Abstract:

To explore the response of soil organic carbon and total nitrogen sequestration to vegetation restoration in loess hilly region is not only of great significance for accurately evaluating the soil carbon sequestration effect of ecological restoration project on the Loess Plateau, but also the scientific basis for the country to actively promote the carbon neutral strategy and the construction of ecological civilization. Taking the typical ecological restoration area of Youyu County in the loess hilly region of northwest Shanxi Province as the research object, with adjacent farmland as a control, the characteristics of soil carbon and nitrogen and the effect of soil carbon (nitrogen) sequestration of different vegetation types under different restoration years were analyzed. The results showed that the SOC and STN sequestration effect of vegetation restoration could be divided into three stages: short-term, medium-term and long-term. In the short term (<20 a), the soil carbon (nitrogen) sequestration effect of returning farmland to woodland (grassland) is not obvious, and even leads to a decrease in the content and reserves of soil organic carbon and total soil nitrogen in the surface layer (0-20 cm),while the effect of returning farmland to shrubland on soil carbon and nitrogen sink is more significant. In the medium term (21-50 a), with the increase of restoration years, the sink effect of soil carbon and nitrogen of the three vegetation types were significant. After 21-35 years of restoration, the SOC and STN sequestration effect of grassland reached the highest value, and then the SOC(STN) content and storage began to decrease, while the carbon and nitrogen sink effect of woodland and shrubland was still significant. In the long term (>50 a), the soil carbon and nitrogen sequestration effect of woodland was still increasing, while the SOC and STN content and reserve of shrubland and grassland were lower than that of the control farmland. The medium and long-term effects of woodland on soil carbon and nitrogen sequestration were remarkable, while the short-term and medium-term effects of soil carbon and nitrogen sequestration in shrubland were significant, while for grassland, only in the mid-term (21-35 a) the soil carbon and nitrogen sequestration effect was shown. From the change of average rate of soil carbon and nitrogen of topsoil of three vegetation types, woodland showed a higher average soil carbon (nitrogen) sequestration rate in the middle and long-term, while the average soil carbon (nitrogen) sequestration rate of shrubland was the highest in the early stage (<20 a), and then gradually decreased with the increase of restoration years, and finally changed to negative value(>50 a); while that of grassland was only in 21-35a,the rate of soil carbon and nitrogen sequestration was positive and negative in other stages. After returning farmland to woodland (shrubland and grassland), the surface SOC content has a significant positive correlation with STN content, C/N and soil moisture content, and a significant negative correlation with soil bulk density. Soil C/N was more affected by SOC than STN. The improvement of soil moisture conditions could promote the accumulation of SOC and STN, and its influence on surface STN was even higher than SOC. As the SOC content of the surface layer increased, the cementation status of the soil particles would be changed, resulting in a decrease in soil bulk density.

Key words: soil organic carbon, soil total nitrogen, vegetation restoration, Northwest Shanxi Province, Loess Hilly Region

摘要：

探讨黄土丘陵区土壤碳氮特征及其蓄积速率对植被恢复的响应规律,不仅对于精确评估黄土高原生态恢复工程的土壤碳氮效应具有重要意义,同时也是国家积极推进碳中和战略和生态文明建设的科学基础。以晋西北黄土丘陵区—右玉县典型生态恢复区为研究对象,以相邻农田为对照,分析了不同植被类型在不同恢复年限下的土壤碳氮特征和固碳（氮）效应。结果表明：以20 a和50 a为界,植被恢复的土壤固碳（氮）效应可分为短期、中期和长期3个阶段。从短期（<20 a）来看,退耕还林（草）的土壤固碳（氮）效应不明显,甚至导致了土壤表层（0—20 cm）SOC和STN含量和储量的降低,而退耕还灌的土壤碳氮增汇效应较为显著;从中期（21—50 a）来看,随着恢复年限的增加,3种植被的土壤碳氮增汇效应显著,其中草地在恢复21—35 a后,土壤固碳（氮）达到最高值,之后其SOC、STN含量和储量开始降低,而林地和灌木地的土壤碳（氮）汇效应表现显著;从长期来看（>50 a）,林地表层土壤碳（氮）增汇效应仍持续增强,而灌木地和草地SOC、STN含量和储量,均低于对照农田。总体来看,退耕还林在中期和长期的土壤碳氮蓄积效应相当可观,退耕还灌在短期和中期的土壤碳氮蓄积效应表现显著;对于草地,仅在退耕中期（21—35 a）表现出一定的土壤碳氮增汇效应。从表层土壤的碳氮平均蓄积速率来看,林地在退耕中期和长期,均表现出较高的平均碳氮蓄积速率,而灌木地在退耕早期（<20 a）,其表层土壤平均碳氮蓄积速率最快,之后,随恢复年限的增加,逐步降低,最终转变为负值（>50 a）;而草地仅在退耕21—35 a,土壤碳氮蓄积速率表现为正值,其余阶段均为负值。退耕还林（草）后的表层SOC含量与STN含量、C/N和土壤含水量呈明显的正相关,与土壤容重之间呈显著的负相关。土壤C/N受SOC的影响较大;土壤水分条件的改善可促进SOC和STN的积累,其对表层STN的影响程度甚至高于SOC。随着表层SOC含量的增加,会改变土壤颗粒的胶结状况,从而导致土壤容重的降低。

关键词: 土壤有机碳, 土壤全氮, 植被恢复, 晋西北, 黄土丘陵区