Study On Soil Organic Carbon Stability In The Deep Soil Layer Of The Hilly Loess Plateau, China

Understanding soil organic carbon （SOC） stability in the deep soil layer is important tosoil carbon sequestration benefits evaluation and certification. Aiming at the key scientificproblems about the stability of organic carbon in deep soil layer on the hilly Loess Plateau,the stability mechanism, stable condition and provide the basis for certification of the deepsoil carbon sinks were explored by studying the characteristics of soil organic carbon （SOC）mineralization, physical components and the factors affecting the stability of SOC in deep soillayer under the major vegetation types. The main conclusions were as follows:1. Considering SOC mineralization rate, SOC stability in deep layer（100-400cm）was similar to the shallow layer. However, considering the SOC mineralization process,the release rate of CO₂in the deep soil layer was slower than that in the shallow layer.（1） There was a downward trend in the total SOC mineralization with the soil depthincreasing. The total SOC mineralization in the sub-deep soil（100-200cm） and deepsoil（200-400cm） were equivalent to approximately88.1%and67.8%of that in the shallowlayer （0-100cm）.（2）Throughout the culturing phase, as the same as the shallow soil, theprocess of mineralization in sub-deep and deep soil could be divided into3stages. In therapid decomposition phase, the ratio of the mineralization to the total mineralization in thesub-deep and deep layer（0-10d） was approximately50%of that in the shallow layer（0-17d）.Inthe slow decomposition phase, the ratio of sub-deep, deep layer（11-45d） was150%of that inthe shallow layer（18-45d）. There was no significant difference of the ratio among three soillayers （46-62d） in the relatively stable stage.（3）There was no significant difference （P>0.05）in the mineralization rate of SOC among the shallow, sub-deep, deep layers. The stability ofSOC in the deep soil layer （100-400cm） was similar to the shallow soil layer.2. Considering the SOC physical form, the content and distribution ratio of soil lightfraction organic carbon （LFOC） in the deep layer were fewer than those in the shallowlayer. Moreover, the content and distribution ratio of soil LFOC in the sub-deep anddeep soil of that in the shallow layer reduced after cropland shifting to shrubland andwoodland. Compared with that in the shallow soil layer, the stability of deep soil organiccarbon was enhanced respectively in the process of revegetation.（1） There was a significant declining trend in the content and distribution ratio of soil light fraction organic carbon along with the increasing of soil depth under the studied land usetypes. The content of soil light fraction organic carbon ranged from0.09to1.76g/kg, whilethe distribution ratio of soil light fraction organic carbon ranged from4.19%to32.24%.Under the three land use types, the content and distribution ratio of soil light fraction organiccarbon in the sub-deep soil and deep soil of that in the shallow layer ranged from12.4%to39.8%,28.7%to66.2%, respectively. The decline rates of the soil light fraction organiccarbon content and distribution ratio were Robinia pseudoacacia woodlands＞abandonedcroplands＞Caragana korshinskii shrublands＞slope croplands.（2） The content anddistribution ratio of soil light fraction organic carbon in the same soil layer among the threeland use types were different. The content and distribution ratio of soil light fraction organiccarbon both were forestlands＞abandoned croplands＞slope lands in these three layers.（3）Both the content and distribution ratio of soil light fraction organic carbon were increasedafter cropland shifting to shrubland and woodland, while those in the sub-deep and deep soilof that in the shallow layer was reduced. Compared with that in the shallow soil layer, thestability of deep soil organic carbon was enhanced respectively in the process of revegetation.3. On the whole soil profile, SOC mineralization was mainly affected by the soillayers. Compared with the shallow layer, the response of SOC mineralization in the deeplayer to moisture variation was consistent, while the response of that to temperaturevariation was greater.（1） On the whole soil profile, effects of soil layers and temperature on SOCmineralization, mineralization rate were great, while effects of vegetation type and soilmoisture on SOC mineralization, mineralization rate were smaller relatively. Effect of theinteraction between temperature and moisture on these was moderate.（2） Compared with theshallow layer, the response of SOC mineralization in the deep layer to moisture variation wasconsistent, while the response of that to temperature variation was greater: When soilmoisture increased from4%to18%, the mineralization, mineralization rate, CO₂-C/SOC inthe deep layer increased firstly and then decreased. And there was no significant difference inamplitude of variation between the deep layer and the shallow layer. When the temperatureincreased from10℃to30℃, similar to the shallow layer, the mineralization, mineralizationrate, CO₂-C/SOC in the deep layer increased gradually. The amplitude of variation in the deeplayer was greater than that in the shallow layer.4. Among the soil physical and chemical properties which was associated with thestability index of SOC, the contribution of LFOC to the variation of SOC mineralizationwas42.3%, clay content and SOC contributed about44.0%of the variation. TheCO₂-C/SOC was mainly affected by SOC and clay content. The contribution of SOC and soil texture to the variation of the content and distribution ratio of soil LFOC were60.0%and67.9%respectively. Total nitrogen content and C/N contributed to thevariationo of the content and distribution ratio of soil LFOC for about32%and30%.