Morphological characteristics, humus and nutrient content of recent and buried soils of the Inozemtsevo mound

. The main objective of the work was to identify the evolutionary changes in the profile of recent and buried soil in the Holocene, the content of humus and the composition of mobile forms of nutrients in the solonetzol-silted black soil in the insufficiently wetted zone of Stavropol Krai. The research was carried out near the village of Inozemtsevo, Stavropol Krai. The soils of the study area are formed on marine sediments. A 1.5-metre-high grave was erected more than 5,000 years ago. At a distance of 150 metres, an incision was made on recent virgin soils. According to morphological characteristics, recent soils are medium saline fused medium powered black soils, while the buried soils evolved into weakly saline fused low powered black soils. They are distinguished by lower profile thickness and brown, rather than black, colouring. There is a BC horizon in recent soils, which is absent in paleosoils. New formation of carbonates and gypsum is noted only in recent soils in B2 and BC horizons. The organic matter content in recent soils is 5-6% and decreases 6-7 times in paleosoils in the upper horizons. Recent soils are depleted in mobile phosphorus, but enriched in mobile potassium. Differences in the content of mobile sulphur are due to some salinity of the paleosoils. In the content of mobile trace elements, the differences between the studied soils are significant, especially zinc, cobalt and molybdenum. Consequently, during the evolutionary period of 5 thousand years, the soil-absorbing complex was impoverished by these microelements.


Introduction
The study of soil genesis is a priority of recent soil science.We need to have an understanding of of not only soil fertility but also the direction of the soil formation process.This will help to predict the rate of soil evolution or degradation and to determine the potential fertility of soils in the future.
Soil evolution is generally associated with climate, vegetation, soil-forming rocks, and topography [1].Time, as a factor of soil formation, is often neglected by soil scientists.But the evolution of the soil passes in a time cycle, if the factors of soil formation remain unchanged.
The study of soils under burial mounds provides a unique opportunity to investigate the composition and properties of soils as they were formed at the time of burial.Joint work of archaeologists and soil scientists allows to make such researches and to compare the buried soils with the recent ones, which continued their development [2][3].
The territory of the Stavropol Territory has been used since ancient times by nomadic cattle-breeding tribes [4].The occupation of cattle-breeding allowed the soil not to be disturbed by cultivation tools.They buried their noble warriors or chieftains under artificial mounds.Thus, the paleosoils have been well preserved and remain intact to the present day [5][6].
The aim of the work is to identify the evolutionary changes in the profile of recent and buried soil, the content of humus, and the composition of mobile forms of nutrients in the saline-weathered black earth in the zone of insufficient moisture in the Stavropol Krai in the Holocene.

Methods and materials
The buried soils were studied near the village of Inozemtsevo.On this territory soils are represented by solonetzemes and common solonetzem fused chernozems, formed on the eluvium of marine deposits of the ancient Maikop Sea.A 1.5-meter-high grave was erected more than 5 thousand years ago.At a distance of 150 metres an incision was made on recent virgin soils.At a distance of 150 metres an incision was made on recent virgin soils.They are virgin soils, on which grasses of legume-mixed herbs-gramineous associations grow.
The mobile phosphorus and potassium compounds were determined by the Machigin method modified by the Central Research Institute of Agrochemical Services of Agriculture, nitrate nitrogen by ionometric method, mobile sulphur by the Central Research Institute of Agrochemical Services of Agriculture, mobile boron by the Berger and Truog method; mobile manganese, cobalt, zinc, copper by the Peyve and Rinx method; mobile molybdenum by the Grig method.

Results
A morphological description of the palaeo-soil section beneath the mound was made.
Section 1 palaeolithic soil Horizon A -0-6cm -Fresh, dark grey, dry.The structure is lumpy and clumpy-lumpy.The pores are thin and few in number.Does not boil over from 10% HCL.
Horizon B2 -26-37 cm -Inhomogeneous in colour, light grey, grey-yellow and bright yellow.Granulometric composition clayey, no structure, no pores visible.It does not boil over at 10% HCL.
Horizon C -37... Inhomogeneous and very patchy in colour, grey, bluish grey and yellow.Layers of presumably limonite, siderite and hydrotroilite are visible.The structure is clayey with no clayey composition.In the lower part of the horizon, granularity of structure and chocolatey, compacted layers of indigenous Maikop sediments appear.Does not boil over from 10% HCL.

Fig. 1. Cut of the buried soil of the Inozemtsevo burial mound
There is some doubt that the upper suprasalt horizon is undisturbed, but has remained so since the origin of the mound.The layer from the buried A palaeolithic horizon to the surface of the recent soil is only 50-60 cm.The infiltration of precipitation moisture into the buried soils is unlikely due to the heavy clay granulometric composition of the soil and the domelike shape of the mound structure itself.This ensures that moisture does not accumulate, but rolls away from the centre of the mound, where the cut was made, towards the periphery.
Morphological description of the recent soil in the virgin soil Section 2 -virgin, recent soil (150 m from the mound).Horizon A -0-8cm -Grey, lighter in colour than the underlying horizon.Poorly wetted, clayey in texture, dense felt of virgin grass root system observed.The structure is rather flaky with signs of dustiness.Porous, but the pores are thin.The transition to the next horizon is sharp in structure and compaction.It does not boil over from 10% HCl.
Horizon B1 8-38cm -Black, with a steely cast.Slightly dampened structureless, clumpy in its facies, broken into columnar vertical separations.The pores are rarely seen and they are very thin.The transition to the next horizon is abrupt in structure.It does not boil up to 10% HCl.
Horizon B2 38-51cm -Dark grey, fresh, clayey.The structure is lumpy.Resuspended from 10 percent HCl in the lower part of the layer.
Horizon BC 51-73cm -Muddy yellow, homogeneous, but with humus interbedded.The structure is lumpy and clumpy-clumpy, sometimes with signs of prismatics.It boils down to 10% HCl.White-eye horizon from a depth of 72 cm.Inclusions of 2-3 mm gypsum are traceable.
Transformation of organic matters in the soil is of big interest from the point of view of its evolution.As it is known in the soil there are labile and rather stable components of organic matter.Actually, humus is relatively stable organic matter and especially its part that is called humic.Humus acids are only 10-15% active in chernozems and chestnut soils [7].
The accumulation of organic material gives us an indirect indication of the level of soil fertility.
The palaeo-soils in the upper horizon show an organic matter content of 0.94% (Fig. 2).In the illuvial horizon there is a slight decrease in the studied value to 0.86%.The soilforming rock has a humus content of 0.34%.For soils that were buried during the second half of the Holocene this organic matter content can be considered high.Most likely, it is connected with the fact that the most inert part of hardly decomposable components remains in the soil.
In recent soils of virgin ecosystems, the content of organic matter in turf horizon is 6.28%.It is almost 7.5 times higher than in paleosoils.In illuvial horizon there is a decrease of the studied value by 0.89%.Down the profile there is also a decrease in this value and it is only 0.47% in the bedrock.The content of macronutrients gives us a basis for inferring the degree of fertility of any soil.As studies have shown, the nitrate nitrogen content in the upper horizon of the mound soils is only 2.9 mg/kg (Table 1).These are rather low values.Down the profile similar values were obtained, with a significant decrease in the C horizon.
In the recent soil this nutrient is more than in the palaeo-soil, but the differences are rather insignificant.
In the content of mobile phosphorus, the differences are more significant.So, in the top horizon of the buried soils its quantity makes 27.5 mg/kg that is average security, but close to the raised.Down the profile there is a decrease in the studied indicator to 18 mg/kg.
In recent soils the value studied is 15.7 mg/kg, which is also an average supply, but close to low.It is almost 12 mg/kg lower than in the buried soils.Down the profile there is also a decrease in the content of this form of phosphorus at lower values than in the mound soil.
The content of mobile potassium in the upper palaeolithic soil horizon is 260 mg/kg (Table 1).This is considered to be an average supply.In the solonetz horizon there is a decrease in this indicator by 40 mg/kg followed by an increase in the B2 and C horizons, also with an average supply of these soils.In recent soils the content of mobile potassium is 358 mg/kg.This is almost 100 mg/kg higher than in the palaeo-soil.The degree of supply of mobile potassium becomes elevated and in the B1 and B2 horizon it becomes high.This difference in the content of mobile potassium is indicative of a different mineralogical composition.Only prevalence of newly formed hydrosludite material in recent soils can guarantee such significant changes.
When studying the content of mobile sulfur (Table 1) we found out that its quantity in the upper horizon of paleosoils is 25.6 mg/kg.The soils are classified as highly supplied according to this indicator.In the solonetz B1 horizon the studied value increases up to 185.5 mg/kg.It is possible only with high content of salts and especially sulphate in these soils.
It is noteworthy that in the B2 horizon the content of mobile sulphur remains very high, while in the parent rock it decreases by 2 times compared to the solonetz horizon and by 3 times compared to the solonetz horizon.Consequently, in the initial stage of soil evolution there was purely chemical weathering of pyrite [8] to iron sulphate and eventually, judging by the colour, to limonite.The sulphuric acid produced in the reaction reacts with the carbonate group minerals with the accumulation of readily soluble sulphates.More significantly this already affects the soil mass.This process is also detected in the rock, with less activity, which indicates the beginning of the transformation of the mineral basis of the indigenous Maikop clays and their transformation into Maikop eluvium.
The content of trace elements may indicate the degree of erosion of both soils and parent rocks.They are also a diagnostic sign of the level of potential fertility of soils.
When carrying out researches it has been established that the contents of boron in buried soils in the top horizon is 3.1 mg/kg (tab.2).The soil is classified as highly fertile according to this indicator.In the B1 horizon there is a decrease of the studied value by 0.5 mg/kg.In the sunflower horizon the amount of mobile boron is only 0.5 mg/kg, which is 6 times lower than in the upper horizon.In the parent rock there is a further decrease of the studied index to the value of 0.05 mg/kg.
In recent soils of virgin lands in suprasalt horizon the content of mobile boron is 4.5 mg/kg.There is an increase of the indicator by 1.4 mg/kg.Down the profile there is a decrease in the studied value similar to the buried soils of the mound.It can be concluded that soils do not undergo significant changes in this indicator during the second half of the Holocene.
The content of mobile manganese in the paleosoils in the upper horizon is 13.8 mg/kg.In the solonetz horizon this indicator is halved.In the solonetz horizon and in the rock the content of manganese decreases, but insignificantly.In recent soils changes in the content of this element of a nutrient along a profile are similar to changes in soils of a mound.
In recent soils in supersaline and solonetz horizons the investigated size slightly differs and makes 0,74 and 0,71 mg/kg accordingly.It is almost 2,5 times lower than in soils under the mound.Such a supply of soils is considered to be low.Consequently, recent soils have experienced depletion in the content of this nutrient element for the period of 5 thousand years.
At research of the content of mobile copper it is established, that its quantity in the top horizon of paleosoils makes 0.39 mg/kg (tab.2).It decreases in the B1 horizon to 0.17 mg/kg and to 0.12 mg/kg in the B2 horizon.In the soil-forming rock this indicator is only 0.06 mg/kg.Consequently, all copper in the soil is concentrated in bulk forms.In recent soils on virgin land in the upper horizon the studied indicator is 0.21 mg/kg.This is 0.18 mg/kg lower than in buried soils.Down the profile of recent soils the decrease in the content of mobile copper is similar to the buried soils.Consequently, in the undisturbed horizons of paleosoils and recent soils on virgin soils the changes are relatively similar.
The content of mobile zinc is somewhat different.Its amount in the upper horizon of paleosoils is 1.64 mg/kg, and in the saline horizon 1.75 mg/kg.Such indicators are close to the average soil sufficiency for this nutrient.In the B and C horizon there is a further decrease in the studied index to 1.15 and 0.20 mg/kg respectively.
The content of mobile cobalt in paleosoils is 0.18 mg/kg, which is 3 times higher than in virgin recent soils.Down the profile the content of this nutrient element in the mound soils is higher, but insignificantly.
The content of mobile molybdenum between researched soils differs only in the top horizons.In the supra-solonts layer of paleosoils this indicator is 0.13 mg/kg and 0.04 mg/kg in recent soils.Reduction of the index more than 3 times.In the solonetz horizon such differences are 2 times.Thus, under the mound of mobile molybdenum 0,08 mg/kg, and on the virgin recent soils 0,04 mg/kg.

Conclusions
Thus, the soils of the zone have undergone a definite evolutionary path over a 5,000-year period in the second half of the Holocene.Morphologically, there is a BC horizon in recent soils, which is absent in paleosoils.The solonetz and podsolonetz horizons are highly compacted in the mound soils.In recent soils, the situation is different.Due to the presence of newly formed gypsum and carbonates, the solonetz horizon is relatively structured.Maternal rocks consist of slabs of bedrock of marine genesis.Layers of siderite and limonite are visible here.
Calcium carbonates and gypsum are absent throughout the buried soil profile.The genetic horizons are less developed, which is visible in their low thickness.Consequently, the soils of the mid-Holocene are classified as mononetzemic black soils in the first stages of evolution.
Recent black soils are solonetzolitic and have a relatively high supply of organic matter.This is due to a fairly rich vegetation cover, rich mineralogical and chemical composition [9].The content of organic matter in paleo-soils in the upper horizons is 6-7 times lower than in recent soils.It can be assumed that the amount of humus in the buried soils was similar to the recent soils at the time of the mound's creation.But over the entire period of the burial the organic matter retained its presence and in rather high values.This is especially true for horizons B1 and B2, which had no contact with pore water that was seen from the upper horizons of the soil.
There is an impoverishment of recent soils with mobile phosphorus, but an enrichment with mobile potassium.The identified differences must be sought in the changes in the composition of clay minerals.Probably the top horizon of recent soils is more sated with illite and the buried soils with smectite.The differences in the content of mobile sulphur are due to the higher degree of salinity of the paleosoils.
In the content of mobile trace elements, the differences between the studied soils can be considered significant.This is especially true for zinc, cobalt, and molybdenum.Consequently, during the evolutionary period of 5 thousand years, the soil-absorbing complex has been impoverished by these microelements.
Thus, according to the results of the morphological profile study, it was revealed that the buried soils are slightly saline fused low-powered black soils, while the recent soils have evolved into medium saline fused medium-powered black soils.

Fig. 2 .
Fig. 2. Total humus content in recent and buried soils of the Inozemtsevo mound, %.

Table 2 .
Microelement content in recent and buried soil of the Inozemtsevo mound, mg/kg.