Boron Dissolved and Particulate Atmospheric Inputs to a Forest

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Boron dissolved and dust atmospheric inputs to a forest ecosystem (Northeastern France) Philippe Roux, Marie Pierre Turpault, Gil Kirchen, Paul-Olivier Redon, and Damien LEMARCHAND Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03226 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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Environmental Science & Technology

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Boron Dissolved and Particulate Atmospheric Inputs to a Forest Ecosystem (Northeastern France)

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PHILIPPE ROUX1.2 ; MARIE-PIERRE TURPAULT1 ; GIL KIRCHEN1 ; PAUL-OLIVIER REDON3

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AND DAMIEN LEMARCHAND*

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*corresponding author : [email protected]

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BEF-INRA, Centre de Nancy, 54280 Champenoux, France LHyGeS/CNRS, Université de Strasbourg, France Andra- Centre de Meuse Haute Marne 55290 Bure, France

1 Environment ACS Paragon Plus


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Abstract

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Boron concentrations and isotopic compositions of atmospheric dust and dissolved depositions

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were monitored over a two-years period (2012-2013) in the forest ecosystem of Montiers

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(Northeastern France). This time series allows the determination of the boron atmospheric inputs to

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this forest ecosystem and contributes to refine our understanding of the sources and processes that

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control the boron atmospheric cycle. Mean annual dust and dissolved boron atmospheric depositions

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are comparable in size (13 g.ha-1.yr-1 and 16 g.ha-1.yr-1, respectively), which however show significant

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intra- and inter-annual variations. Boron isotopes in dust differ from dissolved inputs, with an annual

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mean value of +1 ‰ and +18 ‰ for, respectively.

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The notable high boron contents (190-390 µg.g-1) of the dust samples are interpreted as resulting

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from localized spreading of boron-rich fertilizers, thus indicating a significant local impact of regional

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agricultural activities. Boron isotopes in dissolved depositions show a clear seasonal trend. The

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absence of correlation with marine cyclic solutes contradicts a control of atmospheric boron by

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dissolution of seasalts. Instead, the boron data from this study are consistent with a Rayleigh-like

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evolution of the atmospheric gaseous boron reservoir with possible but limited anthropogenic

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and/or biogenic contributions.

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Keywords: boron, isotopes, atmosphere, dust deposition, dissolved deposition

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Introduction

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Boron is a micronutrient essential for plant growth1. Deficiency symptoms are observed in

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more than 80 countries where 30% of the arable soils are diagnosed with too low boron

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concentration to sustain perennial agriculture2,3. On the contrary, desertification, irrigation,

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fertilizers, waste waters and mining cause boron local accumulations in soils with subsequent

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dramatic effects on their fertility4 but also at wider scale on the quality of groundwaters5,6 and

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possibly, but in a less documented way, on the atmosphere7. Finally, the range between boron

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deficiency and toxicity levels is very restricted8, making boron the second global micronutrient

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challenge after zinc9 and giving a critical interest to the understanding of the boron biogeochemical

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cycle to develop sustainable agriculture practices and water management.

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Boron is a trace constituent in the atmosphere that exists in both particulate and gaseous

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forms with the latter representing about 90-95% of the total10. Published work on boron in

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rainwaters have shown concentrations ranging over more than two orders of magnitude, from 0.2 to

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385 µg.L-1, which contribute to about 20% of the dissolved boron in rivers11-13. Boron isotopic

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compositions in rainwaters also show large variations, from -13‰ to +48‰, spanning approximately

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60% of the total range observed in natural materials

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the atmosphere considered seawater as the main source of boron

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continental contribution evaluated at 0.2%

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determined the global atmospheric boron dissolved deposition at about 2.75 Tg.yr-1. The major part

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of it (2.54 Tg.yr-1) only impact coastal and marine sites leaving a much smaller global boron dissolved

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deposition over lands (0.22 Tg.yr-1). These authors also expect a boron deposition over land as dry

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falls equal to the dissolved ones and inferred a global boron particulate deposition from mobilization

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of soil dust of 0.054 Tg.yr-1. The main conclusion of these authors is certainly that the global boron

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cycle is increasingly impacted by anthropogenic activities, which now induce boron fluxes that about

7,12,14-22

. Early works on the sources of boron in 10,19,23

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with a negligible 13

. In a recent review Schlesinger and Vengosh



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equal the natural ones thus questioning the role of seawater as a net source of boron deposition

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over lands.

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It is now admitted that boron atmospheric wet deposits have three main sources, 1) a marine

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component characterized by high 11B close to the water value (∼+40‰) composed of dissolved

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marine aerosols and/or of evaporated/condensed gaseous boron; 2) a biogenic component with 11B

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close to 0‰ or slightly negative that may, at least partly, derive from biomass burning, pollen or soil

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organic compounds and 3) anthropogenic components deriving from leaching of aerosols including

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coal combustion particles and from fertilizer spreading characterized by more scattered 11B values

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but high SO4/B and NO3/B chemical ratios, which values depends on the fertilizer spread

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7,14,15,17,21,24,25

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combustion and fertilizers to rainwaters are determined from the concentration ratios of dissolved

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sodium/boron, sulfate/boron and nitrate/boron, respectively. The contribution of each of these

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sources is highly dependent on local conditions at the sampling site. Seawater is a major contributor

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to dissolved boron in coastal rainwaters, which contribution rapidly decreases with the distance from

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the coast or change of the wind direction7,14. This so-called marine contribution is inferred from the

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B/Na chemical ratio of the collected rainwaters, assuming that seasalts are the only source of Na to

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the rainwaters, which actually only accounts for the dissolution of marine aerosols. However, The

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high B/Cl or B/Na chemical ratios (sometimes expressed in the literature using a boron enrichment

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factor relative to seawater, EF) and the absence of correlation between boron isotopes in rainwaters

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and the calculated contribution of marine seasalts (e.g. from 20 to 90% in Brest, France ), indicate

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that dissolved B isotope in rainwaters cannot be explained solely by dissolution marine seasalts.

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Alternatively, it has been proposed that evaporation of boric acid from seawater and later

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condensation or interaction with particles in the atmosphere may control the dissolved boron budget

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of rainwaters14,16,17 with a strong temperature dependence of the phase partition of boron in the

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atmosphere23,26. The vapor-liquid fractionation of boron isotopes has been determined

. In all these studies, the respective contributions of marine sea-salts, coal

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experimentally14 or empirically14,16,17, which therefore may explain most of the boron data in

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rainwater following a Rayleigh-like distillation process. A comprehensive dataset including hydrogen

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isotopes in rainwaters from various locations in the worlds also support such a gaseous atmospheric

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transport of boron over long distance14,16,17. Compiling all these studies, it comes that seawater is

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therefore considered as a major source of boron to continental rainwaters, through the combined

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contribution of marine aerosols and water/vapor exchange processes with possible boron recharge

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over lands, which may include biogenic and anthropogenic components.

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Boron in atmospheric dust has received much less interest than rainwaters and has only been studies

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through leachates of urban aerosols15 or water soluble aerosols25. In both studies, boron isotopes

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have been used to trace the sources of particle emissions in two sites (Paris city center and Japan Sea

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coast, respectively) chosen for their expected elevated anthropogenic contribution. Boron is here of

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particular interest because it is abundant in coal and is characterized by relatively low δ11B values .

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To our knowledge, no study has focuses on boron dust depositions and boron sources in rural areas.

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With only few studies published so far, and none conducted in parallel on dissolved and dust

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depositions, the sources and sinks of boron in the atmosphere are still a matter of

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debate7,10,13,14,16,17,19,23. In this study, we present a 2-years (2012-2013) monitoring of boron

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concentrations and isotopic composition in dissolved and dust depositions on a forest site in

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Montiers (rural area located in Northeastern France). The dissolved and dust boron atmospheric

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fluxes are then determined and discussed in terms of possible sources and processes affecting the

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boron atmospheric transport.

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Materials and methods

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Site

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The Montiers sampling site is an even-aged beech stand of approximately 50 years located in

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Lorraine, Northeastern France (48°31'55"N / 5°16'8"E) managed by ANDRA-OPE and INRA-BEF. In this



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region, 48% of the surface is dedicated to crop production (DRAAF, annual report 2014). The use of

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boron fertilizer is widespread in this area, especially onto oleaginous products (colza and sunflower)

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and beets, which corresponds to about 20% of the total cultivated surface. The Montiers area is very

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rural with a low population density ( 70% of the total mineral phases). The rest of the particles (≈ 10-30%) consists in

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interlayered clay minerals, micas, plagioclase, K-feldspar, Fe-oxides and carbonates. Sulfates and

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gibbsite can be found occasionally35. From their chemical and mineralogical compositions, it has been

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determined that dust result mainly from the erosion of regional temperate soil with only rare

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episodes of marked Saharan contribution35. Between 2012 and 2013, dust deposition rate displays a

Mineralogy, organic content and deposition rate of dust.



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large variability ranging from 2.4 mg.m-2.d-1 to 23.3 mg.m-2.d-1. The highest dust deposition rates

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were observed during the summer months and the lowest deposition rates during the winter

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months. Despite seasonal changes of the dust deposition rates, the mineralogical composition of the

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deposited dust has been observed to be about constant, with random occurrence of accessory

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minerals however.

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Major elements in dissolved deposition

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Na and Cl concentrations in Aw range from 0.22 to 1.24 mg.L-1 and from 0.42 to 2.50 mg.L-1,

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respectively (Table S1). The strong correlation between Na and Cl concentrations in our samples with

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a molar Na/Cl = 0.86 (r² = 0.8) is consistent with the generally admitted marine origin of atmospheric

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Na and Cl inputs37,38. NO3 concentrations vary between 1.02 mg.L-1 and 5.27 mg.L-1 with an outlier of

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11.94 mg.L-1 measured in April 2013. Ca and SO4 concentrations vary between 0.18 mg.L-1 and 3.76

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mg.L-1 while Mg concentrations range from 0.04 mg.L-1 in September 2013 to 0.21 mg.L-1 in August

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2012 (Table S1). Boron

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Dust deposition - Boron concentrations in Ap range from 150 mg.kg-1 in August 2013 to 502

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mg.kg-1 in December 2013, with 67% of the data comprised between 300 and 400 mg kg-1 (Table 1

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and Fig 1). These concentrations are very high compared to the mean continental crust (≈10 mg.kg-1)

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but also compared to common soil minerals (≈10-100 mg.kg-1

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between -4.7‰ during the winter months 2012 and +11.4‰ in December 2013 (Table 1). Over the

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two-years sampling period, the δ11B values tend to show a bimodal distribution of boron isotope with

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higher values (δ11B>0‰) between May and July and then between October and December. Boron

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concentrations in Ap are statistically higher in 2012 than 2013. The annual oscillation of δ11B values in

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2013 Ap samples is similar but of higher magnitude than in 2012.

16,31


). The δ11B values of Ap vary

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Dissolved deposition - Boron concentrations in Aw range from 1.1 µg.L-1 to 3.6 µg.L-1 with a mean

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value of 2.2±1.6 µg.L-1 (±1SD, n=24, Table 1, Fig 1) and no clear seasonality is observed. A relative

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high boron concentration is measured in July 2013 (9.1 µg L-1). The δ11B data in Aw show seasonal

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variations over the two years ranging from 8.6‰ in March 2013 to 36.6‰ in July and August 2012.

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December 2012 is characterized by a relative high δ11B value compared to other winter months

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(+27‰). Overall, the ranges of boron concentrations and isotopic compositions in dissolved

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deposition are consistent with previously published works7,12,14-17,21,22,24. Both years display similar

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δ11B trends in Aw characterized by higher δ11B values in the summer months and lower values in

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autumn and winter but we observe that the δ11B values in 2013 are significantly lower than those in

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2012, especially during the summer months. We also observe that the period of highest δ11B value is

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delayed by about one month in 2013 (max. in August-September) compared to 2012 (max. in July-

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August).

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Overall, dust and dissolved boron geochemical signals appear disconnected in our study. The

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absence of correlation between boron data (concentrations and isotope) in Ap and Aw samples

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together with the time lag between the observed seasonal oscillations of boron isotopes in dusts and

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dissolved samples (Fig. 1) suggest that no significant boron exchange occurred between the solid and

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liquid phases after collection. Additionally, the high boron concentrations measured in dust samples

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contrast with the rather low ones in the recovered supernatants. It has however to be noted that our

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sampling procedure actually allows the transfer to solution of most, if not all, the water-soluble

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boron initially present in the collected dusts. In reverse, the slightly acidic rainwater pH makes

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unlikely a significant removal of dissolved boron by adsorption onto the collected dusts.

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Figure 1: Evolution of B concentrations and isotopic compositions (‰) of atmospheric particulate deposition (Ap) and Total dissolved deposition (Aw) in 2012 and 2013. Gray bands correspond to the time of local fertilizer spreading.

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Fertilizers and soils – The δ11B values of the three fertilizers analyzed in this study range from -

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13.5 (F2) to +0.5‰ (F3) but these values may vary from one manufacturer to the other and even over

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time because of the industrial process as well as the fluctuations of the boron market. The soil

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untreated with boron-rich fertilizer S1 shows lower boron concentration and lower δ11B (80 mg.kg-1; -

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7.8‰) than the fertilized soil S2 (160 mg.kg-1; -3.6‰).

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Discussion

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•

Budgets of boron atmospheric inputs During 2012 and 2013, the inputs of atmospheric dust deposition (Ap) range from 1.3 µg.m-

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.d-1 in winter 2013 to 8.0 µg.m-2.d-1 in June 2012 with an mean annual value of 3.9 ± 2.2 µg.m-2.d-1

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(±1SD, Table 1). Based on the measured boron concentrations and dust deposition rates, we

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calculate a yearly-integrated particulate boron deposition of 15.5 g.ha-1.yr-1 with δ11B = -0.3‰ in

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2012 and 10.4 g.ha-1.yr-1 with δ11B = +3.2‰ in 2013. A seasonal cyclicality of the boron dust

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deposition rates was observed (higher during summer months and lower during winter ones), which

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is mostly driven by the cyclicality of the dust deposition rate rather than by seasonal changes of the

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boron concentrations (Fig. 1). The monitoring of the intensity of the rainfall events were used to

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calculate the flux of the boron dissolved deposition. The inputs of atmospheric dissolved boron (Aw)

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are comparable in amount to dust deposition, ranging from 1.8 µg.m-2.d-1 in August 2012 to 9.7 µg.m-

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2

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yearly-integrated dissolved boron deposition of 17.2 g.ha-1.yr-1 with δ11B = +21.7‰ in 2012 and 14.9

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g.ha-1.yr-1 with δ11B = +13.4‰ in 2013. Although the rates of dissolved boron are comparable

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between the two years, we observe a significant discrepancy of δ11B values, which is discussed later.

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By comparison with previous studies, the atmospheric input of dissolved boron in Montiers is similar

.d-1 in July 2012 with a mean annual value of 4.8±2.0 µg.m-2.d-1 (±1SD, Table 1). We calculate a


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to the one in Clermont-Ferrand (5.5 µg.m-2.d-1)

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ecosystems by a factor about 3.5 (1.4 µg.m-2.d-1) .

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but significantly higher than other forest

12

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From our data, we determine that dust depositions contribute to 45% of the total boron

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atmospheric inputs to the Montiers forest. This is consistent with the value determined in a previous

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work in which dust depositions account for half of the total boron atmospheric inputs . The reason

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for this discrepancy has to be found in the unusual high boron concentrations measured in Ap

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samples in Montiers.

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Origin of boron in atmospheric dust

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We observe a general decrease of the boron concentrations in dust with increasing silicon,

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aluminum ones indicating that aluminosilicate minerals are not the main boron carriers. To the

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opposite, boron concentrations globally increase with the complement to 100% of the sum of the

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major cations analyzed (rock forming cations, except C), which complement is expected to be mostly

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composed of organic compounds. In addition, correlations (r>0.75) are observed between B/Zr and

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Na/Zr, P/Zr, Cu/Zr, Mo/Zr and Zn/Zr wt ratios (fig. S1). Here, Zr is considered as an immobile element

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mostly derived from minerals and serve to compare elements without a firm correction of the

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organic compounds. In particular, the lower δ11B values observed in this study correspond to the

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sample showing the extreme Na concentrations and not Cl ones. Since boron is usually present in

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fertilizers as sodium borate and supplied in pair with molybdenum, our results therefore point to

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fertilized-derived organic materials as main boron carriers in dusts in Montiers. Quartz being

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abundant in the dust samples and never showing important boron concentrations, we deduce that

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the boron-bearing phases should be characterized by very high boron contents, likely exceeding

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1,000 mg.kg-1. This value is unusual in geological material except in evaporite, tourmaline or

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palygorskite that are not observed in our samples.



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Examination of boron isotopes indicates that at least three distinct components or physico-

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chemical processes are necessary to explain our data (Fig. 2). One component, noted A, is principally

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defined by dust samples collected in the second half of 2012 and November 2013. It is characterized

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by a moderate boron concentration (400 mg.kg-1)

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but with contrasted δ11B values, one +12‰ for the component noted C has no equivalent amongst the fertilizer samples analyzed in

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this study. The component B is defined by most of the dust samples collected in 2012, except

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November, and the early months of 2013 whereas the component C is defined by the summer

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months of both years and December 2013. No clear relationships have been observed between δ11B

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values and major or trace element concentrations.

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These data tend to indicate that the B-rich fertilizers widely used in the region of Montiers

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dope with boron the cultivated plants and likely the top soil organic and mineral phases, in particular

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those for which boron is known to have great chemical affinity like clay minerals and iron oxides39,40.

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This conclusion is also supported by shift in boron concentration and isotopes observed in the

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fertilized top soil S2. It has to be noted that the fertilized soil S2 has been sieved at 50 µm, a

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granulometric size coarser than found in atmospheric dust. We expect that the finest granulometric

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fraction of S2 is composed of material with higher reactive surface and therefore higher boron

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concentration. The component A would then be most representative of dust from unfertilized soils.

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During growth, plants absorb boron without significant isotopic fractionation12 such that boron

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isotopes in plants are sometimes used to unravel their origin . It is then expected that the organic

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compounds present in fertilized soils have δ11B values close to the fertilizer or the mean of the

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fertilizers spread. We therefore suggest that the component B would reflect mostly fertilized organic

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compounds.

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Figure 2: δ 11B vs 1/[B] for atmospheric dusts (Ap). Dust deposited in 2012 appear mainly influenced by the boron-rich endmember A, whereas dusts deposited in 2013 tend to be influenced by the three endmembers. Endmember C, the less boron concentrated endmember, would then reflect less contaminated soils

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However, neither the fertilizers F1, F2 and F3 nor the top soil samples analyzed in this study

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allow to better constrain the component C characterized by high boron concentrations and high δ11B.

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Since boron-rich fertilizers are mostly used for the culture of oleaginous products (colza and

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sunflower) and beets, the inter-annual variations of boron concentrations and isotopes in Ap might

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reflect crop rotation and possible changes of fertilizers and manufacturers. Note that if synthetic

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fertilizers general present low δ11B values, some can present high values (>+20‰)40, which might be

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consistent with endmember C. Accordingly, dusts deposited in 2012 might be mainly influenced by

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the boron-rich source, whereas dusts deposited in 2013 would be influenced by a more balanced

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contribution of the three sources. Contribution of the source C (high concentrations and higher δ11B

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values) seems to be more important during the summer months and possibly at end of the year,

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coincident or just after fertilizer spreading (fig. 1 and 2). Identification of the boron sources in the

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dust collected in Montiers is therefore not straightforward and requires more extensive field

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investigations including collection of larger sample size to allow chemical and isotopic analyses of

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separate mineral and organic phases and a larger sample set, with spreading schedule, of the boron-

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rich fertilizers used in the region. Table 1: B concentration (µg L-1), flux (µg m-2 d-1) and isotopic composition (‰) of total dissolved deposition (Aw) and dust deposition (Ap). Analytical errors (±2SD) are 1% for boron concentrations, and 0.5‰ for boron isotope analyses.

353 354 355 356

Origin of boron in dissolved atmospheric inputs

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•

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Assuming that all the sodium in dissolved atmospheric inputs derives from seasalt dissolution,

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which is the upper limit, the maximum seasalt contribution to rainfall chemistry can be calculated for

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each element X using the following equation:



[] =

[]

[ ]

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× [ ] 1

361

Where [X]mar is the concentration of the element X in Aw derived from seasalt dissolution;

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([X]/[Na])seawater is the concentration ratio of X and Na in seawater and [Na]Aw is the concentration of

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Na in Aw. The relative seasalt contribution to the total atmospheric deposition of dissolved Ca, Mg,

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SO4 and B is illustrated in Fig. S2. It is calculated that 33 to 100% of the total dissolved Mg can be

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explained by dissolution of seasalts whereas only a small contribution is calculated for Ca and SO4,

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about 12 % and 8 %. Most of the dissolved Ca deposition occurs in spring and summer months coeval

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with the increase in dust deposition rate, which may reflect the dissolution of carbonate dust. SO4 is

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mainly emitted as gas and fine particles by industrial, urban and agricultural activities41. From these

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observations, we conclude that the rainwaters collected in Montiers in 2012-2013 have chemical

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compositions consistent with a marine air mass with variable geologic and anthropic contributions,

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which relative magnitudes depend on the elements. For boron, the contribution of seasalt

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dissolution does not exceed 29% of the total dissolved boron during winter and decrease down to 8%

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during summer. Since seawater is characterized by a high and invariable δ11B value (≈+40‰), the

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calculated relative seasalt contributions are inconsistent with the lower δ11B values observed in

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winter and the highest ones in summer, indicating that the greater seasalt injection to the

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atmosphere during winter months is unlikely the major control of dissolved boron. Additionally, the

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boron enrichment factor (EFsw) in each rainwater sample, defined as the B/Na chemical ratio

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normalized to the B/Na seawater ratio, is systematically above unity (3.4 ≤ EFsw ≤ 80.5, Fig. S3).

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Noteworthy, the seasonality of EFsw (lower values in winter and higher ones in summer) is driven by

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seasonal variations of the sodium concentrations, in line with previous studies

381

these observations contradict the hypothesis of boron degassing after seawater bubble bursting or

382

any physico-chemical reaction at the seasalt particle surface as proposed earlier10,16 because mass

383

balance would lead Esw factors to be alternately higher and lower than unity depending on the


14,42

. In Montiers,

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relative boron and sodium depletion in the air masses. Alternatively, possible explanations may

385

include incorporation of either mineral, biogenic or anthropogenic materials with B/Na ratio above

386

the seawater value, gradual boron depletion by rainfalls of the air masses as well as effects of

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temperature variations on the boron partition between atmospheric vapor and liquid phases.

388

Based on SO4/B and NO3/B ratios in rain waters, previous studies have identified the leaching

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of coal burning ashes and the spread of fertilizers as possible sources of boron to the

390

atmosphere15,21,24,43. Our samples are globally consistent with this conclusion, as shown in Fig. S4

391

and by the correlation observed between EFsw and (NO3+SO4)/Na (Fig. S3). Compared to the available

392

boron dataset in rainwaters, our samples appear particularly rich in NO3 suggesting that atmospheric

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dissolved boron in Montiers is more affected by fertilizer spreading than by leaching of coal ashes.

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However, the low δ11B values are frequently, but not always, associated with low boron

395

concentrations, which is inconsistent with a simple addition of anthropogenic boron to the

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atmosphere. The combination of low boron concentration with low δ11B has been earlier observed

397

and explained by evaporation of gaseous boric acid from the seawater and subsequent integration in

398

rains of isotopically fractionated boron. Boron isotopes in rains are then modeled by a Rayleigh-like

399

distillation process:

400

=

Eq 2

401

With R and R0 being the isotopic ratios of the rainwater and the original atmospheric gaseous boron

402

reservoir, respectively. ƒ is the mass fraction of residual gaseous

403

fractionation factor between the gaseous reservoir and the rainwaters. Note that α actually

404

integrates a series of reactions including evaporation/condensation and interactions with aerosol

405

surface and thus may vary from one site to the other. Using the empirical α value (α=1.031)

406

empirically determined by Rose-Koga et al.17 and assuming the coastal rains from Brest (Western

407

France) as representative of the initial air mass, the model can explain most of the boron data

408

measured in Montiers. However, a Rayleigh-like model alone cannot explain neither the existence of

10


B and α is the boron isotopic


409

boron-rich rainwaters with low δ11B values (Fig. 3) nor the systematic difference of the mean δ11B

410

values measured in 2012 and 2013 because it would imply important variations of the atmospheric

411

conditions (temperature, relative humidity)17,45, which are not observed.

412

Finally, we propose a model of boron sources in the dissolved atmospheric deposits

413

combining a condensation of gaseous boron, most likely originating from seawater evaporation, a

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small contribution of seasalts dissolution and episodic inputs of anthropogenic boron, mostly

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deriving from the nearby fertilizer spreading. Condensation of marine-derived gaseous boron would

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be the dominant process during summer months when the higher temperatures promote the

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evaporation of boron. This will lead to high δ11B values and moderate boron concentration. In winter

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months, the dissolution of seasalts may also partially contribute to the dissolved boron deposition

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thus maintaining δ11B values around +10‰. Fertilizer spreading episodically affects the rain

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chemistry by adding significant amount of boron with an isotopic composition close to +10‰. Taken

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from published data that anthropogenic sources are characterized by boron concentrations ranging

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from 20 µg.L-1 to 50 µg.L-1 and δ11B from 0‰ to +10‰ 7,14,15, a mass balance calculation leads to an

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global anthropogenic contribution below 10% of the total dissolved boron with the exception of

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fertilizer spreading periods, when it may reach 25%. Additionally, the summer δ11B maximum in Aw is

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delayed by 1 or 2 months compared to Ap. The difference of residence time in the atmosphere

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between particulate (few days) and gaseous boron (about one month)1010 combined to the time of

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fertilizer transfer from soil to atmospheric dust deposits may explain this delay.

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Figure 3: δ11B values plotted as a function of B concentration (µg L-1) in rainwaters of Montiers (this study), Brest (West France) 7, Clermont Ferrand (Center France) 7, Dax (South-West France)7, Orleans (Center France) 7, Paris (Center France) 15, Guiana 14, Reunion 45, Nepal 18, Quebec 19. This figure also displays the two mechanisms controlling boron in Aw: a Rayleigh distillation model (α = 1.031, after Rose-Koga et al.17) using coastal rains as the origin of air masses and a secondary contribution (biogenic/anthropogenic). The latter has been roughly evaluated to contribute up to 10% to the overall Aw boron budget. The dashed lines illustrate trends corresponding to the Rayleigh model using extreme rainwater values measured in Brest.

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Page 19 of 28


Environmental implications

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•

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With regard to the critical physiological role of boron, this study suggests that the atmospheric

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boron fluxes need to be considered to establish conditions of a sustainable management of

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ecosystems. In particular, an advance of this work is to recognize that the boron input by dust

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deposition to forest ecosystem about equals the boron dissolved flux by rainwaters. Additionally,

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boron concentrations and isotopes point to a significant impact at regional scale of fertilizers on the

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boron deposition by dust. By comparison, the boron deposition by dust represents about 1/10 of

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boron recycled each year by the vegetation of the nearby Vosges mountains12.

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Beyond the evidence of a regional fertilizer imprint on boron atmospheric depositions, this study

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emphasizes the transfer of contaminants through clay-size dusts. Although dust deposition

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constitutes a small flux at the global scale compared to rivers or aquifers it could affect a wide area.

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The schematic boron transfer in the atmosphere presented in Fig. 4 could therefore serve as a

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framework for geochemical models including a more detailed description of the physico-chemical

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properties of the gas/water/particles exchanges.

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Fig 4: Conceptual model of both dissolved boron (Aw) and particulate boron (Ap) dynamics in the atmosphere in the forest ecosystem of Montiers. Aw seem to mostly derive from condensated gaseous boron with a contribution (

Boron Dissolved and Particulate Atmospheric Inputs to a Forest

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