树干径流对梭梭"肥岛"和"盐岛"效应的作用机制

为探明荒漠灌木个体周围土壤养分和盐分的空间分布及其成因,以荒漠灌木梭梭(Haloxylon ammodendron Bge.)为研究对象,对其周围土壤pH值,电导率,有机碳,全氮和有效磷的空间异质性以及树干径流的化学性质进行了研究。结果表明:树干周围出现显著的"肥岛"效应,且土壤有机碳,全氮和有效磷的"肥岛"作用范围发生在距主根大约20—40 cm以内。土壤pH值和电导率在主根周围25 cm以内的值却显著低于外围,而在离主根25 cm处出现了"盐岛",即在梭梭主根中心形成了一种高养分、低盐和低pH值的环境。树干径流和自然降雨化学性质的对比研究表明:除pH值和CO23-外,树干径流中其他化学离子的含量均显著高于自然降雨中的含量,说明这种高养分、低盐分的环境是由树干径流引起的,也就是树干径流带给土壤养分的同时将盐分带走。

The effects of stemflow on the formation of "Fertile Island" and "Salt Island" for Haloxylon ammodendron Bge

Spatial heterogeneity is considered to be a ubiquitous feature in natural ecosystems. A typical example of spatial heterogeneity in desert ecosystems is "fertile islands". Fertile islands are formed around small shrubs and are important local and regional nutrient reserves that influence community structure and ecosystem function. They alter soil water and nutrient contents which are the primary limiting factors for vegetation structure and production in arid ecosystems. These fertile islands are formed where nutrients accumulate under the shrub canopy. In addition, soil pH and salinity have also been shown to have significant differences between shrub canopies and interspaces. The development of fertile islands is related to a number of factors but in arid land, the stemflow of shrubs could be a significant contributor in their development as well as playing a major role in salinity redistribution. In this study, the distribution patterns of soil salinity and nutrients around individual shrubs and the major influencing factors was investigated using Haloxylon ammodendron Bge. shrubs. The heterogeneity of soil pH, electrical conductivity (EC), soil organic carbon (SOC), total nitrogen (TN), and available phosphorus (AP) was investigated. Soil samples were collected in circles of 2 cm, 10 cm, 20 cm, 30 cm, 40 cm and 50 cm radius around the taproot, with eight sampling points in each layer (48 points in total). Results demonstrated that significantly enriched SOC, TN and AP content at the shrub center, which extended to 20-40 cm from the taproot. Conversely, soil pH and EC were significantly lower at 0-25 cm from the taproot, indicating that the fertile island is also an area of low alkalinity/salinity. Water has been regarded as the focus of soil nutrient reduction and redistribution in arid and semiarid regions. Stemflow is considered to be a major source of soil moisture in desert ecosystems, and also a significant influence on runoff generation, soil erosion, groundwater recharge, spatial patterning of soil resources and the distribution of understory vegetation and epiphytes. Therefore, the chemical properties of the shrub stemflow and bulk precipitation were also evaluated in this study to determine the contribution of stemflow to the formation of fertile islands and the distribution of soil salinity. The results revealed the stemflow funneling ratio of H. ammodendron was 46.4 which indicated large amounts of rainwater were accumulated through the shrub stem and consequently, moisture was higher at the shrub center. The chemical properties of stemflow were significantly higher (P<0.05) than the bulk precipitation except pH and CO₃^2-. Compared with bulk precipitation, the accumulation ratios of PO₄^3-, NO₃^-, NH₄⁺, K ⁺, Na⁺, Ca²⁺, Mg²⁺, HCO₃^-, Cl^- and SO^2-₄ in stemflow were from 3.30 to 34.87. This indicates the much higher nutrient and salinity content in stemflow compared with bulk precipitation. There was no difference in pH between bulk precipitation and stemflow. These results demonstrated the fertile island effect of higher nutrients and lower salinity around the taproot likely formed by stemflow. It also demonstrated that stemflow will input nutrients and remove salinity from the soil with increased water infiltration.