黄土丘陵区土壤有机碳固存对退耕还林草的时空响应

研究了黄土丘陵区土壤有机碳固存对退耕还林草的时空响应特征,分析了退耕还林草对土壤有机碳的近期影响和长期效应。结果表明,1)从黄土丘陵区退耕还林草的土壤固碳效应整体而言,相对于坡耕地,退耕还林和退耕撂荒具有显著的土壤碳增汇效应,而退耕还草、退耕还果没有明显土壤碳增汇效应。以天然草地土壤有机碳密度为目标,撂荒地表层土壤有机碳增汇潜力可达8.3 t/hm2。2)以10a为界,退耕还林草的近期土壤碳增汇效应不明显,而10a后土壤碳增汇效应逐渐明显,退耕还林、还灌、撂荒和坡耕地的固碳效应差异显著。3)在评估黄土丘陵区退耕还林草的土壤固碳效应时应当注重长期固碳效应。4)退耕还林草的土壤固碳效应主要受还林草方式及年限的影响,二者分别可解释55.6%和24.1%的有机碳变异性;地形因子可解释8.5%的有机碳变异性。在评估该区退耕还林的土壤固碳效应时应当充分考虑退耕年限和地形因子的影响。5)人工刺槐林地、人工柠条林地以及撂荒地深层土壤(100—200 cm)有机碳密度占2 m土体有机碳密度的35%—40%,而且随着植被恢复深层土壤有机碳密度显著增加。6)在估算黄土丘陵区退耕还林土壤固碳效应时应该考虑深层碳累积。如果按1 m土层的土壤有机碳密度计算,会严重低估退耕还林草的土壤固碳量。

Response of soil organic carbon sequestration to the "Grain for Green Project" in the hilly Loess Plateau region

Land use change is one of the major factors affecting soil organic carbon(SOC) and global carbon balance.The Grain for Green Project,implemented in the hilly Loess Plateau region,has remarkably changed the land use types in this region.However,it is not clear how this has affected SOC sequestration.Our objectives were(Ⅰ) to determine the temporal and spatial response of SOC sequestration to the land use changes,(Ⅱ) to understand the critical factors impacting the SOC sequestration.The research was conducted on the central of the hilly Loess Plateau region,which covered an area of 707km2.Soil samples were taken in five catchments with different revegetation types and revegetation chronosequence.The results showed that at the regional scale,natural grasslands,planted shrub lands,planted wood lands and naturally revegetated grasslands showed remarkable SOC sequestration compared with slope croplands.However,no significant increase in SOC sequestration was found in planted grasslands and orchards compared with slope croplands.Setting the SOC density in natural grassland as the target,the naturally revegetated grassland had a SOC sequestration potential of 8.3t/hm2 in top soil(0—20cm).We found no increase in SOC sequestration in the first 10 years after the Grain for Green Project implemented.However,after 10 years,we saw a significant increase in SOC sequestration in planted woodlands,planted shrub lands,revegetated grasslands and slope croplands.The effect of the Grain for Green project on SOC sequestration was mainly influenced by land use type and age,which explained 55.6% and 24.1% of the variation of SOC.Topographic factors,including slope aspect and slope position explained 8.5% of the variation in SOC.The SOC density in the deep soil layer(100—200 cm) in the planted woodland(dominated by Robinia pseudoacacia),planted shrub land(dominated by Caragana Korshinskii) and revegetated grassland was about 35%—40% of that in the 0—200 cm soil profile.The SOC sequestration in deep soil layer(100—200cm) was significantly increased after revegetation.Therefore,our study showed that evaluation of the Grain for Green project needs to account for 1) land use type and age,2) SOC sequestration in the deep soil layer and 3) long-term effects(>10 years) rather than short-term effects.Otherwise,the effect of this program on SOC sequestration will be significantly under-estimated.