Effects of Early Stages Tree Line Shift on Soil Microbial Biomass and Catabolic Diversity of Reserved and Grazed Subalpine Meadows

The tree line is moving up the mountain slopes. Reasons include global warming and reduced grazing intensity. The effect of changes in plant cover on soil microbial abundance and diversity in subalpine meadows during the first stage of tree line shift is poorly understood. Soil samples from subalpine meadows on 3 reserved and 3 grazed slopes were mixed with chopped plant residues: 1) subalpine grass (GR) and 2) mixture of GR and birch leaves (BL) in a 1:1 ratio, mimicking early tree line shift. Soil samples with plant residues were placed in vessels, sealed and incubated for 1 year in dark with intermittent air exchange. The carbon, nitrogen (MBN) and phosphorus content of the soil microbial biomass and catabolic diversity index were assessed at the experiment’s beginning, after 6 months and after 1 year. The addition of BL with GR residues resulted in decrease in soil microbial biomass but increase in catabolic diversity. The increase in portion of recalcitrant fractions of plant residues (acid-insoluble and non-polar extractables) contributed to decrease in soil MBN content. It also decreased the proportion of soil microbial response to carbohydrates in total substrate-induced respiration, resulting in a more balanced and catabolically diverse microbial community.