Long-term management effects on soil organic matter in two cold, high-elevation grasslands: clues from fractionation and radiocarbon dating

Grassland covers large areas at the treeline of the Alps, but management effects on the amount, distribution and turnover of soil organic matter (SOM) in this region are largely unknown. Our aim was to elucidate such effects for two management types using soil fractionation in combination with radiocarbon dating. Two Swiss grasslands sites at approximately 2020 m elevation (MAT +2.2 degrees C, MAP 1050 mm) were used, which were hay meadows until 60 years ago when one was changed to grazed pasture. The sites are located in close vicinity to each other and have similar climate and parent material. Soils at each site were shallow (maximum 20 cm) and had high stone contents and thus a limited carbon (C) storage capacity. Total soil organic carbon (SOC) stocks in the two soils were similar (6.11 +/- 0.69 and 6.07 +/- 0.91 kg C m(-2), respectively), as were proportions of readily available SOC and Q(10) values measured by means of respiration in the laboratory. Grazing caused lower soil nutrient contents and slight soil compaction. Carbon in labile fractions and its radiocarbon age were particularly useful to distinguish management effects. Significantly more SOC was located in the pasture topsoil (0-4 cm) with a higher share of particulate organic C. Particulate and microaggregate-associated C was slightly younger under pasture, indicating incorporation of recent plant tissue by cattle. At both sites, (14)C-derived mean residence times of SOC increased in the order particulate < micro-aggregate < silt and clay-associated matter and were greater below 4 cm than in the topsoil. Age differences in old silt and clay C could not be attributed entirely to management effects. We discuss pitfalls for turnover estimates of bulk soils introduced by choosing non-unique fractions. Together, the data suggest that a change from haying to cattle grazing may cause a strong decoupling of microbial processes within the soil's architecture and that in such ecosystems SOM under pastures may be more vulnerable to environmental disturbances.