3.1. Studies on Soil CO2 Emissions, Moisture and Temperature
Studies on soil CO
2 emissions are abundant around the world, but the results are highly controversial. Some authors find similar CO
2 emissions from direct sowing, no-till and conventional tillage, others find higher CO
2 emissions from direct sowing on untilled land, while others argue that direct sowing on untilled land only results in higher CO
2 emissions in certain periods and lower CO
2 emissions in other periods [
35,
36]. Some researchers argue that CO
2 emissions from the soils of direct sowing are generally lower compared to conventionally ploughed soils for a short period after cultivation [
37].
Measurements taken 1 month after sowing winter oilseed rape (15.09.2021) showed that CO
2 emissions were significantly lower on uncultivated land with cover crops and uncultivated land with no cover crops (
Figure 1). Compared to conventional deep ploughing, CO
2 emissions were 29% and 28%, and 24% and 23% lower in both fields without straw and with straw, respectively. However, subsequent measurements at the beginning, middle and end of the winter oilseed rape growing season (21.10.2021, 8.10.2022 and 25.7.2022) did not reveal any significant differences in CO
2 emissions from the soil. At that time, neither the tillage systems investigated nor the use of straw had any effect.
In winter wheat (
Figure 2), the same trends were observed at the beginning, middle and end of the growing season as in winter oilseed rape.
Our results are in line with those of other authors [
38,
39]. Between tillage and sowing, before the soil is covered with new plants, tillage can have a significant impact on CO
2 emissions from the soil. More intensive loosening and mixing tillage practices significantly increase CO
2 emissions from the soil in the first 2 weeks compared to no-tillage.
The thermal exchange process in the soil depends on meteorological conditions, the thermal conductivity of the soil, the thermal capacity, the water content of the soil and other soil properties. One of the main factors influencing the thermal process of the soil is tillage and the covering of the soil surface with various plants or their residues. However, in our field experiment, the temperature of the topsoil was not significantly influenced by the tillage system studied or using straw (
Figure 3 and
Figure 4).
Soil moisture conservation is becoming increasingly important in a changing climate. Under dry conditions, direct sowing into uncultivated land allows better moisture retention in the 0–10 cm soil layer and is considered a moisture-saving measure [
40]. However, under the 2022–2023 meteorological conditions, neither the tillage systems studied nor the use of straw had a significant impact on the soil moisture content in the surface layer (
Figure 5 and
Figure 6).
Changes in soil CO2 emissions, temperature and moisture under different tillage systems and the use of straw in winter oilseed rape and winter wheat production. The results show that CO2 emissions from the soil may vary depending on the tillage technology, but that these differences are not constant and may change throughout the plant growing season.
Direct sowing on uncultivated land, both with and without cover crops, immediately after tillage reduces CO2 emissions from the soil compared to conventional tillage. However, in subsequent measurements during the growing season, no significant differences in CO2 emissions were found between the different tillage systems, indicating that the initial effect of the tillage method evens out over time.
Soil temperature was not found to be significantly influenced by tillage system or straw application. This suggests that soil temperature is more dependent on other factors, such as meteorological conditions, rather than directly on the tillage method.
A very important aspect is soil moisture retention, which is particularly important in arid conditions. Although direct sowing on uncultivated land has traditionally been considered a moisture conserving measure, this study found that, under specific meteorological conditions, the use of different tillage systems or straw did not have a significant impact on soil moisture content.
In summary, the results of the study reveal a complex interaction between tillage and plant growth on soil CO2 emissions, temperature, and moisture. Although in some cases direct sowing on uncultivated land can reduce CO2 emissions and help to conserve soil moisture, these effects are not the same at all stages of plant growth or under different environmental conditions. It is therefore important to consider complex factors when designing tillage strategies and applying practices that focus on sustainability and environmental protection.
3.2. Soil Organic Carbon Stocks
Soil organic carbon (SOC) stocks in 2003 and 2023 across two soil depths (0–10 cm and 10–25 cm) and under various straw management and tillage practices reveal significant trends in SOC accumulation over 20 years (
Table 3). The experimental setup included two main variables: straw management, with one practice involving the removal of straw (R) and the other involving spreading chopped straw (S), and tillage methods, which comprised conventional ploughing (CP), using cover crops for green manure without tillage (GMNT), and no-tillage (NT).
Over the two decades, SOC stocks increased across all treatments and depths, demonstrating the soil’s enhanced carbon stock potential under both improved straw management and reduced tillage practices. Specifically, the spread of chopped straw (S) resulted in higher SOC accumulation than straw removal (R), indicating the beneficial impact of straw retention on soil carbon levels. In terms of tillage, the no-tillage (NT) and green manure no-tillage (GMNT) practices showed the most significant increase in SOC stocks, surpassing conventional ploughing (CP), especially in the upper soil layer (0–10 cm). This suggests that minimising soil disturbance and incorporating green manure are highly effective strategies for enhancing SOC.
After 20 years, the increase in SOC was most pronounced under the no-tillage (NT) and green manure no-tillage (GMNT) methods, with the NT method showing the highest increase in the upper soil layer (from an initial 25.57 t ha-1 to 37.17 t ha-1). Similarly, the GMNT method demonstrated a substantial increase, reaching 36.49 t ha-1 from an initial 23.53 t ha-1 in the upper soil layer. These changes underscore the critical role of tillage management in soil carbon dynamics and highlight the potential of conservation agriculture practices for sustainable soil health and carbon sequestration.
The correlation regression analysis showed to strong correlations. In 2023, a linear very strong positive and statistically significant correlation was found in the straw-removed fields with no-till between the CO2 release from the soil (12.05.2023) r = 0.99, y = -2.464 + 2.22x, P < 0.05 and the soil organic carbon stock in the 0–10 cm soil layer.
3.3. Crop Yields for Winter Oilseed Rape and Wheat
The experimental data examines the impact of straw management and tillage methods on the productivity of winter oilseed rape in 2022 and winter wheat in 2023.
In winter oilseed rape in 2022 (
Figure 7), the use of green manure and no-tillage (GMNT) method yielded the highest productivity regardless of straw management, with a peak productivity of 3.52 t ha
-1 when the straw was incorporated. This suggests a synergistic effect of green manure and conservation tillage practices on oilseed rape yield. In contrast, traditional deep ploughing (CP) had the lowest yield when straw was removed, although yields substantially improved with the incorporation of straw.
For winter wheat in 2023, the trends were slightly different. The highest yields were observed with the GMNT method without straw and with the CP method when straw was incorporated. This indicates that while green manure and no-tillage practices are generally beneficial, the incorporation of straw can offset the lower yields associated with traditional ploughing, possibly due to the added organic matter and nutrients.
The data suggests that integrating green manure with no-tillage is generally the most productive practice for both crops, with straw incorporation offering additional benefits in certain cases. However, the variation in response between the two crops suggests that the effectiveness of these methods is crop-specific and may depend on other environmental and management factors not detailed in the experiment. The results underscore the importance of adopting tailored agronomic practices for different crops to optimize yield and potentially enhance sustainability.
The yield of winter wheat depended on the organic carbon stock. Correlation regression analysis showed a moderate correlation. In the topsoil layer (0-10 cm), there was a linear very strong positive and statistically significant relationship between organic carbon stocks and the yield in 2023. r = 0.71; P = ≤ 0.05.