Biological characteristics of forest soils and seepage waters during simulated rainfalls of high intensity

Dissolved organic carbon (DOC), biodegradable dissolved organic carbon, hydrolytic activity and bacterial numbers were measured in two forest soils (a mottled brown soil and a podzolic pseudogley) and in their seepage water collected during 3-day experiments simulating high-intensity rainfall. Differences were observed in seepage water released from the two soils. Because of the high intensity of the artificial rain, water was drained through preferential pathways. Seepage water contained DOC, bacteria and free enzymes but did not reflect the general soil composition. Biological activity of the mottled brown soil increased with soil moisture. On the contrary, biological activity and bacterial counts measured in water seeping from this soil varied in the opposite direction. Hydrolytic activity measured in seepage water during the first day of artificial rain was low and depended on the initial soil water content. During the second and the third day of the experiment, peaks of enzymatic activity were observed in seepage water because biological activity was stimulated by the higher water content of the soil. Whatever the season, DOC measured under the mottled brown soil was not different from one day to another. However, the biodegradable portion decreased after the first day of artificial rain. Such a phenomenon was not observed for the podzolic pseudogley. These results suggest that, during storms, soils are able to release organic matter (OM) such as DOC, enzymes and bacteria, which, in part, can be a food source for aquatic organisms. If the released OM is not significantly different from one soil to another on a quantitative basis, its quality seems to be linked to the soil type, the former water content of the soil and the intensity and duration of storms.