Radiocarbon dating reveals different past managements of adjacent forest soils in the Campine region, Belgium

The soils of adjacent first generation monospecific stands of Scots pine (Pinus sylvestris L.) and pedunculate oak (Quercus robur L.) in the Campine region, Belgium, apparently developed under the same forming factors, were studied for carbon dynamics to disentangle eventual different past land uses. In fact, visual observations suggested that the soil under pine experienced substantial addition of organic matter and ploughing, such to be considered a plaggen, opposite to the soil under oak, which is inexplicably much poorer in C. In order to prove this hypothesis, the soil organic carbon was quantified by horizons and, both bulk soil organic matter (SOM) and the least mobile SOM fractions - the humic acid and the unextractable fractions - were radiocarbon dated. Surprising was the marked difference between the mean SOM age from the two stands. In fact, while under oak this age is a few years or decades, under pine it amounts to more than a millennium, so confirming the hypothesis of a confined C supply occurred mainly in the Middle Age, or later using partly humified matter. The mean residence time (MRT) of SOM in the organic layers matches almost perfectly with that estimated via a mass balance approach and, as expected, was much lower in the oaks than in the pines. The humic acid fraction, generally the most stable fraction of SOM, in terms of both mobility and degradability, reflects the behaviour of the bulk SOM, showing higher radiocarbon ages under pine. The findings of this work indicate that the large human-induced additions of organic material in the area now occupied by the pine stand, probably occurred in the Middle Age and it continues to strongly affect the present soil C pools and their dynamics. Any study dealing with budgets and dynamics of C in soil should avail itself of a careful reconstruction of the land uses and management history, in order to provide reliable conclusions about the real role of the current vegetation on soil carbon.