Temporal changes in soil organic C under Mediterranean shrublands and grasslands: impact of fire and drought

Abstract

Forest areas have increased in the Mediterranean basin over the last two decades, due to the abandonment of agriculture. This and the occurrence of intense drought periods have led to an increase in the frequency and intensity of fires. Fire and drought can increase short-term soil organic C accumulation as a result of increased plant residues. In this study, we examined the changes in the soil organic C and the effects of fire and drought during a 12-year period in two Mediterranean grasslands and a shrubland. Thus, we established 6 plots for each of the three vegetation type and we set 18 experimental fires. Soils were sampled 3 days, 9 months, 6 years and 12 years after the fires and were analyzed for organic C. We used the RothC-26.3 model to help interpret the changes we observed. Three days after the fire, the amount of organic C was higher in burned plots than in unburned plots down to a depth of 5 cm. This was true in all plant communities under study and was probably due to burned plant deposition after the fires. However, these differences disappeared in the following years. In some cases, organic C from burned and unburned plots showed a large increase between years 6 and 12, which coincided with an extended 4-year drought period. Our results indicate that in Mediterranean shrublands and mixed shrub-grasslands the influence of drought periods could produce transient pulses of C that are much larger than the pulses produced by fire. The pulses of C caused by drought should be considered when studying the soil organic C dynamics in the frame of global warming.