What deep-soil profiles can teach us on deep-time pH dynamics after land use change?

Soil profiles keep records of the legacies of historical land uses on soil physicochemical properties with deep-soil (3-m depth) profiles providing information on centuries-old dynamics. By combining geohistorical archives on past land uses and management practices together with soil pH data from 19 plots scattered across five study areas in North France, we analyzed the effect of two contrasting historical land-use change trajectories (afforestation of former arable land vs. cultivation after deforestation) on the acidification and alkalinization dynamics along 3-m-depth loessic Luvisols profiles. An analysis of covariance and a generalized additive model were used to test the interaction effect between depth and the two studied chronosequences levels on soil pH. Results show two contrasting alkalinization dynamics for formerly acidic forest soils converted to arable land depending on past management practices. One century of liming practices based on hydrated lime is enough for a complete deprotonation, whereas alkalinization dynamic was much slower (two to four centuries are necessary to neutralize the relictual acidity below 2 m) under liming based on chalk-blocks supply. Per contra, afforestation of former arable land cultivated and limed during at least 2-4 centuries keep an alkaline 'memory' effect over more than a millennium. The acidification process is progressive through time from the surface to deeper soil horizons. This suggests that soil pH profiles can be used to estimate time since afforestation of former arable land. Linking deep-soil pH profiles with chronosequences is a powerful tool to understand the potential impact of future land-use change trajectories on soil physicochemical properties.