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Determining precisely the soil organic carbon (SOC), below-ground biomass carbon (BGC), and their spatial distribution and influence factors is a prerequisite for accurately estimating the function of ecosystem carbon sequestration in alpine grasslands. Normalized differential vegetation index (NDVI), meteorological data and field investigation data from 2013 were used to determine the SOC, biomass carbon allocation and spatial distribution of alpine grassland in the upper reaches of the Shule River Basin, northeastern margin of the Qinghai-Tibet Plateau. Results indicate that a linear relationship between NDVI and biomass carbon can be used to accurately determine regional biomass carbon, particularly for below-ground biomass carbon (BGC) (BGC=16.11 NDVI-2.65, R2 =0.59, P <0.001). The BGC is a significant resource of soil carbon, accounting for approximately 93.72% of the total carbon biomass. BGC can be used to explain most variations in SOC density at different soil depths (55% for 0-10 cm, 44% for 0-30 cm and 62% for 0-50 cm). The distribution of BGC and SOC had spatial heterogeneity across the study region, being greater in the south and east, and less in the north and west. A liner relationship between BGC and NDVI was found, which could be used in future prediction models and carbon budget assessments of the study area and vicinity. [ABSTRACT FROM AUTHOR]

Conversion of cropland to alfalfa grassland is a traditional practice widely used for soil and water conservation in the semi-arid region of northwest China. The purpose of this study is to evaluate the effect of alfalfa grassland on soil properties at different elevations and soil textures, compared with normal cropland. Twenty-four sites (including twelve sandy loam and silty loam soil sites) were selected where alfalfa had been grown annually for eight years. Paired soil samples from adjacent croplands and alfalfa grasslands were sampled to a depth of ' 5 cm. The soil particle size fraction, pH, electrical conductivity, soil organic carbon (SOC), total nitrogen (N), easily oxidizable carbon (EOC), microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) were analyzed. The results show that: compared with adjacent cropland, alfalfa grassland samples showed that electrical conductivity and soil bulk density decreased in sandy loam soil, while the pH value increased significantly at both sandy loam and silty loam sites. Compared with the adjacent cropland, there were greater increases in the contents of SOC (2.0' g kg-1) and total N (0.14 g kg-1) in the alfalfa grassland at sandy loam sites than those at the silty loam sites (1.78 g kg-1 for SOC and 0.16 g kg-1 for total N). In addition, the mean contents of EOC, MBC and MBN in the alfalfa grassland increased by 103.90%, 59.81% and 136.71% respectively at the silty loam sites and increased by 58.53%, 40.09% and 98.90% at the sandy loam sites. The increment level and rates of EOC and MBC at sandy loam sites were greater than those at silty loam sites. There were positive relationships between elevation and SOC, EOC, MBC, and between elevation and increment ratios of SOC, EOC and MBC after conversion of cropland to alfalfa grassland at both silty loam and sandy loam sites. These results imply that the SOC accumulation potential at a high elevation region was larger than at low elevation in the study area. The increase content of silt and clay at sandy loam sites can be attributed to irrigation water which is silt-laden. This plays an important role in the SOC accumulation at sandy loam sites. This outcome could provide basic information for the suitable land management in the Hexi corridor of northwest China. [ABSTRACT FROM AUTHOR]