SEASONAL NITRIFICATION MEASUREMENTS WITH DIFFERENT SPECIES OF FOREST LITTER APPLIED TO GRANITE-SAND-FILLED LYSIMETERS IN THE FIELD

The biodegradation of litter from Festuca silvatica, Abies pectinata, Fagus silvatica, Calluna vulgaris, Picea abies associated with forest brown acid soils or with podzolic soils was studied in field lysimeters filled with granite sand. Analysis of the leachates collected during 2 years made it possible to determine NO3-, NH4+, and soluble organic N production in order to investigate the specific influence of the different species of litter on the mineralization of organic N and the variations in nitrification. With Festuca silvatica (grass), active nitrification was observed after the addition of fresh litter in autumn (fall of leaves). Nitrification remained significant in winter, reached a maximum in spring until early summer, and then decreased after mineralization of the easily mineralizable organic N. Nitrification was the major N transformation process in this litter. The addition of fresh litter of Abies pectinata (fir), Fagus silvatica (beech), Calluna vulgaris (heather), and Picea abies (spruce) in autumn induced an inhibition of nitrification during winter and spring. With these litter species, nitrification started again by the end of spring and was at a maximum in summer and autumn until leaf fall. By comparison with Festuca, inhibition observed in winter and spring with the other litter species was definitely due to the chemical composition of the leaves. Simultaneously, a lower C mineralization of these plant material occured. These litter species, in particular Calluna and Picea released leachates containing significant amounts of soluble organic N that were only slightly decomposed. We conclude that NO3- production outside of the plant growth period can definitely be involved in soil acidification and weathering processes.