Natural grassland conversion to agriculture or pine plantations: Effects on soil methane uptake

Upland soils are the only known biological sink for methane (CH4) by methanotrophic bacteria consumption. This process is mainly limited by the diffusion processes related to the soil's physical characteristics, which can be modified because of changes in land use depending on the soil type, the original system and the new land use converted. Our study focused on determining the differences in soil CH4 uptake because of changes in land use (from natural grassland to agricultural land and two Pinus radiata afforestation, differing in thinning management) and on determining which are the main drivers that control CH4 uptake in the studied soil type (Hapludoll), with focus on the diffusion process. CH4 fluxes were measured 12 times with the static chamber technique between October 2015 and April 2019. Also, CH4 gradient concentration in the soil profile and physical and chemical variables were measured on the same dates. All land uses studied acted as net CH4 sinks. Land-use change from grassland to agriculture decreased soil CH4 uptake (similar to 37% +/- 19), whereas afforestation increased (similar to 85% +/- 73) this environmental service related to natural grassland. We found that the main drivers that control CH4 uptake in this soil are water and air-filled pore space, variables that govern soil CH4 diffusion; they are mostly related to differences in bulk density (compaction) among land uses. Organic matter was also an important driver, mainly related to soil structure. Land-use change affected all of these drivers. CH4 concentration presented differences at deeper soil layers only in the two afforestations, which differed in management (pruning and thinning vs. no management). However, CH4 uptake did not present significant differences between them, suggesting that there is no need for a high tree cover to increase the CH4 sink of the soil. This mixed tree and herbaceous cover may result in a similar environmental service output, increasing the options of land uses.