藏东南地区不同土地利用方式下土壤有机碳组分及周转变化特征

Impact of Land Use Type on Soil Organic Carbon Fractionation and Turnover in Southeastern Tibet

【目的】自然植被转变为农业用地显著影响土壤有机碳储量。青藏高原东南部地区森林或草地转换为农田的面积逐年增加,但其对土壤有机碳组分及周转特征的影响尚不明确。因此,阐明藏东南地区不同土地利用方式对土壤有机碳储量的影响程度和作用机制,可为该地区农业土地资源合理利用提供科学依据。【方法】采集藏东南地区长期耕作的农田(50年以上)及毗邻的自然森林和草地土壤,采用物理-化学联合分组技术以及稳定性碳同位素测定,分析3种土地利用方式下土壤有机碳组分的数量、碳含量的差异,探究不同有机碳组分周转差异及其对农田耕作的响应规律。【结果】农田0—20 cm表层土壤有机碳储量为(39.4±2.0) Mg C·hm^(-2),比自然森林的(81.5±8.5)Mg C·hm^(-2)和草地的(71.4±7.3)Mg C·hm^(-2)分别降低了约52%和45%。农田耕作导致粗颗粒有机质(cPOM)数量相对于自然植被降低了63.4%—70.8%,微团聚体(μagg)和黏粉粒(dSilt+Clay)的数量分别增加了10.0%—25.9%和65.7%—86.2%。农田土壤的有机碳含量与森林和草地土壤相比降低了51.7%—58.1%,其中不稳定性、物理稳定性和生物化学稳定性有机碳库分别降低79.8%—86.3%、72.4%—73.1%、32.4%—39.8%,且与总有机碳的变化显著正相关,但化学稳定性有机碳库没有显著变化。土地利用方式不同导致不同有机碳组分的C/N值和δ^(13)C值差异明显。农田土壤cPOM组分的C/N值(10.0±0.5)显著低于森林(13.5±0.4),而δ^(13)C值(-21.6±0.5)‰则显著高于森林土壤(-23.6±0.4)‰。微团聚体保护的颗粒有机质(iPOM)和难酸解组分(NH-dSilt+Caly和NH-μSilt+Clay)具有较低的δ^(13)C值(-25.3‰—-27.2‰),并且其C/N在农田土壤为8.4—9.4,显著低于森林土壤(13.5—15.9)。【结论】藏东南地区长期耕作的农田土壤有机碳储量相比于自然植被降低了约50%。农业耕作显著加速了不稳定颗粒有机质的周转,减少了稳定性有机碳组分如微团聚体保护的有机碳组分的形成,是导致土壤有机碳库明显下降的关键原因。因此,为有效降低农业耕作对土壤有机碳储量的负面影响,免耕和保护性耕作或可成为藏东南农耕区固碳增汇、维持该地区土地资源可持续利用的技术选择之一。

【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(δ^13C)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 andδ13C 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 itsδ13C value((-21.6±0.5)‰)was significantly higher than forest soils(-23.6±0.4)‰.Theδ13C 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.