Short-time effects of land-use changes on O-horizon in subalpine grasslands

Due to economic needs, a process of abandonment of unfavourably situated areas and intensified cultivation on favoured areas has taken place in the alpine region. This abandonment not only means the loss of a cultural landscape based on centuries of care but also interference with an established ecological balance. We wanted to investigate the impact of land-use, such as mowing, fertilisation and irrigation and the effects of abandonment on the O-horizon type, quantity and quality. Therefore, we selected areas with different types of land use but with equivalent site conditions in the Passeier Valley (Bolzano-South Tyrol province) for investigation. The areas studied were as follows: an organically fertilised and irrigated, intensively managed hay meadow mown once a year; an organically fertilised, intensively managed hay meadow mown once a year; a lightly managed hay meadow mown every second year; a lightly managed hay meadow mown every 3–4 years; an area densely covered with dwarf shrubs abandoned 10 years ago; and a reforested area abandoned 30 years ago. These selected areas represent different intensities of land use or stages of succession. On these areas, vegetation, phytomass, litter quantity, humus type, O-horizon quantity, litter decomposition and C and N storage, were analysed in detail. Abandonment causes the percentage of coverage with grasses and herbs to decrease from 100 to 10%, while lignified species increase from 0 to 90%. This leads to growth of the phytomass pools, which in turn causes an increase in the annual litter quantity (from 117 to 525 g m−2). Significant changes in litter quality in the case of reduced use or abandonment increases the litter’s resistance to decomposition, decreasing overall decomposition. Litter quantity and quality then have a significant impact on the O-horizon quantity. This increases strongly with decreasing intensity of land use or longer times since abandonment (115–1,180 g m−2). This leads to a significant increase of C and N storage and a change in O-horizon type ranging from no definite O-horizon or mull O-horizons on regularly used areas to moder on abandoned areas.