Silva Balcanica, 20(Special issue 1)/2019

HUMIC SUBSTANCES, PHYSICOCHEMICAL PROPERTIES AND AGROCHEMICAL CHARACTERISTIC OF STRONGLY LEACHED CHERNOZEM FROM NORTHEASTERN

Toma Shishkov1, Ekaterina Filcheva1, Margarita Nankova2, Milena Kercheva1, Emil Dimitrov1 1N. Poushkarov Institute of Soil Science, Agrotechnology and Plant Protection – Sofia 2Dobrudzha Agricultural Institute – General Toshevo

Abstract

This paper discusses interpretation of variables on study of organic matter, physicochemical properties and agrochemical characteristic of strongly leached chernozem, formed in the region of in NE Bulgaria. The composition of organic matter is used as evidence of advanced and specific balance of soil processes in the steppe zone as well as biological activity. Total organic carbon (%) was recognizable even at the depth of profile. With regard to the agrochemical characteristic the study of the potential nitrogen-supplying capacity of the soil (nitrification capacity), was carried out to determine the capabilities of the particular site (plot) under optimal nitrification conditions, yield available nitrogen in nitrate form. For this reason, it was followed in dynamics (7th, 14th, 28th, 56th days). The highest value of mineralized nitrogen (an average of 29.67

NO3 mg/1000 g) was reached at 56th days of incubation period. Key words: organic matter, cation exchange capacity, chernozems, nitrogen, phosphorous

INTRODUCTION

The research site is located in Northeastern Bulgaria and covers the eastern part of Dobrudzha plateau in an area with a pronounced influence of the Black Sea on the climate. To the north it is bounded by the village of St. Nicholas, to the northeast by the village of , in the eastern part with the Black Sea and to the west by the village of . The unique feature of Dobrudzha coastline is the steppe areas. Geomorphologically, the area is in the eastern part of the Misesian platform, as a major morpho-structural unit, and according to physical geographic zoning it falls into Dobrudzha subregion of the Eastern Danube Plain. In hypsometric terms, the area is referred to the lowland up to 200 m a. s. l. The terrain has a undulating relief plain to plain-hilly, with prevailing slopes from 3o to 5o. In geological terms, this region of Dobrudzha plateau is built on the lower and the middle-Sarmatian sediments, made up of alternating sandy limestones and marls with the aquiferous flow that run through in them. The most common surface karst forms are known in the region as kayryatsi. They are located at the edge part of Dobrudzha plateau and raised up to 110 m heigh near Kavarna town. Among the old abrasive flat forms, the one most well-known is the Sarmatian-Pontic with an altitude of 230-250 m.

29 The survey area is refered to the temperate continental climate, which is formed under the direct influence of the Black Sea. The localclimate is determined by the influx of oceanic air from the northwest and northeast and continental air from the East European Plain. Relatively level terrain and its bare location facilitate the influx of humid, warm air in the spring, summer and autumn, as well as that of polar air in the winter. The Black Sea also exerts an influence over the region’s climate, particularly within 40–60 km from the coast. The small amount of rainfall and the wind with cold continental northern air masses during winter are typical for the region. There is wind during about 85–90% of all days which usually comes from the north or northeast. Winter seems harsh due to the constantly winding winds. The average wind speed is about twice higher than the average in the country. The wind speed in the area is the highest in winter and spring, when it reaches 6.5-8.0 m/sec and the lowest is in summer and autumn less than 5 m/sec. The average annual temperatures range from 11°C inland and to 11.8°C on the coast. The area is relatively cold for its geographic location, with average temperature of 0.8oC in January and 22.3oC in July. The annual temperature amplitude is 21.7-21.9°C. The coastal region of Southern Dobrudzha is the most arid part of Bulgaria, with the average annual rainfall is in the range of 411-480 mm. The maximum rainfall for the area is in autumn, the minimum in spring and summer. In hydrological terms, the area is poor on surface water, the sparse rivers are usually short and with low discharge. Zoning the territory referred it to into the Euxin province of the European broad- leaved forest region and the Lower Danube province of the Eurasian steppe and forest- steppe region (Bondev, 2002). The vegetation is characterized by the prevalence of grass species and scarce involvement of tree and shrub species. In the larger range, the plain terrains distributed on non-cultivated land are dominated by the vegetation, represented predominantly by xerothermic species – bulbous grass (Poa bulbosa L.), pastoral grass (Lolium perenne L.), Bermuda grass (Cydon dactylon L.), belichma (Dichantium ischaemum (L.) Roberty), sadda (Chrysopogon gryllus (L.) Trin.), wheat grass (Agrobyron cristatum), brome grass (Bromus commutatus), and other dry-resistant grass species, including steppe elements such as feather grass (Stipa lessingiana), wheat grass (Agropyrum brandzae), corncrake (Astra galus cornutus Pall.), (Jurinea albicaulis ssp. kilaea), cornflower (Centaurea salonitana Vis.), and others (Kitanov, Penev, 1980). The legumes occupy about 20% of the grassland and include mainly annual or heather species such as Astragalus hamosus, Medicago minima, Vicia sativa, Vicia peregrina, and the most well-represented perennials are a common star (Lotus corniculatus), Medicago lupulina, M. falcata, and Melilotus officinalis. There are separate forest-protection belts on chernozems dominated by oak (Quercus cerris L.), mixed with rust colored oak (Quercus pedunculifloraeL.), summer oak (Quercus robur L.), maple (Acer campestre L.), (Carpinus orientalis) and flowering ash (Fraxinus ornus), and communities of Christ’s thorn (Paliurus spina-Christi) and common smoke-tree (Cotinus coggygria Scop.) Along the gullies and drylands, there are some forms of elm Ulmus minor, Fraxinus oxycarpae, Quercus pedunculifloraeL. and willows Salix alba, S. fragilis. The natural forest ecosystems occupy local areas and fields of protection, dominated by the mixed deciduous forests.

30 In the area among Balgarevo village, Cape and Eni Kulak takes place the last and best preserved steppe habitats in Bulgaria. They are result from the combination of specific relief, soils and climatic conditions. Most of the plants belong to the xerothermic type formations similar to the Crimean peninsula.

MATERIALS AND METHODS

Soil organic carbon content was determined by modified Turin’s method (Filcheva, Tsadilas, 2002; Filcheva, 2015) (dichromate digestion at 125oC, 45 min, in presence of

Ag2SO4 and (NH4)2SO4 FeSO4 6H2O titration, phenyl anthranilyc acid as indicator). According to the method of Ganev, Arsova (1980) the total acidity is determined by the solutions of 1.0 N sodium acetate and 0.2 N potassium maleinate at pH 8.25, the value with maximum saturation of soil colloids with strong alkaline bases. Agrochemical characteristic was carried out on the territory of AGROSPECTOR LTD in the region of Kavarna. The analytical data for the mobile and total forms of the basic macroelements as well as the mineralization capacity of the soil was carried out in the Laboratory of Agrochemistry and physical characteristics of the soil at Dobrudzha Agricultural Institute, General Toshevo. Soil samples were prepared according to the requirements of each specific analysis, as follows: • Soil pH was determined in water and salt suspension of 1 N KCl (Palaveev, Totev, 1979). • The sum of the mineral nitrogen forms in soil was determined by extraction with

1% K2SO4 (mg/1000 g soil).

• The identification of NO3-N in the filtrate was done using the disulphophenol method; and ammonium nitrate was determined by the phenol method (Agrochemical methods for soil investigation, 1975). • The potential nitrogen supplying ability of soil was determined through incubation under constant temperature of 30oC at 60% humidity from its total moisture absorption capacity in order to develop optimal conditions for nitrification. The incubation was done in thermostat to investigate its dynamics at the 14th, 28th and 56th days. The samples were analyzed to determine the amount of nitrate

nitrogen in 1% K2SO4 extract. The ability of NO3-N to form intensive yellow

coloration when interacting with disulphurphenolic acid [C6H3OH(HSO3)2] in alkaline media was used (Agrochemical methods for soil investigation, 1975). • Available phosphorus and the exchangeable potassium were determined by AL-method (modification of Ivanov, 1984). Phosphorus available to plants

(mg P2O5/100 g soil) was determined colourimetrically, and the amount of

exchangeable potassium (mg C2O/100 g soil) through flame photometry. • The total nitrogen content in soil (mg/100 g) was determined by the method of

Kjeldahl after wet burning with concentrated H2SO4 diluted in distilled water at ratio 1:10, followed by distillation of nitrogen in Parnas-Wagner distillation apparatus.

31 • Total phosphorus content (%) was determined according to Bone method (Agro chemical methods for soil investigation, 1975). • The mathematical analysis of the obtained results was made by means of the software SPSS 16.0 (2007). Mean was separated by Least Significant Difference test (LSD) and the post-hoc analyses were presented through the Waller-Duncan’s Multiple Range Test (DMRT (Р < 0.05). The location of soil is nearby Krupen village, municipality of Kavarna, with coordinates N 43o34’22.43’’ E 28o14’45.59’’ and elevation 173 m. Relief is plane to undulating, parent materials are loess deposits. Soil moisture and temperature regime is mesic-xeric. Soil classification is as follows: the Bulgarian classification (1976): Strongly leached chernozem, none eroded and slightly eroded; according to the WRB (2015): Endocalcic Phaeozem (Pachic, Profundihumic, Siltic); according to the Soil Taxonomy USDA (2014): Pachic Haploxerolls; according to the FAO (1990): Haplic Chernozem. Morphological description is as follows: Ah 0-8 cm Colour 10 YR 3/2 (dry) and 10 YR 3/1 (wet), structure granular moderate friable medium size (2-5 mm diameter 55%), coarse strong friable (5-10 mm 40%) and very coarse (> 10mm 5%), with low packing density < 1.40 g cm3, abundance of fine roots, smooth boundary; A1 8-24 cm Colour 10 YR 3/2 (dry) and 10 YR 3/1 (wet), structure granular moderate friable medium size (2-5 mm diameter 40%), coarse strong friable (5-10 mm 55%) and very coarse (> 10 mm 5%), with low packing density < 1.40 g cm3, abundance of fine roots, smooth boundary; A2 24-42 cm Colour 10 YR 3/2 (dry) and 10 YR 3/1 (wet), structure granular moderate friable medium size (2-5 mm diameter 30%), coarse moderate firm (5-10 mm 60%) and very coarse (> 10 mm 10%), with low packing density < 1.40 g cm3, abundance of fine roots, smooth boundary; A3 42-67 cm Colour 10 YR 3/2 (dry) and 10 YR 3/1 (wet), structure subangularblocky strong friable very fine size (< 5 mm diameter 10%), fine moderate firm (5-10 mm 70%) and medium strong firm (10-20 mm 20%), with low packing density < 1.40 g cm3, abundance of fine roots, smooth boundary; AB 67-84 cm Colour 10 YR 3/2 (dry) and 10 YR 3/1 (wet), structure subangularblocky strong firm fine size (5-10 mm diameter 30%), medium size moderate firm (10- 20 mm 50%) and firm strong coarse (20-50 mm 20%), with low packing density < 1.40 g cm3, few fine roots, smooth boundary; B1 84-105 cm Colour 10 YR 3/3 (dry) and 10 YR 3/2 (wet), structure subangularblocky strong friable fine size (5-10 mm diameter 10%), medium size moderate firm (10- 20 mm 40%) and firm strong coarse size (20-50 mm 20%), with medium packing density 1.40-1.75 g cm3, very few roots; clear boundary; Bk2 105-120 cm Colour 10 YR 4/3 (dry) and 10 YR 3/4 (wet), structure subangularblocky firm strong fine size (5-10 mm diameter 20%), firm strong medium size (10-20 mm 50%) and firm strong coarse size (20-50 mm 30%), with medium packing density 3 1.40-1.75 g cm , strong effervescence, CaCO3 pseudo mycelium clear boundary;

32 BCk 120-152 cm Colour 10 YR 5/3 (dry) and 10 YR 4/3 (wet), structure subangularblocky strong friable medium size (10-20 mm diameter 30%), coarse moderate firm (20- 50 mm 40%) and very coarse (> 50 mm 30%), with medium packing density 3 1.40-1.75 g cm , strong effervescence CaCO3 concretions and pseudo mycelium, smooth boundary; Ck1 152-178 cm Colour 10 YR 6/3 (dry) and 10 YR 5/4 (wet), structure subangularblocky firm strong medium size (10-20 mm diameter 40%), coarse moderate firm (20- 50 mm 40%) and very coarse (> 50 mm 20%), with medium packing density 3 1.40-1.75 g cm , strong effervescence, CaCO3 concretions and pseudo mycelium, smooth boundary; Ck2 178-195 cm Colour 10 YR 6/4 (dry) and 10 YR 5/4 (wet), structure subangularblocky medium size moderate firm (10-20 mm 20%), coarse size strong firm (20-50) mm diameter and massive, with medium packing density 1.40-1.75 g cm3, strong

effervescence and CaCO3 concretions.

RESULTS AND DISCUSSION

The area is entirely occupied by chernozems and with their varieties. Soils have a well developed humus horizon, with a weak alkaline to neutral soil reaction. In the northern part of the region, the most common soil type is strongly leached as well clayey chernozems (or karasoluk). Rendzinas (or humus-calcareous soils) occupy a strip along the sea. These shallow skeletal soils developed on Sarmatian limestone parent rock are pastures of semi-steppe nature. The studied strongly leached chernozem (none eroded and slightly eroded) is characterized with non-differentiated profile, which is confirmed by the data of textural fractions composition (Table1). According to the texture data the soil is classified as Silty Clay Loam (according to USDA) and Fine Loam (according to Kachinsky).

The measured data on physical properties – bulk density (db), particle density (ds) and total porosity (Pt) in genetic horizons show no evidence of soil compaction, strength structure due to the organic matter content and porosity that ensure favourable moisture and air regime (Table 2). The surface horizons can be determined as with high 2.59% amount of organic carbon, which is 1.81-1.97% in the subsurface soil horizons. The amount of carbon decreases gradually downward, but it is about one and a half times less as compared to that at the surface (Table 3).

The amount of the extractable organic carbon (extracted with 0.1 M Na4P2O7+0.1 M NaOH) in the surface horizons is 27-30% from the total organic carbon to the depth of 24 cm. In the humus horizons that amount is respectively 30 to 33% to the depth 42 cm, while downward increases to 38-41%. This difference shows that in the upper horizons the organic matter is more stronger bounded to the soil mineral part. Humic acids amount is 21-23% in the upper humus accumulative horizons, determining it as such with low degree of humification and gradually increases to

33 Table 1. Textural fractions and classes measured according to FAO (2006), USDA and Kachinsky method (revised) of strongly leached chernozem (none eroded and slightly eroded)

Textural Particle size (mm) and According fractions fractions (%) to USDA after Textural Textural class Kachinsky class Depth, Sand according to according (cm) Silt Clay

Horizon (2000- Sand USDA to (2-63 (< 2 < 10 < 1 63 (0.05-2 Kachinsky μm), μm), μm μm μm), μm), % % % % Ah 0-8 2.0 60.4 37.5 3.1 55.5 32.7 Fine loam A1 8-24 1.2 60.2 38.6 2.2 Silty Clay Loam 55.1 33.9 Fine loam A2 24-42 1.5 60.8 37.7 3.1 Silty Clay Loam 55.7 29.0 Fine loam A3 42-67 0.8 60.7 38.5 1.8 Silty Clay Loam 56.0 33.8 Fine loam AB 67-84 0.8 61.2 38.0 3.7 Silty Clay Loam 55.4 33.3 Fine loam B1 84-105 1.0 62.1 36.9 2.7 Silty Clay Loam 54.2 32.6 Fine loam Bk2 105-120 1.1 63.1 35.8 3.1 Silty Clay Loam 52.9 31.5 Fine loam BCk 120-152 1.1 63.4 35.5 2.8 Silty Clay Loam 52.8 30.3 Fine loam Ck 152-178 2.4 60.2 37.4 4.5 Silty Clay Loam 55.4 31.7 Fine loam Ck2 178-195 4.0 59.7 36.3 5.4 Silty Clay Loam 52.5 30.2 Fine loam

Table 2. Bulk density (db), particle density (ds) and total porosity (Pt) of strongly leached chernozem (mean ± standard deviation of the replicates)

Depth, Core sampling Horizon d , g.cm-3 d , g.cm-3 P , % (cm) depth, (cm) b s t Ah 0-8 0-5 1.28 ± 0.04 2.643 51.5 ± 1.5 A1 8-24 15-20 1.46 ± 0.02 2.677 45.3 ± 0.7 A2 24-42 30-35 1.43 ± 0.03 2.700 47.2 ± 1.1 A3 42-67 55-60 1.38 ± 0.06 2.703 49.0 ± 2.4 AB 67-84 73-78 1.44 ± 0.04 2.710 46.8 ± 2.5 B1 84-105 93-98 1.36 ± 0.02 2.722 50.1 ± 0.8 Bk2 105-120 - - 2.728 - BCk 120-152 135-140 1.39 ± 0.03 2.730 48.9 ± 1.1 Ck 152-178 - - 2.724 - Ck2 178-195 185-190 1.41 ± 0.09 2.729 48.2 ± 3.2

32.14% in the transitional (AB) horizon at the depth 70-85 cm, determining it as such with medium degree of humification.

The ratio Cha/Cfa in the range 3 to 5 is an evidence for humic type of humus, according to Kononova (1966). The sharp decrease of this ratio to 1.23 at the depth 84-105 cm in the transitional (AB) horizon determines humus to be of fulvic-humic type, due to the slight increasing of fulvic acids from 6 % up to 18 %. The soil performs 100 % humic acids Ca-complexed, forming insoluble microstructure fragments. The

34 (%) 0.23 8.88 0.15 7.61 0.11 6.08 0.10 6.67 0.11 7.86 0.07 7.53 0.07 9.33 0.06 9.38 0.04 0.04 11.11 11.76 Org. C Org. NaOH, , ------ha C (%) Free ) 6 /E 4 Optic Optic (E , ha 3.51 3.89 3.36 3.30 3.30 3.33 3.34 3.27 2.86 3.24 characteristic C (%) Total 4 SO extr. extr. 2 (%) fa with 0.07 2.70 0.06 3.05 0.04 2.21 0.04 2.67 0.04 2.86 0.03 3.23 0.03 4.00 0.03 4.69 0.02 5.56 0.02 5.88 0.1 N 0.1 N H C (%) 1.88 1.38 1.21 0.92 0.86 0.55 0.48 0.40 0.22 0.20 72.59 70.05 66.85 61.33 61.43 59.14 64.00 62.50 61.11 58.82 Org.C, Unextr. Ca 100 100 100 100 100 100 100 100 100 100 complexed 3 Humic acids Humic O 2 Free 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 R Organic C, (% ) of Organic compl.. fa /C - ha 3.73 3.21 3.00 3.46 5.00 1.23 3.50 3.00 2.50 C (Pachic, Profundihumic, Siltic) WRB (2015) Siltic) Profundihumic, (Pachic, ) humic acids; ) humic acids; ha +0.1 +0.1 7 0.00 0.15 5.79 0.14 7.11 0.15 8.29 0.13 8.67 0.09 6.43 0.17 0.06 8.00 0.06 9.38 0.04 acids 18.28 11.11 Fulvic Fulvic O 2 P Organic matter content and composition 3. Organic Table 4 Phaeozem Endocalcic 0.56 0.45 0.45 0.45 0.45 0.21 0.21 0.18 0.10 0.14 acids Strongly leached chernozem, none eroded and slightly eroded (1976); (1976); and slightly eroded none eroded leached chernozem, Strongly 21.62 22.84 24.86 30.00 32.14 22.58 28.00 28.12 27.78 41.18 Humic Humic M NaOH free) alkaline extract free) b ext., Organic carbon, (%) Organic а ha (%) 0.24 0.14 0.59 0.60 0.58 0.54 0.38 0.27 0.14 0.71 37.50 41.18 29.95 33.15 38.67 38.57 40.86 36.00 38.89 27.41 Org. C Org. Extracted with 0.1M Na Extracted ) optic characteristic, ) optic characteristic, 6 C, Org (%) 0.93 0.64 0.36 0.34 0.75 1.97 1.81 1.50 1.40 2.59 Total /E 4 m) B1 A1 A2 A3 Ah Ck AB 0-8 Bk2 Ck2 BCk 8-24 24-42 42-67 67-84 84-105 120-152 152-178 178-195 105-120 total) pyrophosphate extract, (C total) pyrophosphate ) fulvic acids; (E depth, ( с Horizon and Horizon fa ha (a) % of dry mass;(b) % of total carbon; (c) humic acids; (C (C (C

35 Fig. 1. Profile of strongly leached chernozem, none eroded and slightly eroded (Yolevski, Hadzhiyanakiev, 1976); according to the WRB (2015): Endocalcic Phaeozem (Pachic, Profundihumic, Siltic) unexrtactable organic carbon of 59-72 % is strongly bound not only as organic-clay compounds in soils but as CaCO3 humate mixed sediments.

Soil is with well developed adsorbent throughout the profile (T8.2=44.7-43.9 cmol/kg-1 of soil) to depth of 67 cm (Table 4). According to the crystallochemical characteristic, the share of strongly acidic (TSA) ion-exchange positions is 84.88-83.56% of (T8.2) in surface horizons at a depth of 24 cm and 87.70% deeper, which confirms the presence of clay minerals with high isomorphic substitution. The degree of base saturation is very high (90-100%) (Fig. 3). The values (exch. H8.2=7.67-9.49 % of T8.2) indicate a weak acidification in the surface horizons, but it affects mostly the weakly acidic exchange positions (TA) of adsorbent, while the strongly acidic (TSA) are almost completely saturated with strong exchange bases. There are no destructive processes in the soil adsorbing complex. Soil is with moderately humified mineral colloids, where the dominant clay mineralogy is homogeneous through the profile of montmorillonite-illite composition of clay minerals.

The values of pH (H2O) 6.3-6.8 indicate the leaching of carbonates and presence of the hydrolytically alkaline salt, composed of hydrogen ions and weakly acidic ion-

36 Humic acids,%

Fulvic acids,% 45 3 Total carbon,% 40 2,5 35 30 2 25 1,5 20 15 1 10 of total organic carbon organic total of 0,5 5

Content of organic substances, % substances, organic of Content 0 0 Ah A1 A2 A3 AB B1 Bk2 BCk Ck1 Ck2 Content of total organic carbon, % carbon, organic total of Content

Fig.Fig 2. Content. 2. Content and distribution and distribution of organic carbon of in organic the profile carbon of strongly in leached the profilechernozem of strongly leached chernozem (none eroded and slightly eroded) (none eroded and slightly eroded).

100 90 80 Exch. H8.2, % 70 60 Exch. Ca, % 50 40 Exch. Mg, %

30 Tca, % ofT8,2 20 Percentage , % of T8,2 10 Base saturation, % 0 Ah A1 A2 A3 AB B1 Bk2 BCk Ck1 Ck2 Fig. 3. Content of exchange cations and base saturation in the profile of strongly leached chernozem (none eroded and slightly eroded) Fig. 3. Content of exchange cations and base saturation in the profile of strongly leached chernozemexchanger. The (none chemical eroded reactivity and slightly of adsorption eroded) system. is determined by the hydrogen ions (exch. H8.2

37

2

3

Table 4. Cation exchange capacity (meq/100g soil = cmol/kg-1) and base saturation (%) after the method of Ganev, Arsova (1980).

Base satu- Exchange cations, T (%) 8.2 ration, O) 2 (%) Horizon pH (H Depth (cm) Depth T8.2 TSA TA H8.2 Ca Mg TSA TA H8.2 Ca Mg

Strongly leached chernozem, none eroded and slightly eroded; Endocalcic Phaeozem (Pachic, Profundihumic, Siltic) WRB Ah 0-8 6.35 43.2 36.1 7.1 4.1 33.0 6.1 83.56 16.44 9.49 76.39 14.12 90.51 A1 8-24 6.50 43.0 36.5 6.5 3.3 34.0 5.6 84.88 15.11 7.67 79.07 13.02 92.33 A2 24-42 6.65 44.7 39.3 5.4 1.6 37.2 5.8 87.92 12.08 3.58 83.22 12.98 96.42 A3 42-67 6.80 43.9 38.5 5.4 1.3 36.8 5.6 87.70 12.30 2.96 83.83 12.76 97.04 AB 67-84 6.70 39.5 33.5 6.0 2.5 23.2 5.5 84.81 15.19 6.33 81.52 13.92 93.67 B1 84-105 6.60 39.5 33.4 6.1 2.7 31.0 5.5 84.56 15.44 6.84 78.48 13.92 93.16 Bk2 105-120 7.35 37.4 - - 0 33.6 3.8 - - 0 89.84 10.16 100 BCk 120-152 7.35 37.3 - - 0 33.5 8.8 - - 0 89.81 10.19 100 Ck 152-178 7.60 30.5 - - 0 27.0 3.5 - - 0 88.52 11.48 100 Ck2 178-195 7.90 31.0 - - 0 27.5 3.5 - - 0 88.71 11.29 100

Т8.2 – total sum of exchange cations

ТSА- cation-exchange capacity of soil strongly acidic ion-exchanger

ТА- soil slightly acidic ion-exchanger

The analysis of variants shows a high level of statistical reliability of the established values for the studied parameters identified in the genetic horizons of strongly leached chernozem (Table 5). During September 2018 after a hot summer season the total nitrogen content and data on mineral forms of nitrogen are typical for the studied uncultivated chernozems. The content of mineral forms of nitrogen is not high, particularly at the moment of sampling where dominant nitrate form of nitrogen was established throughout the depth of profile (Table 6). The average total nitrogen content is 207.951 mg/100 g of soil in the humus-accumulative horizons (0-67 cm); and is 102.193 mg/100 g of soil in the transitional horizons (from 67 cm to 152 cm); and in the Ck horizons (from 152 cm and below 200 cm) it is 42.725 mg/100 g of soil. The high content of total nitrogen is the main preconditions for the high potential nitrogen supply capacity of the studied soil in dynamics depending on the incubation period. With regard to the agrochemical characteristic the study of the potential nitrogen-supplying capacity of the soil (nitrification capacity), was done to determine where the capabilities of the particular site (plot) under optimal nitrification conditions, yield available nitrogen in nitrate form. For this reason, it was performed in dynamics, i.e.7th, 14th, 28th and 56th days.

38 Table 5. Analysis of the variances of the investigated indices (values of parameter P), where MA – mineralization ability of soil; PMA – pure mineralization ability of soil Type III Sum Mean Source Dependent Variable Df F Sig. of Squares Square рН – KCl 6.787 9 .754 930.963 .000

рН – H2O 4.696 9 .522 599.768 .000

NH4 10.634 9 1.182 10.750 .000

NO3 16.334 9 1.815 68.590 .000 Sum 46.933 9 5.215 42.565 .000

P2O5 54.682 9 6.076 361.437 .000

K2O 341.635 9 37.959 154936.510 .000 Horizons Total N 104 173.363 9 11 574.818 384.106 .000 of strongly Total P .006 9 .001 103.208 .000 leached chernozem, MA – 14 days period 7850.428 9 872.270 6066.908 .000 none eroded PMA – 14 days period 7230.288 9 803.365 5660.891 .000 and slightly МA – period 28 days 11 715.996 9 1301.777 10177.689 .000 eroded PMA – period 28 days 10 963.893 9 1218.210 7894.821 .000 Super PMA – period 14-28 518.942 9 57.660 124.030 .000 days МA – period 56 days 10 820.954 9 1202.328 591.626 .000 PMA – period 56 days 10 104.459 9 1122.718 570.178 .000 Super PMA – period 28-56 94.930 9 10.548 3.938 .022NS days Super PMA – period 10 104.459 9 1122.718 570.178 .000 0-56 days

Table 6. Soil pH and composition of mobile forms of mineral and total nitrogen in strongly leached chernozem (none eroded and slightly eroded). (a, b, c, d, ab, cd) mark similarity and difference between the values of the indicators ranges from a small ‘a’ to a large one ‘z’, after Waller-Dunkan test.

Soil reaction Forms of mineral N, Horizons Depth, cm Total N, mg/1000 g mg/100 g рH KCl pH H2O NH4 NO3 ∑ Ah 0-8 5.91 a 6.92 a 4.30 d 7.16 f 11.46 f 275.975 h A1 8-24 6.07 c 7.21 b 3.41 c 5.26 e 8.67 e 194.765 g A2 24-42 6.03 bc 7.31 c 3.12 bc 4.84 cd 7.96 de 197.455 g A3 42-67 6.22 d 7.53 e 2.08 a 4.93 de 7.01 b 163.610 f AB 67-84 5.99 b 7.34 c 2.04 a 4.69 bcd 6.73 b 140.880 e B1 84-105 5.93 a 7.43 d 1.97 a 4.66 bcd 6.63 b 106.230 d B2k 105-120 6.93 e 7.82 f 2.22 a 3.37 a 5.59 a 91.225 c BCk 120-152 7.17 f 8.21 g 2.22 a 4.39 b 6.61 b 70.435 b Ck 152-175 7.32 g 8.37 h 3.21 bc 4.57 bc 7.78 cd 43.880 a C 175-200 7.33 g 8.35 h 2.64 ab 4.51 bc 7.15 bc 41.570 a The error term is Mean .001 .001 .110 .026 .123 30.134 Square (Error)

39 60,00

50,00

40,00

30,00

20,00

10,00

0,00

-10,00 AH (0-8 cm) A1 (8-24 cm) A2 (24-42 cm) A3 (42-67 cm) AB (67-84 cm) B1 (84--105 cm) B2K (105-120 cm) BCk (120-152 cm) Ck (152-175 cm) C (175-200 cm) 0-14 day 14-28 day 28-56 day Fig. 4. Pure soil mineralization capacity between individual incubation periods, NO3 mg/ 1000 g soil

Fig. 4. Pure 400,0soil mineralization capacity between individual incubation250,0 periods, NO3 mg/ 1000 g soil 350,0 200,0 300,0

150,0 250,0

200,0 100,0 Total N, g/m2 N, Total P,Total g/m2 150,0 50,0 100,0

50,0 0,0 Ah A1 A2 A3 A4 B1 B2k BCk Ck Ck2

Total N Total P

Fig. 5. Stock of the nitrogen and phosphorus (P O ), g/m2 in the strongly 2leached chernozem Fig. 5. Stock of the nitrogen and phosphorus2 5 (P2O5), g/m in the strongly leached chernozem (none eroded and slightly eroded) (none eroded and slightly eroded). The incubation period of 56th days gives the most accurate idea of the potential soil mineralization capacity and ability of the soil to supply plants with nitrogen. The highest value of mineralized nitrogen (an average of 29.67 NO mg/1000 g) was reached 3 at 56th days of incubation period. Significantly higher content of mineralized nitrogen is established in the upper part at the humus-accumulative horizons that gradually decreases in the depth of profile (Fig. 4). This trend also is valid for the other two incubation periods.

40

4

5

1,600 1,400 1,200 1,000 0,800 0,600 0,400 0,200

Mineral forms, nitrogen g/m2 0,000 Ah A1 A2 A3 A4 B1 B2k BCk Ck Ck2

NH4 NO3 Σ mineral nitrogen forms

Fig. 6. StockFig. of6. Stockthe mineral of the mineral nitrogen nitrogen forms forms ( g/m(g/m22)) in thethe strongly strongly leached leached chernozem chernozem (none (none eroded and slightly eroded) eroded and slightly eroded). 8,0 35,0

7,0 33,0 31,0 6,0 29,0

5,0 27,0

4,0 25,0

3,0 23,0 21,0 2,0 19,0

Available phosphorus, g/m2Available 1,0

17,0 Exchangeable potassium, g/m2 0,0 15,0 Ah A1 A2 A3 A4 B1 B2k BCk Ck Ck2

P2O5 K2O

2 Fig. 7. Stock of the available phosphorus (P2O5) and exchangeable potassium (K2O), g/m 2 Fig. 7. Stock of the available phosphorus (P2O5) and exchangeable potassium (K2O), g/m

The incubation period of 14th days releases an average of 56.75% and 67.37% respectively for the 28th days of incubation versus the 56th day period. During the last incubation period, the amount of mineralized nitrogen exceeds the sum of the mineral nitrogen forms by 297.47% on average for the depth of the profile (Table 7). In the humus accumulative horizon (0-67 cm) after 14th days of incubation

an additional 115.81 mg NO3/1000 g of soil was released and after another period of 41 6

7

Table 7. Soil mineralization dynamics and capacity depending on the incubation period of 14, 28, 56

days, NO3 mg/1000 g soil

Profile Mineralization capacity, NO3 mg/1000 g 14 days period 28 days period 56 days days days Total Total Total 14-28 14-28 28-56 Horizons Depth, cm Depth, 0-14 days 0-28 days 0-56 days Ah 0-8 66.81 i 59.65 g 83.23 g 75.57 g 15.93 e 87.56 f 4.33 a 80.40 f A1 8-24 37.61 h 32.35 f 39.90 f 34.64 f 2.29 b 51.99 e 12.09 b 46.73 e A2 24-42 25.73 g 20.89 e 32.38 e 27.54 e 6.65 d 44.08 d 11.70 b 39.24 d A3 42-67 7.85 f 2.92 d 13.22 d 8.29 d 5.37 d 23.87 c 10.65 b 18.94 c AB 67-84 6.84 e 2.15 c 5.70 b 1.01 b -1.14 a 14.14 ab 8.44 b 9.45 a B1 84-105 5.67 d 1.01 b 3.86 a -.81 a -1.82 a 15.66 ab 11.80 b 11.0 ab Bk2 105-120 4.00 a .63 b 7.94 c 4.57 c 3.94 c 16.72 b 8.78 b 13.35 b BCk 120-152 4.15 ab -.24 a 5.64 b 1.25 b 1.49 b 14.59 ab 8.96 b 10.20 a Ck 152-175 4.96 cd .39 ab 3.99 a -.59 a -.97 a 14.44 ab 10.46 b 9.87 a Ck 175-200 4.78 bc .27 ab 4.06 a -.46 a -.73 a 13.68 a 9.63 b 9.87 a The error term is Mean Square .144 .142 .128 .154 .465 2.032 2.678 1.969 (Error)

Table 8. Correlation between soil mineralization capacity depending on the incubation period and some basic agrochemical parameters

Indices Period of 14 days Period of 28 days Period of 56 days ∑ N mineralization forms 0.927** 0.912** 0.905** Total N 0.868** 0.880** 0.890** ** ** ** Total P2O5 0.903 0.913 0.905 ** ** ** pH H2O - 0.714 - 0.709 - 0.723

Table 9. Content of available forms of phosphorus and potassium and total phosphorus in the strongly leached chernozem, none eroded and slightly eroded;

Depth, cm Available forms, mg/100 g soil Total P O , Horizons 2 5 P2O5 K2O % Ah 0-8 6.14 f 26.88 f .1782 g A1 8-24 1.84 c 22.77 e .1545 f A2 24-42 1.11 a 18.63 c .1436 e A3 42-67 1.00 a 18.63 c .1361 cd AB 67-84 1.00 a 20.70 d .1390 de B1 84105 1.08 a 18.63 c .1309 b Bk2 105-120 1.47 b 18.63 c .1361 cd BCk 120-152 1.93 c 16.56 b .1332 bc Ck 152-175 2.81 d 12.42 a .1153 a Ck2 175-200 4.54 e 12.42 a .1165 a The error term is Mean Square (Error) ,017 .000 6.41E-006

42 14th days a new 30.24 mg NO3/1000 g was released. During the longest incubation period, the values of nitrified nitrogen attained 207.5 mg NO3/1000 g of soil, as between 28th and 56th days from the start of the incubation additional 38.77 mg of

NO3/1000 g were nitrified at a depth of 67 cm (horizons Ah-A1-A2-A3). The observed net nitrified nitrogen content at the end of the 56th day of the incubation was 185.31 mg NO3/1000 g soil, and the total amount in the 0-200 cm depth was 249.05 mg

NO3/1000 g soil. In the transition horizons (84-152 cm) and deeper, the processes of denitrification associated with nitrogen loss occur during the studied incubation periods.

CONCLUSIONS

Significant positive correlative relationship was recognized: between the mineralization capacity of soil with different incubation periods; (i) with the total content of nitrogen and phosphorus (Fig. 5); and, (ii) with the sum of the mineral nitrogen forms (Table 8, Fig. 6). The pH correlation is inversely proportional with a high level of statistical reliability. In the upper horizons the content of available forms of phosphorus is low and increased with depth (Table 9). The amount of total phosphorus is quite high throughout the soil. The content of available forms of potassium is high in the profile (Fig. 7).

REFERENCES

Agrochemical methods for investigation of soil. 1975. Moscow, Nauka (Science in Russian), 656 Bondev, I. 2002. Biogeographic regions of Bulgaria. Physical Geography of Bulgaria, ForKom, ISBN 978- 954-464-123-8, 760 (In Bulgarian). Ivanov, P. 1984. A new acetate-lactate method for identification of plant available phosphorus and potassium in soil. Soil science and agrochemistry, 4, 88-98 (In Bulgarian, English summary). FAO. 2015. World Reference Base for Soil Resources. International soil classification system for naming soils and creating legends for soil maps. World Soil Resources Reports No 106, FAO, Rome, 191, ISBN 978-92-5-108369-7 (print) E-ISBN 978-92-5-108370-3. Filcheva, E. 2014. Humus formation, composition of organic matter and organic carbon stocks in soil groups and types. – In: Soil organic matter and fertility of Bulgarian soils. (S. Krastranov, E. Filcheva, M. Teoharov, N. Artinova, R. Ilieva, I. Pachev, R. Mitovska, Z. Petkova, V. Koutev, T. Mitova, M. Nankova, D. Stoicheva, I. Dimitrov, S. Rousseva, I. Marinov, P. Bouzhinova, N. Dinev, E. Velizarova, K. Chakalov). BHSS, ISBN 978-619-90189-1-0, 470 (In Bulgarian). (88-106). Filcheva, E., C. Tsadilas. 2002. Influence of Cliniptilolite and Compost on Soil Properties. Commun. of Soil Sci. and Plant Analysis, 33 (3/4), 595-607. Filcheva, E. 2015. Characteristics of Soil Organic Matter of Bulgarian Soils. Lap Lambert Academic publishing, 177, ISBN 978-3-659-51204-9. Ganev, S., A. Arsova. 1980. Methods for determination of strongly acidic and weakly acidic cation-exchange in the soil. Soil science and Agrochemistry, Sofia, 15 (3), 22-33 (In Bulgarian, English summary). Kitanov, B., I. Penev. 1980. Flora of Dobrudzha. Geography and Earth Science. Publ. house Science and arts, Sofia, 632 (In Bulgarian) Kononova, M. 1966. Soil Organic matter. 2nd ed. Pergamon press, Inc., M.V., 544. Orlov, D. S. 1985. Soil Chemistry. Moscow State University, 376 (In Russian).

43 Palaveev, Т. D., T. P.Totev. 1979. Soil acidity and agro methods for its neutralization. Sofia, Zemizdat, 132- 137 (In Bulgarian) Soil Survey Staff. 2014. Keys to Soil Taxonomy, 12th Edition, Washington, DC, U.S. Department of Agriculture, Natural Resources Conservation Service, 372. Yolevski, M., Hadzhiyanakiev, A. 1976. Agro-grouping of soils in Bulgaria. Extended Systematic List, Code of groups and soil varieties, Institute of Soil Science ‘N. Poushkarov’, Sofia, 91 (In Bulgarian)

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