Influence of long-term intensive use of irrigated prairie-chernozem soil on biological activity and topsoil productivity
Application of mineral fertilizers to irrigated soil stimulated the numbers of nitrifying bacteria and fungi the most, 27-52 and 77%, respectively. The effect of mineral fertilizers on soil microflora depended on the combined influence of soil moisture and temperature factors, and the type of crop grown (Table 1).
Optimization of mineral nutrition when growing perennial grasses on an irrigated bottom in 2015 and 2016 stimulated the growth of the number of nitrifying bacteria the most – 61.6 and 72.6% for the control, respectively. An increase in the number of nitrifiers indicates an intensive process of nitrification in the soil, and the formation of nitrates, the main source of nitrogen fertilization of plants, therefore indicates an improvement in the nitrogen nutrition of the cultivated crop. Soil moistening in June-July 2016, when, in addition to the norm, 87.7 mm of precipitation fell, contributed to an increase in the number of soil fungi (68.6-77.7 thousand CFU/ g). In 2017, under the influence of fertilizers, the number of microorganisms on MPA and SAA significantly increased by 60 and 87% compared to the control, respectively. An increase in the number of microorganisms in the soil of the fertilized variants associated with mineral nutrient enrichment, an increase in the amount of root litter during the growing season and a large number of post-harvest residues of crops on fertilized plots26.
On average, over three years of research, the application of fertilizer under irrigated perennial grasses stimulated the number of nitrifying bacteria and fungi the most, by 51.5 and 76.6% in the control. In the references it was noted that an increase in the number of fungi can occur when the soil is acidified with fertilizers during their long-term use.26. Since the main function of fungi is the decomposition of plant organic residues in the soil, it can be assumed that on fertilized bottoms the process proceeds more intensively due to an increase in the amount of decomposed substrate.
The intensity of cellulose decomposition under perennial grasses did not differ significantly in the variants over the years of research. The difference between the variants was 2.3 to 11.0%. Cellulolytic activity was highest (48.3 to 59.3%) in 2016, compared to 2015, 2017 years, which was associated with temperature increase and heavy rains in June-July (163 to 187% of standard).
Soil nitrification capacity in the fertilized variants only exceeded the 35.4% witness in 2015; in the following years, the potential amount of nitrate-nitrogen was lower concerning the control from 17 to 24%. One of the possible reasons was a significant excess of the bluegrass component over the legume in agrocenosis. In the control without nitrogen fertilizers, the share of the legume component was higher and the botanical composition of the grass mixture was more balanced. Under such conditions, the influence of perennial grasses on the biological properties of the fertilized soil compared to the control has a less positive effect. When decomposing residues of bluegrass crops, the number and activity of microorganisms are lower and lower than in legumes28. Apparently, for the same reason, the intensity of cellulose decomposition in the fertilized variant compared to the control tended to decrease. However, the number of microorganisms in the soil during the fertilizer application was higher compared to the control, which therefore influenced the total biological activity of the soil. The total biological activity of the perennial grass fertilized variants was 6–26% higher than the control, depending on conditions over the years of research.
The use of nitrogen-phosphorus fertilizers for spring barley under irrigated conditions had a stimulating effect on the number of various groups of microorganisms (Table 2).
In 2015, the number of nitrifiers doubled compared to the control in a fertilized variant, in 2016-2017 years – mushrooms by 107 and 52%, respectively. In 2017, the number of saprophytic bacteria on MPA increased by 78%, the microorganisms on AAS increased by 100% (twice) regarding control without fertilizer. On average, over the years of research, the application of mineral fertilizers significantly increased (by 66.6%) the intensity of cellulose decomposition in the soil, by 44.4% – accumulation of nitrates under favorable conditions (nitrification capacity of the soil). At the same time, the total biological activity of the soil increased by 16%.
Thus, the long-term use of mineral fertilizers in stationary experiments at moderate doses (N30-60P60) for different crops had a stimulating effect on groups of soil microorganisms. In the average of the years of research, the number of nitrifying bacteria and fungi (from 52 and 76% to the control), and the number of saprophytic bacteria on the MPA and immobilizing NH3 on the ASA have been extended (in 2017, the year was 78 and 100% respectively).
The application of mineral fertilizers had a positive effect on the nitrification capacity of the soil, i.e. the amount of nitrate nitrogen, which is formed due to the internal nitrogen resources of the soil, which has made possible the potential to supply plants with available nitrogen (21 –34 N-NON3 mg/kg). Under natural conditions, the supply of nitrogen to plants depends on the intensity of the nitrification process.
The intensity of cellulose decomposition in the irrigated soil when applying mineral fertilizers fluctuated according to Zvyagintsev DG from medium to strong, which indicates high biological activity of the irrigated soil2. The calculation of total biological activity under perennial grasses was 6-26% higher, under spring barley 10-41% higher than the control.
One of the important indicators of the activity of biological processes in the soil is the ratio of microorganisms growing on starch ammonium agar (SAA) and on meat peptone agar ( MPA). An increase in this ratio indicates the predominance of the mineralization process in the soil and the intensive use of soil nitrogen, while its decrease indicates an increase in humification processes. The intensity of the microbiological processes of transformation of nitrogenous compounds in the soil was estimated by the coefficients of mineralization (SAA/MPA) and immobilization (MPA/SAA)23.
The number of microorganisms on starch ammonium agar is associated with the consumption of mineral nitrogen. On average, over the years of research in the soil under cultivation, the processes of immobilization or fixation of nitrogen by micro-organisms have prevailed, the MPA/SAA ratio > 1. The soils under the pedoclimatic condition of Siberia have a great ability to fix introduced nitrogen. mineral nitrogen, contributing to its preservation and accumulation in the root system zone. The continuous transformation of the nitrogen immobilized in the mineral products of decomposition makes it possible to supply the plants with assimilable forms of nitrogen in a more homogeneous way.29.
The highest organic matter transformation coefficient (LM= (MPA + SAA) * MPA/SAA), which potentially reflects the intensity of the accumulation of humic substances in the soil, was under barley in variants with the application of mineral fertilizers (Table 3).
It should be noted that during the operation of the stationary experiment there was a reliable decrease in the content of humus due to the removal of macronutrients due to the mineralization of processes, incl. mineralized nitrogen from humus because a moderate level of chemicalization in the previous period only partially compensated for the elimination of nitrogen by the crop (soil content has been monitored since 1972). For the preservation of the positive balance of humus with the use of perennial grasses in crop rotations, in particular bluegrass, on irrigated soils it is necessary to apply organic fertilizers30.
The level of crop yields ultimately determines the effectiveness of using one or another farming technique. In 2015, the crop yield in the crop rotation was higher in the variants with the use of fertilizers, additional 2.97 t/ha of barley grain and 1.34 t/ha of grass dry matter perennials were obtained for two cuts compared to the order (fig. 1).
In 2016 and 2017 during barley cultivation, the same regularity was observed: on the fertilized variants, the grain yield increased significantly from 0.76 to 2.28 t/ha. On average, over the 2015-2017 period, optimizing soil moisture and plant mineral nutrition conditions contributed to an increase in barley yield of more than 100%, up to 3.88 t/ha. In the cultivation of perennial grasses, systematic nitrogen fertilization has shifted the Galega orientalis with Bromine and the dry matter yield of perennial grasses was on average 0.25 t/ha lower. The share of crucifer crop—Bunias orientalis was negligible and did not affect the yield of the grass mixture.
Correlation analysis showed the presence of medium and strong positive associations between various indicators of soil biological activity and crop yield in a crop rotation with long-term irrigation (Table 4).
In 2016, a strong link was obtained between cellulose decomposition intensity indices in the soil, nitrification capacity and crop yield (r = 0.67 ± 0.28 and 0.91 ± 0, 16, respectively). A similar link between crop yield and nitrification capacity was confirmed in 2017 (r = 0.68 ± 0.28). This was explained by the fact that during the process of nitrification, organic compounds containing nitrogen under the action of microorganisms were mineralized into nitrated nitrogen, which was the main source of plant nutrition in Siberia.29.31.