Pyrogenic carbon quantity and ages in tropical soils under shrubland and dry deciduous forest in small-scale, sub-humid and semi-arid watersheds in southwestern India

Abstract

<p>Wildfires burn 4 % of the Earth’s vegetated land surface annually and thus are key to global biochemical cycles such as the terrestrial organic carbon (OC) cycle. The incomplete combustion of biomass during fires produces a thermochemical-altered and environmentally more persistent form of OC than its unburnt precursor material, namely pyrogenic carbon (PyC). PyC accounts for one of the largest and oldest terrestrial OC pool and constitutes substantial parts of the total soil organic carbon (SOC) pool. Tropical ecosystems are a key component of the terrestrial OC cycle, with highest annual primary production and largest pool of OC in vegetation biomass; however, these ecosystems are prone to disturbances such as global warming, land use changes or wildfires. In addition, tropical soils store up to one third of the terrestrial OC, and substantial amounts are held in deeper soil horizons (>30cm). However, our understanding of the quantities, ages and dominant processes of PyC (and SOC) cycling in tropical soils and in the landscape (e.g. watersheds) are still limited.</p><p>We sampled soils (0-60 cm depth) in tropical semi-arid watersheds under shrubland (Maddur and South Gundal watershed) and an experimental, sub-humid watershed under dry deciduous forest (Mule Hole Critical Zone observatory) in southwestern Karnataka, India. We included prevalent soil types (Lixisol, Vertisol and Ferralsol) under different geology in order to determine PyC (and SOC) stocks and radiocarbon ages. Along soil cores, we sampled bulk density cores to calculate accurate PyC (and SOC) stocks, and used the chemo-thermal oxidation (PyC<sub>CTO</sub>) and hydrogen pyrolysis (PyC<sub>HyPy</sub>) methods to separate the PyC. The bulk SOC and the PyC<sub>HyPy</sub> were further radiocarbon dated. Our aim was to quantify PyC (and SOC) stocks and identify major processes of PyC (and SOC) in prevalent soil types under shrubland and forest and in the landscape of in tropical watersheds.</p><p>SOC stocks were lowest for Lixisols (58-98 Mg ha-1), intermediate for Ferralsols (85-129 Mg ha-1) and highest for Vertisols (118-169 Mg ha-1), and all soils stored substantial proportions of OC in subsoils (up to 40 % below 30cm depth). Radiocarbon data of bulk SOC indicates fast turnover of OC in these tropical topsoils (modern ages in 0-15cm for shrublands and forests: between 99.7 to 107.0 % modern 14C (± 0.5 %)); however, the 14C of bulk SOC shows much older ages in subsoils (>40cm), from 1530 to 8490 calibrated radiocarbon years BP in Ferralsols and from 1155 to 8125 calibrated radiocarbon years BP in Vertisols with increasing depth.</p><p>PyC<sub>CTO</sub> stocks accounted for 0.3-0.9 Mg ha-1 (Lixisols), 0.4-0.7 Mg ha-1 (Ferralsol) and 0.9-1.9 Mg ha-1 (Vertisol). This means that PyC<sub>CTO </sub>represented approximately 1.1-3.4 % of SOC in the topsoil (0-15 cm) and increased with depth to 2.7-5.6 % in 45-60 cm depth on average over all sites. Conclusively, PyC can make up a substantial proportion of the SOC in tropical soils, and it may represent one of the oldest OC pools of bulk SOC.</p>