摘要
小叶锦鸡儿(Caragana microphylla Lam.)为锦鸡儿属(Caragana Fabr.)多年生豆科落叶灌木,是锦鸡儿属植物(又统称柠条)在我国黄土高原地区的主要栽培种之一。它既能防风固沙、保持水土,又是优质的灌木饲料植物资源,具有较高的生态经济价值。自20世纪60年代起,小叶锦鸡儿即在晋西北黄土丘陵区大量种植,成为当地退耕还林、植被恢复和水土保持的重要灌木树种之一。但在种植和管理过程中,一些地方出现了小叶锦鸡儿灌丛土壤肥力退化现象,还有部分地区因对其营养状况不了解,利用和管理措施不当,导致小叶锦鸡儿灌丛木质化严重、植被退化,使其生态和经济效益没有得到充分发挥。针对上述问题,本研究对晋西北黄土丘陵区小叶锦鸡儿人工灌丛的植物营养特征和土壤肥力状况进行了系统研究,以揭示小叶锦鸡儿灌丛对环境的响应和影响特征,提出对小叶锦鸡儿灌丛应采取的管理措施,为晋西北黄土丘陵区小叶锦鸡儿灌丛的可持续经营和生态环境重建提供理论依据。
1.对小叶锦鸡儿灌丛地上部生物量和植株矿质养分含量的动态监测研究表明,其生物量和营养成分含量呈现随林龄增大先逐年增大而后逐年降低的态势。小叶锦鸡儿生物量(地上部干重)随生长年限的延长逐渐增大,30年时生物量最大,其后生物量开始减少。就植株不同器官(根、枝、叶)的各种矿质养分而言,以叶部的含量最高。与生物量趋势相同,小叶锦鸡儿不同器官的养分含量基本呈现先逐年(5龄、10龄、20龄)增高而后(30龄以后)有下降的趋势。
对小叶锦鸡儿作为饲料的品质研究表明,其营养成分在平茬后随着生育期(从休眠期到开花期)的变化,主要营养成分含量逐步增加,呈浓缩型发展趋势,有利于家畜利用。随着生长年限延长体内粗蛋白、粗脂肪逐年下降,粗纤维含量上升,木质化程度高,适口性下降。综合考虑枝条生物量的大小和小叶锦鸡儿正常生长的需求,平茬后第3年应是小叶锦鸡儿作为饲料的最佳利用时间。
2.在小叶锦鸡儿灌木林地,土壤的有机质、全氮、速效钾在植株根际土中均有一定程度的“富集现象”。由于小叶锦鸡儿根系的固氮作用和大量根系分泌物的存在,与土体土相比,根际土壤的全氮、有机质含量提高,pH下降,EC提高,养分有效性提高。
在晋西北石灰性土壤上,磷是制约小叶锦鸡儿生长的关键因子。小叶锦鸡儿的生物量随磷肥用量的增加而显著增加。施磷能显著降低土壤pH,增加EC值,提高土壤各项速效养分的含量。
3.营造小叶锦鸡儿人工灌丛有利于土壤团粒体形成,可降低土壤容重,改善土壤物理性状,增强土壤的抗冲蚀能力。随着小叶锦鸡儿灌丛生长年限的延长,0～40cm土壤容重逐渐减小,土壤总孔隙度提高；土壤机械组成中砂粒(>0.05mm)比例下降,粉粒(0.002～0.05mm)比例提高,土壤持水性、通气性等物理性状得以改善。
随生长年限的增加,小叶锦鸡儿人工灌丛的土壤全氮、有机质和硝态氮含量均呈现先逐渐增大(30年生最大)随后降低的趋势；而全磷和全钾含量则无明显变化。与此同时,速效养分(尤其是有效磷和速效钾)则呈逐渐下降趋势。不同生长年限的小叶锦鸡儿灌丛的土壤全氮、有机质、有效磷和速效钾均具有明显的“表聚效应”,即0～20cm土层土壤养分含量显著大于20～40cm土层土壤。
不同土地利用方式对土壤的物理性质影响较大。在0～20cm土层,土壤容重依次为小叶锦鸡儿人工林<混交林<农田<杨树林<撂荒地。0～20cm土层的土壤孔隙度则表现为小叶锦鸡儿人工林>混交林>农田>杨树林>撂荒地。小叶锦鸡儿杨树混交林的土壤有机质和全N含量最高,小叶锦鸡儿人工林和杨树林次之。在几种利用方式中,农田的有效磷含量最高,小叶锦鸡儿人工林和混交林的有效磷含量次之,而杨树林最低。农田有效磷含量较高与当地农民施磷肥有关。
不同生长年限小叶锦鸡儿灌丛下各部位土壤有机质及土壤速效养分含量均呈现明显的“肥岛效应”,即丛心>丛中>丛边>带间。
4.小叶锦鸡儿灌丛的土壤碱性磷酸酶和过氧化氢酶活性随着林龄的增长,酶活性先逐渐提高,20年达到最高,随后便下降。酶活性同时与土层深度有关,随土层加深土壤酶活性逐渐降低。
根际土壤各种酶活性均高于根外土,但其活性与灌丛生长年限的关系则不明显。土壤磷酸酶活性自丛心、丛中、丛边、到带间依次减小,表现出明显的根际效应。
不同土地利用类型之间的土壤(0～20cm)过氧化氢酶活性没有显著差异,但是其碱性磷酸酶活性的差异则很明显,即小叶锦鸡儿人工林>混交林>杨树林>撂荒>农田。
在灌丛的不同部位,0～20cm土层土壤的微生物总数均大于20～40cm土层土壤。其中细菌数量占微生物总数的79%以上,尤其是丛心土壤细菌数达总数的90%以上。由丛心、丛中、丛边到带间,土壤微生物数量逐渐减少。土壤微生物区系随着距丛心距离的增加,自丛心、丛中、丛边到带间,细菌、真菌占总菌数的比例逐渐下降,放线菌的比例则增加。
以上结果表明,在晋西北黄土丘陵区,30龄以上的小叶锦鸡儿灌丛已出现明显退化,土壤地力也开始衰退。结合试验区较为贫瘠的土壤以及管理粗放等现实情况,为充分发挥小叶锦鸡儿在退耕还林(草)和生态建设中的积极作用,我们初步提出以下几点建议：
(1)对老龄林进行适当刈割、平茬
小叶锦鸡儿萌蘖力极强,地上部分被破坏后会产生补偿或超补偿。对30年生以上灌丛,可以采取割条和樵采等方式进行合理利用。这些合理的刈割、平茬有利于老龄植株复壮更新,是促进其地上生物量快速恢复的有效方式。
(2)在林地内酌情放牧
当地畜牧业比较发达,尤以牧羊为主。由于在幼林内放牧会直接影响幼林的发育和郁闭度。建议禁止在幼龄林内放牧。在老龄林地内,每年开花期后可适当放牧。这样牲畜的啃食不仅能起到局部刈割的效果,而且牲畜活动的粪便等残留物也能起到培肥土壤的作用。
(3)合理利用土地,营造灌乔混交林
鉴于小叶锦鸡儿与杨树混交林对土壤的培肥效果强于小叶锦鸡儿灌木林。建议今后适当扩大此类灌乔混交林的营造。
Little leaf Peashrub (Caragana microphylla Lam.), a species of the genus Caragana Fabr. of Legume Family, is a deciduous and drought-resistant shrub. It is the main species cultivated in the Loess Plateau in northwestern Shanxi, China. Since1960s, this shrub has been widely planted and plays important roles in ecological environment management, such as windbreak, sand fixation, soil and water conservation, and as valuable forage in northwestern Shanxi. However, few studies have been carried out to study the dynamic changes of nutrient contents in the shrub-soil system which provides foundation information for the ecological recovery program (Grain-for-Green) in the Loess Plateau in northwestern Shanxi. The objectives of this study are (1) to characterize the dynamics of biomass and nutrient contents of the shrub;(2) to characterize the dynamics of nutrient contents at the rhizospheres of the shrub;(3) to determine the spatial and temporal changes of nutrient contents in soils under different land uses; and (4) the dynamics of soil enzyme activities in the soil-shrub system under different land uses.
1. The content of nitrogen in the roots of the shrub was higher than the leaves because of the nitrogen fixing by the legumes in the roots. The contents of other mineral nutrients in the leaves of the shrub were higher than stems and roots.
The fresh above-ground biomass of the shrub and nutrient contents of the shrub organs showed increase followed by a decrease with the increase of growth age and the peaks occurred at its age of30years old. As the increase of growth age, the contents of raw protein and raw fat of the shrub decreased and the contents of raw cellulose increased.
2. The contents of soil organic matter and total nitrogen at rhizosphere of the shrub were higher than the bulk soil. This result may be interpreted by the two processes at root zone, nitrogen fixing by the legumes in the large root system and exudation of low molecular weight organic acids from roots.
The content of soil available phosphorus at rhizosphere of the shrub was higher than the bulk soil. This result may be interpreted by the replacement of adsorbed phosphorus to CaCO3by the low molecular weight organic acids released into root zone. The biomass of the shrub increased when external addition of phosphorus fertilizer was applied, and the possible reason is that the shrub need a large amount of phosphorus to meet their growth requirement and phosphorus is a limiting factor in calcareous soil.
The content of soil available potassium at rhizosphere of the shrub was higher than the bulk soil.
3. The soil physical and chemical properties changed as the establishment of the shrub vegetation. As the growth of the shrub, there were increases in soil bulk density in the depth of0-20cm, soil porosity, field water holding capability, and soil aerating condition, and the content of sand with diameter>0.05mm decreased while the content of silt with diameter between0.002and0.05mm increased.
As the growth of the shrub, the contents of soil total nitrogen, organic matter, and nitrate showed increases followed by decreases with the peaks at the shrub age of30years old. The contents of soil total phosphorus and potassium showed non-significant change, but the contents of soil available nutrient, especially phosphorus and potassium, decreased with the increase of the age of the shrub. The contents of soil total nitrogen, organic matter, available phosphorus and available potassium showed to be higher at the layer0～20cm than those at the layer20～40cm.
Soil physical properties showed significant difference between land uses in this study. The order of soil bulk density in depth of0～20cm was shrub plantation
shrub, shrub mixed with Poplar> Poplar. The highest content of soil available phosphorus in farmland was due to the large amount of phosphorus fertilizer applied.
There was an obvious spatial distribution of soil chemical properties. The contents of soil organic matter and available nutrients decreased with the increase of distance from the center of the shrub, and this phenomenon was coincident with the fertile island effect in literature.
4. As the increase of the age of the shrub, the soil enzyme activities in the shrub showed gradual increases followed by decreases with peaks at the shrub age of20years old. During the growth of the shrub, the soil enzyme activities decreased with the increase of soil depth and the horizontal distance from the center of the shrub.
There was no significant difference of catalase activities between land uses. The order of soil alkaline phosphatase activities in the soil depth0～20cm was shrub> forest> Poplar> fallow> farmland.
The soil microbial populations gradually decreased with soil depth in the shrub. The abundance of bacteria was dominant (>79%) of the total organisms, and it was as high as90%at the center of the shrub. The abundance of organisms decreased with the increase of distance from the center of the shrub. The community of soil organisms showed spatial distribution, i.e. decreases of the percentages of bacteria and fungi and increases of the percentage of actinomycetes with the increase of the distance from the center of the shrub.
This study of the dynamics of nutrient contents in the shrub-soil system can be implemented in the practice follows.
(1) Based on the observed temporal change of the biomass and nutrient contents in the shrub-soil system in this study, it is feasible to cut back the shrub with age older than30years in order to restore the shrub vegetation. New roots and shoots will sprout and the biomass of the new vegetation will be equal to or greater than the old one. Thus, mowing and cutting back of the shrub with age older than30years, such as cutting bars, is an effective means of rejuvenation. According to the comprehensive criteria in this study including above-ground biomass and the contents of nutrients, raw protein, raw fat and cellulose, the optimum time is recommended at the age of three years old for the shrub to be used as forage.
(2) Proper grazing in the old shrub. Since animal husbandry especially herding in the region is well developed, sheep's eating young leaves and flowers of the shrub might have partial effect of mowing and cutting back old shrub mentioned above. Additionally, the manure of the sheep will improve soil physical and chemical condition for shrub growth. This grazing is not allowed in young shrubs because the young shrub will be simply destroyed or the establishment of its crown will be delayed.
(3) The plantation of the shrub mixed with Poplar is recommended as an alternative way of land use because it showed improvements in soil physical and chemical properties and enhancement of soil enzyme activities in this study.
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