Abstract: 【Objective】 Land use conversion from native vegetation to cropland has a great impact on soil organic carbon (SOC) storage. Recently, the area of croplands that converted from native forests or grasslands increases gradually in the southeastern part of Tibetan Plateau, yet its influence on SOC fractionation and turnover remains unknown. It is therefore in great need to understand the extent and mechanisms that difference in land use type has exerted on SOC storage, which will provide scientific basis for sustainable management of agricultural soils in southeastern Tibet. 【Method】In the present study, soil samples were collected from cropland that had been cultivated for more than 50 years, as well as adjacent native forest and grassland. A combination of physical and chemical method was conducted to partition the SOC into different fractions. For each SOC fractions, the present in soil mass, SOC content and stable carbon isotope composition (δ¹³C) were analyzed to investigate the differences in SOC fractionation and turnover under different land use types. 【Result】 The SOC that stored in top 20 cm soil of cropland was (39.4±2.0) Mg C·hm^-2, which was 52% and 45% lower than native forest ((81.5±8.5) Mg C·hm^-2) and grassland ((71.4±7.3) Mg C·hm^-2), respectively. Compared to native forests and grasslands, long-term cultivation led to a decrease of 63.4%-70.8% in the mass of coarse particulate organic matter (cPOM), whereas the mass of microaggregate (μagg) and easily dispersed silt and clay (dSilt+Clay) increased 10.0%—25.9% and 65.7%-86.2%, respectively. The C content in each SOC fraction was significantly lower in cropland soils than that in forest and grassland soils. SOC content in cropland soils was 51.7%-58.1% lower than that in forest and grassland soils. The SOC contents of unprotected C, physically protected C and biochemically protected C pool reduced 79.8%-86.3%, 72.4%-73.1% and 32.4%-39.8% in cropland soils, respectively, and were positively correlated with changes in total SOC content following land use conversion. The SOC content in chemically protected C pool, however, was not affected by land use conversion. The C/N ratio and δ13C differed among different SOC fractions and three land use types. The C/N ratio of cPOM in cropland soils (10.0±0.5) was significantly lower than forest soils (13.5±0.4), whereas its δ¹³C value ((-21.6±0.5) ‰) was significantly higher than forest soils (-23.6±0.4) ‰. The δ¹³C values of microaggregate-protected particulate organic matter (iPOM) and non-hydrolyzable fraction (NH-dSilt+Clay and NH-μSilt+Clay) were comparably lower (-25.3‰- -27.2‰) than other SOC fractions, and had significantly lower C/N ratio in cropland soils (8.4-9.4) compared to forest soils (13.5-15.9). 【Conclusion】Results of the present study indicated that long-term cultivation had resulted in c.a. 50% reduce in SOC stock compared to native vegetation in southeastern Tibet. Agricultural cultivation strongly promoted the turnover of unprotected particulate organic matter, and suppressed the formation of more stabilized SOC, such as microaggregate-protected SOC fraction, which contributed great proportions to the dramatic decrease in SOC storage. Therefore, implement of no-tillage or other protective management would be necessary to reduce the negative influence of agricultural land use on SOC storage and to maintain the sustainable utilization of natural soil sources in southeastern Tibet.

Key words: soil organic carbon, land use conversion, croplands, soil organic carbon fractionation, stable carbon isotope, Tibetan Plateau