Balancing Greenhouse Gas Emissions and Yield through Rotational Tillage in the Cold Rice-Growing Region

Abstract

Tillage practices are of critical importance in maintaining soil quality on cropland and for food production, with rice cultivation representing a significant portion of the world’s food production and greenhouse gas (GHG) emissions. While numerous studies have examined the effects of reduced and no-tillage on soil GHG emissions and rice yields, the impact of adopting a rotational approach to tillage practices on the rice cultivation cycle remains uncertain. In this study, we conducted a four-year (2017–2020) field experiment in a single rice-growing area in Northeast China with the aim of investigating the effects of different tillage practices on GHG emissions from paddy fields and rice yields under full straw return conditions. We set up three experimental treatments: rotary tillage, plowing, and rotational tillage (i.e., a combination of one year of plowing and one year of rotary tillage). The results showed that averaged across all treatments, average methane (CH₄, 302.6 ± 51.1 kg ha⁻¹) and nitrous oxide (N₂O, 0.86 ± 0.361 kg ha⁻¹) emissions and rice yield (9.0 ± 0.9 t ha⁻¹) did not exhibit significant inter-annual variability during the entire experimental period and were comparable to the average for the region. The ranking of GHG emissions during the rice-growing season was as follows: rotary tillage > plowing > rotational tillage. Across the experimental period, CH₄ and N₂O emissions were 9.1% and 8.5% lower in the plowing treatment and 21.2% and 13.1% lower in the rotational tillage treatment compared to the rotary tillage treatment. During the experimental period, there was no significant effect of tillage treatments on rice yield. This reduction in emissions may be attributed to changes in soil penetration resistance. In the rotational and plowing treatments, soil penetration resistance was in a range more adapted to rice growth and GHG emissions reduction compared to the rotary tillage treatment. The yield-scale GHG emission intensity was reduced by 12.7% and 26.1% in the plowing and rotational tillage treatments, respectively, in comparison to the rotary tillage treatment. This suggests that rotational tillage is a management practice that can achieve greenhouse gas emission reductions in paddy fields and stabilize or possibly increase rice yields. Consequently, the results demonstrated that a rotational alternation of multiple tillage practices is a synergistic strategy for achieving low carbon and high yield in rice in the cold rice-growing region of Northeast China.

1. Introduction

While long-term mono-tillage in agricultural production, such as traditional tillage and rotary tillage, can result in increased land production in the short term, it can also lead to the destruction of soil structure, shallow tillage, a reduced water and fertilizer holding capacity, and a thickening of the subsoil layer of the plow [1,2]. These problems impede the deep distribution of crop roots and the water and fertilizer absorption functions, thus threatening crop growth. Long-term monoculture tillage not only affects crop germination and soil quality but can also have a negative impact on the ecological environment [3,4]. To address these issues, the concept of “rotational tillage” has been proposed [5]. Rotational tillage practice, a combination of at least two types of tillage practices from shallow tillage (e.g., rotary tillage or no-tillage) and deep tillage (e.g., moldboard plowing tillage or subsoiling), represents an effective solution or mitigation of the problems caused by long-term monoculture tillage [6,7]. Rotational tillage practices can enhance the organic carbon content of the tillage soil, facilitate the uniform distribution of organic carbon and nitrogen (N) in the tillage soil, and play a pivotal role in optimizing the efficiency of fertilizer use [8,9,10]. The incorporation of appropriate tillage with organic materials can effectively enhance the soil environment, which is conducive to the growth and development of crops, and can achieve the objective of promoting stable, high yields and sustainable development in agriculture [11,12,13,14].

The differential effects of tillage practices on soil properties can further modify emissions of greenhouse gases (GHGs), i.e., methane (CH₄) and nitrous oxide (N₂O), from agricultural sources [15,16,17]. The majority of existing studies have focused on the effects of conventional tillage and no-tillage on GHG emissions from rice fields [18]. A number of studies have reported the effects of no-tillage versus conventional tillage practices, such as plowing or rotary tillage, on GHG emissions from rice cultivation in China [19,20,21,22], Korea [23], Brazil [24], India [25], and Spain [26]. The effects of no-tillage on CH₄ and N₂O emissions observed in these studies are inconsistent. A recent meta-analysis demonstrated that no-tillage significantly reduced CH₄ emissions and combined GHG emissions in rice fields by 23% compared to conventional tillage [18]. The results of the meta-analysis indicated that no-tillage reduced N₂O emissions from rice fields by 19% when the duration of the experiment was greater than or equal to three years, but the effect was not significant for shorter-duration treatments (less than three years), suggesting that the effect on N₂O emissions was dependent on the duration of the experiment [18]. To date, the majority of studies have focused on the comparison of reduced and no-tillage with conventional tillage practices [15,18,27,28] due to the inconsistency in the definition of rotational tillage and the limitation of the study duration. Consequently, there is still a lack of information on how rotational tillage would affect GHG emissions from rice fields.

The effects of tillage practices on rice production are complex, and the effects of different tillage practices on rice yields remain inconclusive. A long-term double-crop rice experiment conducted in Hunan, China, over two years revealed that rice yields in the early rice season were marginally lower in the no-tillage and plow tillage treatments than in the rotary tillage treatment [21]. Conversely, rice yields in the late rice season were significantly lower in the plow tillage treatment than in the other two tillage treatments [21]. In contrast, a three-year consecutive field experiment conducted in Jiangxi, China, found that early season yields were lowest in the no-tillage treatment, followed by plow tillage, and the highest in the rotary-tillage treatment, with a consistent trend in late season yields [19]. A comparison of three major rice cropping systems, namely monoculture in Northeast China, the rice–wheat rotation system in the lower reaches of the Yangtze River, and double-cropping in central China, revealed that rotary tillage treatment exhibited slightly but insignificantly lower rice yields than the plowing treatment, although none of these differences reached statistically significant levels [29]. The meta-analysis demonstrated that the combined effect of no-tillage treatments on rice yield was not significant [18]. There have been few recent reports on the effect of rotational versus conventional tillage on rice yield. Nevertheless, numerous studies have demonstrated that rotational tillage treatments can enhance soil microbial activity by improving soil structure and aeration, thereby enhancing the water use efficiency of dryland crops and increasing crop yield [30].

Northeast China is an important rice-producing region in China, accounting for approximately 9% of the total rice area and 16–20% of the national rice production [31]. The practice of returning straw to the field as a means of improving soil fertility and sustainable agriculture presents a number of challenges, including the question of how to effectively return straw to the soil in order to promote decomposition while reducing GHG emissions. Different tillage practices will have different effects on decomposition and nutrient release following the return of straw to the soil. This study hypothesizes that rotational tillage practices are more effective in reducing GHG emissions from single-season rice cultivation compared to long-term single plowing or rotary tillage. This hypothesis is grounded in the premise that rotational tillage improves the soil structure, thereby promoting rice growth and increasing yield. Previous research has demonstrated that rotational tillage under dryland conditions in Northeast China enhances soil aggregation, thereby improving the soil microenvironment [32,33]. Additionally, rotational tillage significantly enhances soil infiltration capacity, thereby improving the effective use of soil moisture and rainfall [9,34]. Therefore, a four-year field experiment was conducted in a single rice-growing area in Northeast China under straw return conditions to compare the effects of continuous rotary tillage, plowing, one-year plowing, and one-year rotary tillage rotational cropping patterns on CH₄ and N₂O emissions from rice fields and rice yields. The findings of this study can provide data to support the low-carbon green development of single rice-growing area in Northeast China and inform the formulation of more effective agricultural policies and management strategies.

2. Materials and Methods

2.1. Description of Experimental Sites

The experiment was conducted in the field of the National Modern Agricultural Demonstration Area of Heilongjiang Academy of Agricultural Sciences, located in Democracy Township, Daowai District, Harbin City, Heilongjiang Province, China (45°49′ N, 126°48′ E, 117 m above sea level) (Figure 1a). This area is situated within the northeastern single rice paddy region, characterized by a temperate continental monsoon climate. The average annual sunshine duration is 2669 h, and the frost-free period ranges from 131 to 146 days. Annual precipitation varies between 508 and 583 mm, and the cumulative effective temperature, defined as temperatures greater than or equal to 10 °C, is 2600–2700 °C.

The soil at the experimental site is classified as black chernozem. The main physicochemical properties of the soil are as follows: soil organic matter (SOM) at 25.5 g kg⁻¹, total nitrogen at 0.9 g kg⁻¹, total phosphorus at 0.5 g kg⁻¹, total potassium at 20.9 g kg⁻¹, alkaline hydrolyzable nitrogen at 86.5 mg kg⁻¹, available phosphorus at 24.9 mg kg⁻¹, available potassium at 114.3 mg kg⁻¹, and a pH of 8.3.

2.2. Field Experiment Design

The experiment was conducted during the rice-growing season from 2017 to 2020. The experiment was a randomized block design with three treatments and three replications for each treatment, with each plot area being 300 m². The treatments were as follows: rotary tillage, plowing, and rotational tillage (Figure 1b–d). After rice harvesting in the fall, all straw was mechanically chopped (≤10 cm) and returned to the field at an approximate dry weight of 10–11 t ha⁻¹. The rotary tillage treatment entailed rotating the surface soil to a depth of ≤15 cm after rice harvest, while the tillage depth of the plowing treatment was 18–22 cm. The rotational tillage treatment entailed rotating the plots for one year with plowing and one year with rotary tillage. The water management system involved the initial soaking of the field with shallow water in the spring, followed by a non-driven churn to create a uniform surface, and finally, the filling of the field with deep water. During the early stages of the rice-growing season, the field was maintained in a shallow layer of water (1–3 cm) and drained in the middle of the season. This was followed by the re-watering of the field to alternate between wet and dry conditions until the field was dried out naturally before rice harvest. Rice seedlings were transplanted at a density of 30 cm × 13.3 cm at the beginning of late May each year (18 May in 2017, 2018, and 2020, and 19 May in 2019). The N fertilizer application rate was 180 kg ha⁻¹, applied in the ratio of 4:5:1 as basal, tillering, and spiking fertilizers. The phosphorus fertilizer was 70 kg ha⁻¹ of P₂O₅ applied as the basal fertilizer. The potassium fertilizer was 60 kg ha⁻¹ of K₂O, applied in the ratio of 1:1 as basal and spiking fertilizers. The rice variety was Longdao 21, which is the main variety for local large-scale production.

2.3. Gas Sampling and Flux Calculations

The gas sampling and determination during the experimental period were conducted using static opaque chamber-gas chromatography [35,36]. The sampling chamber and base were constructed from a PVC opaque plastic sheet. The base was 50 cm × 50 cm × 10 cm in size, while the sampling chamber was 50 cm × 50 cm × 50 cm in length × width × height. With the increase in rice plant height, two additional sampling boxes were added for gas sample collection. To minimize temperature fluctuations caused by solar irradiation, the exterior of the chamber was wrapped with a 2–3 cm thick sponge and aluminum foil fiberglass cloth to maintain a stable temperature within the chamber during the sampling period. A small opening was created at the top of the sampling chamber to accommodate an alcohol thermometer and a silicone tube, which were used to monitor the temperature within the chamber and to collect gas samples. The other end of the silicone tube was connected to a three-way valve, which allows for the collection of gas samples through a syringe. A fan was installed inside the chamber to provide uniform airflow during the sampling process. The sampling bases were buried in the soil after the rice was transplanted, with one base per plot. Once the base had been buried and stabilized for one week, periodic sampling commenced with a sampling frequency of once a week and a sampling time period of 9:00 a.m. to 11:00 a.m. local time. During the sampling process, water was injected into the base groove to seal the base to the sampling chamber. After the chamber enclosure, four gas samples were collected at 5 min intervals with a 50 mL syringe while the temperature inside the chamber was recorded. The collected gas samples were returned to the laboratory for analysis to determine the concentrations of CH₄ and N₂O in the samples using a gas chromatograph (Agilent 7890A, Agilent Technologies, Santa Clara, CA, USA). The determination of the gas samples was completed within 24 h. The gas flux was calculated using the following equation:

F = ρ × 273/(273 + T) × H × dC/dt,

(1)

where F is the emission flux (mg or µg m⁻² h⁻¹); ρ is the density of CH₄ or N₂O at standard atmospheric pressure; T is the average temperature (°C) inside the chamber during the sampling process; H is the net height of the sampling box (m); and dC/dt is the rate of change in the GHG concentration inside the chamber. The CH₄ concentrations of the four time periods were fitted linearly, and the slope of the regression coefficient R² was greater than or equal to 0.9. In the event that R² was less than 0.9, any three concentration data points were combined according to time. The slope of the data point with the largest correlation coefficient was then used to indicate the dC/dt of the group. This required that R² be greater than or equal to 0.9. Otherwise, the data were excluded from the group. Cumulative emissions of CH₄ and N₂O during the growing season were calculated by averaging and summing two adjacent results.

The integrated global warming potential (GWP) was employed to assess the collective impact of GHGs on climate change [37]. This was accomplished by applying the following formula:

GWP = 25 × E_CH4 + 298 × E_N2O,

(2)

where GWP represents the integrated GWP in kg CO₂-eq ha⁻¹, E_CH4 is the cumulative seasonal emissions of CH₄ (kg ha⁻¹), and E_N2O is the cumulative seasonal emissions of N₂O (kg ha⁻¹). The integrated GWP is expressed in kg CO₂-eq ha⁻¹. On a 100-year scale, the GWP of a single molecule of CH₄ and N₂O is 25 and 298 times that of CO₂, respectively [38].

Greenhouse gas emission intensity is expressed in terms of GWP per unit of production, which is a comprehensive evaluation index that combines environmental and production benefits [37]. It is calculated using the following formula:

GHGI = GWP/Yield,

(3)

where GHGI represents the GHG emission intensity at the yield scale in kg CO₂-eq ha⁻¹. GWP is the global warming potential in CO₂-eq ha⁻¹ as calculated above, and yield is the rice yield in t ha⁻¹.

2.4. Measurement of Other Indicators

Following the harvest of rice in each season, soil penetration resistance was measured in the field at a depth of 0–45 cm in each plot using a portable soil compaction meter (Field Scout SC 900, Spectrum Technologies, Aurora, IL, USA). After the maturation of the rice, 1 m² of rice plants was harvested from each plot to determine the seed yield and converted to standard moisture content (14.5%) to determine the actual yield.

2.5. Statistical Analysis

Microsoft Excel 2019 software (Microsoft Corporation, Redmond, WA, USA) was employed for the initial processing and statistical analysis of the data. The data are presented in graphs and tables as mean ± standard error. Prior to the analysis of variance (ANOVA), the data were tested for normality and variance using the chi-squared test. The effect of treatments on gas emissions and yields was tested using one-way analysis of variance (ANOVA) with SPSS 27.0 (IBM SPSS Statistics, Somers, NY, USA). Multiple comparisons were analyzed using the least significant difference (LSD) method. Test results were considered statistically significant at the 0.05 probability level.

3. Results

3.1. CH₄ Emissions

Overall, CH₄ emissions exhibited a comparable pattern throughout the experimental period, although slight differences were observed between years (Figure 2). For 2017 and 2018, the seasonal variation characteristics of CH₄ emission fluxes from different tillering practices exhibited a single peak pattern, with a trend of first increasing and then gradually decreasing. This indicates that the peak in CH₄ emission fluxes occurred in the tillering stage with rice growth, followed by a gradual decrease until the pre-harvest stage. The magnitude of the average CH₄ emission fluxes of different tillage practices was as follows: rotary tillage > plowing > rotational tillage. In the 2017 rice season, the average CH₄ emission fluxes of rotary tillage, plowing, and rotational tillage treatments were 12.57 mg m⁻² h⁻¹, 11.44 mg m⁻² h⁻¹, and 9.82 mg m⁻² h⁻¹, respectively, which were comparable to the values of 12.14 mg m⁻² h⁻¹, 10.27 mg m⁻² h⁻¹, and 10.04 mg m⁻² h⁻¹ observed in 2018. For the years 2019 and 2020, the seasonal variation in CH₄ emission fluxes from different tillage practices exhibited a bimodal trend, with an initial increase, followed by a decrease, and then an increase, before finally reaching a gradual decrease (Figure 2). This pattern can be observed during the growth of rice, with the first peak occurring in the tillering stage and then decreasing, and the second peak occurring in the nodulation stage and then gradually decreasing until the rice harvest. In 2019, the average CH₄ emission fluxes of different tillage practices were in the order of rotary tillage > plowing > rotational tillage, with corresponding average emission fluxes of 13.10 mg m⁻² h⁻¹, 11.17 mg m⁻² h⁻¹, and 7.41 mg m⁻² h⁻¹, respectively. In 2020, the average CH₄ emission fluxes of plowing, rotational tillage, and rotary tillage were 9.46 mg m⁻² h⁻¹, 8.30 mg m⁻² h⁻¹, and 7.15 mg m⁻² h⁻¹, respectively.

For all treatments, seasonal CH₄ emissions ranged from 227.4 to 417.9 kg ha⁻¹ (

Table 1. Effects of different tillage practices on greenhouse gas emissions and yields of single rice-growing area in the cold region of Northeast China.

Year	Treatment	CH4 (kg ha−1)	N2O (kg ha−1)	GWP (kg CO2-eq ha−1)	Yield (t ha−1)	GHGI (kg CO2-eq kg−1)
2017	Rotary tillage	354.7 ± 30.1 a 1	1.31 ± 0.33 a	9260 ± 820 a	7.67 ± 0.59 a	1.21 ± 0.14 a
	Plowing	317.8 ± 35.5 ab	1.45 ± 0.25 a	8379 ± 957 a	8.00 ± 1.51 a	0.93 ± 0.09 b
	Rotational tillage	280.2 ± 5.7 b	1.49 ± 0.20 a	7450 ± 92 a	8.50 ± 0.05 a	0.88 ± 0.01 b
2018	Rotary tillage	314.5 ± 10.8 a	0.81 ± 0.39 a	8104 ± 88 a	8.28 ± 0.06 a	0.98 ± 0.01 a
	Plowing	267.8 ± 21.2 b	0.79 ± 0.15 a	6932 ± 420 b	7.80 ± 0.81 a	0.89 ± 0.05 b
	Rotational tillage	270.9 ± 23.1 b	0.68 ± 0.15 a	6974 ± 419 b	8.77 ± 0.12 a	0.80 ± 0.05 c
2019	Rotary tillage	417.9 ± 106.7 a	0.68 ± 0.12 a	10,650 ± 2650 a	9.35 ± 0.06 a	1.14 ± 0.28 a
	Plowing	344.6 ± 9.9 ab	0.44 ± 0.04 b	8744 ± 258 ab	9.57 ± 0.68 a	0.91 ± 0.05 ab
	Rotational tillage	227.4 ± 34.2 b	0.48 ± 0.05 b	5828 ± 866 b	9.67 ± 0.49 a	0.60 ± 0.09 b
2020	Rotary tillage	276.7 ± 17.2 a	0.81 ± 0.20 a	7159 ± 308 a	9.53 ± 0.06 a	0.75 ± 0.03 a
	Plowing	294.1 ± 22.1 a	0.76 ± 0.15 a	7577 ± 509 a	10.12 ± 1.63 a	0.76 ± 0.07 a
	Rotational tillage	265.1 ± 16.9 a	0.64 ± 0.07 a	6819 ± 405 a	10.19 ± 0.08 a	0.67 ± 0.04 a

¹ The values represent the mean ± standard error (n = 3). The lowercase letters indicate significant differences between the treatments in the same year for each column (p < 0.05).

). Seasonal CH₄ emissions were lower for the rotational tillage treatment than the rotary tillage treatment in three of the four years tested (2017–2019). Seasonal CH₄ emissions were significantly lower in the plowing treatment than rotary tillage only in 2019. Across four years, CH₄ emissions from the plowing and rotational tillage treatments were on average 9.1% and 21.2% lower than those of the rotary tillage treatment. Furthermore, CH₄ emissions from the rotational tillage treatment were 13.6% lower than those from the plowing treatment.

3.2. N₂O Emission

The patterns of soil N₂O emission fluxes during the rice-growing season exhibited variability across different years (Figure 3). In 2017, the seasonal variation characteristics of N₂O emission fluxes from different tillage methods demonstrated a trend of first increasing and then decreasing. In 2018, the seasonal change characteristics of N₂O emission fluxes under different tillage practices with straw return exhibited a trend of first increasing and then decreasing. In 2017, the average N₂O emission fluxes from rotary tillage, plowing, and rotational tillage were 40.56 μg m⁻² h⁻¹, 53.88 μg m⁻² h⁻¹, and 54.61 μg m⁻² h⁻¹, respectively. In 2018, the corresponding average emission fluxes were 27.14 μg m⁻² h⁻¹, 26.97 μg m⁻² h⁻¹, and 23.24 μg m⁻² h⁻¹, respectively.

In 2019 and 2020, the seasonal variation in N₂O emission fluxes from different tillage practices exhibited a “sawtooth” pattern (Figure 3). In 2019, the average N₂O emission fluxes from rotary plowing, plowing, and rotational tillage were 21.41 μg m⁻² h⁻¹, 13.57 μg m⁻² h⁻¹, and 15.04 μg m⁻² h⁻¹. In 2020, the corresponding average emission fluxes were 28.00 μg m⁻² h⁻¹, 25.50 μg m⁻² h⁻¹, and 23.92 μg m⁻² h⁻¹, respectively.

For all treatments, seasonal N₂O emissions ranged from 0.44 to 1.49 kg ha⁻¹ (Table 1). Overall, seasonal N₂O emissions in the rotary tillage treatment were significantly higher than the other two treatments in 2019, but they did not differ from each other in the remaining three years. On average, N₂O emissions were 8.5% and 13.1% lower for the plowing and rotational tillage treatments, respectively, compared to rotary tillage. Overall, the GWP of the rotary tillage treatment was significantly higher than the other two tillage treatments in 2018 and 2019. The GWP of the rotary tillage treatment was, on average, 9.0% and 20.9% higher than the plowing and rotational tillage treatments, respectively. The GWP of the plowing treatment was 13.5% higher than that of the rotational tillage treatment.

3.3. Rice yield and GHGI

Overall, rice yields of the treatments did not reach the level of significant differences throughout the experimental period (Table 1). In all treatments, the rice yield showed an increasing trend from year to year.

The yield-scale GHG emission intensity indicator, GHGI, exhibited the lowest values in the rotational tillage treatment, followed by plowing and then rotary tillage (Table 1). On average, the GHGI was 12.7% and 15.6% lower in the rotational tillage treatment than in the rotary tillage and plowing treatments, respectively. The GHGI was 26.1% higher in the rotary tillage treatment than in the plowing treatment.

3.4. Soil Penetration Resistance

A four-year monitoring period revealed that the change pattern of the soil penetration resistance for the three tillage practices was essentially identical, with rotary tillage exhibiting the greatest resistance, followed by rotational tillage and plowing. The change pattern for the soil layer below 22.5–25 cm was more pronounced (Figure 4). The mean soil penetration resistance of the rotary tillage treatment was 10% higher than that of the rotational tillage treatment, while the rotational tillage treatment was 11% higher than that of plowing. If the soil penetration resistance is too high, it may impede downward water infiltration, reduce the fertilizer utilization rate, and negatively impact the growth of the rice root system. Conversely, if the soil penetration resistance is too low, it is susceptible to the downward loss of soil water and nutrients, which is also detrimental to rice growth. Consequently, the plots subjected to rotational tillage exhibited a more suitable soil penetration resistance than those treated with rotary tillage or plowing. This enhanced the downward rooting and growth of the rice root system, as well as the inter-root oxygen secretion.

4. Discussion

The objective of this study was to investigate the effects of different tillage practices on GHG emissions from rice fields and rice yields. To this end, we carried out a four-year field experiment in a single-season rice-growing area in Northeast China. The results of our study demonstrated that the practice of alternating rotary and rotational tillage was beneficial in reducing carbon emissions from rice production in Northeast China compared with rotary or plow tillage. Although the differences were not statistically significant, rice yields were slightly higher in the rotational tillage treatment than in the other two tillage treatments, resulting in the lowest carbon emission intensity per unit of yield in the rotational tillage treatment. Thus, our study demonstrated that rotational tillage is a tillage technique that can achieve synergistic carbon emission reductions and stable or increased rice yields in the northeastern single rice-growing region.

The observed GHG emissions and rice yields were found to be reliable. The range of CH₄ emission fluxes during the rice-growing season from 2017 to 2020 across all treatments was 2.5 ± 0.5 kg ha⁻¹ d⁻¹, which is higher than the IPCC National Greenhouse Gas Inventory Guidelines default value of 1.19 kg ha⁻¹ d⁻¹ [39]. This discrepancy may be partly attributed to the impact of straw return, which is not accounted for in the IPCC default value [40]. Our findings are consistent with the most recent CH₄ emission fluxes in China (2.41 kg ha⁻¹ d⁻¹) derived from the updated global CH₄ dataset of rice fields [41]. The seasonal CH₄ emissions over the four-year observation cycle were found to be 302.6 ± 51.1 kg ha⁻¹, which is significantly higher than the recommended value of 168 kg ha⁻¹ (range: 112.6–230.3 kg ha⁻¹) for the northeast region in the guidelines for preparing provincial-level GHG inventories in China [42]. This discrepancy could be due to the CH₄ emission factor for rice fields in the national recommendations being based on observations made in 2005 under conditions of no or very little straw return. Additionally, varietal differences might also contribute to differences in CH₄ emission intensity [43,44]. The average rice yield in this study was 9.0 ± 0.9 t ha⁻¹, while in the 2000s, the average rice yields in China and Northeast China were 6.5 and 6.9 t ha⁻¹, respectively [31]. The rice yield obtained in this study is comparable to the results of 8.0–8.7 t ha⁻¹ for this variety, as reported in a regional trial (https://www.ricedata.cn/variety/varis/615640.htm (accessed on 12 March 2024)). The N₂O emissions during the rice-growing season in 2017–2020 were 0.86 ± 0.36 kg ha⁻¹ in all treatments, which is within the estimated range of regional default emission factor values recommended in China’s provincial GHG inventory guidelines (mean: 2.05 kg ha⁻¹, range: 0.38–4.64 kg ha⁻¹) [42]. Our results are also comparable to those estimated under a specific water management regime (i.e., F-D-F-M) (1.31 ± 0.40 kg ha⁻¹) when the specific water management regime is considered [45].

Our study demonstrated that the implementation of rotational tillage treatments in the northeast single rice-growing region can result in a reduction in carbon emissions. Through two rounds of comparative observational studies conducted over a four-year period, we found that both rotary tillage and plowing practices stimulated GHG emissions from rice fields. In particular, the GWP increased by an average of 21% and 13% under rotary tillage and plowing, respectively, compared to rotational tillage. Considering the implementation of rotational tillage in paddy fields in Northeast China and based on previous estimates of greenhouse gas emissions from rice cultivation in this region [46,47], we roughly estimate that rotational tillage could reduce emissions by 3.0 Tg CO₂-eq yr⁻¹ compared to plowing and by 4.7 Tg CO₂-eq yr⁻¹ compared to rotary tillage. This phenomenon is primarily attributed to the fact that the continuous practice of single tillage management promotes both CH₄ and N₂O emissions, particularly in the context of continuous rotary tillage treatment (Table 1). The impact of rotary tillage practices on soil structure was more pronounced than conventional tillage. Rotary tillage can lead to greater soil bulk density by destroying soil macroaggregates, ultimately leading to poorer soil aeration [6,32]. The results demonstrated that rotary tillage led to a 10% increase in soil tightness in comparison to rotational tillage (Figure 4). Additionally, Yang et al. [32] observed a significant decrease in soil bulk density and compactness in rotational tillage treatments in comparison to continuous rotary tillage in a corn experiment in Northeast China. The increase in soil compactness may promote CH₄ production and/or inhibit CH₄ oxidation under flooded conditions, as well as promote soil N₂O emission from paddy fields during non-flooded periods. In comparison to rotary tillage, plowing has been demonstrated to effectively reduce CH₄ and N₂O emissions from single-season rice cultivation. This is attributed to alterations in the chemical and biological properties of the soil, promotion of oxygen availability in the soil, acceleration of organic matter decomposition in the soil, and improvement in soil structure and aeration. Consequently, although soil compactness was higher in the rotational tillage treatment than in the continuous plowing treatment, the adoption of an alternating management of rotary tillage and plowing could somewhat reduce the stimulating effect of continuous rotary plowing on carbon emissions. These findings are consistent with those of Li et al. [48] who conducted two years of field observations in a rice–cotton cropping system in the lower reaches of the Yangtze River. Their observations demonstrated that CH₄ emissions from rice paddies in the rotary-tillage treatment were, on average, 34% higher than those in the combination of plowing and rotary tillage. In a nine-year double-cropped rice experiment in Hunan Province, Chen et al. [21] observed GHG emissions in the last two years and found that that for the plowing treatment was lower than that for the rotary tillage. Similarly, observations of three consecutive years of double-cropped rice in Jiangxi Province showed that conventional tillage had lower carbon emissions compared to rotary tillage [19]. Although there are relatively few observational studies comparing rotational tillage and continuous mono-tillage practices on GHG emissions from paddy fields, the result that no-tillage significantly reduces CH₄ and N₂O emissions from paddy fields compared with conventional tillage practices as revealed by the integrated analysis study also partly supports our findings and speculations [18].

During the experimental period, no significant effect of different tillage treatments on rice yield was observed. However, rotational tillage and plowing treatments yielded slightly higher results than rotary tillage. This outcome can be attributed to the differing effects on soil structure produced by the successive application of different tillage treatments. In general, soil compaction can impede downward water infiltration, reduce fertilizer utilization, and hinder rice root growth [22,49]. Conversely, soil compaction can also result in the downward loss of soil water and nutrients, which is similarly detrimental to rice growth. The increase in soil compaction caused by continuous rotary tillage can affect the downward migration of nitrogen, which in turn affects the uptake and growth of the rice root system [50,51]. Although plowing may result in less soil compaction than rototilling, it may also facilitate downward migration and the loss of soil water and nutrients, which may be detrimental to rice uptake and utilization. Therefore, maintaining soil structure with appropriate soil compaction may be an effective way to improve the efficiency of soil nutrient utilization and thus crop yield. These results are consistent with those of Han et al. [52], who found that continuous harrowing for three to four years resulted in the formation of a harrowed subsoil layer with a high degree of compactness in the 15–20 cm soil layer. This resulted in a reduction in soil permeability and moisture retention capacity and seriously hindered the normal rooting process. In contrast, the cyclic rotation pattern of plowing for one year and harrowing for two years could promote rooting and thus improve crop yields [52]. Rotational tillage can improve the pore condition of the soil and reduce soil bulkiness and compactness, thus forming a suitable seedbed for the crop, promoting seed germination and growth and development, and ultimately improving the yield traits of the crop [6,8,34]. Furthermore, our results demonstrated that the adoption of rotational tillage or plowing treatments can effectively reduce the carbon emissions intensity per unit of production compared to continuous rotary tillage (Table 1). Consequently, this study demonstrated that the adoption of alternating rotary tillage and plowing may be an effective measure to achieve a win–win situation in terms of increased rice production and GHG emission reduction in the single-season rice-growing area in Northeast China.

This study, through multi-year observations, explored the impact of different tillage practices on GHG emissions in paddy fields in Northeast China. However, several limitations remain. Key parameters such as soil redox potential, water layer depth during flooding, and the abundance and composition of methane-producing and methane-oxidizing bacteria were not monitored concurrently, which are crucial for understanding the impact of tillage practices on greenhouse gas emissions. Additionally, the number of replications when assessing soil structure impacts (e.g., soil penetration resistance) was insufficient, limiting statistical significance. Future research should focus on monitoring more environmental and biological parameters, increasing the number of replications and scale of experiments for data reliability, and investigating the mechanisms by which tillage practices affect greenhouse gas emissions. This will provide a scientific basis for developing low-carbon and high-yield agricultural management strategies.

5. Conclusions

This study examined the impact of different tillage practices on GHG emissions and rice yields in paddy fields through a four-year field experiment in a monoculture rice-growing region in Northeast China. The findings indicated that alternating rotary and plowing tillage practices were more effective than rotary or plowing in reducing carbon emissions from rice production in cold regions of Northeast China. Although there was no significant difference in terms of rice yield, the rotational tillage treatment produced slightly but insignificantly higher rice yields than the other two tillage treatments, resulting in the lowest carbon emission intensity per unit of yield in the rotational tillage treatment. The results of this study also indicated that the continuous practice of a single tillage management practice (e.g., rotary or tillage) increased the GHG emissions from paddy fields. Conversely, rotational tillage was found to be an effective method of reducing carbon emissions, particularly in terms of reducing CH₄ and N₂O emissions. Furthermore, rotational tillage treatments have been shown to improve soil structure and enhance soil aeration, thereby promoting rice growth and yield. In conclusion, this study demonstrated that the implementation of alternative management practices with multiple tillage treatments in the northeast monoculture rice-growing area is an effective strategy for achieving both increased rice yield and carbon emissions reduction.