黄土丘陵区不同退耕还林地土壤有机碳、氮密度变化特征

探讨了黄土丘陵区退耕种植10～40a的柠条、侧柏及刺槐林地0—60cm不同土层有机碳及全氮密度随退耕年限及在土层分布上的变化特征。结果表明:不同土层相比,退耕栽植柠条、侧柏、刺槐10～40a后0—20cm土层有机碳密度平均比20—60cm增加4.20,6.87,4.46Mg/hm2;0—20cm土层的全氮密度比20—60cm平均增加0.08,0.02,0.07Mg/hm2。与坡耕地比较,0—20cm土层在退耕30a中固碳速率为侧柏0.33Mg/(hm2·a)]>刺槐0.28Mg/(hm2·a)]>柠条0.17Mg/(hm2·a)],固氮速率则为刺槐0.03Mg/(hm2·a)]>侧柏0.02Mg/(hm2·a)]>柠条0.01Mg/(hm2·a)],且碳氮固定速率均显著高于深层土壤。10～30a不同退耕还林地增加的有机碳、氮平均分别有57%和51%来自0—20cm的土层。不同退耕还林地土壤C/N随土层深度的增加而减小。综上,退耕还林土壤表现出显著的提升土壤碳氮的效应,且以侧柏林地固碳能力较佳,刺槐林地固氮效果较好。

Change Characteristics of Density of Soil Organic Carbon and Nitrogen under Land Shifted into Forestland in Hilly Loess Region

This study was conducted to determine the change characteristics of the density of soil organic carbon and total nitrogen and distribution in soil layers of 0—60 cm with 10~40 years of conversion of cropland to forest in Hilly Loess Region.The results showed that: compared different soil layers in forestland shifted from cropland for 10~40 years,the average organic carbon density in 0—20 cm soil layer of caragana,orientalis and robinia was 4.20,6.87,4.46 Mg/hm2,which was higher than that in 20—60 cm soil layer in caragana,orientalis and robinia forest land,respectively,and average nitrogen density was increased 0.08,0.02,0.07 Mg/hm2,which was higher than that in 20—60 cm soil layer,respectively.Compared with slope farmland,the organic carbon sequestration rate in 0—20 cm soil layer within 30 years of conversion of cropland to forestland follow the order of orientalis(0.33 Mg·hm-2·a-1)>robinia(0.28 Mg·hm-2·a-1)>caragana(0.17 Mg·hm-2·a-1),and nitrogen sequestration rate follow the order of robinia(0.03 Mg·hm-2·a-1)>orientalis(0.02 Mg·hm-2·a-1)>caragana(0.01 Mg·hm-2·a-1).The carbon and nitrogen sequestration rate of 0—20 cm soil layer was higher than that in deep soil layer.Average 57% of increased soil organic carbon and 51% of increased nitrogen were contributed by 0—20 cm soil during 10~30 years of conversion of cropland to forest.The ratio of C/N decreased with soil depth under all lands of conversion of cropland to forest.In conclusion,conversion of cropland to forestland could significantly increase soil carbon and nitrogen stocks,and orientalis was better for carbon sequestration and robinia was better for nitrogen sequestration.