Short and long-term fire effects on soil C and N in an African savanna

Fires have been an important ecosystem driver in the Earth system for millions of years. It drives ecosystem composition, diversity and structure, thus rendering numerous terrestrial ecosystems dependent on fires for maintaining ecosystem health and functioning. As an important ecological driver in many ecosystems, fires have been shown to lead to substantial impacts on soil carbon (C) and nitrogen (N). Although many global studies have looked at either short-term or long-term fire effects on soil C and N, they have contradicting outcomes. This study aimed at investigating the effect of both short- and long-term fire effects on soil C and N, C:N ratios and soil C stocks, as well as how fire-induced changes to vegetation may influence C and N distribution. Soil samples (0-5 cm) were collected under tree and shrub canopies and open, grassy areas from Annual Burn and No Burn (fire exclusion) plots at a 68-year-old long-term fire experiment. Samples were collected pre-burn, 1 m and 1 yr post-fire to determine short-term fire impacts, and from the No burn plot to determine long-term fire impacts (compared to the Annual Burn). Soils were analysed for total C, total N, C:N ratios and C stocks. Our results found that fires led to significantly higher soil C and N within the first month post-fire (p < 0.05) and remained significantly higher even up to a year later. Fires also altered soil total C and N in the long-term which saw decadal increases in C and N due to fire exclusion (p < 0.001). C:N ratios improved shortly after burning to similar C:N ratios seen in the long-term No Burn plot. Even though soil C stocks were greatest in the No Burn plot, there was also an increase in soil C stocks on the Annual Burn plot a month after the fire (p < 0.05). Lastly, trees and shrub were found to increase soil C and N below their canopies compared to open, grass-dominated areas, suggesting further indirect effects of fires on soil C and N by means of changing woody cover. These findings have implications for fire management in African savannas when considering fire suppression policies, bush encroachment and global tree-planting initiatives.