Cropland Water-Soluble Selenium, Groundwater Silicon, Atlantic Rain – Agro-Geological Assessments

role of the water-soluble Se [Se.H 2 O], “plant available Se”, and its atmospheric Se supply needs clarification. The main aim is to compare regional associations of [Se.H2O] and groundwater silicon [Si] with erosion associated Latitude [Lat], longitude [Long] and Altitude [Alt], as well as with Atlantic rain by its change in oxygen isotope ratio ( from its Atlantic value: [δ18O]}, groundwater hardness [Ca+Mg] and [Mg+K] (as biotite/clay indicator) in order to find differences in their terrestrial and atmospheric associations. Results: [Si] was better explained by [Lat], [Long], [Alt] and [δ18O] than by [Se. H2O]. and [Mg+K] associated stronger with O] (p < 0.001, by both) than with [Si] (p < 0.01, by both). Combined regressions by [Lat;Long;Alt] explained [Se. H2O] , [Si] and [δ18O] by 65.2 – 95.2 % (p < 0.001, by all), all coefficients were strongly negative by [Si.gw], but coefficients of [Alt] were positive by [Se.H 2 O] and [δ18O]. [Si] explained [Se.H2O] by 61.9 % (p < 0.001). “hint-like” that the altitude-erosion could been by Se. High association between [Si] and [Se.H 2 O] can be explained by their associations with humus. Se fertilization seemed to have influence on [Se.H 2 O] and its ability to predict plant Se.


Introduction
This study is treating hot water extractable selenium of Finnish croplands in 1978-80, before the era of Se fertilizers. There are many methods for cropland Se determination [1], even "total selenium" can be determined by different selections and orders of strong acids. Se values are usually expressed by mg/kg, but watersoluble Se by mg/l or µg/l. Total Se in plough layer has reported to have been 0.201 mg/kg (N=250) [2], 0.209 mg/kg (N=93) [3], 0.229 mg/kg in 1998 [4]. Generally, Se content has been highest in clays and organic soils (org), lowest in coarse mineral soils (coms).
Colloidal elemental selenium is electrically charged and adsorbed by clay minerals [5], which explains Se clay association.
Water-soluble Se can be determined by shaking or boiling the water [8], the exact method is not always clearly expressed, e.g.
Atmospheric Se: Volatilization of selenium from selenates and selenites in Finland according to [10] for 3 months has been very scanty: generally, < 1 %, anyhow from Carex peat Se losses could reach ca 3 % by treating soils with both lime and organic matter [10]. Metylated selenium compound can volatilize more easier, e.g. even 30 % of the selenium added to fine sand in the form of trimethylselenonium chloride (Se 2,5 mg/kg) volatilized from the soil during 42 days (the trimethylselenonium ion is an important urinary metabolite of dietary Se) [11]. Atmosphere is a great reservoir of Se [12], composed from anthropogenic (62.5 %) and natural (37.5 %) sources. Vicinity of ocean can increase Se content in soil [13]. It is expected that the amount of Atlantic rainfall could effect on atmospheric emission of Se.

Comments on
µg/l in 2006 (estimated mean by author from [1], where Se content in sand was 7 µg/kg and in clay 13 µg/kg (by rough volume weight estimate 1 by [4], ca 10-12 months after Se fertilization). Data in [16], as well as in [22] are results of commercial analyses.
There are several studies on Se content in soil and plants. But Additionally, water extraction can be performed by shaking or boiling Sampling time-point could affect on results during Se fertilization: e.g. 1 or 12 months after fertilization [24,1]. Label "solube Se" is non-precise and misleading, e.g., in [4]. N of samples in [16] (1340) is higher than in [2] [16,24] could not predict the possible 30-500-fold increase in plant Se [25] caused by fertilizers.
In Finland there are no satisfactory studies on regional airborne Se depositions. In 1990 Finnish total "anthropogenic" Se fallout from precipitation was approximatedly 18.4 t/a (0.54 g/ha) [26].
Se content in rain was 118 ng/l, and in snow 63.1 ng/l, suggesting on moderate inaccuracy, because consumption of coal and oil was obviously higher during snowing than raining [26]. Support on (some part of) Atlantic Se fallout from precipitation gives Danish rain water (250 ng Se/l) in 1971 [26]. Se association with sulfur is known [26]. So higher sulfur emissions in 1978-80 to 1991 [27] together with inaccuracy in Se determination [26] could suggest on availability of airborne upto Se 1 g/ha/a in 1978-80, cf. 12 g/ ha/a via fertilizers in 1992-2004 [4]. The separate role of airborne Atlantic Se, possibly upto 1/3 of Se fallout [12], could not be determined. Anyhow all atmospheric Se via (common) southwest wind could have had compensated the Se losses -better than by Si -of the hills, which are impoverishing by erosion Table 2. Low content of molybdate-reactive silicon (1 %) in biotite extracts by oxalate [7] can be dependent on aluminium-silicon interactions [29] in acid solution (pH 0.65).

Conclusions
During the time before Se supplementation "plant available Se", [Se.H2O], obviously worked well. Humus is the home of soil biota, amount of "plant available Se" can be increased 10-15-fold by mycorrhizae [28]