Degradability of soils under oak and pine in Central Spain

The purpose of this research was to study the influence of the vegetation on the soil C pool of forests of pines (Pinus sylvestris) and oaks (Quercus pyrenaica), located in Central-Western Spain. Horizons from selected soils located in these forests were sampled, and the soil organic C (SOC) was determined. In addition, in vitro incubation experiments were carried out, under controlled conditions, to monitor the stability of SOC against the microbial activity. Soil humus fractions were isolated following a classical procedure of chemical fractionation using alkaline solutions, before and after the incubation experiment. A deeper O horizon was found under the pine forest than under oak one; however, higher SOC content was found in the oak site than that under pine one. During the in vitro mineralization process, a lower CO₂ production by the soil sample from pine forest was observed, in relation to that emitted by the oak soil. In addition, a lower humification degree was estimated for the soil humus under pines than for that under oaks. In conclusion, replacement of oaks by pines produced a decrease in SOC accumulation and a lower quality of humus in the forest soils.