Effects of heat on stable carbon fractions in repellent and non-repellent forest soils.

Abstract

Soil organic carbon (C), the main component of soil organic matter, consists of two major fractions as the labile C fraction (LCF) and the stable C fraction (SCF). Soil water repellency (SWR) is caused by the presence of organic materials and has important consequences for C mineralization. The proportion of SCF in water-repellent (WR) and non-repellent (NR) soils might be different due to the differences in content and composition of organic substances present in these soils. Forest fires can significantly impact soil C due to generated heat, and might change the amount of SCF. This study aimed to examine the effects of heat (heating temperature, TH; duration of exposure to heat, DH) on SCF in WR and NR forest soils. Soil samples were collected from Mahogani Forest in Mapalana and Casuarina Forest in Hambantota, representing NR and WR soils, respectively. Soils samples were separately exposed to eight heating temperatures (50, 100, 150, 200, 250, 300, 350, and 400°C) for 20 and 40-min durations in triplicates, and then kept for 24 hours before testing. Total organic C (TOC) content (Walkley-Black method), SCF (size fractionation method), LCF (numerical method), and SWR (water drop penetration time, WDPT and molarity of ethanol droplet, MED tests) of the samples were determined before and after heating. SWR, TOC, and LCF in both WR and NR forest soils decreased with increasing TH and DH. The SCF in NR soil increased from 61% of initial value to 89%, whereas that of WR soil increased from 71% of initial value to 95%, after heating at 400°C for 40 minutes. Although the SCF% in WR soils was higher than that in NR soils, the increase in SCF and the decrease in LCF with increasing temperature did not show any significant difference between NR and WR soils. Results revealed that the SCF increases when the soil is exposed to higher temperatures, showing that the increasing temperature removes labile C more profoundly from the soil compared with stable C.