Beryllium-7 atmospheric deposition and soil inventory on the northern Loess Plateau of China
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Atmospheric Environment 77 (2013) 178e184Contents lists availableAtmospheric Environment
journal homepage: www.elsevier .com/locate/atmosenvBeryllium-7 atmospheric deposition and soil inventory on thenorthern Loess Plateau of China
Fengbao Zhang a,b, Bo Zhang b, Mingyi Yang a,b,*a State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Northwest A&F University,Yangling, Shaanxi 712100, Chinab Institute of Soil and Water Conservation, Chinese Academy of Science and Ministry of Water Resources, Yangling, Shaanxi 712100, Chinah i g h l i g h t s 7Be concentrations in rainfall were artificially high due to 7Be dry deposition. 7Be deposition input showed the highest in summer and the lowest in winter. 7Be inventories in undisturbed soil were obviously unimodal over the year.a r t i c l e i n f o
Article history:Received 11 August 2012Received in revised form12 April 2013Accepted 3 May 2013
Keywords:Loess Plateau7BeDeposition fluxSoil inventory* Corresponding author. State Key Laboratory of Soilon the Loess Plateau, Institute of Soil and Water Consversity, No. 26 Xinong Road, Yangling District, ShaaTel.: 86 (0)29 87012884; fax: 86 (0)29 87016082.
E-mail address: firstname.lastname@example.org (M. Yang).
1352-2310/$ e see front matter 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.atmosenv.2013.05.002a b s t r a c t
Beryllium-7 is a potentially powerful tracer of soil erosion, but information on 7Be atmospheric depo-sition and associated soil inventories on the Loess Plateau of China is not readily available. In the studyreported in this paper, we measured the 7Be inventories in undisturbed soil at different sampling timeson the northern Loess Plateau of China for three years, between 2010 and 2012, and estimated the 7Bedeposition fluxes and the daily 7Be inventories in undisturbed soil. The annual 7Be depositionfluxes during this period varied between 1303 119 and 2222 147 Bq m2, with a mean of1759 416 Bq m2. There is a marked seasonality for the 7Be deposition fluxes with the maximum insummer, approximately 50% to the annual deposition flux, and the minimum in winter, approximately 5%to the annual deposition flux. Precipitation amounts can explain more than 70% of the variation in 7Bedeposition flux. 7Be deposition in the form of dustfall, dew and frost make a significant contribution tothe 7Be deposition flux in the study region. The daily 7Be inventories in undisturbed soil varied markedlythrough time and ranged between 89.2 and 941.8 Bq m2 with a mean of 392 210 Bq m2. Theydemonstrated a unimodal distribution over the year, with the highest values in August or September andthe lowest in late winter or early spring.
2013 Elsevier Ltd. All rights reserved.1. Introduction
Cosmogenic 7Be (T1/2 53.3 d) is produced in the uppertroposphere and mainly in the stratosphere as a product of thespallation reaction of oxygen and nitrogen nuclei with high-energy cosmic ray particles (Lal et al., 1958). After 7Be is pro-duced, it rapidly forms BeO or Be(OH)2 by ionic reactions andbecomes associated with sub-micrometre aerosol particles(Papastefanou and Ioannidou, 1995; Cho et al., 2007). Subse-quently, 7Be enters the marine and terrestrial environmentsErosion and Dryland Farmingervation, Northwest A&F Uni-nxi Province 712100, China.
All rights reserved.through wet and dry deposition processes (Wallbrink and Murray,1994; Ioannidou and Papastefanou, 2006; Doering and Akber,2008). Many investigators (Olsen et al., 1985; Dibb, 1989; Harveyand Matthews, 1989; Todd et al., 1989; Caillet et al., 2001;Graham et al., 2003) have suggested a linear relationship betweenthe amount of precipitation and 7Be deposition flux and haveshown that precipitation plays a dominant role in 7Be depositionflux. Annual, seasonal and monthly variations of 7Be depositionfluxes, which are determined by the 7Be production rate in theatmosphere, the extent of stratospheric to tropospheric air ex-change, tropospheric circulation patterns, latitude, and the effi-ciency of the removal of 7Be by wet and dry deposition, have alsobeen reported for many areas of the world (Narazaki et al., 2003;Gonzlez-Gmez et al., 2006; Baskaran and Swarzenski, 2007; JuriAyub et al., 2009). Upon reaching the land surface, 7Be is rapidlyand strongly fixed by soil particles and other ground cover and
F. Zhang et al. / Atmospheric Environment 77 (2013) 178e184 179activities are readily measured by gamma spectrometry (Bondiettiet al., 1984; Wallbrink and Murray, 1996; Papastefanou et al., 1999;Zhang et al., 2011). 7Be in bare soil is restricted to an upper surfacelayer of approximately 20 mm, predominantly within the first10 mm, and it decreases exponentially with increased depth(Wallbrink and Murray, 1996; Blake et al., 1999; Walling et al.,1999; Wilson et al., 2003; Yang et al., 2006). 7Be has been usedsuccessfully to document soil erosion occurring on bare soils sincethe 1990s in various places around the world (Wallbrink andMurray, 1993; Blake et al., 1999; Walling et al., 1999; Wilsonet al., 2003; Yang et al., 2006; Sepulveda et al., 2008; Wallinget al., 2009; Schuller et al., 2010).
Soil erosion is a major environmental and agricultural problemon the Loess Plateau of China. There is an urgent need to quantifyamounts of soil loss and 7Bemeasurement offer a valuable means ofquantifying short-term erosion rates. For such studies, a good un-derstanding of the local deposition flux of 7Be and the temporalvariation of the 7Be soil inventory is necessary. However, infor-mation on 7Be deposition flux and soil inventories is relativelyscarce on the Loess Plateau. Although a large amount of data on 7Beatmospheric deposition is available for other areas of the world(Walling, 2013), 7Be deposition fluxes are site specific and stronglydependent on location, and particularly latitude and local meteo-rological conditions. The underlying objectives of the study re-ported here were to document 7Be deposition fluxes and toreconstruct the temporal distribution of 7Be fallout inputs andassociated daily 7Be inventories in undisturbed soil on the northernLoess Plateau by measuring 7Be inventories in undisturbed soil atregular intervals and to examine the short-term variations and thefactors that may possibly influence them. No previous publicationhas reported the 7Be deposition fluxes for this region and this studytherefore provides information on inputs of 7Be to the northernLoess Plateau to add to existing databases.
2. Methods for determining 7Be deposition flux
Generally, 7Be deposition flux has been determined by collectingthe rainfall and dry fallout for individual rainfall events or at regularintervals, recovering the 7Be from the collected rainfall by chemicalmeans, measuring the 7Be activity by gamma spectrometry, andthen calculating the 7Be fallout input as the product of the 7Beconcentration in the collected rainfall and the amount of precipi-tation during the rainfall event or sampling interval (Wallbrink andMurray,1994; Caillet et al., 2001). However, the chemical procedurerequired to separate 7Be from rainfall is complex and timeconsuming. It is known that the 7Be inventory in undisturbed soildirectly reflects the 7Be fallout input and is controlled by the 7Befallout input and the decay of 7Be. Variations of the 7Be inventory inundisturbed soil can reflect the variations of 7Be fallout input.Previous investigators have also shown that undisturbed soil sitesserve as effective natural repositories for atmospherically derivedradionuclides and that soil inventories from such sites can be usedto estimate past atmospheric fluxes (Hardy et al., 1973; Moore andPoet, 1976; Nozaki et al., 1978).Walling et al. (2009) calculated themean 7Be concentration in rainwater (Cm in Bq l1) using Eq. (1),based on measurements of the 7Be inventories in undisturbed soiland records of daily rainfall. They then used this value to calculatethe 7Be deposition flux and reconstructed the temporal distributionof 7Be fallout input during the periods between inventory mea-surements. Eq. (1) was expressed as:
(1)where Aref (Bq m2) is the 7Be inventory in undisturbed soil, I(l m2) is the daily rainfall, l is the daily decay constant for 7Be, andT (d) is the number of days in the study period. In this equation,Walling et al. (2009) assumed that the 7Be concentration in rain-water at a given site is constant at the weekly or monthly timescale,that all the 7Be fallout is delivered as wet deposition and that drydeposition can be ignored during the inter-sampling period.Walling et al. (2009) also demonstrated that it is reasonable toassume that the mean 7Be concentration in rainfall remainsessentially constant on the weekly or monthly timescale. However,dry deposition (dustfall, dew and frost) may be significant in semi-arid and arid regions and will influence the results obtained usingthis approach. Further details are provided below.3. Study site and experimental procedures
3.1. Study site
The study site was located in the Dunshan watershed at theAnsai Research Station of Soil and Water Conservation, ChineseAcademy of Science, situated within Ansai County, Shaanxi Prov-ince, on the northern Loess Plateau region of China (1091902300E,365103000N). The climate is warm and semi-arid. The mean annualprecipitation is approximately 510 mm, most of which falls duringthe period July to September and causes severe soil erosion. Theannual mean temperature is 8.8 C, and the annual evaporationranges from 1500 to 1800 mm. The soil type is Huangmian soil(Calcaric Cambisols, FAO), developed by wind deposits and char-acterized by a yellow colour. Dustfall occurs frequently in this re-gion, particularly in spring and early summer.3.2. Soil sampling and laboratory procedures
Five adjacent experimental plots with a 0 slope in the AnsaiResearch Station were selected as our experimental sites for sam-pling soil. On December 31, 2009, these plots were manually tilledto mix the upper 25 cm layer. Extraneous materials such as stones,roots or grass fragments were removed, and the surface wassmoothed by replicating local cultivation practices. Aref (t 0) wasassumed to be zero after tilling. To maintain the surface of theseplots bare, herbicide was sprayed on to prevent the growth ofvegetation during the study period. Further disturbance of the plotswas prevented. Soil samples were routinely collected from April 1,2010, to December 25, 2012. In each plot, three soil samples werecollected using a scraper-plate to a depth of 2.5 cmwithin an area of10 10 cm2 and these were mixed to provide a single compositesoil sample. A total of five mixed soil samples were sent back to thelaboratory to measure the 7Be activity on each sampling occasion.Few samples were collected in winter and early spring because ofthe low amount of precipitation. All soil samples were air-dried,weighed, dispersed and passed through a 1 mm sieve. The sam-ples of sieved soil were then packed into identical plastic column-shaped boxes prior to detecting 7Be activity.
Measurements of the 7Be activity in soil were undertaken bygamma spectrometry using a high-resolution, low-background,low-energy, hyperpure n-type germanium coaxial r-ray detector(EG&G ORTEC, Oak Ridge, TN, USA). Further details of the 7Bemeasurement procedure are provided by Yang et al. (2006) andZhang et al. (2011). The measured 7Be activity was always correctedto the sampling day using the decay constant. The mean inventoryof the five soil samples was used to represent the 7Be inventory inundisturbed soil on each sampling occasion. Statistical analyseswere performed using SPSS PASW Statistics (Version 18.0)software.
F. Zhang et al. / Atmospheric Environment 77 (2013) 178e1841803.3. Meteorological data
A record of daily precipitation during the study period wasobtained from the Ansai Research Station of Soil and Water Con-servation, Chinese Academy of Science.Fig. 1. Relationship between measured 7Be inventories in undisturbed soil anddecayed cumulative precipitation.4. Results and discussion
4.1. 7Be atmospheric deposition
The measured 7Be inventory in undisturbed soil during thestudy period varied between 92.5 15.6 and 860.8 38.5 Bq m2(Table 1). The 7Be inventory in undisturbed soil reflects the cu-mulative input of 7Be after decay. If we assume that the cumulativeprecipitation has the same half life as 7Be, a significant linearrelationship is obtained between the measured 7Be inventories inundisturbed soil and the decayed cumulative precipitation (Fig. 1).The significant linear relationship (r2 0.88) implies that 7Be wetdeposition by precipitation controlled the 7Be deposition input andit is reasonable to use Eq. (1) to estimate the 7Be concentration inrainfall at the study site for the timescale of the individual samplinginterval.
The mean 7Be concentrations in rainfall during the samplingintervals which ranged from 19 to 161 days were calculatedusing Eq. (1) and are presented in Table 1. The calculated values ofmean 7Be concentration in rainfall for each sampling interval variedwidely within the range1.2 0.2 to 10.1 1.7 Bq l1, with oneexceptionally high value of 20.8 2.6 Bq l1. The higher valueswere related to periods with low precipitation, which may occur inany month from November to June at the study site. The calculatedmean 7Be concentration in rainfall quickly decreased withincreasing precipitation during the study period (Fig. 2), showing asimilar response to those reported by Caillet et al. (2001) andBaskaran et al. (1993) who studied the 7Be concentration in rainfallduring individual rainfall events. After monthly averaging, theTable 1Data for the 7Be inventories in undisturbed soil, 7Be deposition fluxes, and the calculate
Sampling date Samplinginterval (d)
Aref (Bq m2 SD)a
1-Apr-2010 90 92.5 15.61-Jun-2010 61 265.7 20.220-Jun-2010 19 352.8 28.610-Jul-2010 20 396.5 30.431-Jul-2010 21 354.8 28.520-Aug-2010 20 539.8 50.211-Sep-2010 22 455.3 44.530-Sep-2010 19 383.9 36.45-Nov-2010 36 308.4 27.315-Apr-2011 161 140.2 20.424-May-2011 39 331.5 18.326-Jun-2011 33 355.1 29.511-Aug-2011 46 503.0 45.25-Sep-2011 25 853.0 44.225-Sep-2011 20 860.8 38.515-Oct-2011 20 724.7 40.830-Nov-2011 40 617.4 39.507-Mar-2012 98 206.5 23.212-May-2012 66 234.5 27.307-Jun-2012 26 387.0 32.401-Jul-2012 24 592.9 35.720-Jul-2012 19 552.2 30.511-Aug-2012 22 493.4 41.503-Sep-2012 23 594.2 50.226-Sep-2012 23 697.6 48.515-Oct-2012 19 622.7 25.625-Dec-2012 71 302.2 32.4a The data in the grey column were influenced by manual tilling.lower values of 7Be concentration in rai...