Shifting Mountain Tree Line Increases Soil Organic Carbon Stability Regardless of Land Use.

Climate and land use changes are causing trees line to shift up into mountain meadows. The effect of this vegetation change on the partitioning of soil carbon (C) between the labile particulate organic matter (POM-C) and stable mineral-associated organic matter (MAOM-C) pools is poorly understood. Therefore, we assessed these C pools in a 10 cm topsoil layer along forest-meadow ecotones with different land uses (reserve and pasture) in the Northwest Caucasus of Russia using the size fractionation technique (POM 0.053-2.00 mm, MAOM < 0.053 mm). Potential drivers included the amount of C input from aboveground grass biomass (AGB) and forest litter (litter quantity) and their C/N ratios, aromatic compound content (litter quality), and soil texture. For both land uses, the POM-C pool showed no clear patterns of change along forest-meadow ecotones, while the MAOM-C pool increased steadily from meadow to forest. Regardless of land use, the POM-C/MAOM-C ratio decreased threefold from meadow to forest in line with decreasing grass AGB (R2 = 0.75 and 0.29 for reserve and pasture) and increasing clay content (R2 = 0.63 and 0.36 for reserve and pasture). In pastures, an additional negative relationship was found with respect to plant litter aromaticity (R2 = 0.48). Therefore, shifting the mountain tree line in temperate climates could have a positive effect on conserving soil C stocks by increasing the proportion of stable C pools.

Contribution of microbial activity and vegetation cover to the spatial distribution of soil respiration in mountains.

The patterns of change in bioclimatic conditions determine the vegetation cover and soil properties along the altitudinal gradient. Together, these factors control the spatial variability of soil respiration (RS) in mountainous areas. The underlying mechanisms, which are poorly understood, shape the resulting surface CO2 flux in these ecosystems. We aimed to investigate the spatial variability of RS and its drivers on the northeastern slope of the Northwest Caucasus Mountains, Russia (1,260-2,480 m a.s.l.), in mixed, fir, and deciduous forests, as well as subalpine and alpine meadows. RS was measured simultaneously in each ecosystem at 12 randomly distributed points using the closed static chamber technique. After the measurements, topsoil samples (0-10 cm) were collected under each chamber (n = 60). Several soil physicochemical, microbial, and vegetation indices were assessed as potential drivers of RS. We tested two hypotheses: (i) the spatial variability of RS is higher in forests than in grasslands; and (ii) the spatial variability of RS in forests is mainly due to soil microbial activity, whereas in grasslands, it is mainly due to vegetation characteristics. Unexpectedly, RS variability was lower in forests than in grasslands, ranging from 1.3-6.5 versus 3.4-12.7 µmol CO2 m-1 s-1, respectively. Spatial variability of RS in forests was related to microbial functioning through chitinase activity (50% explained variance), whereas in grasslands it was related to vegetation structure, namely graminoid abundance (27% explained variance). Apparently, the chitinase dependence of RS variability in forests may be related to soil N limitation. This was confirmed by low N content and high C:N ratio compared to grassland soils. The greater sensitivity of grassland RS to vegetation structure may be related to the essential root C allocation for some grasses. Thus, the first hypothesis concerning the higher spatial variability of RS in forests than in grasslands was not confirmed, whereas the second hypothesis concerning the crucial role of soil microorganisms in forests and vegetation in grasslands as drivers of RS spatial variability was confirmed.

Temperature Sensitivity of Topsoil Organic Matter Decomposition Does Not Depend on Vegetation Types in Mountains.

Rising air temperatures caused by global warming affects microbial decomposition rate of soil organic matter (SOM). The temperature sensitivity of SOM decomposition (Q10) may depend on SOM quality determined by vegetation type. In this study, we selected a long transect (3.6 km) across the five ecosystems and short transects (0.1 km) from grazed and ungrazed meadows to forests in the Northwest Caucasus to consider different patterns in Q10 changes at shift of the vegetation belts. It is hypothesized that Q10 will increase along altitudinal gradient in line with recalcitrance of SOM according to kinetics-based theory. The indicators of SOM quality (BR:C, respiration per unit of soil C; MBC:C, ratio of microbial biomass carbon to soil carbon; soil C:N ratio) were used for checking the hypothesis. It was shown that Q10 did not differ across vegetation types within long and short transects, regardless differences in projective cover (14-99%) and vegetation species richness (6-12 units per plot). However, Q10 value differed between the long and short transects by almost two times (on average 2.4 vs. 1.4). Such a difference was explained by environmental characteristics linked with terrain position (slope steepness, microclimate, and land forms). The Q10 changes across studied slopes were driven by BR:C for meadows (R2 = 0.64; negative relationship) and pH value for forests (R2 = 0.80; positive relationship). Thus, proxy of SOM quality explained Q10 variability only across mountain meadows, whereas for forests, soil acidity was the main driver of microbial activity.