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

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

In the complex of agrotechnical measures aimed at increasing soil fertility, the leading role should belong to the application of organic and mineral fertilizers. It is also necessary to study the nutritional 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 the Tomsk Polytechnic Institute, we conducted a field experiment in the spring of 1965.

The composition of the applied granular fertilizers is given in Table 1. The fertilizers were applied in an amount of 30 kg a.i./ha. To study the dynamics of nutrients in the soil under investigation, soil samples were taken from the arable layer. In all, four observation dates were used.

Let us consider the data of the field experiment presented in Table 2.

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

At the first date (28.VI) the amount of ammonia in all variants was low; only in variant IIv did slightly more ammonia appear 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 fertilizers of compositions 1a, 1b, IIv. It is possible that at this time the process of ammonification proceeded intensively, especially in the variants with the application of urea, but because of the dryness of the soil the ammonia was not absorbed by the soil and volatilized; this is also indicated by the low nitrate content in all variants.

At the third observation date (30.VII) field moisture increased somewhat, but the soil had been so dried out that this amount of moisture was insufficient 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.

At the next date (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 variants, but the difference among variants is insignificant.

No less important 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.

Table 1

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

Note. Moisture content of simple superphosphate — 14,25%, double superphosphate — 7,00%, potassium chloride — 2,12%, potassium salt — 2,20%.

The results of studies on enzyme activity and the release of soil CO₂ are given in Table 2. These data show that the release of carbon dioxide from the soil during the growing season undergoes seasonal fluctuation: at the beginning of summer (June) soil “respiration” is maximal in all treatments. In mid-summer (July) and at the beginning of autumn (September), the intensity of soil “respiration” subsides, increasing slightly on the fertilized plots.

A decrease in soil “respiration” is also underscored 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, the higher the intensity of “respiration”; the wetter the soil, the

Axis labels in the figure:
Vertical axis: Ammonia content, mg/kg of soil
Horizontal axis: Dates — 28/VI, 13/VII, 30/VII, 7/IX

Fig. 1. Dynamics of ammonia in light gray forest soil:

1 — composition Ia (Table 1)
2 — composition Ib (Table 1)
3 — composition IIId (Table 1)
4 — composition IIId (Table 1)
5 — NPK (mineral control)
6 — control (without fertilizers)

less the soil “respiration.” The influence of soil moisture on gas exchange is associated, 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. 2. Dynamics of nitrates in light-gray forest soil:

1 — composition Ia (Table 1)
2 — "— Ib —"—
3 — "— IIc —"—
4 — "— IIId —"—
5 — NPK (mineral control)
6 — control (without fertilizers)

Vertical axis: nitrate content, mg/kg of soil.
Horizontal axis: dates.

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 soil regime, the influence of the applied fertilizers on the course of biochemical 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. The experiment showed that in June enzyme activity was fairly high in all variants; in summer (July) the enzymatic activity of the soil subsided and revived somewhat at the beginning of

autumn. The dynamism of enzyme activity in the soil is evidently determined by temperature and water conditions.

From the data in Table 2 it is evident that, in the biological activity of light-gray forest soil, there is a correlation between the “respiration” of the soil

Fig. 3. Field moisture of light-gray forest soil:

1 — composition Ia (Table 1)
2 — composition Ib
3 — composition IIв
4 — composition IIIд
5 — NPK (mineral control)
6 -x- control (without fertilizers)

Axis labels in the figure: vertical — Moisture, %; horizontal — Dates. Dates shown: 28/VI, 13/VII, 30/VII, 7/IX.

Table 2

Dynamics of mobile forms of nutrients and the biological activity of light-gray forest soil

and catalase, and between sucrase activity and water-soluble humus, but there is no parallelism in the action of the enzymes either by time period or by variant.

Summarizing all of the above, the following conclusions may be drawn:

1. Of all the tested organo-mineral granular fertilizers on light gray forest soil supplied with phosphorus, during the dry growing season of 1965 a slight effect in improving the nutrient regime of the soils was obtained at individual dates from the application of formulations I a, II v, I b: at date I, accumulation of nitrates and ammonia was noted; at dates II and IV, accumulation of ammonia.

2. The intensity of soil “respiration” in all fertilized plots was higher than in the control.

3. Soil moisture in the plots with the application of peat-mineral granules was higher than in the control and than with scattered mineral fertilizers, even during the dry period of vegetation (dates I and II).

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

5. The processes of hydrolysis of nitrogen-containing substances (urease enzyme) proceeded intensively at date I with the application of formulation IIv, NPK, and at date II with the application of fertilizers of formulation IIId. Hydrolysis of nitrogen-free substances (sucrase enzyme) proceeded intensively at date I in plots with fertilizers Ia, IIv, IIId, NPK. By the end of vegetation the hydrolysis process decreased.

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.