Effects of spatio-temporal changes of land-use on soil organic carbon in yangjuangou watershed in Loess Plateau, China

Land use change is an important driving factor for changes of storage and spatial distribution of soil organic carbon (SOC). In order to study effects of land use change on SOC, we analyzed the spatio-temporal changes of land use and their effects on SOC content, density and storage of topsoil based on soil sampling data and land-use map from 2006 to 2011 in Yangjuangou watershed. The main conclusions include that, (1) Land use types in this watershed changed significantly from 2006 to 2011. The area of grass land, which mainly transformed into shrub land and arboreal land, decreased the most with 64.030 hm², following by sloping farmland mainly transformed into terrace; the area of shrub land and arboreal land increased rapidly by 39.697 hm², and 46.404 hm²respectively, which were converted from the grass land, terrace, sloping farmland, dam land and construction land; the area of terrace was so small that the transformation was not obvious. (2) Land use change caused change of density and storage of SOC. The increase of carbon storage due to land type changes was 587.25 kg, among which the contribution from changed land types converted from grass land was 441.64 kg. Within all the land use transformations, ‘grass land to shrub land’, ‘grass land to arboreal land’, ‘sloping farmland to grass land’, ‘sloping farmland to shrub land’, ‘sloping farmland to arbor land’, ‘sloping farmland to terrace’, ‘sloping farmland to terrace’, ‘terrace to grass land’, ‘terrace to shrub land’, ‘terrace to arboreal land’, ‘terrace to dam land’, ‘dam land to grass land’, ‘dam land to shrub land’, and ‘dam land to arboreal land’showed higher SOC densities than those of the original land use types. It indicated that these land use transformations were conducive to increase of surface SOC and reduction of carbon emissions, which were beneficial to formation of surface soil carbon sink. However, other land use transformations, which caused increase of surface soil organic carbon emissions, should be paid more attention to. (3) The content, density and storage of SOC didn’t increase so much in Yangjuangou watershed from 2006 to 2011. Nevertheless, soil carbon sequestration should focus on the long-term effects, and frequent transformations of land use types may reduce the intercepting effect of soil carbon. The long-term utilization and maintain of vegetation reconstruction are much more conducive to accumulation of SOC in the Loess hilly-gully regions. Different transformations of land use types led to different regional changes of SOC storage. Those land use types which converted into shrub land and arboreal land contained the most SOC storage. Therefore, it is of extremely importance to protect forests to mitigate global warming, and the attention should be paid to afforestation and plantation tending management. Only seeking out a suitable mode of land resources management, which is beneficial to increase of sequestration of atmospheric CO₂ through soil and could reverse land degradation process, can it be the best choice for mitigating rising levels of CO₂.