Effects of decomposing beech (Fagus sylvatica) logs on the chemistry of acidified sand and loam soils in two forest reserves in Flanders (northern Belgium)

We studied the effect of coarse woody debris (CWD) on the soil nutrient status in two beech (Fagus sylvatica) dominated forest reserves in Flanders, Belgium: Wijnendale Forest, on a sandy soil and Kersselaerspleyn in Sonian Forest, on a loamy soil. More specifically, we looked at the chemical composition of beech logs of different stages of decay. In addition, we examined the chemical composition of the organic and the mineral soil at five distances from the decomposing logs. We considered the concentrations of the following elements: C, N, P, S, Ca, K, Mg, Mn, Fe, and Al. The results indicate a difference in wood and soil chemical composition between the two forest sites. The soil and the aboveground biomass of Wijnendale had the highest total N concentration and the lowest concentrations of P and base elements (Ca, K, Mg, and Mn). There is an increase in element concentrations in CWD of both forests during the decomposition, except for K and C. The higher N concentration in Wijnendale, explained by high atmospheric nitrogen deposition in this forest, persisted during decomposition. By contrast, the concentrations of P, Ca, K, and Mg in dead wood of both forests became similar when decomposition proceeded. The effect of CWD was more pronounced in the organic soil layer than in the mineral soil. The organic soil in the proximity of CWD had a higher pH and higher concentrations of C, N, P, Ca, Mg, and Mn (in Sonian forest) and a lower Al concentration (in Wijnendale forest) and this is highly significant for Ca, a limiting nutrient in moderate to highly acidic forests. The percentage of the soil surface impacted by the logs is 0.92% and 0.36% for Sonian and Wijnendale respectively, which is expected to increase with time, considering the fact that both reserves are only recently left unmanaged. The results of this study highlight the contribution of CWD in sustaining the nutrient status and buffering capacity of forest sites, in particular on soils sensitive to acidification and exposed to high nitrogen deposition.