Influence Of Elevation And Land Use On Soil Organic Carbon And Microbial Community On Southeast Tibetan Plateau

Soil microorganisms play a key role in regulating ecosystem functioning and biogeochemical cycling, including the decomposition of soil organic matter (SOM). Environmental factors vary dramatically over a relatively short spatial scale in mountain ecosystems; hence it provides an excellent opportunity to investigate changes in microbial community composition and associated ecosystem functions. The Tibetan Plateau stands as the highest and largest plateau in the world with an averaged altitude of 4000 m above sea level (a.s.l.).The inventory of soil organic carbon (SOC) on the Tibetan Plateau is enormous but sensitive to climate change and anthropogenic disturbance. The present study selected Mount Segrila as the study region, and field samplings were conducted in combination with incubation experiments to explore the influence of altitude and land use type on SOC and soil microbial community on southeastern Tibetan Plateau. The present study aims to explore the potential mechanisms by which soil microbial communities regulate C cycling in high-altitude region, which can provide deep insight into terrestrial C cycling in the frigid and high-altitude ecosystems. The main results are as follows:(1) Total SOC stock in top-60 cm profile varied from 77.67 to 281.73 Mg C ha-1 along an elevational transect from 3000 to 4600 m on the west slope of Mount Segrila. Generally for each profile depth, the variations in SOC stock with altitude were shown as a unimodal pattern with the highest SOC stock appeared around 3900 to 4100 m. The differences in SOC storage on the west slope of Mount Segrila were controlled by combined effects of vegetation types, mean annual temperature and soil water content.(2) The carbon stable isotope composition (δ13C) of plants and litters sampled from west slope of Mount Segrila varied from -25.40 to -31.68%o. The variations in δ13C values of plants and litters with altitude were both shown as a unimodal pattern with the lowest δ13C value appeared around 3900 m. Compared to plants and litters, soils sampled at different profile depths were 13C-enriched with δ13C values ranged from -21.55 to -29.17%o. The carbon stable isotope fractionation factor (a) during SOM turnover process was estimated from 1.0005 to 1.0024, and also presented a unimodal pattern with altitude. The a value was the highest in frigid dark coniferous forests but lowest in alpine frigid meadow and temperate broad-leaved and coniferous mixed forests. The variation in a value was tightly correlated with soil water content and litter nitrogen (N) content, suggesting their primary controls over the turnover of SOM on the west slope of Mount Segrila.(3) Along an elevational transect from 3105 to 4556 m on the west slope of Mount Segrila, there was no consistent elevational pattern observed for soil microbial community composition, as indicated by phospholipid fatty acid (PLFA) profiles. Whereas the activity of carbon-source utilization by the microbial community decreased significantly with increasing altitude. Soil microbes at higher altitudes had the potential to metabolize diverse carbohydrates and relatively more recalcitrant carbon components, whereas microbes at lower altitudes preferred labile carbon sources. The increase in soil O-alkyl C and decrease in alkyl C content at higher altitudes indicate slow SOM decomposition, which was in line with carbon-source utilization pattern by microbial community. Soil pH is the predominant factor determining the structure and activity of microbial communities across the elevational gradient. Mean annual temperature and vegetation type also exerted great impact on soil microbial community via their effects on soil pH and substrate quality.(4) Both topsoil and subsoil sampled from along an elevational transect from 3356 to 4590 m on the west slope of Mount Segrila were incubated at 15℃ and 25℃ to investigate the responses of soil microbial community and SOC decomposition to experimental warming. For soils collected from low altitude (El), increasing temperature did not have a significant effect on microbial PLFAs in topsoil, whereas those in subsoil increased significantly at 25 ℃. The responses of microbial PLFA in topsoil and subsoil sampled from intermediate (E3) and high altitude (E5) to warming were contrary to E1. The relative changes in microbial PLFA suggested a more sensitive response of microbial community to warming in topsoil than in subsoil, and the composition of microbial community differed significantly after incubated at different temperatures. Increasing temperature enhanced the decomposition of SOC. The mineralization rate and cumulative SOC mineralization was significantly affect by altitude and soil layer. The temperature sensitivity (Q10) of SOC decomposition increased progressively during the decomposition period, indicating a higher Q10 value of recalcitrant SOC than that of labile SOC. The Q10 value of subsoil in the late period of SOC decomposition was significantly higher (3.04) than that of topsoil (1.94). The elevational patterns of Q10 value varied with soil layer, and were significantly correlated with climate factors and the quality of SOC. There were negative correlations between Q10 value and microbial PLFA (especially Gram-positive bacteria and fungi), suggesting the positive effect of these microbial groups in reducing the sensitivity of SOC decomposition to experimental warming.(5) The conversions of forests and grasslands to croplands resulted in an averaged loss of 43-58% in SOC storage, which was reflected in significant reduces in the labile C pool, recalcitrant C pool, unprotected C pool and physically protected C pool. The variations in C content and C:N ratio of each SOC fraction were affected by changes in soil pH, aggregation and soil texture. Total PLFA concentration extracted from soils was significantly higher in forests and grasslands compared to croplands, and the composition of soil microbial community significantly differed among three land use types. Changes in microbial biomass and community structure (F/B ratio and G+/G-ratio) exerted great impact on the characteristics of SOC fractions. The δ13C value of coarse non-protected particulate OM (cPOM) was significantly higher in croplands (-21.63±0.52%) compared to forests (-23.57±0.44‰) and grasslands (-23.99±.43‰), suggesting an increase in microbial turnover of unprotected SOC and a decrease in formation of more stable SOC fraction (e.g. microaggregate-associated SOC) after cultivation, which led to the significant reduce in SOC storage.