粤北石漠化地区土壤侵蚀的137Cs法研究

据2005年10月的野外调查及对TM卫星影像解译分析结果,研究了粤北不同程度的石漠化地区、不同利用类型土地的¹³⁷Cs区域分布以及土壤侵蚀速率特征。结果表明:粤北石漠化地区土壤¹³⁷Cs活度的平均值为(6.54±0.57) Bq·kg^-1,土壤中¹³⁷Cs水平随石漠化程度增加有先降低然后升高的趋势,表明在重度石漠化阶段,过高的基岩裸露率导致其周围土壤对¹³⁷Cs的集中吸附,¹³⁷Cs在某些沉积小环境内逐渐富集。研究区¹³⁷Cs背景值为(1 433.60±131.40) Bq·m^-2,低于北半球的平均值,¹³⁷Cs剖面分布基本符合指数分布,最高值出现在2~4 cm深度范围内。在轻度、中度石漠化地区,非耕作土壤侵蚀速率的平均值分别为1 369.0、1 833.5 t·km^-2·a^-1;耕作土壤侵蚀速率远高于非耕作土壤,坡度对土壤侵蚀影响非常明显。     

基于GIS与137Cs技术的土壤侵蚀微观研究

坡地侵蚀的研究是进行流域侵蚀研究的基础，影响坡地土壤侵蚀的因素很多，但最主要的是地形因子。通过^137Cs技术和GIS技术，以黄土高塬沟壑区的多个自然汇流坡面为研究对象，从微观角度探讨了在自然坡面条件下不同地形因子对坡面土壤侵蚀的影响，根据土壤侵蚀在坡面上的垂直分异规律．对不同部位土壤侵蚀量的差异及不同地形因子对土壤侵蚀的影响大小进行了初步探索性研究。结果表明：在微观尺度内，坡度、坡长等常规因子对土壤侵蚀的影响并不像径流小区测定的那样明显，似乎已经退居到次要位置。借助GIS对土壤侵蚀进行微观研究，可以发现一些用常规方法很难发现的规律，有助于我们更加深刻地认识土壤侵蚀的变化规律。     

Validating the use of Cs measurements to estimate rates of soil redistribution by wind

Wind erosion has degraded over one-half billion hectares of land worldwide. ¹³⁷Cesium (¹³⁷Cs) has been used as a tracer to study long-term rates of soil redistribution by water and, to a lesser extent, by wind. Early studies assumed that the decline in ¹³⁷Cs activity for a potentially eroded soil relative to that for an uneroded soil was linearly proportional to soil loss. More recently, models have emerged that consider the effects of soil cultivation and the particle surface area-dependent partitioning of ¹³⁷Cs on soils. We investigated the partitioning of ¹³⁷Cs in wind-eroded sediments and with soil surface samples sieved into contiguous ranges of particle sizes. We also compared the ¹³⁷Cs activities and stratification of several adjacent soils with known wind erosion and deposition histories. Finally, we tested ¹³⁷Cs-based soil loss models with measured data from sites with documented histories. ¹³⁷Cs activities and mean particle diameters of aeolian samples agreed well with the ¹³⁷Cs activities and respective mean diameters of the sieved surface soil samples. Good agreement between model estimations and measured data indicated that ¹³⁷Cs models developed to estimate soil redistribution by water were also applicable to soil redistribution by wind provided that the models contained an appropriate particle size correction parameter.     

Erosion and deposition history derived by depth-stratigraphy of Cs and soil organic carbon

There are a number of uncertainties in the use of ¹³⁷Cs as a marker for deriving soil erosion rates. However, this should not limit other potential uses of this anthropogenic radionuclide in the study of soil landscape processes. This study outlines a sampling methodology which aids in the assessment of the history of erosion and depositional processes within a landscape unit. The depth distribution of ¹³⁷Cs and soil organic carbon (SOC) was utilized as a means of determining the erosion and depositional history of a conventionally tilled agricultural field in southern Ontario, Canada. Three transects oriented along the slope of a large field had five soil profiles excavated at the summit, sideslope, shoulder slope, footslope and toeslope landscape positions. The soils were sampled in 5 cm increments, and ¹³⁷Cs and SOC were determined on the samples. The results show that soil redistribution within landscape units of agricultural fields has been substantial both before and after fallout of ¹³⁷Cs to the soil surface. Soils in depositional areas contained significant ¹³⁷Cs and SOC at depths beyond which the plow can attain at present. This implies that a significant amount of carbon is being sequestered beneath the present plow layer, and the characterization of this pool must be considered in deriving the dynamics of SOC in agroecosystems.     

137Cs和210Pbex双核素示踪"三北"防护林区退耕前后坡地土壤侵蚀变化

为查明"三北"防护林建设前后农耕地和退耕地土壤保持效益变化，利用137Cs和210Pbex双核素示踪技术，选择了防护林建设较为成功的张家口坝上地区（风力侵蚀区）作为典型区，研究了农耕地以及退耕地土壤137Cs和210Pbex的剖面变化规律及其示踪的土壤侵蚀变化。结果表明：1）由于耕作的混匀作用，农耕地土壤剖面中137Cs和210Pbex均呈均匀态分布；退耕地土壤剖面中137Cs和210Pbex则表现为表层（0～5cm）浓度最高、下层（5～25cm）浓度均相对较低且分布相对均匀的形态，这表明退耕后坡地土壤137Cs和210Pbex剖面形态均会发生一定变化，退耕驱动土壤137Cs和210Pbex剖面变化导致运用土壤核素估算侵蚀模型在该区域难以适用；2）基于土壤137Cs和210Pbex剖面变化规律，利用210Pbex质量平衡方程，提出了退耕地土壤210Pbex土壤侵蚀估算模型；3）利用137Cs比例模型估算退耕地土壤侵蚀速率为27.94±11.92 (t/hm2·a)，农耕地侵蚀速率为29.11±14.42 (t/hm2·a)，而利用修正后的210Pbex转换模型估算得到"三北"防护林区退耕地造林前平均侵蚀速率为82.16±14.36 (t/hm2·a)，造林后平均侵蚀速率为-41.28±33.91 (t/hm2·a)；农耕地造林前平均侵蚀速率为68.55±22.11 (t/hm2·a)，造林后平均侵蚀速率-8.52±47.32 (t/hm2·a)。这表明137Cs示踪技术主要表征了1963年以来该区坡地土壤侵蚀和沉积的平均结果，而210Pbex示踪技术则可以较好地示踪防护林建成前后的土壤侵蚀变化。此外，研究结果也表明，相比于"三北"防护林建成之前，建成之后该区农耕地和退耕地的土壤侵蚀速率均呈显著下降趋势，且均由前期的风沙侵蚀转变成了风沙沉积。     

Estimation of Deposition Velocities for 85Sr, 131I, 137Cs on Spinach,Radish and Beans Leaves in a Tropical Region Under Simulated Fallout Conditions

Poly dispersive aerosols of SrCl₂, NaI, CsCl having a size distribution of 2.33 m (Activity Median Aerodynamic Diameter, [AMAD]) with a geometric standard deviation of 1.83 m are used for the determination of deposition velocity for ⁸⁵Sr, ¹³¹I and ¹³⁷Cs aerosols on tropical spinach (Spinicia olericia), radish (Raphanous sative) and beans (Phasolous valugeries) plant leaves. The experiments were carried out in a specially designed exposure chamber to simulate deposition under accidental releases of radioactivity in tropical environment. The rates of particles deposition are expressed as a function of plant surface area and of plant dry weight. The deposition velocities obtained are in the range of 10^–6 to 10^–7 m/s. These values differ significantly from those in temperate regions. The lower values for deposition velocities (v _g) for the tropical environment is attributed due to high humidity. Therefore, the deposition of suspended aerosols on leaf surfaces caused by impaction and Brownian diffusion becomes slower in tropical environment and resulted into lower value of deposition velocities.     

Die Aufnahme von137Cs und 90Sr durch verschiedene sommerrapsgenotypen aus dem Boden*

Die Aufnahme von ¹³⁷Cs und ⁹⁰Sr durch verschiedene Sommerrapsgenotypen aus kontaminiertem Boden wurde geprüft. Statistisch gesicherte Unterschiede bei der Aufnahme von Radionukliden zwischen den Genotypen konnten festgestellt werden. Die Aufnahme von ¹³⁷Cs und ⁹⁰Sr ist bei den einzelnen Sorten nicht gleichsinnig. Nur ein geprüfter Genotyp nahm sowohl geringe Mengen von ¹³⁷Cs, als auch von ⁹⁰Sr auf. Die mittleren jährlichen Schwankungen in der Radionuklidaufnahme waren größer, als die Unterschiede zwischen den Genotypen.     

龙门山地震带坡耕地土壤侵蚀对有机碳迁移的影响

坡耕地土壤再分布对土壤有机碳(SOC,soil organic carbon)迁移的作用机制研究已成为土壤侵蚀学研究的热点,然而目前极少有研究关注地震后生态脆弱的龙门山地震带坡耕地土壤侵蚀机理及其导致的土壤有机碳再分布规律。该研究选择龙门山地震带内(都江堰市)一块陡坡耕地和一个梯田系列,采用137Cs法和野外调查,对比分析强震导致田埂垮塌和未受损情况下坡耕地土壤侵蚀空间变化特征和有机碳运移变化机理。结果表明,该区黄棕壤有效137Cs背景值为1 473 Bq/m2;坡度较小的坡式梯田内部上坡表现为侵蚀,下坡表现为沉积,同时,上部梯田的侵蚀速率高于下部梯田,但整个梯田系列净侵蚀量非常小,这表明梯田之间由于缺乏田埂的保护,水力也起着侵蚀、搬运上坡梯田土壤的作用,但是整个坡式梯田系列可以起到较好的保土作用,同时,坡式梯田内部主要以耕作侵蚀为主,是造成梯田上部坡位土壤流失严重的主要原因;陡坡耕地的地形为复合坡,由于田埂垮塌导致其土壤侵蚀速率显著高于坡式梯田系列,在整个坡面上,除了坡顶土壤侵蚀速率高之外,下坡坡度变大(曲率较大)的部位土壤侵蚀速率也非常高,同时,土壤沉积也发生在2个坡位(中下坡坡度较缓的部位和坡脚部位);在梯田系列和陡坡耕地上,SOC与土壤137Cs的空间变化规律较为一致。研究结果表明,在龙门山地震带,质量较好的石埂梯田仍然发挥着较好的土壤保持效果,同时,耕作侵蚀是该区坡耕地上一种重要的土壤侵蚀形式,在制定相应的土壤保持措施时,必须充分考虑耕作侵蚀的作用,才能有效地控制土壤侵蚀,此外,该研究结果还表明采用137Cs核素示踪技术可以比较科学地解释该区域的土壤侵蚀速率和SOC的空间变异规律。     

西汉水流域典型林草地侵蚀特征分析

为了说明西汉水流域典型林草地的侵蚀特征,为农业工程和流域治理提供依据,采用137Cs土壤侵蚀核素示踪技术,按野外采样、核素检测、模型计算三步开展研究。结果表明,6个本底区域的137Cs面积活度为1 600～2 402 Bq/m2,平均值为2 022 Bq/m2;本底区域和未扰动的林草地,剖面核素活度随深度呈指数递减分布;采样荒草地为轻度侵蚀,林地为沉积或无明显流失,退耕还林用地为中度侵蚀;侵蚀强度：林草地＜荒草地＜退耕还林用地;对于同坡面的不同坡位,侵蚀强度：坡面上部＞中部＞下部;植被发育程度影响侵蚀量的大小,对于同一土地利用类型,通常植被覆盖度越高,侵蚀速率越小,反之侵蚀速率越大。     

黔西北麻窝山岩溶盆地沉积物断代的质疑

  据谢良胜等的2篇文章报道,黔西北麻窝山岩溶盆地近29年来平均沉积厚度1.33 m,沉积速率4.6 cm/a,流域平均侵蚀模数为2 900.55 t/(km2.a)。而根据笔者的研究成果,该岩溶盆地中央部位沉积物剖面中,表征1963年沉积的137Cs峰值深度为40 cm,减去犁耕层深度,1963年以来的沉积厚度为20 cm,年均沉积速率为0.44 cm/a,此值仅为谢良胜等研究结果4.6 cm/a的1/10。