within regions related to current vegetation or sample depth, likely due to the long residence time of PyC in the soil. δ¹³C_PyC values were strongly correlated with δ¹³C_OC values but were systematically 1–2‰ higher even in C₃ only regions.