3.1. Topsoil Coverage by Precrop Residues
The quantity of crop residue remaining on the soil surface under each tillage technology is crucial, as it affects the quality of tillage and sowing, soil characteristics, the state of weed and disease and, finally, the productivity and quality of crops [
11,
12]. For example, we found a strong correlation between soil stability in water (at the depths of 0–15 and 15–25 cm) and the content of precrop residues (
r = 0.897 and 0.906 at
P ≤ 0.05) [
26]. In this experiment the smallest amount of crop residues, both before and after the spring sowing, was found in plowed plots (
Table 2).
In deeply and shallowly cultivated plots, the amount of plant residues on the topsoil was similar and decreased approximately twice during the tillage and sowing operations. On average, in no-tilled plots, the composition of stubble was higher than 50 percent. Differences of topsoil coverage depended on precrop growth, development, and harvesting conditions during vegetations. For example, if precrop winter wheat was laid down by wind or rain at the time of harvest, the combine shredded the straw poorly and their projection coverage was lower. Another reason is sparse precrop cultivation, especially in not-tilled plots.
3.2. Soil Chemical Composition
In 2016, before the tillage and faba bean sowing in spring, the soil pH of the soil was similar in all plots. The soil pH ranged from 6.7 to 7.4 at the depth of 0–15 cm and from 7.3 to 7.5 at the depth of 15–25 cm (
Table 3 and
Table 4). In [
9] global meta-analysis, no-tillage reduced the soil pH on average by 2.8% compared to conventional tillage.
The content of available phosphorus was higher in the soil surface (0–15 cm) than in the deeper layer (15–25 cm), except for the plowed plots. In our earlier experiments we found a similar differentiation into an upper layer with higher concentrations of P and K and a bottom layer with lower concentrations of P and K. In the present experiment with faba bean, at the depth of 0–15 cm, most of the phosphorus was found in shallowly cultivated (disked) plots of soil but the differences were insignificant. At the depth of 15–25 cm, most of the phosphorus was found in plowed plots with no significant differences. Similar trends were observed when studying the availability of potassium, but the differences varied significantly. In the 0–15 cm soil layer, most of the available potassium was found in shallowly cultivated plots, while in the 15–25 cm layer it was found in plowed plots. The differences between the treatments were significant. More magnesium was also found in shallowly and deeply cultivated plots in the 0–15 cm soil layer but the differences were insignificant. Like for other macronutrients, higher content of magnesium was found in deeply and shallowly plowed plots at the depth of 15–25 cm. Before the experiment of growing the faba beans the content of soil nitrogen was not high. Essentially, most nitrogen was found in the 0–15 cm soil layer in no-tilled soil and in the 15–25 cm layer in deeply and shallowly plowed plots.
After the harvesting of faba bean, a tendency of soil alkalinity was observed at the depth of 0–15 cm, while at the depth of 15–25 cm the soil pH remained almost unchanged (
Table 3 and
Table 4). During the bean vegetation, available phosphorus increased at the depth of 0–15 cm in plowed and no-tilled plots, while in deeply cultivated plots it decreased. No significant differences were found between the treatments. The opposite tendencies were observed at the depth of 15–25 cm. Similar trends were observed for available potassium and magnesium, but the levels of available potassium remained almost unchanged at the depth of 15–25 cm in unplowed plots (treatments 3–5). Annual changes in total nitrogen during the faba bean vegetation were insignificant. In their experiment [
30] showed that in a two-year period while faba bean was used in crop rotation, the total nitrogen in soil increased 1.3 times compared to the free rotation of faba bean.
In 2017, prior to spring tillage, the soil pH did not significantly differ. At the depth of 0–15 cm, the soil pH ranged from 7.1 to 7.4 and at the depth of 15–25 cm it ranged from 7.4 to 7.5 (
Table 5 and
Table 6). The amount of available phosphorus was higher in the surface (0–15 cm) than in the deeper layer. Most of phosphorus in the soil was found at the depth of 0– 15 cm in no-tilled and shallowly cultivated plots but there were no significant differences. At the depth of 15–25 cm, essentially most of the phosphorus was found in deeply and shallowly plowed plots. Similar trends were observed regarding available potassium. In the 0–15 cm soil layer, more magnesium was found in shallowly cultivated and no-tilled plots, but the differences were not significant. Unlike other macronutrients, magnesium at the depths of 15–25 cm was found in deeply cultivated and no-tilled plots. The soil nitrogen content was not high. In the 0–15 cm soil layer it varied from 0.120% to 0.149%. Deeply plowed plots contained the smallest content of nitrogen or significantly less than in the other plots of the experiment. In the 15–25 cm soil layer differences in soil nitrogen content were insignificant but most of the nitrogen (0.164%) was found in shallowly plowed soil.
It was found a strong correlation between soil chemical composition and the amount of plant residues. A positive strong correlation was found between the content of crop residues after sowing and the content of phosphorus, potassium, and magnesium in soil in the 0–15 cm soil layer (r = 0.882*, 0.852, 0.813).
Even though the pH of the 0–15 cm soil layer did not change significantly after harvesting, decreasing tendencies were observed (
Table 3 and
Table 4). The amount of available phosphorus decreased in most cases, except for deeply plowed plots. The content of available potassium increased during the faba bean vegetation in all the plots, especially in shallowly cultivated and no-tilled plots. The contents of magnesium varied, and the amounts of nitrogen increased significantly in cultivated plots compared to control.
At the depth of 15–25 cm the soil pH slightly decreased while the levels of phosphorus and potassium varied between the treatments. In deeply and shallowly cultivated plots, the amount of phosphorus and potassium increased in the soil during vegetation. The content of magnesium in the soil at the depth of 15–25 cm decreased in all the plots, although an increase in its content was observed in the 0–15 cm layer. Except for the control plots, the decrease in nitrogen was mostly insignificant.
The trends observed in 2018 were similar as in previous experimental years. However, at the depth of 0–15 cm, soil pH values were significantly different in no-tilled plots both at the beginning and at the end of vegetation. The amount of available potassium in no-tilled plots was essentially the highest and reached the average contents for this criterion (
Table 3 and
Table 4). The amount of magnesium in the soil increased mostly but not significantly during vegetation. The results of nitrogen testing were similar to those of 2016 and 2017, but more pronounced. Any of the tillage alternatives studied yielded significant amounts of nitrogen compared to control. The highest amounts of nitrogen were found in no-tilled plots.
At the depth of 15–25 cm, the elemental composition of soil tended to deteriorate during the vegetation period, with rare exceptions. Nitrogen increased only in plowed plots. In general, there was less nitrogen in the 15–25 cm layer than in the topsoil one.
Positive strong and moderate correlations were found between the content of precrop residues on topsoil and the contents of potassium and nitrogen in the 0–15 cm layer (
r = 0.702, 0.535 at
P ≥ 0.05). Similarly, refs. [
30,
31] concluded that the residue return increased the concentration and the stock of N in the soil. Other scientists established that addition of bacteria and/or melatonin significantly increased growth parameters and yield components, and that bacteria inoculation and melatonin application enhanced N, P, and K concentrations, the proline content, RWC%, and the K
+/Na
+ ratio when Na
+ and Cl
− concentrations were decreased significantly in salt-stressed faba beans.
3.3. Soil Temperature and Moisture Content
Soil moisture content and temperature have a strong influence on gas emissions from soil [
25]. The temperature of soil surface usually depends on the ambient temperature, but in spring the soil covered with more plant residues is warming up more slowly. In our experiment, in 2016, no-tilled soil warmed up more slowly and was significantly cooler than the tilled one (
Table 5). Measurements in the middle and at the end of vegetation did not reveal any significant differences between the treatments.
In 2017, the differences between the surface temperatures of differently tilled soil were mostly insignificant but the temperature of deeply plowed soil was the highest. In 2018, the measurements revealed significantly lower soil temperature (4.2% to 5.6%) in shallowly cultivated and no-tilled plots. The highest soil temperatures were found in deeply plowed plots both at the beginning and in the middle of faba bean vegetation. During the whole vegetation period the lowest soil temperature was found in the no-tilled soil. In our experiment the temperature of soil surface (0–10 cm) layer at the beginning of vegetation was highly correlated with moisture content (r = –0.912 at P ≤ 0.05).
The moisture content in the topsoil layer is mostly dependent on the amount of precipitation, while in the deeper layer, on the rate of precipitation and capillary rising moisture from the groundwater. In our experiment, in the 0–15 cm topsoil layer in no-tilled plots, we found significantly higher (from 24% to 33%) moisture content compared to deeply plowed plots (data are not presented). These findings are similar to the results of our earlier investigations. At the time of the measurements of soil temperature, the CO
2 e-flux, and concentration in 2016, the soil moisture did not significantly differ between the tillage systems (
Table 5). Moisture content was highest at the end of vegetation because approximately 1.3 % usual precipitation rate fell in August. In 2017, as in 2016, there was no significant difference in the soil moisture content between the tillage systems. It was highest in July, with the precipitation rate about 20 mm above the norm. In 2018, at the beginning and in the middle of faba bean vegetation, all the reduced tillage systems tended to increase the soil moisture content (from 0.1% to 0.6%) compared to deep plowing. At the end of vegetation, the soil moisture content was similar and did not differ significantly. Higher differences between the soil moisture content in differently tilled soils were observed in the period between the sowing and germination of faba bean. Similarly, Li et al. [
9] found that conservation tillage systems increased available water capacity in the soil.
3.4. Soil CO2 e-Flux and Concentration
In our experiment, in 2016, CO
2 emission flux (e-flux) varied during the faba bean vegetation. It was the lowest at the beginning of vegetation and increased afterwards (
Table 6). While the most active gas flow in plowed and shallowly cultivated soil was found in the middle of faba bean vegetation, the e-flux from deeply cultivated and no-tilled soil intensified at the end of vegetation. No significant differences between the tillage systems were found. [
32] found that a sudden increase in the CO
2 emission from the soil was caused by the destruction of soil structure during the tillage. It lasted approximately three hours after the tillage and the emission of the CO
2 was compositional to the level of tillage intensity. The highest CO
2 respiration was found in the deeply plowed soil. The CO
2 emissions from no-tilled soil were consistently lower throughout the experiment. Conversely, [
2] tested the CO
2 emissions from the soil for 330 days after applying conventional tillage and no-tillage technologies and found no difference in the CO
2 emissions for 41% of days. The CO
2 emissions from conventionally tilled soil were higher for 6% of days and from no-tilled soil, for 53% of days. In our experiment, the measurement of the CO
2 concentration above the soil surface showed that it was quite stable during the faba bean vegetation and did not differ significantly between the tillage treatments (
Table 6). The reason for this is that July and August were cold and excessively humid. However, in general faba bean reduces the CO
2 emissions [
19].
In 2017, at the beginning and at the end of the faba bean vegetation, the e-flux of CO
2 was the lowest, but it was higher in the middle of vegetation due to higher ambient temperatures. A similar situation was described by [
3]. In our experiment, there were no significant differences between the treatments (except for exclusive cases). The CO
2 concentration above the soil surface during the faba bean vegetation was relatively constant and there was little difference between the measurements. In the first half of vegetation, the concentration of CO
2 above shallowly cultivated and no-tilled soil plots was generally significantly higher than above deeply plowed plots. This was not the case at the end of vegetation (
Table 6). Correlation analysis showed that the concentration of CO
2 at the beginning of vegetation make significantly effect for the density of the faba bean crop (
r = 0.906 at
P ≤ 0.05) and during the second half of the vegetation, on the content of precrop residues on topsoil in spring (
r = 0.804 and –0.876 at
P ≥ 0.05).
In 2018, during the vegetation period the CO
2 emission from the soil varied depending on meteorological conditions and agricultural activity. The highest CO
2 emission from the soil was found at the end of vegetation when the air temperature increased (
Table 6). Higher temperatures promote the growth of microorganisms and increase enzymatic activity, organic matter decomposition, and plant root respiration [
33,
34]. In our experiment, at the beginning of faba bean vegetation, a very low CO
2 emission was found in almost all the tillage systems. Similar trends were observed when determining the CO
2 concentration from the soil surface by using shallow plowing and deep cultivation methods compared to conventional tillage. In other tillage systems, the distribution of CO
2 was uneven throughout the vegetation period. Correlation analysis showed that CO
2 e-flux in the middle of vegetation was for the temperature of soil surface (
r = 0.792 at
P ≥ 0.05), moisture content (
r = 0.841 at
P ≥ 0.05), and PAR (photosynthetic active radiation) conditions above the soil surface (
r = 0.743 at
P ≥ 0.05). At the end of vegetation period, CO
2 e-flux was dependent on the topsoil temperature (
r = 0.659 at
P ≥ 0.05). Mikša et al. [
35] found that the CO
2 e-flux in agroecosystems (maize and oilseed rape) changed during the vegetation (June–August) and is correlated with temperature (r = 0.8) and soil moisture content (
r = 0.6) as in our experiment.