Humus profile degradation as influenced by decreasing earthworm activity

Three humus profiles were sampled in moist grasslands on loamy sand, repres enting a chronosequence of an acidifying ecosystem. Over the chronosequence , earthworm activity decreased, from high activity to complete absence of e arthworms. We studied the organic matter at three observation levels: the h umus-profile, the aggregate and the soil organic matter (SOM)-fraction leve l. With decreasing earthworm activity a trend in separation of organic and mineral elements can be discerned. Humus forms changed from Vermimull via V ermic Rhizomull to Rhizic Mullmoder. With decreasing earthworm activity, pa rticulate organic matter (POM) first changes from intra- to extra-aggregate POM, and accumulates completely on top of the mineral soil in the absence of earthworms. The trend in vegetation that follows this changes in organic matter morphology indicates that less nutrients become available with incr easing acidification. The decrease in earthworm activity thus seems to be r elated with an overall decrease of SOM and nutrient dynamics.