Revegetation with artificial plants improves topsoil hydrological properties but intensifies deep-soil drying in northern Loess Plateau,China

Knowledge about the effects of vegetation types on soil properties and on water dynamics in the soil profile is critical for revegetation strategies in water-scarce regions, especially the choice of vegetation type and human management measures. We focused on the analysis of the effects of vegetation type on soil hydrological properties and soil moisture variation in the 0–400 cm soil layer based on a long-term(2004―2016) experimental data in the northern Loess Plateau region, China. Soil bulk density(BD), saturated soil hydraulic conductivity(Ks), field capacity(FC) and soil organic carbon(SOC) in 2016, as well as the volumetric soil moisture content during 2004–2016, were measured in four vegetation types, i.e., shrubland(korshinsk peashrub), artificial grassland(alfalfa), fallow land and cropland(millet or potato). Compared with cropland, revegetation with peashrub and alfalfa significantly decreased BD and increased Ks, FC, and SOC in the 0–40 cm soil layer, and fallow land significantly increased FC and SOC in the 0–10 cm soil layer. Soil water storage(SWS) significantly declined in shrubland and grassland in the 40–400 cm soil layer, causing severe soil drought in the deep soil layers. The study suggested that converting cropland to grassland(alfalfa) and shrubland(peashrub) improved soil-hydrological properties, but worsened water conditions in the deep soil profile. However, natural restoration did not intensify deep-soil drying. The results imply that natural restoration could be better than revegetation with peashrub and alfalfa in terms of good soil hydrological processes in the semi-arid Loess Plateau region.     

Soil hydraulic conductivity as affected by vegetation restoration age on the Loess Plateau,China

The Loess Plateau of China has experienced extensive vegetation restoration in the past several decades,which leads to great changes in soil properties such as soil bulk,porosity,and organic matter with the vegetation restoration age.And these soil properties have great effect on the soil infiltration and soil hydraulic conductivity.However,the potential changes in soil hydraulic conductivity caused by vegetation restoration age have not been well understood.This study was conducted to investigate the changes in soil hydraulic conductivity under five grasslands with different vegetation restoration ages(3,10,18,28 and 37 years)compared to a slope farmland,and further to identify the factors responsible for these changes on the Loess Plateau of China.At each site,accumulative infiltration amount and soil hydraulic conductivity were determined using a disc permeameter with a water supply pressure of –20 mm.Soil properties were measured for analyzing their potential factors influencing soil hydraulic conductivity.The results showed that the soil bulk had no significant changes over the initial 20 years of restoration(P0.05);the total porosity,capillary porosity and field capacity decreased significantly in the grass land with 28 and 37 restoration ages compared to the slope farmland;accumulative infiltration amount and soil hydraulic conductivity were significantly enhanced after 18 years of vegetation restoration.However,accumulative infiltration amount and soil hydraulic conductivity fluctuated over the initial 10 years of restoration.The increase in soil hydraulic conductivity with vegetation restoration was closely related to the changes in soil texture and structure.Soil sand and clay contents were the most influential factors on soil hydraulic conductivity,followed by bulk density,soil porosity,root density and crust thickness.The Pearson correlation coefficients indicated that the soil hydraulic conductivity was affected by multiply factors.These results are helpful to understand the changes in hydrological and erosion processes response to vegetation succession on the Loess Plateau.     

Changes in soil carbon stocks and related soil properties along a 50-year grassland-to-cropland conversion chronosequence in an agro-pastoral ecotone of Inner Mongolia, China

Land use change significantly influences soil properties.There is little information available on the long-term effects of post-reclamation from grassland to cropland on soil properties.We compared soil carbon(C) and nitrogen(N) storage and related soil properties in a 50-year cultivation chronosequence of grassland in the agro-pastoral ecotone of Inner Mongolia.Field surveys on land use changes during the period of 1955-2002 were conducted to build a chronosequence of cropland of different ages since the conversion from grassland.The results showed that soil C and N storage,soil texture,and soil nutrient contents varied with land use types and cropland ages(P<0.01).In the 0-30 cm soil layer,the soil organic carbon(SOC) density was significantly lower in the croplands(3.28 kg C/m2 for C50 soil) than in the grasslands(6.32 kg C/m2).After 5,10,15,20,35,and 50 years of crop planting(years since the onset of cultivation),the SOC losses were 17%,12%,19%,47%,46%,and 48%,respectively,compared with the grasslands.The soil total nitrogen(TN) density of the grasslands was 65 g N/m2,and TN density of the cropland soil was 35 g N/m2 after 50 years of crop planting.Both the SOC and TN densities could be quantitatively determined by a negative exponential function of cropland age(P<0.0001,R2=0.8528;P<0.0001,R2=0.9637).The dissolved organic carbon(DOC) content,soil available potassium(AK) content,clay content,and pH value were decreased;and the soil bulk density and sand content were increased since the conversion of grassland into cropland during the 50-year period.Our results show soil nutrients were higher in grassland than in cropland.The conversion of grasslands to croplands induced a loss of soil C storage and changes of related soil properties.The reclamation time of cultivated soil(cropland age) had significant effects on soil properties in the study area.     

Effects of land-use types on the vertical distribution of fractions of oxidizable organic carbon on the Loess Plateau,China

The oxidizability of soil organic carbon(SOC_) influences soil quality and carbon sequestration. Four fractions of oxidizable organic carbon(very labile(C_1), labile(C_2), less labile(C_3) and non-labile(C_4)) reflect the status and composition of SOC_ and have implications for the change and retention of SOC_. Studies of the fractions of oxidizable organic carbon(OC_) have been limited to shallow soil depths and agroecosystems. How these fractions respond at deep soil depths and in other types of land-use is not clear. In this study, we evaluated the vertical distributions of the fractions of oxidizable organic carbon to a soil depth of 5.0 m in 10 land-use types in the Zhifanggou Watershed on the Loess Plateau, C_hina. Along the soil profile, C_1 contents were highly variable in the natural grassland and shrubland I(C_aragana microphylla), C_2 and C_4 contents were highly variable in the natural grassland and two terraced croplands, respectively, and C_3 contents varied little. Among the land-use types, natural grassland had the highest C__1 and C_2 contents in the 0–0.4 m layers, followed by shrubland I in the 0–0.1 m layer. Natural grassland had the highest C_4 contents in the 1.0–4.5 m layers. Natural grassland and shrubland I thus contributed to improve the oxidizability of SOC_ in shallow soil, and the deep soil of natural grassland has a large potential to sequester SOC_ on the Loess Plateau.     

Dynamics of ecosystem carbon stocks during vegetation restoration on the Loess Plateau of China

In the last few decades, the Loess Plateau had experienced an extensive vegetation restoration to reduce soil erosion and to improve the degraded ecosystems. However, the dynamics of ecosystem carbon stocks with vegetation restoration in this region are poorly understood. This study examined the changes of carbon stocks in mineral soil(0–100 cm), plant biomass and the ecosystem(plant and soil) following vegetation restoration with different models and ages. Our results indicated that cultivated land returned to native vegetation(natural restoration) or artificial forest increased ecosystem carbon sequestration. Tree plantation sequestered more carbon than natural vegetation succession over decades scale due to the rapid increase in biomass carbon pool. Restoration ages had different effects on the dynamics of biomass and soil carbon stocks. Biomass carbon stocks increased with vegetation restoration age, while the dynamics of soil carbon stocks were affected by sampling depth. Ecosystem carbon stocks consistently increased after tree plantation regardless of the soil depth; but an initial decrease and then increase trend was observed in natural restoration chronosequences with the soil sampling depth of 0–100 cm. Moreover, there was a time lag of about 15–30 years between biomass production and soil carbon sequestration in 0–100 cm, which indicated a long-term effect of vegetation restoration on deeper soil carbon sequestration.     

Effects of fencing on vegetation and soil restoration in a degraded alkaline grassland in northeast China

In order to restore a degraded alkaline grassland, the local government implemented a large restoration project using fences in Changling county, Jilin province, China, in 2000. Grazing was excluded from the protected area, whereas the grazed area was continuously grazed at 8.5 dry sheep equivalent（DSE）/hm2. In the current research, soil and plant samples were taken from grazed and fenced areas to examine changes in vegetation and soil properties in 2005, 2006 and 2008. Results showed that vegetation characteristics and soil properties improved significantly in the fenced area compared with the grazed area. In the protected area the vegetation cover, height and above- and belowground biomass increased significantly. Soil pH, electrical conductivity and bulk density decreased significantly, but soil organic carbon and total nitrogen concentration increased greatly in the protected area. By comparing the vegetation and soil characteristics with pre-degraded grassland, we found that vegetation can recover 6 years after fencing, and soil pH can be restored 8 years after fencing. However, the restoration of soil organic carbon, total nitrogen and total phosphorus concentrations needed 16, 30 and 19 years, respectively. It is recommended that the stocking rate should be reduced to 1/3 of the current carrying capacity, or that a grazing regime of 1-year of grazing followed by a 2-year rest is adopted to sustain the current status of vegetation and soil resources. However, if N fertilizer is applied, the rest period could be shortened, depending on the rate of application.     

Impact of land use/cover changes on carbon storage in a river valley in arid areas of Northwest China

Soil carbon pools could become a CO_2 source or sink, depending on the directions of land use/cover changes. A slight change of soil carbon will inevitably affect the atmospheric CO_2 concentration and consequently the climate. Based on the data from 127 soil sample sites, 48 vegetation survey plots, and Landsat TM images, we analyzed the land use/cover changes, estimated soil organic carbon(SOC) storage and vegetation carbon storage of grassland, and discussed the impact of grassland changes on carbon storage during 2000 to 2013 in the Ili River Valley of Northwest China. The results indicate that the areal extents of forestland, shrubland, moderate-coverage grassland(MCG), and the waterbody(including glaciers) decreased while the areal extents of high-coverage grassland(HCG),low-coverage grassland(LCG), residential and industrial land, and cultivated land increased. The grassland SOC density in 0–100 cm depth varied with the coverage in a descending order of HCGMCGLCG.The regional grassland SOC storage in the depth of 0–100 cm in 2013 increased by 0.25×1011 kg compared with that in 2000. The regional vegetation carbon storage(S_(rvc)) of grassland was 5.27×10~9 kg in2013 and decreased by 15.7% compared to that in 2000. The vegetation carbon reserves of the under-ground parts of vegetation(S_(ruvb)) in 2013 was 0.68×10~9 kg and increased by approximately 19.01%compared to that in 2000. This research can improve our understanding about the impact of land use/cover changes on the carbon storage in arid areas of Northwest China.     

Fractal features of soil profiles under different land use patterns on the Loess Plateau,China

Fractal theory is becoming an increasingly useful tool to describe soil structure dynamics for a better understanding of the performance of soil systems. Changes in land use patterns significantly affect soil physical, chemical and biological properties. However, limited information is available on the fractal characteristics of deep soil layers under different land use patterns. In this study, the fractal dimensions of particle size distribution(PSD) and micro-aggregates in the 0–500 cm soil profile and soil anti-erodibility in the 0–10 cm soil profile for 10 typical land use patterns were investigated in the Zhifanggou Watershed on the Loess Plateau, China. The 10 typical land use patterns were: slope cropland, two terraced croplands, check-dam cropland, woodland, two shrublands, orchard, artificial and natural grasslands. The results showed that the fractal dimensions of PSD and micro-aggregates were all significantly influenced by soil depths, land use patterns and their interaction. The plantations of shrubland, woodland and natural grassland increased the amount of larger micro-aggregates, and decreased the fractal dimensions of micro-aggregates in the 0–40 cm soil profile. And they also improved the aggregate state and aggregate degree and decreased dispersion rate in the 0–10 cm soil profile. The results indicated that fractal theory can be used to characterize soil structure under different land use patterns and fractal dimensions of micro-aggregates were more effective in this regard. The natural grassland may be the best choice for improving soil structure in the study area.     

Influence of vegetation parameters on runoff and sediment characteristics in patterned Artemisia capillaris plots

Vegetation patterns are important in the regulation of earth surface hydrological processes in arid and semi-arid areas. Laboratory-simulated rainfall experiments were used at the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Yangling, northwestern China, to quantify the effects of Artemisia capillaris patterns on runoff and soil loss. The quantitative relationships between runoff/sediment yield and vegeta- tion parameters were also thoroughly analyzed using the path analysis method for identifying the reduction mechanism of vegetation on soil erosion. A simulated rainfall intensity of 90 mm/h was applied on a control plot without vegetation(C0) and on the other three different vegetation distribution patterns: a checkerboard pattern(CP), a banded pattern perpendicular to the slope direction(BP), and a single long strip parallel to the slope direction(LP). Each patterned plot received two sets of experiments, i.e. intact plants and roots only, respectively. All treatments had three replicates. The results showed that all the three other different patterns(CP, BP and LP) of A. capillaris could effectively reduce the runoff and sediment yield. Compared with C0, the other three intact plant plots had a 12%–25% less runoff and 58%–92% less sediment. Roots contributed more to sediment reduction(46%–70%), whereas shoots contributed more to runoff reduction(57%–81%). BP and CP exhibited preferable controlling effects on soil erosion compared with LP. Path analysis indicated that root length density and plant number were key pa- rameters influencing runoff rate, while root surface area density and root weight density were central indicators affecting sediment rate. The results indicated that an appropriate increase of sowing density has practical signifi- cance in conserving soil and water.     

Vertical root distribution and root cohesion of typical tree species on the Loess Plateau,China

Black locust(Robinia pseudoacacia L.) and Chinese pine(Pinus tabulaeformis Carr.) are two woody plants that are widely planted on the Loess Plateau for controlling soil erosion and land desertification. In this study, we conducted an excavation experiment in 2008 to investigate the overall vertical root distribution characteristics of black locust and Chinese pine. We also performed triaxial compression tests to evaluate the root cohesion(additional soil cohesion increased by roots) of black locust. Two types of root distribution, namely, vertical root(VR) and horizontal root(HR), were used as samples and tested under four soil water content(SWC) conditions(12.7%, 15.0%, 18.0% and 20.0%, respectively). Results showed that the root lengths of the two species were mainly concentrated in the root diameter of 5–20 mm. A comparison of root distribution between the two species indicated that the root length of black locust was significantly greater than that of Chinese pine in nearly all root diameters, although the black locust used in the comparison was 10 years younger than the Chinese pine. Root biomass was also significantly greater in black locust than in Chinese pine, particularly in the root diameters of 3–5 and 5–10 mm. These two species were both found to be deep-rooted. The triaxial compression tests showed that root cohesion was greater in the VR samples than in the HR samples. SWC was negatively related to both soil shear strength and root cohesion. These results could provide useful information on the architectural characteristics of woody root system and expand the knowledge on shallow slope stabilization and soil erosion control by plant roots on the Loess Plateau.     

黄土丘陵沟壑区退耕植被恢复地土壤水稳性团聚体的变化特征

以黄土高原典型丘陵沟壑区—吴旗县为例,研究了退耕地不同植被恢复年限、不同植被类型和不同恢复方式下的土壤水稳性团聚体含量的变化。结果表明,>0.25mm土壤水稳性团聚体含量,在0～60cm土层随着退耕地恢复年限的延长而不断增加,乔木地相对较高,草地相对较低,灌木地介于两者之间。自然恢复方式>自然 人工恢复方式>人工恢复方式,坡耕地的团粒含量最低;在0～20cm土层,恢复前期土壤水稳性团聚体含量增幅较大,后期增幅较小,而40～60cm土层的变化规律则相反。对同一种植被类型,>5mm的土壤水稳性团聚体含量从0～20cm到40～60cm土层均呈下降趋势,并且在较浅土层灌木地的降幅相对较大,在较深土层乔木地的降幅相对较大。人为干预使得土壤水稳性团聚体从大粒径向小粒径转化,这种转化在较深土层表现的较明显。可见,在退耕地植被恢复中,具有较长恢复年限的乔木地,土壤的抗蚀性较强;自然 人工的恢复方式既缩短了植被恢复时间,也提高了土壤水稳性团聚体含量,从而改善了土壤结构。     

黄土高原森林草原区退耕还林还草土壤保持效应评估

黄土高原地处生态过渡带和环境脆弱区,区内大范围的土壤侵蚀严重影响了当地的生态环境。以黄土高原森林草原区为研究对象,应用修正通用土壤流失方程,根据2000、2005、2010年气象数据及土地利用等数据,从不同坡度、植被覆盖度、土地利用类型评估了黄土高原森林草原区退耕还林还草工程的土壤保持效应。结果表明,(1)随着退耕还林还草工程的实施,林地和草地面积明显增加,分别增加2 219.41 km~2、2 205.27 km~2,研究区植被覆盖度逐渐改善。(2)2000—2010年土壤保持量增加2.41亿t,单位面积土壤保持量由3 033.15 t·km~(-2)·a~(-1)增加至5 114.86 t·km~(-2)·a~(-1),土壤保持效应显著提升。(3)研究区土壤保持效应与植被覆盖度呈正相关关系,在不同土地利用类型中,林地、草地和耕地具有较高的土壤保持效应,单位面积土壤保持量分别为5 405.57、3 598.41、3 078.81 t·km~(-2)·a~(-1)。退耕还林还草工程的实施提升了区域的土壤保持效应,但是,区内东北部由于矿产资源开采导致的植被破坏、地表塌陷以及土壤侵蚀问题亟待解决。     

红砂植被盖度对土壤不同形态碳、氮及细菌多样性的影响

以自然恢复的红砂群落为研究对象,探讨黄土高原红砂植被不同盖度对土壤不同形态碳、氮及细菌多样性的影响,为该地区的人工生态恢复措施提供理论支撑.在兰州市南北两山植被恢复技术研究与示范基地,按照5级盖度分类法设置红砂植被盖度梯度,重点对土壤养分碳氮、微生物量碳氮和细菌多样性开展研究.结果表明:随着植被恢复,土壤有机碳 (SOC)和全氮(TN)、土壤微生物碳 (MBC)和微生物氮(MBN)逐渐提高,并且增加比较快 ,但是当总盖度达到48.73%之后,增加比较缓慢,而且增加的差异不显著.相同的植被盖度对土壤有机碳、全氮和土壤微生物碳、氮的影响趋于一致.土壤细菌多样性随植被盖度有所增加,在植被盖度达到48.73%后多样性维持在彼此接近的水平,尽管微生物多样性群落结构有差异 .在植被稀疏、物种多样性较低的干旱坡地,红砂植被盖度增加明显改善了土壤生态功能, 但是片面追求植被盖度的增加,对土壤特性改善有限.     

退耕年限与方式对土壤团聚体稳定性及有机碳分布的影响

以黄土高原南部退耕还林年限6 a(FL06)和15 a(FL15)刺槐林地、退耕还草年限6 a(GL06)和15 a(GL15)紫花苜蓿草地为研究对象,以临近长期耕作坡耕地(CK)作为对照,采用湿筛法,分离出2 mm、1~2 mm、0.5~1 mm、0.25~0.5 mm和0.25 mm 5个粒级的水稳性团聚体,研究了退耕年限与方式对团聚体稳定性和不同粒径团聚体有机碳分布的影响。结果表明:在0~20 cm土层,退耕还林还草与未退耕相比能显著提高2 mm和1~2 mm粒径团聚体含量,显著减少0.25 mm粒径团聚体含量,其中对于2 mm和1~2 mm粒径团聚体在不同退耕年限与方式下含量表现为GL15GL06FL06FL15CK和GL15FL06GL06FL15CK;退耕还林和还草增加了两个土层的团聚体稳定性,GL15的平均重量直径(MWD)值和几何平均直径(GMD)值均最大,土壤结构最稳定,其次为GL06;不同退耕年限,2 mm粒径下退耕还林地和还草地、1~2 mm粒径下退耕还草地团聚体有机碳含量均随退耕年限的延长而增加。20~40 cm土层中,团聚体含量均值随粒径的减小而增加;MWD和GMD值均小于0~20 cm层;各粒径范围内退耕还林与还草后的团聚体有机碳含量与坡耕地相比总体表现出减小的趋势。研究结果表明,退耕改善了土壤结构,对各粒径团聚体有机碳含量分布的影响随退耕年限与方式不同效应各异,且GL15相较于其它退耕年限和方式下的样地有更好的土壤团聚体稳定性和更多的团聚体有机碳积累。     

黄土高原丘陵沟壑区不同土地利用的土壤养分特征

以黄土高原丘陵沟壑区第3副区典型流域为研究区,采用实地调查、土壤采样分析和数理统计相结合的方法,基于33个样点的土壤养分数据,研究了林地、草地、果园、梯田、坡耕地5种土地利用的土壤养分特征。结果表明：土地利用方式对黄土高原丘陵沟壑区典型流域土壤有机质、全N和速效P含量的影响达到显著水平,对其他养分指标影响不明显。土壤有机质和全N含量从高到低依次为：林地＞坡耕地≈梯田＞果园＞草地,林地明显高于其他土地利用类型,而草地土壤养分最差。速效P含量从高到低依次为：果园、坡耕地、梯田、林地和草地,农业耕作植被下的速效P含量明显高于自然生态植被。研究表明：梯田作为传统的水土保持措施,在研究区具有良好的土壤保肥作用,而分布于陡坡的草地,养分保持效果在5种土地利用中最差。     

黄土高原退耕还林措施对深层土壤含水率的影响

深层土壤水分分布状况对于黄土高原植被恢复和农业可持续发展意义重大。对陕北黄土区不同退耕还林措施(1979年种植的侧柏林、油松林和苹果林)及农地(对照)20 m深土壤剖面的土壤水分、根系分布状况及土壤粘粒含量进行了取样分析。结果表明:退耕还林措施显著降低了土壤含水率,总体表现为农地(11.30%)苹果林(6.66%)≈油松林(6.48%)≈侧柏林(5.92%)。深层土壤含水率同时受植物根系和土壤质地的影响,植物根系是影响4~10 m土壤含水率的主要因素,根系通过吸水作用降低了土壤含水率,但随着深度的增加,根系对土壤含水率的影响逐渐减弱,土壤质地对土壤含水率的影响逐渐增强,土壤含水率与土壤粘粒含量之间呈正相关关系。侧柏人工林根系影响土壤水分深度可达18 m,油松林的影响深度为16 m,35 a树龄的苹果林耗水深度为19 m左右。