Soil Organic Carbon Storage And Sources In Deep Soil In The Hilly Loess Plateau Region

Vegetation restoration based mainly on conversion of cropland to forest andgrassland was an important ecological construction project in loess hilly region,which not only changed the land use and topsoil carbon sequestration status inthe region, but also affected the deep layer soil carbon storage as well asecosystem carbon cycle. At present only a few research were carried out on deepsoil organic carbon. The accumulation process and sequestration mechanism isunclear, which may lead to bias estimates of soil carbon storage. To meet therequirement of soil organic carbon research and assessment of the benefits ofsoil carbon sequestration on the Loess Plateau, through field investigation andsampling, using13C isotopes method, this paper was designed to explore deepsoil organic carbon storage, its distribution in profile, accumulation dynamicsand its response to LUCC under different land use types and land use years. Thesources of deep soil organic carbon were also discriminated. T he results couldprovide scientific basis for estimating regional soil carbon storage andcertification of carbon sequestration in the Loess Plateau. The main conclusionswere as follows:(1) The distribution of soil organic carbon storage in soil profile underdifferent vegetation types in the loess hilly region was: the storage decreasedsignificantly as the depth increased and ranged from5.10to12.85t hm-2in the0–100cm layer; but the storage just varied from5.12to6.13t hm-2, and therewere only slight fluctuations among different soil layers.(2) In0-100cm soil profile, the organic carbon storage decreasedsignificantly with the increase of soil depth. However, in deep soil, the organiccarbon storage had a slight fluctuation. The total organic carbon storage in100-400cm soil profile was considerably high, accounting for approximately 60%of that in0-400cm soil profile. The organic carbon storage in80-100cmsoil layer had a significant linear correlation with that in100-200cm and200-400cm soil layers, and among the organic carbon storages in the five layersin0-100cm soil profile, the organic carbon storage in80-100cm soil layer hadthe strongest correlation with that in100-400cm soil profile, being able to beused to estimate the soil organic carbon storage in deep soil in this region.(3) The organic carbon storage in0-20cm soil layers in the three types ofrevegetation lands was significantly higher than that in slope croplands, but theorganic carbon storage in deep soil had no significant difference among the landuse types. The organic carbon storage in deep soil increased with the increasingyears of revegetation. In R. pseudoacacia woodlands and C. korshinskii shrublands, the average increasing rate of the organic carbon storage in100-400cmsoil layer was0.14and0.19t hm-2a-1, respectively, which was comparable tothat in the0-100cm soil layer. It was suggested that in the estimation of the soilcarbon sequestration effect of revegetation in the hilly Loess Plateau region, theorganic carbon accumulation in deep soil should be taken into consideration.Otherwise, the effect of revegetation on soil carbon sequestr ation could besignificantly underestimated.(4) The δ13C value of soil organic carbon increased as the depth increasedin the soil profile of R. pseudoacacia woodlands, whereas in slope croplands,theδ13C value had a slight fluctuation. The proportion of organic carbon thatcome from the plant residues of R. pseudoacacia woodlands in0-20cm was69.6%, which was much higher than the proportion that between7.0~17.0%in100-300cm soil layer.