FOCUS OF MINERALIZATION–SYNTHESIS PROCESSES OF THE ORGANIC MATTER IN THE LEACHED CHERNOZEM WHILE CULTIVATING POTATOES ON DIFFERENT FERTILIZATION BACKGROUNDS

It is known that in natural phytocenosis soils the processes of mortmass mineralization and synthesis of de novo organic matter are balanced, which ensures maintaining the stable humus content in them. On the contrary, due to anthropogenic impact arable soils are characterized by negative changes in both the resources of humus substances therein and their agronomic properties (Waksman S.A., 1952; Brock C. et. al, 2013). The fertilization system of agricultural crops is a special factor, impacting the humus state of agriculISSN: 2312–3370, Agricultural Science and Practice, 2020, Vol. 7, No. 1


INTRODUCTION
It is known that in natural phytocenosis soils the processes of mortmass mineralization and synthesis of de novo organic matter are balanced, which en-sures maintaining the stable humus content in them. On the contrary, due to anthropogenic impact arable soils are characterized by negative changes in both the resources of humus substances therein and their agronomic properties (Waksman S.A., 1952;Brock C. et. al, 2013). The fertilization system of agricultural crops is a special factor, impacting the humus state of agricul-FOCUS OF MINERALIZATION-SYNTHESIS PROCESSES OF THE ORGANIC MATTER tural lands. For instance, the excessive use of mineral fertilizers does not envisage the introduction of fresh organic matter (except for crop residue) which is the substrate for humus synthesis, has negative effect on the fraction-wise ratio of humic acids, conditions the weakening of their bond with Ca 2+ , leads to the increase in the motility and mineralization of humus constituents. The abovementioned results in gradual deterioration of soil fertility (Doran J.W. et al., 1996;Margdoff F. et al., 1997, Baliuk S. et al., 2014Purtova L.N. et al., 2017). In some conditions, the additional application of organic materials (manure, straw, mass of cover crops, etc.) to fertilize agricultural crops and replenish soils with carbon promotes the domination of organic matter synthesis processes over its mineralization (Raupp J. et al., 2001;Baliuk et al., 2016;Purtova L.N., Kiseleva I.V., 2019). However, the consequences of the impact of agrotechnical technologies on the optimization of mineralization-synthesis processes and humus content are widely separated in time which hinders timely conclusions on the effi ciency of specifi c measures.
Therefore, the express method of determining the focus of mineralization-synthesis processes of organic matter was suggested (Volkogon et al., 2019). Its essence lies in determining the emission of N 2 O and CO 2 depending on the agrotechnology and subsequent calculations of specifi c losses of nitrogen gas (per unit of emission carbon dioxide) as well as mineralizationsynthesis indices for organic matter which take into account the intensity of processes in the soil of agrocenosis as compared to the "reference" soil (fallow, virgin land). Gas chromatography analysis of emission losses of nitrogen and carbon allows to determine timely and accurately which of the two processes (mineralization of organic matter or synthesis de novo) dominates in the soil.
Taking the abovementioned into consideration, the aim of our present research was to study the focus of mineralization-synthesis processes in leached chernozem while cultivating potatoes using the abovementioned methodological approaches (Volkogon et al., 2019).

MATERIALS AND METHODS
The study was conducted in 2016-2018 at the fi eld long-term experiment of the Institute of Agricultural Microbiology and Agroindustrial Manufacture, the National Academy of Agrarian Sciences of Ukraine, initiated in 2009 on leached chernozem. The agrochemical characteristic of soil, determined by standard methods (Fomin G.S., Fomin A.G., 2000), demonstrates the fol-lowing indices: рН sal -5.3; humus content -3.03 %; easily hydrolysable nitrogen -95 mg/kg of soil; mobile phosphorus compounds (Р 2 О 5 ) -150 mg/kg of soil; exchange potassium (К 2 О) -108 mg/kg of soil.
The following agricultural crops were cultivated during the experiment on 1 ha of short crop rotation: potatoes -spring barley -peas -winter wheat.
The potatoes were cultivated on different agrobackgrounds for the purposes of the study. This crop is a convenient study object, as its fertilization systems may be based on the application of both organic, mineral, and organo-mineral fertilizers which may ensure introduction of different fresh organic matter into soil and thus different focus of biological processes.
The absolute control ("reference" plot) in the study was a fallow (since 2009) with the area of 0.75 ha, located next to the long-term fi eld experiment.
In the corresponding variants of the experiment, 5 t/ha ground straw of winter wheat were incorporated into to the soil by disking immediately after harvesting. To receive green manure mass, blue lupine seeds were sown immediately after wheat harvesting and disking. In late autumn, the obtained green manure mass (13 t/ha) was incorporated to the soil by disking and subsequent ploughing. Cattle manure in the amount of 40 t/ha was incorporated to soil by tillage at the same time. The depth of tillage was 15 cm.
Complex mineral fertilizers (the content of the active substance -N 16 P 16 K 16 ) were introduced in spring prior to cultivating potatoes according to the experiment scheme (40, 80 and 120 kg/ha of the active substance).
The experiment was arranged in a completely randomized design with four repeats and the area of the experimental plot of 44.6 sq.m.
The closed chamber method (Hutchinson G.L. et al., 2000;Kusa K. et al., 2008;Zviagintzev, 1991) was used to estimate the emission of nitrous oxide and carbon dioxide in the "soil-plant" system after our modi-fi cation (Volkogon et al., 2019). This method involved the use of plastic 10-liter buckets, the bottom of which was drilled to install a rubber plug to isolate gases, and 50 ml syringe was then used to take air samples.
The chambers were "installed" in soil at the depth of 5 cm. A "water plug" was made around the chamber for better isolation of the system and prevention of the loss of gases, accumulated in the chambers. A weighing cup with water was put inside the chamber with the addition of 20 g calcium acetylide prior to the exposition (the reactions of calcium acetylide with water result in the formation of acetylene, inhibiting the enzyme of nitrous oxide reductase and terminating the process of dissimilation of NO 3ˉ and NO 2ˉ at the stage of nitrous oxide reduction (Zviagintsev D.G., 1991). The exposition lasted three hours. The selected gas samples were placed into previously vacuumed vials with rubber plugs, taken to the laboratory and analyzed using gas chromatography.
The amount of N 2 O in the samples was detected using gas chromatograph Tsvet-500 M (Dzerzhinsk, Russia) with the electron capture detector. The 3meter-long steel sorption columns were fi lled with sorbent Paropak Q 60-80 mesh (Water Corporation, USA). The column temperature was 40 ºС, the evaporator temperature -120 °С, that of the detector -330 °С. The amount of used carrier gas (argon with methane 95/5) was 35 cc/min.
The investigation on the content of СО 2 in gas samples was conducted using gas chromatograph Tsvet-500 M with the thermal conductivity detector (bridge current of 130 mA). The steel sorption columns were fi lled with sorbent Paropak Q 60-80 mesh. The temperature of columns was 25 °С and that of the detector -40°С. The amount of used carrier gas (helium) was 20 cc/min. The emission of N 2 O was estimated according to the formula: where E -amount of nitrous oxide in the sample under analysis, nmol N 2 O; V 1 -volume of the chamber, cc; V 2 -volume of the sample, introduced to the chromatograph, cc; S -area of cross section of the chamber, sq. m.; t -exposition time.
The same formula was used to determine the emission of CO 2 , but E was estimated as the amount of carbon dioxide in the sample under analysis, nmol.
The indices of gas chromatography determination of the emission of N 2 O and CO 2 were used to estimate the losses of nitrogen and carbon, taking into consideration the molecular mass of N 2 O and CO 2 and the velocity of gas emission from one hectare in 24 h.
The specifi c losses of N-N 2 O were determined as the ratio of g N-N 2 O/kg С-СО 2. Based on the indices obtained, the mineralization-synthesis indices were estimated as the ratio of the module of the difference between the emission ratio g N-N 2 O/kg C-СО 2 of soil of the "reference" plot (fallow) and the soil of the agrocenosis to the emission ratio index g N-N 2 O/kg C-СО 2 of the "reference" plot: where І m-s -mineralization-synthesis index; Е r.p.emission ratio of g N-N 2 O/kg C-СО 2 of the "reference" plot; Е agro -emission ratio of g N-N 2 O/kg C-СО 2 in the agrocenosis.
According to our method (Volkogon et al., 2019), the negative values of І m-s index represent intense mineralization processes in soil. The approximation of index values to the zero index demonstrates the state of balance for the mineralization-synthesis processes. And index values above zero prove that synthetic processes in soil prevail over the mineralization processes.
The statistical processing of experimental data was conducted using disperse analysis and Microsoft Excel 2010.

RESULTS OF INVESTIGATIONS
The determination of gas emission from soil under potatoes during the emergence phase demonstrated the highest losses of N 2 O in variants with the introduction of manure into soil, its combination with green manure mass, organo-mineral fertilization (manure + N 40 P 40 K 40 ), and the highest dose of mineral fertilizers in the experiment (Table 1).
The lowest indices of nitrous oxide emission were registered in the fallow and the control variant (without fertilizers). However, the difference between the indices of the control and fallow is rather considerable which demonstrates the imbalance of biological processes in soil of agrocenosis even without the use of fertilizers.
First of all, CO 2 emission increases after the introduction of fresh organic matter into soil in the form of manure, straw, green manure and their combinations. It is quite logical, considering the mineralization of the abovementioned organic materials. Noteworthy is another issue: the emission of carbon dioxide increases along with the increase in the doses of mineral fertilizers. For instance, at the introduction of N 40 P 40 K 40 into soil, the amount of СО 2 , released into the atmosphere, increases by 37.7 % as compared to the index for the control, while using N 80 Р 80 К 80 -by 62.7 %, and in case of the highest dose of fertilizers in the experiment the emission increases by 80.0 %. As there is no suffi cient amount of fresh organic matter in soil (according to the scheme of the experiment it may be in these variants only in the form of roots or afterharvest residues of previous crops in the rotation), it may indicate the mineralization of conservative organic substances.
However, while assessing the dependence of the emission of investigated gases from soil depending on agrobackgrounds, the most interesting issue may be related to the emission ratio of N-N 2 O/C-CO 2 , i.e. specifi c losses of nitrous oxide per unit of carbon dioxide. Thus, the estimation of absolute losses of N 2 O may bring us to the conclusion about ecological risks of manure. However, specifi c losses of nitrogen gas in this variant are some of the lowest in the experimentat the level of the control variant.
After the introduction of 5 t/ha of straw, the specifi c losses of nitrous oxide are lower as compared to the indices of the control variant and even the "reference" soil (fallow). It indicates intense immobilization of nitrogen compounds under these conditions. There is some increase in specifi c losses of N 2 O in the variant with lupine mass which may be explained by rather rapid mineralization of this organic material and rather a high content of nitrogen compounds therein. However, the combination of lupine and straw eliminates these losses.
The highest specifi c losses of nitrous oxide are observed after the introduction of mineral fertilizers. It demonstrates that under defi ciency of fresh organic matter in soil, the nitrogen fertilizers applied may not be effectively immobilized by microorganisms.
This conclusion is confi rmed with the indices of specifi c losses of nitrogen gas on condition of combining mineral fertilizers with straw and green manure mass. For instance, after the introduction of low and medium doses of mineral nitrogen on the background of "straw + cover crop", specifi c losses of N 2 O decrease sharply and are even lower than the indices of the control variant. Nitrogen losses also decrease in case of combining the highest experimental dose of mineral fertilizers (N 120 Р 120 К 120 ) and fresh organic matter, but they still exceed control indices which evi- dently demonstrates the excess of nitrogen fertilization for potatoes.
The indices of mineralization-synthesis (I m-s ), as an integral index of these processes compared to the data of both the control and the fallow [8], demonstrate the formation of predominantly synthetic processes (indices with "+") in the variants with the introduction of straw and in case of using N 40 P 40 K 40 in the combination with straw and green manure. The indices of I m-s , close to the value of the "reference" soil, are also present in the variants "straw + green manure", "manure + green manure", "N 40 P 40 K 40 + straw + green manure", as well as "manure + green manure + N 40 P 40 K 40 " and "manure + N 40 P 40 K 40 ".
After the introduction of mineral fertilizers only, the indices of mineralization-synthesis of organic matter demonstrate the prevalence of mineralization processes (Table 1).
During the second term of observations (the phase of potato fl owering), we observe a similar character of mineralization and synthesis processes in soil depending on the fertilization for the crop (Table 2).
At the same time, in the variants with the introduction of straw, the combination of N 40 P 40 K 40 with straw and green manure and in case of using manure along with the cover crop and the introduction of N 40 P 40 K 40 , the increase in synthesis processes was observed (indices of I m-s with "+" increase). There was also a decrease in the intensity of mineralization processes in the variants with the introduction of mineral fertilizers (I m-s with the value "-" decreased as compared to the indices, registered in the initial phases of plant organogenesis).
It should be noted that during this period in the variant "N 80 Р 80 К 80 + straw + green manure" the index of mineralization-synthesis of organic matter had almost no differences from the index for the fallow (-0.01 against 0), which demonstrated the optimization of mineralization-synthesis processes under these conditions.
Positive changes can also be observed in the variant with the introduction of the highest experimental dose of fertilizers, but even the combination of fertilizers and additional organic matter was not capable of ensuring optimal ratio of mineralization and synthetic processes.
At the completion of vegetation period of potatoes, the determination of the emission of N 2 O and CO 2 , and estimation of their emission ratio and the indices of mineralization-synthesis of organic matter generally yielded fi ndings, similar to the abovementioned (Ta- ble 3). However, during this period there is a tendency towards domination of mineralization processes in the variants with previously determined prevalence of organic matter synthesis.
In case of introducing the additional organic matter in the form of straw into soil, while combining the lowest experiment dose of fertilizers and green manure, or combination of manure and green manure with the lowest experiment dose of mineral fertilizers, during fl owering phase the indices of mineralization-synthesis of organic matter are negative (although the difference from the fallow index is insignifi cant).
At the same time, negative values of I m-s in the variants with the introduction of fertilizers increase considerably. It should be noted though that the combination of fertilizers, straw and green manure changes the index under investigation considerably (Table 3).
Therefore, the application of mineral systems of fertilization while cultivating potatoes on leached chernozem leads to initiating mineralization processes in soil. The combination of fertilizers and additional organic matter in the form of straw and green manure improves the situation considerably. At the same time, the lowest (N 40 P 40 K 40 ) and medium (N 80 Р 80 К 80 ) experiment doses of mineral fertilizers are characterized by optimal in-dices of the balance of mineralization-synthesis processes of organic matter.
The highest dose of mineral fertilizers (N 120 P 120 K 120 ), even in combination with 5 t/ha straw and 13 t/ha green manure, activates mineralization processes.
There is a noted optimization of the investigated processes in case of combining manure and N 40 P 40 K 40 and especially when these fertilizers are combined with lupine mass.
The investigations, conducted in other periods (2016 and 2018), confi rm the abovementioned dependences in the focus of the processes of mineralizationsynthesis of organic matter depending on the agrobackground.
Improving the nutrition for potato plants using orga-nic, mineral and organo-mineral fertilization has positive impact on the productivity of the crop (Table 4).
At the same time, mere introduction of 5 t/ha of straw does not ensure a signifi cant surplus of crop productivity. Still, the combination of straw and green manure promotes a considerable increase in agrocenosis productivity. The most effective ways to impact the productivity of potatoes were found to be as follows: the introduction of manure -gain of 9.8 t/ha, combined application of manure and green manure -productivity increase by 11.0 t/ha; the introduction of N 80 P 80 K 80 (gain of 11.6 t/ha) and especially the combination with straw and lupine mass (16.1 t/ha); the introduction of N 120 P 120 K 120 (increase in productivity by 16.2 t/ha) and especially in the combination with fresh organic matter (gain to the control at the level of 21.2 t/ha).
High indices of productivity were also obtained in case of combining manure with mineral fertilizers (gain of 16.4 t/ha), and combining manure with green manure mass and mineral fertilizers in the dose of N 40 P 40 K 40 (gain of 18.6 t/ha).
However, the comparison of crop productivity rates against the indices of the focus of mineralization-synthesis process demonstrates the compromise between economic effect and ecologic requirements only for the use of organic fertilizers and organo-mineral fertilization. After the introduction of mineral fertilizers, there are considerable losses of nitrogen compounds and high emission of CO 2 which may demonstrate enhanced dehumifi cation processes under these conditions. At the same time, the addition of organic fertilizers to mineral fertilization of the crop ensures the optimization of investigated indices. Only in case of using the highest experiment dose of fertilizers (N 120 Р 120 К 120 ) this addition was not capable of changing the focus of the investigated biological processes in soil completely.

DISCUSSION OF RESULTS
It should be highlighted that a famous agrochemical statement (Doran J.W. et al., 1996;Baliuk S. et al., 2014) regarding the decrease in soil fertility due to limited introduction of fresh organic matter thereinto is supplemented by our fi ndings of determining the focus of biological processes of mineralization-synthesis of organic matter. In addition, the discussion point about the conclusions of some researchers (Doran J.W. et al., 1996;Margdoff F. et al., 1997;Purtova L.N. et al., 2016) regarding the destructive impact of mineral fertilization systems for agricultural crops on the humus state of soils is confi rmed with the data of gas chromatography determination of the emission of N 2 O and CO 2 and subsequent calculations of the specifi c losses of N-N 2 O and indices of mineralization--synthesis of organic matter. Contrary to classic agrochemical technologies, the application of which allows determining the prevalence of synthetic (or mineralization) processes in soil on condition of adhering to specifi c fertilization systems for quite a long time (sometimes the verifi cation of this conclusion requires decades), the calculations of I m-s ensure timely and accurate results (it should be noted that gas chromatography (with electron capture detector) determination of the emission of N 2 O allows detecting changes at the level of 10 -12 mol of nitrogen). We do not reject the need for classic agrochemical methods, but believe that the methodological approaches, suggested by us, may be a reliable addition to the existing ones and may be useful for express determination of ecologic perspectives of fertilization systems for agricultural crops.

CONCLUSIONS
According to our fi ndings, the use of solely mineral fertilizers in the technology of potato cultivation is inappropriate in terms of environmental reasons. Organo-mineral fertilization ensures the optimization of the course of the processes of mineralization-synthesis of organic matter, but exceeding the dose of N 80 Р 80 К 80 is unfavorable.
The methodological technologies, used in the investigations, may serve as a relevant supplement to current agrochemical methods of assessing agronomic measures, whose results are remote in time. Considering rather low sensitivity of determining the carbon content in soil, the statements about its quantitative changes below 5 % may be rather doubtful. The calculations of the indices of mineralization-synthesis of organic matter while using modern high-sensitivity gas chromatographic methods to determine the emission of N 2 O and CO 2 allow to determine the focus of biological processes in the soil timely and accurately, depending on fertilization, and to estimate ecologic sustainability of types and doses of fertilizers.