Evolution of soil organic matter composition and edaphic carbon effluxes following oak forest clearing for pasture: climate change implications

Abstract

This research encompasses soil CO2 emission studies from forest and pasture couplets located in northwestern Spain, where two mature oak forest ecosystems partially cleared for pasture 5 or 50 years ago were selected to monitor soil C effluxes during 2 years. The CO2 exchanges in the soil–atmosphere interphase of forest and pasture plots were seasonally determined using portable infrared gas analysers. At the same time, soil samples from both forest and pasture ecosystems were used to carry out long-term incubations under laboratory conditions. Solid-state 13C-NMR with cross-polarization/magic angle spinning was applied to determine the deforestation effects on soil organic matter (SOM) composition. Pasture implantation caused a notable decline of the labile C pool and a decrease in the total soil C, with an increase in both the SOM humification and the relative concentration of phenolic and carboxyl C. After only 5 years, the deforestation caused a general decrease in the soil CO2 emissions with reduced seasonal fluctuations, these effects being even more intense 50 years after clearing. The correlation observed in oak forests between the CO2 measured in situ and the soil temperature, is masked in pasture ecosystems by the high summer soil dryness. After the partial disappearance of soil C stocks caused by deforestation, a new long-term C input/output equilibrium seems to be established, probably due to the joint adaptation of both SOM and microbial communities in the old pasture soil; however, the entire soil C retention capacity remains still degraded as compared with the original uncleared forest ecosystem.