The Impact of the Joint Application of Biochar and Mineral Nitrogen Fertilizers on Oat Plants and the Biological Properties of Soils

Sewage sludge is hard to dispose waste, not only due to its large volumes. It contains heavy metals, pathogenic microorganisms, and helminth eggs. For this reason, although it contains substances valuable for plant nutrition, sewage sludge cannot be inserted into the soil without prior treatment. Before being inserted into the soil, the sewage sludge must undergo appropriate treatment to comply with environmental and sanitary requirements. Pyrolysis is one of the most promising methods for processing sewage sludge, although this method is one of the least used at present. Biochar is the solid product of sewage sludge pyrolysis. It usually contains high amounts of phosphorus and potassium and some amount of available nitrogen. The data about the impact of biochar application on plant productivity and activity of soil microbiota are contradictory. The effect of combined use of biochar and mineral nitrogen fertilizers has not been studied enough. The study was aimed to investigate the effect of application of various amounts of sewage sludge biochar in combination with mineral fertilizers to poor sandy soils on the productivity of oat plants (Avena sativa L.) and the microbiological and biochemical properties of the soil. The laboratory vegetative experiment was carried out for 42 days. Soil and biochar mixture (400 g) was placed in vegetation vessels. The soil was soddy-podzolic sandy (according to the USSR classification) or Glossic Retisols (Arenic) (according to WRB (2014)). Biochar from sewage sludge was obtained at a temperature of +500±20°C. The proportions of the applied biochar was 2%, 5% and 10% by weight. Mineral nitrogen fertilizers were applied at rates of 0.1 g, 0.2 g, and 0.3 g of active ingredient per 1 kg of soil. A common oat (Avena sativa L.) were sown in the amount of 12 seeds per vegetation vessel. After seed germination, the plants were thinned out, leaving 8 plants in each vessel. On the 14th day, half of the plants (4 units) in each pot were cut off, and the average height and dry biomass were measured. The experiment with the remaining 4 units lasted up to 42 days. The experiment was carried out in four replications. The soil without biochar and without mineral fertilizers was used as a control. The number of trophic groups of microorganisms was determined by sowing dilutions of soil suspensions on solid media. In addition, urease, catalase and in-vertase activities were identified. The results showed that the combined application of biochar and fertilizers to poor sandy soil affected the number of surviving oat (Avena sativa L.) plants by day 42 of the experiment (See Table 2). The best result - all plants survived - was obtained when applying 10% of biochar without additional mineral fertilizers. At biochar pro- portion of 10%, additional nitrogen fertilizers turned out to be inexpedient and led to a decrease in the survival rate of plants. The proportions of 2% and 5% of biochar without fertilizers has led to an increase of the number of surviving plants compared to the control (clear soil). Additional application of mineral nitrogen fertilizers increased the number of surviving plants at proportions of 0.1 g/kg and 0.2 g/kg in combination with 2% of biochar and at a proportion of 0.1 g/kg in combination with 5% of biochar. The average height and biomass of plants on day 42 of the experiment in all variants did not have statistically significant differences from the control (See Table 3). The joint and separate applications of fertilizers and biochar to poor sandy soil in most cases have led to an increase in the content of alkaline hydrolysable nitrogen compared to the control (See Fig. 1). However, the correlation between the number of surviving plants and the content of alkaline hydrolysable nitrogen turned out to be weak and negative, r = -0.36. The application of biochar led to a statistically significant alkalinization of the reaction of water extract, but at the same time the reaction remained in the gradations of neutral and slightly alkaline (See Fig. 2). With the joint application, the alkalizing effect of biochar decreased with an increase in the proportion of nitrogen fertilizers. No relationship between the number of surviving plants and the reaction of the water suspension was found; the correlation coefficient was r = -0.004. The conducted study allowed establishing that on day 42 of the experiment, the total microbial number (TMC) increased with an increase of the biochar proportions. The soil biological activity was highest at maximal proportions of biochar and nitrogen fertilizers, although two-way analysis of variance showed that only the concentration of biochar had a significant effect on the total number of microorganisms (See Table 5). The lowest number of surviving plants was observed in the variants with the highest microbiological activity. The correlation between the number of surviving plants and TMC turned out to be medium and negative (r = -0.53). The relationship between the number of other trophic groups of microorganisms and the survival rate of plants and between the activity of enzymes and the survival rate of plants was not found. Two-way analysis of variance showed that the proportions of nitrogen fertilizers and biochar, and also their interaction, had a statistically significant effect on the number of living plants on day 42 of the experiment, the final content of alkaline hydrolyzable nitrogen in the soil, soil acidity, the content of amylolytic microorganisms and micromycetes, and all parameters of enzymatic activity (See Table 5). The content of actinomycetes was significantly affected primarily by the concentration of biochar; the effect of nitrogen fertilizers was only observed when combined with biochar. The paper contains 2 figures, 5 Tables and 18 references. The Authors declare no conflict of interest.

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