Abstract
Spatial heterogeneity is considered to be a ubiquitous feature in natural ecosystems.A typical example of spatial heterogeneity in desert ecosystems is 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(P0.05) than the bulk precipitation except pH and CO2-3.Compared with bulk precipitation,the accumulation ratios of PO3-4,NO-3,NH+4,K +,Na+,Ca2+,Mg2+,HCO-3,Cl-and SO2-4 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.
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