Biotechnology of land reclamation in rice crop rotations

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Introduction
Research relevance concerns the need in modern ecological, economic and social conditions for innovative technologies development on rice irrigation systems which will provide increase in rice yield, rice massifs soil fertility and simultaneous fish bioresources replenishment. One of the new and promising rice cultivation areas is land reclamation biotechnology, which uses the effect of bionts on the environment to improve the ameliorative soil state [1,2].
In the Russian Federation rice plants are sown on the area of up to 210 thousand hectares, grain production volume is on average 1055.5 thousand tons with a maximum value in 2020 -1141.8 thousand tons, the minimum in 2013 -935 thousand tons.  The largest rice grain producer in Russia is the Krasnodar region, where about 80% of the total gross harvest is grown. On average in the region sowing rice area is 127 thousand hectares with the maximum value of this indicator in 2016 136.1 thousand hectares, the minimum in 2018. -117.2 thousand hectares [3].
Rice farming is the environment-forming agriculture sector in Russia, because the cessation of rice production on lands developed with huge investments will lead to their complete elimination from the agricultural turnover, deterioration of the environmental situation in the region.
Rice cultivation in the Russian Federation is carried out by intensive technologies, which remain unchanged in recent decades and each year requires more and more capital investment per hectare of rice fields to maintain the achieved yields [4]. The main reasons for this trend can be considered: 8-field crop rotations use with 62.5 % saturation with rice, in which rice is grown for 3 or more years, introduction of mineral and organic fertilizers increased doses to the fields, aimed at soil fertility loss compensation; use of chemicals to control weeds; exclusion of agromeliorative fields and pure fallows from crop rotations; salinization and waterlogging of soils [5,6].
With the rice systems' construction there was a Kuban River flow's redistribution, which led to soils water and salt regimes change [7], and the use of rice cultivation technologies using pesticides significantly worsened the ecology of the Lower Kuban [8].
Therefore, the purpose of the present research is to develop a biotechnology for land reclamation of rice crop rotations to improve water resources use in the fields efficiency after fish fallow in the rice systems of the Krasnodar Region.

Materials and methods of research
The research was carried out at the Kuban State Agrarian University's vegetation site. Vegetation vessels were filled with pre-sifted alluvial-meadow soil. Soil with 6 kg mass occupied 2/3 of vessel's volume. Soil was delivered from the checks of Slavyanskiy district of Krasnodar Region. This check was bread for fish fallow, on which juvenile carp were reared. Before sowing rice, soil samples were taken from the vessels. Soil samples were analyzed according to the conventional methods in 4-fold repetition. Phosphorus and potassium were determined according to Machigin, nitrates according to Grandwal-Lyazh Rice was sown on 12 th May with seeds embedded in the soil to a 2 cm depth. Short-grain rice seeds, which were obtained using herbicide-free technology, were used as planting material. Before sowing rice, seeds were treated with 1 % hydrogen peroxide solution. Sixteen seeds were sown into each vessel. After the seeds germinated, 5 rice plants were left in each vessel.
During the experiments, plants growth rate from seedlings to full seed maturity was monitored. Biological yield was determined for each option (Table 1). In the grain and straw in 3-fold repetition macronutrients content was determined by conventional methods. Variance data analysis was obtained. Taking into account Rapan rice variety's biological resources and considering that rice plant can develop well without water layer creation from seedlings to tillering phase, 4 options of irrigation regimes were investigated. The variant with shortened flooding was the control, as this method is widely used in the studied region.

Results and findings
Studies showed that the rice plants' growth depended on irrigation regimes. The best results were obtained in options 1 and 3 ( Table 2). Under periodic 5 cm layer flooding with full soil soaking in the vessel to the tillering phase (option 3), from 2 to 3 productive shoots were observed on the plants. According to the number of bolls on plants the best variant was the short flooding regime, and the worst was the option of rice flooding with a water layer of 5 cm after sowing.
In the 4 th option the plants' growth rate was better than in the 2 nd option, but worse than in the first and third options. Different irrigation regimes had different effects on rice yield. Rice yield with periodic flooding with 5 cm layer and shortened flooding was higher than in other options. The total grains weight of an average plant in options 1 and 3 was 3.35 and 3.36 g, respectively, and exceeded the grains weight of option 2 by 27 % and 4 by 36 %.
In addition to quantitative indicators qualitative indicators in the grain and rice straw were determined. In grain and straw of rice the nitrogen, phosphorus and potassium content were found. The macronutrients content in each option is given in table 3. Indicators on nitrogen are the best at option 2, and at the combined regime the highest content of phosphorus in grain and straw of plants was received.  In parallel qualitative grain and straw of rice plants indices were determined, macroelements' content in soil before and after experiments was determined. Table 4 shows comparison of macronutrients in vessels' soil before and after laying of experiments by options. From the above tabulated data, it can be concluded that with an increase in phosphorus and potassium in the grain and straw of rice plants, the number of macronutrients in the soil after fish fallow decreased. In all options water supply to vegetation vessels was controlled. The assumption was made that evaporation and transpiration in all experiments were the same, as the vessels were of the same shape and the number of plants in each vessel was 5. Atmospheric precipitation did not affect plant growth, because a canopy of polyethylene film, located at a height of 2 m, was arranged over the vessels. There was no drainage outflow.
It was found that the highest water supply was in option 1 under shortened flooding (control) and was about 8.46 thousand m3/ha. In option 2 water supply was less than in option 1 by 20%, and in option 3 and 4 it was less by 27 and 32% respectively. Table 5 shows efficiency of rice irrigation regimes depending on water supply volume. From the experimental data in Table 5, it is clear that the most rational irrigation regime is option 3, which has the highest percentage of water volume reduction for the formation of 1 ton of rice yield relative to the control. Option 2 practically did not differ from the control, because the percentage of water volume reduction to form one ton of yield was within the error of the experiment.
As a result, it was found that after fish fallow the highest rice yield is obtained at shortened and periodic flooding with 5 cm layer up to tillering phase. At shortened flooding regime qualitative rice grain and straw indices in terms of macronutrients' content are the best in comparison with other regimes.
The most expedient rice irrigation on the soil after fish fallow regime was set with periodic flooding with 5 cm water layer, at which the percentage of water volume reduction to form 1 ton of rice yield relative to the control was 27.3%.
Consequently, in the presence of fish fallow in the rice crop rotation it is necessary to apply the most rational rice irrigation regimes, which, in the studied conditions, are the short flooding regime and regime with periodic flooding with 5 cm water layer.