DYNAMICS OF NUTRIENTS AND ENZYMATIC ACTIVITY OF LIGHT-GRAY FOREST SOIL WITH THE APPLICATION OF ORGANO-MINERAL GRANULAR FERTILIZERS

S. I. SMOLYANINOV, N. E. SAGACHENKO, G. P. GOBOVA

Light-gray forest soils occupy a significant place in the soil cover of Tomsk Oblast.

In the complex of agrochemical measures aimed at increasing the fertility of these soils, the leading role should belong to the application of organic and mineral fertilizers. It is also necessary to study the nutrient regime of the indicated soil when organo-mineral fertilizers are applied.

For this purpose, together with staff members of the Department of Fundamentals of Agriculture of TPI, in the spring of 1965 we conducted a field experiment on the territory of the Tomsk Regional Agricultural Exhibition.

The composition of the applied granular fertilizers is given in Table 1. To study the dynamics of nutrients in the soil under investigation, soil samples were taken from the arable layer. In all, four observation periods were carried out.

The growing season of 1965 was characterized by a small amount of precipitation and a rather high temperature in comparison with long-term data.

Let us consider the data from the field experiment. The results of the determinations are presented in Figs. 1–6.

The data show that, during the growing season, sharp fluctuations are observed in the dynamics of nitrates and ammonia, in all variants and at all dates correlating with soil moisture.

At the first date (28.VI), the amount of ammonia in all variants was low; only in the variant with the application of organo-mineral fertilizers of compositions Ia and IIb was somewhat more ammonia released than in the control. At this date there were quite a lot of nitrates in all variants.

At the second date (13.VII), a high accumulation of ammonia is noted in the variants with the application of fertilizers of compositions Ia and IIb. It is possible that at this date the process of ammonification was proceeding intensively, especially in the variants with the application of urea.

At the third observation date, field moisture increased somewhat, but the soil had been so dried out that this amount of moisture was not sufficient to revive microbiological activity and increase the rate of biochemical processes. Therefore, the processes of ammonification and nitrification were suppressed to such an extent that the application of fertilizers had almost no effect.

In the next period (7. IX), with an increase in field soil moisture to 25–27% as a result of frequent rains, the microbiological activity of the soils revived. In connection with the increased moisture, an increase in absorbed ammonia is observed in all treatments, but the differences among treatments are insignificant.

Table 1

Composition of peat-mineral granular fertilizers applied to the experimental plots

Note: Moisture content: simple superphosphate—14.25%, double superphosphate—7.00%, potassium chloride—2.12%, potassium salt—2.20%.

Of no less importance in the study of plant nutrition is the question of the biological activity of soils.

The biological activity of soil, determined by the vital activity of microorganisms, plays an enormous role in soil formation and soil fertility.

Observations of this kind are especially interesting when organic and mineral fertilizers are introduced into the soil, since biological activity makes it possible to judge the direction of biochemical processes occurring in the soil.

The results of the study on enzyme activity and the release of soil CO₂ are given in Fig. 4.

From these data it is evident that the release of carbon dioxide from the soil during the growing season undergoes seasonal fluctuation: at the beginning of summer (June), soil “respiration” in all treatments is maximal.

![Graph: Ammonia content, mg/kg of soil, over time. Vertical axis: “Ammonia content, mg/kg of soil”; horizontal axis: “Dates” with marks 28/VI, 13/VII, 30/VII, 7/IX.]

Fig. 1. Dynamics of ammonia in light-gray forest soil.
1 — composition 1a (Table 1),
2 — composition 1b (Table 1),
3 — composition IIb (Table 1),
4 — composition IIId (Table 1),
5 — NPK (mineral control),
6 — x — control (without fertilizer).

In mid-summer (July) and at the beginning of autumn, the intensity of soil “respiration” subsides, increasing only slightly on the fertilized plots.

The decrease in soil “respiration” is also emphasized by the attenuation of the processes of ammonification and nitrification. The release of carbon dioxide from the soil is closely dependent on soil moisture. The drier the soil,

Fig. 2. Dynamics of nitrates in light-gray forest soil.
1 — composition Ia (Table 1),
2 — composition Ib (Table 1),
3 — composition IIv (Table 1),
4 — composition IIId (Table 1),
5 — NPK (mineral control),
6 — x — control (without fertilizers).

the higher the intensity of “respiration”; the more moist the soil, the lower the soil “respiration.” The effect of soil moisture on gas exchange is connected, on the one hand, with changes in the biochemical processes of the soil and, on the other hand, with changes in the volume of pores occupied by air.

In the complex biochemical reactions occurring in the soil and in the living organisms inhabiting it (bacteria, fungi, higher plants, and animals), enzymes play an important role.

Fig. 3. Field moisture content of light-gray forest soil.
1 — composition Ia (Table 1),
2 — composition Ib (Table 1),
3 — composition IIv (Table 1),
4 — composition IId (Table 1),
5 — NPK (mineral control),
6 — × — control (without fertilizers).

Vertical axis: Moisture content, %
Horizontal axis: Dates: 28/VI, 13/VII, 30/VIII, 7/IX.

Fig. 4. Relationship between catalase activity and soil “respiration.”

1 — composition 1a (Table 1),
2 — composition 1b (Table 1),
3 — composition IIc (Table 1),
4 — composition IIId (Table 1),
5 — NPK (mineral control),
6 — x — control (without fertilizers).

Vertical axis, upper plot: Catalase content, ml of $O_2$ per 30 min.
Vertical axis, lower plot: Release of $CO_2$, mg of $CO_2$ per 1 h / 100 g soil.
Horizontal axis: Dates: 28/V, 13/VII, 30/VII, 7/IX.

As a result of the experiment it was established that the activity of enzymes (catalase, sucrase, urease) undergoes seasonal fluctuations. The data indicate that, under conditions of an unstable hydrothermal regime of the soil, the effect of the fertilizers applied on the course of biological processes was weak. The most characteristic indicator of the biological activity of the soil when fertilizers are applied is the activity of catalase and sucrase (Figs. 4, 6). The experiment showed that in June the activity

Fig. 5. Urease activity in light gray forest soil.
1 — composition 1a (Table 1),
2 — composition 1b (Table 1),
3 — composition 1c (Table 1),
4 — composition 5-NPK (mineral control),
5 — control (without fertilizers)

of the enzymes was fairly high in all variants; in summer (July) the enzymatic activity of the soil subsides and revives somewhat at the beginning of autumn. The dynamics of enzyme activity in the soil are evidently determined by temperature and moisture conditions.

From the figures it is evident that, in the biological activity of light gray forest soil, a correlation is observed between soil “respiration” and catalase (Fig. 4), and between sucrase activity and water-soluble humus (Fig. 6), but there is no parallelism in the action of the enzymes either by dates or by variants.

Summarizing all that has been said above, the following conclusions may be drawn:

![Graph with two panels. Y-axes: “Sucrose content, mg/20 g soil” and “Content of water-soluble humus, %.” X-axis: “Dates,” with ticks 28/VI, 13/VII, 30/VIII, 7/IX. Curves are numbered 1–6 as described in the caption.]

Fig. 6. Relationship between saccharase activity and the content of water-soluble humus.

1 — composition 1a (Table 1).
2 — composition 1b (Table 1),
3 — composition IIb (Table 1),
4 — composition IIId (Table 1),
5 — NPK (mineral control),
6 — x — control (without fertilizers)

1. Of all the organomineral granular fertilizers tested on light gray forest soil supplied with phosphorus, during the droughty growing season of 1965 a slight effect in improving the soil nutrient regime was obtained at particular times from the application of composition IIв, Ia: first sampling period—an accumulation of nitrates and ammonia was noted; in the second and fourth periods—much ammonia.

2. The intensity of soil “respiration” in all fertilized plots was higher than in the control. Soil “respiration” was high in the first period; by the end of the growing season it subsided. Among all fertilized plots, the plot with composition IIв stands out in terms of respiration intensity in the first, second, and third periods.

3. Soil moisture in the plots where peat-mineral granules were applied was higher than in the control and than in plots with loose mineral fertilizers, even during the dry period of vegetation (first and second periods).

4. Oxidation-reduction processes were somewhat suppressed, but in the fertilized plots they were higher than in the control.

5. The hydrolysis processes of nitrogen-containing substances proceeded intensively in the first period with the application of composition IIв and NPK, and in the second period with the application of fertilizers of compositions Ia and IIIд. The hydrolysis of nitrogen-free substances (the enzyme saccharase) proceeded intensively in the first period in plots with fertilizers Ia, IIв, and IIIд; by the end of the growing season the hydrolysis process decreased.

6. The absence of parallelism in changes in enzyme activity during the growing season confirms the proposition that enzyme activity reflects only the transformation processes of a particular group of substances and serves as an indicator of the direction of biochemical processes in the soil.

At the same time, data on the biological activity of soils, together with data on the content in soils of nutrients available to plants, make it possible to gain a deeper understanding of the biological properties of soils and their fertility.