Magnesium in soils and plants of a sloping agrolandscape in the south-east of West Siberia
Authors: Nechaeva T. V. , Gopp N. V. , Savenkov О. А. , Smirnova N. V. 
Published: October 2021

The aim of the study. To carry out a comparative assessment of the magnesium status in soils, situated on the various hypsometric levels of sloped agrolandscape and with different soil organic matter content by measuring total, mobile (exchangeable) and water-soluble magnesium in soils and to compare different analytical methods for determining magnesium in soils and plants.
Location and time of the study. The study site was located in the Cis-Salair drained plain in the south-east of West Siberia (55°02΄20˝N; 83°50΄00˝E), administratively being within the boundaries of the Toguchin district of the Novosibirsk region, Russia. Altitude increments were marked on the slope positions of the investigated plot of 225 hectares in area and 4 km in length: the upper part of the slope BCI (280-310 m a.s.l. ), the midslope BCII (260-280 m) and BCIII (220-260 m), and the lower slope BCIV (190-220 m) (see figure 1). Soil and plant samples were collected in July 2013.
Materials and methods. Prevailing soils were podzolized and leached chernozems, dark-gray and gray-forest soils according to the classification of soils of the USSR (1977), which correspond to clayey-illuvial eluvialed and dark-tongue agrochernozems, agro-dark gray soils and agro-gray soils to according of the Russian soil classification (2004, 2008), or Luvic Greyzemic Chernozems, Haplic Chernozems, Luvic Greyzemic Phaeozems, Luvic Retic Greyzemic Phaeozems by WRB Soil Classification (2014) (see table 1, figure 1). Individual soil samples (n=55) were taken with a drill from 0-30 cm layer (ploughed horizon) according to an irregular grid. The aboveground phytomass stock of oat-pea mixture (n=38) was collected using the hay cutting method on the 0.25 m2 area. The geographical coordinates of the soil and plant sampling sites were determined using the geopositioning system (GPS, Garmin eTrex Vista).
The soil samples were analyzed for organic carbon content by dichromate digestion. Overall the studied soils were grouped according to soil organic matter (SOM) content into high-SOM (5-8%) and medium-SOM soils (3-5%). Total magnesium (Mgtot) was measured in dry powdered samples by atomic emission spectrometry; mobile (exchangeable) magnesium was measured by atomic absorption spectrometry using 1 M KCl, 1 M CH3COONH4, 0.1 M C4H4O5(NH4)2 and 0.5 M CH3COOH as extracting solutions. The same extracts were used to measure water-soluble magnesium atomic absorption spectrometry and capillary electrophoresis. The content of total magnesium in plants was determined by atomic absorption spectrometry using two methods of sample digestion, namely wet ashing in a mixture of sulfuric and perchloric acids and dry ashing followed by quantitative transfer of the digest with 1 M HCl. Magnesium content in soils and plants was presented per element and calculated on the air-dry mass basis. Statistical analyses (descriptive statistics, correlation analysis, calculation of student criterion and Mann-Whitney U-test) were performed using Microsoft Office Excel 2007 and Statistica v.6.1.
Results. The average content of total magnesium in the slope soils varied from 0.79 to 0.88%. The proportion of mobile magnesium in its total content in soils averaged 3.3-6.0%, whereas that of water-soluble magnesium was 0.06-0.13%. In medium-SOM soils down along the slope the total magnesium content gradually decreased: agrochernozems → agro-dark gray soils → agro-gray soils (see table 2, figure 2). The similar pattern was revealed an earlier for the content of total phosphorus and total potassium in the slope soils. The average content of mobile magnesium in the high-SOM soils on the upper part of the slope (BCI) varied from 3.2 to 4.0 cmol(+)ꞏkg-1, depending on the extractant used. The medium-SOM soils (BCII-IV) contained 2.2 to 3.2 cmol(+)ꞏkg-1 of mobile magnesium. Atomic absorption spectrometry and capillary electrophoresis produced very close values water-soluble magnesium content in soils: on average, 0.06-0.08 and 0.04-0.09 cmol(+)ꞏkg-1, respectively. However, capillary electrophoresis proved to be more sensitive to the decreased content of water-soluble magnesium in the midslope medium-SOM soils. Magnesium content in aboveground phytomass of an oat-pea mixture, collected in the tillering phase of its cereal component, determined by dry ashing, averaged 0.21-0.26%, which corresponded to the optimal level of the element; estimated by wet ashing it was 1.3-1.6 times lower (see table 2).
Conclusions. Under the conditions of the sloping agrolandscape of the Cis-Salair in the south-eastern part of West Siberia, high-SOM soils in the upper part of the slope are characterized by higher content of mobile magnesium as compared with the medium-SOM soils mid- and downslope. We believe that for assessing magnesium status of soils it is more expedient to use 1 M KCl or 1 M CH3COONH4 to determine mobile (exchangeable) magnesium. Moreover, these extractants can be used for simultaneous determination of other soil properties, i.e. exchangeable calcium and potassium content, pHsalt, etc.