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Through a three-year investigation, we found that a moderate decrease of nitrogen from 280 to 140–210 kg N ha−1 did not markedly influence the populations of cereal aphids or the parasitism rate. However, a moderate decrease of nitrogen input from 280 to 140–210 kg N ha−1 maximized the fitness of two predominant Aphidiinae parasitoid species, suggesting parasitoid control of cereal aphid would get benefit from the moderate decrease of nitrogen fertilizer. Those results showed that moderately decreasing nitrogen fertilizer could boost the parasitoid control of cereal aphids. Our research suggests that moderately decreasing nitrogen input is qualitatively beneficial to parasitoids but would not control cereal aphids quantitatively.
Effect of decreasing nitrogen fertilizer on the cereal aphid population
This study demonstrated that nitrogen fertilizer has the potential to positively influence densities of S. avenae and R. padi among all manipulated nitrogen fertilizer levels (70–280 kg N ha−1) (Fig. 1). Similar conclusions have been documented in research linked with aphids, including cereal aphids5,17,24. First, the plant usually responds monotonously and positively to nitrogen fertilizer. The percentage of nitrogen in the dry weight of tobacco leaves was positively associated with fertilizer levels25. Nitrogen fertilizer in the range of 0–225 kg N ha−1 improved nitrogen concentration of canola throughout the growing season26. It has been reported that fertilization has a positive influence on plants, indicating a cascading effect on herbivorous pests24,26,27. Nitrogen input could enhance the nutritional quality of the host, as nitrogen input increases sugars and amino acids availability for aphids, thereby accelerating the population growth of the herbivores28,29. Second, fertilization negatively affects plant defensive responses to herbivores and lessens the amounts of toxins in host plants27. For example, nitrogen fertilizer employed for walnut seedlings decreased the allocation to defensive toxins such as juglone, thereby lowering resistance to walnut aphids30. Third, fertilization alters the microclimate of crops and thereby contributes to the population growth of aphids17,31.
However, only the lowest nitrogen level manipulated in our experiment (70 kg N ha−1) significantly reduced the population of cereal aphids compared with the conventional nitrogen level (280 kg N ha−1) in 2016 and 2017 (Fig. 1). Those results showed that the magnitude of decreasing fertilizer input from the conventional level (280 kg N ha−1) to a moderate level (140–210 kg N ha−1) was insufficient to contain the population of cereal aphids. The performance of cereal aphids could remain unaffected when fertilizer input was decreased to a low level, as aphids could adapt to the pressure of deficient nutrition by sucking more strongly10. Therefore, to reduce the population of cereal aphids, the nitrogen level should be decreased to 70 kg·N·ha−1 or lower. Similarly, as fertilizer was applied to tobacco in the range of 0–200 ppm N, the nymph weights of whiteflies on tobacco plants did not diminish markedly until the nitrogen concentration level was reduced from 200 to 0 ppm N25.
Nevertheless, cereal yield responds to nitrogen levels as a negatively accelerating curve based on previous studies7,9. Far lower nitrogen input sharply reduces grain yield, and moderate nitrogen fertilizer is always imperative in agricultural production2,7. Therefore, the tradeoff between maintaining the essential grain yield and reduction of the pest population would not have been optimized solely by decreasing nitrogen input.
The wheat variety adopted in our experiment was susceptible to cereal aphids. The landscape around our field employed in this experiment was predominated by winter wheat, and thus the landscape was extremely simplified. By comparison, use of a resistant variety and intercropping wheat with another crop mediated the impact of nitrogen input on densities of cereal aphids10,12. If these factors are taken into consideration, it then seems more unlikely that the pest population can be controlled solely by decreasing nitrogen input in complex realistic agricultural environments.
Effect of decreasing nitrogen fertilizer on the densities of parasitoids and parasitism rate
The results showed that the parasitism rate remained unchanged with nitrogen input (Fig. 2), similar to the results of Garratt, who pointed out that fertilizer levels did not affect the parasitism rate in a cereal-aphid-parasitoid system, as the densities of aphids and their parasitoids increased synchronously with the amount of fertilizer18. Similar findings were observed in a walnut aphid-Aphidiinae parasitoid system24. Mixed results were reported in previous studies5,11. The densities of cereal aphids and parasitoids increased when input of nitrogen fertilizer increased from 115 to 170 kg N ha−1, while the parasitism rate increased steadily5.
Parasitoids are subject to pressures derived from higher trophic level. Coincidental intraguild predation is ubiquitous in the form of parasitized aphids suffering from predation. The effect of coincidental intraguild predation on biocontrol and the abundance of parasitoids remains controversial32,33. Importantly, the Aphidiinae parasitoids have the potential to identify the odors of ladybird beetles and reduce searching efficiency by themselves and their offspring, a trait-mediated indirect effect unrelated with the densities of ladybird beetles34. It is possible that the behavior of Aphidiinae parasitoids and the parasitism rate could have been mediated indirectly by ladybird beetles and other predators. Furthermore, the hyperparasitoids also could have relieved biocontrol by Aphidiinae parasitoids35. Hence, the higher trophic level could relieve the effects of nitrogen levels on densities of parasitoids and the parasitism rate.
Effect of decreasing nitrogen fertilizer on the body size of Aphidiinae parasitoids
This research has shown that nitrogen fertilizer application impacted the body sizes of the two Aphidiinae parasitoids (Figs. 3, 4). It has been reported that the body sizes of parasitoids increased monotonically with nitrogen fertilizer under low densities of aphids in the laboratory18,22, meanwhile the dispersion capacity of parasitoid adults, the fecundity of adult females, the emergence rate, the adult longevity of parasitoids, and the parasitism rate increased with the body sizes of parasitoids19,20,22. In contrast to previous reports, this field study found that a moderate decrease in nitrogen application from 280 to 140–210 kg N ha−1 maximized the body sizes of parasitoids. The body sizes of parasitoids depend negatively on the abundance of parasitoids and positively on the hosts diversity19,36,37. Hence, combining the positive effect of the abundance of aphids and of the nitrogen input with the negative effect of parasitoid abundance, it is assumed that an equilibrium should emerge balancing the positive effect of abundance of aphids and the negative effect of abundance of parasitoids. Analogously, It has been reported that an optimized nitrogen level maximized the ratio of predators to prey in a canola-mustard aphid-predatory gall midge system26.
Manipulating nitrogen fertilizer to maximize the fitness of parasitoids plays a crucial role in natural pest control. Increasing the body sizes of parasitoids means greater fertility and dispersal ability of adults20,21, higher fitness of offspring38, and the resulting greater capacity to control the aphid. Thus, decreasing nitrogen fertilizer from the conventional level to more environmentally-friendly magnitudes (140–210 kg N ha−1) could increase the fitness of Aphidiinae parasitoids and boost the biocontrol by parasitoids. Regrettably, this research study did not validate such a viewpoint since the parasitism rate was not maximized under the moderate nitrogen levels. First, there may be hysteresis effects. The parasitoids that were measured for body sizes came from mummies that were sampled in the flowering phases. These parasitoids came into play and mummified cereal aphids after more than ten days. The mummies remained scarce before the flowering phase. Thus, a notable lag occurred and the effect of parasitoid fitness on the parasitism rate could have been unobservable in this study. Second, apart from affecting parasitoid fitness, nitrogen application affected pest fitness. A moderate amount nitrogen maximized the performance of the green peach aphid and the Bertha armyworm23,39. A positive relationship between aphid weight and hind tibia length of parasitoids has been reported18. Combined with the finding in this study that the body sizes of parasitoids were maximized by moderate nitrogen levels, these results imply that the fitness of cereal aphids also benefited from moderate nitrogen levels. However, the densities of cereal aphids in moderate nitrogen levels were similar to those under higher nitrogen levels, suggesting that there could be a compensation between the effect of nitrogen input on fitness of cereal aphids and the effect of nitrogen input on fitness of parasitoids. Currently, long-term agricultural intensification limited biocontrol of parasitoids5. Previous study has reported that the parasitoids were more strongly influenced by agricultural intensification compared to cereal aphids5,13,14. If serious agricultural intensification had mediated, for example decreasing nitrogen fertilizer to an optimized extent, the equilibrium between the impact of moderate decreasing nitrogen fertilizer on parasitoids and the counterpart on cereal aphids would be reshaped. Thus, the positive influence of decreasing nitrogen fertilizer on parasitoids would prevail. Coincidentally, such a magnitude of decreasing nitrogen application would maintain the current wheat yield and lessen the potential environmental risks9.
Relationship between the parasitism rate and the population growth of cereal aphids
From flowering to milking phase, the population of the cereal aphid R. padi that escaped from Aphidiinae parasitoids increased substantially in both 2017 and 2018, while the population of the cereal aphid S. avenae decreased markedly in both 2016 and 2017 (Table 1). Combining the differences between dynamics of the two cereal aphid species with the fact that the Aphidiinae parasitoids rarely parasitize R. padi in China40, it is apparent that the Aphidiinae parasitoids play a pivotal role in suppressing the cereal aphid S. avenae. Furthermore, a higher parasitism rate had a greater suppression effect on the population of the cereal aphid S. avenae, in line with previous research6,14,41.
Year-to-year fluctuation of the cereal aphids-Aphidiinae parasitoids interaction
Obvious fluctuations in the cereal aphids-Aphidiinae parasitoids interaction across years have been documented in this study. Such population fluctuations of aphids and their natural enemies are ubiquitous14,17,42. It has been assumed that a disadvantageous climate accounted for the fluctuations17. The climate changes could not have been manipulated in our study, but they play essential roles in population fluctuations43. Climate warming induced an outbreak of the cereal aphids, but the parasitism rate remained unchanged43,44. Lack of Aphidiinae parasitoids caused higher populations of the cereal aphid S. avenae in a simulated warmed wheat field. However, abundant Aphidiinae parasitoids retained effective suppression of the cereal aphids even when the wheat field was warmed45. The synchronization of parasitoids with pests is vitally important for maintaining biocontrol46, while climate change has the potential to mismatch the pests with parasitoids and cause strong population fluctuations of pests and natural enemies47.
In this study, the parasitism rate was evaluated according to the densities of discernible mummies, a conventional method widely adopted5,6,24. We keep in mind that this method neglects the fact that the symptomless aphids that have been parasitized. Consequently, the parasitism rare was underestimated and the annual fluctuations of abundance of the parasitoids and the parasitism rate were magnified, especially early in the season. Molecular detection, which has the capacity to evaluate whether symptomless aphids have been parasitized and if so by which parasitoid species, presents an exceedingly promising alternative for exploring the aphid-parasitoid interaction11,33. This burgeoning method should be employed to more accurately evaluate the aphids-parasitoids interaction. More

According to7, identifying fragile ecological areas is imperative for ecological protection and environmental organization and management. Therefore, assessing ecological vulnerability is crucial for the study of ecosystem vulnerability45. Based on the current conditions and previous predictions, the EVI was classified from the lowest vulnerability (potential) to the highest vulnerability (high), as shown in Table 4. Overall, this study obtained three main results, which are highlighted below.
The first result concerned the spatial variation in EVI. In the composite system, the EVI (EVIPCA) varied from north to south, with Littoral being a vulnerable province and Alibori being a stable province. In the additive system, EVI (EVIad), both southern and northern Benin were identified as vulnerable, especially northern Benin, and Littoral (which was identified as vulnerable by the composite system) and central Atacora (which was identified as potentially vulnerable by the composite system), respectively, were identified as vulnerable.
The second result was the calculation of the spatial autocorrelation coefficients (Moran’s I) of each EVI, which were IPCA = 0.955256 and IAD = 0.989222 for the composite and additive systems, respectively. Both of these values are very high and are better than those reported in46. Although the spatial variations in these systems were obviously different, their Moran’s I values remained very high. However, according to Moran’s I, the spatial autocorrelation of the additive system was higher than that of the composite system. The principal component analysis approach assumes no prior relationship between the different factors and allows their relationships to develop from the statistical analysis, thus indicating the regional spatial variability of the components8. The observed discrepancies in spatial variation outcomes did not mean that there was a lack of spatial organization between the components. Therefore, graphic dissimilarities (differences in spatial distributions) do not challenge the spatial layout of the components or notably, their correlations.
The third result was from the cluster analysis, showing high-high clusters in the south for the composite system and in the north for the additive system. We deduce that regardless of the system used to calculate vulnerability, ecosystems in central Benin are still relatively stable. Central Benin has a moderate population density and moderate soil organic carbon levels. Littoral has a high population density rate, while Borgou has a high soil organic carbon level. These outcomes reveal that southern Benin is seriously threatened according to the composite system and that northern Benin is seriously threatened according to the additive system. These findings were explained and discussed with reference to available studies.
We used IDW interpolation, as opposed to41, who used kriging interpolation. We note that the indicators used in that study were slightly different from those in this study and were not classified similarly; in addition, different analysis assumptions were applied. His results show a strong positive correlation between sensitivity and the additive EVI (EVIAD), which is slightly different from the results of our study. In this study, we found a moderate correlation between these two factors. This difference in the outcomes can be attributed to the difference in the indicators and their distribution in the system. Nonetheless, that study showed that additive vulnerability is primarily influenced by adaptation, exposure and sensitivity; our study led us to put these elements in the order of adaptation, sensitivity and exposure. Both studies placed adaptation in the same position. Although the considered variables were different, we reached the same conclusion regarding adaptation, which can be considered a strength of our additive system.
Densely populated areas were determined to be very vulnerable47. High sensitivity rates were detected in southern Benin, including in Littoral, Atlantique, and Oueme. Housing and density indicators were classified as sensitivity variables, which means that density is still a threat to ecosystem stability. Littoral Province, the economic capital of Benin, which has the highest population density (more than 8000 inhabitants per square kilometer, according to the averaged raw data), and Atlantique and Oueme provinces, newly developed residential areas, were classified as extremely vulnerable. Alibori Province, the largest and least populated province, was classified as the most stable area in the composite system. We can deduce from this analysis that the population density also has a great impact on the composite system. In the additive system, Littoral remained an extremely vulnerable area, and central Atacora and Collines were the most stable areas. This outcome confirms that density in Littoral is a serious challenge to stability according to both systems.
However, the composite system than the additive system is more credible since it is based on SPSS, a statistical software, and is therefore empirical. In contrast, the additive system can be unreliable, since the indicators, as a whole, are classified according to the user. This classification method is subjective, and therefore theoretical (here, we based our indicators on expert advice and IPCC recommendations); hence, it leaves room for doubt. This study found that coastal zones, i.e., Littoral, are the most vulnerable33,34,48. This finding indicates the reality for our study. The extremely vulnerable areas identified by the composite system were high per capita density areas, which emphasized that density was a decisive indicator in our composite system. This analysis uncovered significant spatial variation in population vulnerability in southern Benin. According to the raw data we collected, the average density per capita in Borgou is 35.909%, while in Littoral, it is 8003.636%, i.e., 223 times higher than that in Borgou. Borgou is made up of several communes, while Littoral consists only of Cotonou, the economic and administrative capital of Benin, which is a highly desirable area. The demand for buildings has forced people to occupy some natural drainage channels, making this commune vulnerable to flooding. Southern Benin is less spacious but has more inhabitants than northern Benin because almost the entire administrative system of the country is located there, as well as one of the largest markets in West Africa. There is a need for an efficient decentralization process according to the determined standards. Our study revealed that regions with lower density per capita were the least vulnerable.
The additive system found that the areas with high bush fires and soil organic carbon rates were the most vulnerable. Thus, vulnerability is specific to the context34, since the factors that make a region or a community vulnerable can vary among different regions and community. The vulnerability of the northern area that was highlighted by the additive system can be explained by the practice of intensive agriculture (soil organic carbon) and the bush fires involved in these practices. Northern Benin is an agricultural area, and cotton cultivation is common; hence, there are high levels of pesticide use. Agriculture is very important for the Beninese economy and hence pesticides are used. Vulnerability in southern Benin is related to climate, flooding, and the high population density, while vulnerability in northern Benin is related to bush fires and soil organic matter levels. Although the systems and indicator groupings were different, they reached the same conclusion about Littoral Province. In the additive system, the vulnerable areas corresponded to areas with high soil organic carbon.
It is important to point out that this study suffers from certain limitations38. For example, data for all the indicators from the same time period were not always available, some required data were inaccessible and some data were gathered from the public domain. This can be interpreted as a weakness of our system. Since public-domain data are not accurate, they can result in biased outputs, which should not be ignored. The determined spatial and temporal variation, as well as the type of degradation under consideration, depends on the input data sets for the analysis and modeling39. Using automatic linear modeling model building (ALMMB), our results were improved.
The main objective of automatic linear modeling model building (ALMMB) was to improve the present study outcomes by enhancing the accuracy of the established system based on the adjusted chi-square Pearson correlation. Using automatic linear modeling regression combined with the best subsets method in SPSS 23, we tried to enhance each observed vulnerability level. Table 7 displays both the observed and enhanced rates for each EVI, and Fig. 6 displays the map of the enhanced values. We note that the potentially vulnerable areas32 increase or decrease in size less than the highly vulnerable areas.
Table 7 Observed and enhanced rate for EVI.
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Figure 6

Improving composite and additive EVI map.

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Based on Table 8, in the composite system, increases in both the potentially and highly vulnerable areas were highlighted. The observed potentially vulnerable area was 48,600 km2, and the enhanced potentially vulnerable area was 60,269 km2. The observed highly vulnerable area was 3729 km2, and the enhanced highly vulnerable area was 4812 km2; the differences in these values were 11,669 km2 and 1083 km2, respectively. A decrease in the potentially vulnerable area and an increase in the highly vulnerable area were noted in the additive system. In the additive system, the observed potentially vulnerable area was 36,450 km2, and the enhanced potentially vulnerable area was 32,119 km2, for a difference of 4331 km2. The observed highly vulnerable area was 3007 km2, and the enhanced highly vulnerable area was 6977 km2, for a difference of 3970 km2, i.e., more than the double the observed value. However, according to the enhanced composite model, much attention should be paid to all southern provinces, especially Zou, Oueme and Plateau. Figure 6 displays the enhanced vulnerability mapping for a) the composite system and b) the additive system. Figure 7 summarizes the different classified areas and their differences.
Table 8 Observed and enhanced vulnerability areas. Note Classif. = classification, Dif. = difference, Qualif. = qualification, Inc. = increase and Reg. = regression.
Full size table

Figure 7

Synthesis of different classified areas.

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In summary, the composite system was vulnerable to climate and flooding (and to some extent to population density as well, as in Littoral), while the additive system was vulnerable to bush fires and soil organic matter. Littoral was identified as a vulnerable area in both systems. Finally, to improve the accuracy of our results, we used ALMMB. The results showed both increases and decreases in the size of vulnerable areas. The present study used a combination of GIS, PCA and ALMMB to accurately assess the vulnerability of terrestrial ecosystems in Benin. More

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The results of ANOVA tests were summarised in Table 1. None of the interaction effect was significant. Therefore, the main effects of disc type and working depth were presented in the following sections.
Table 1 Summary of ANOVA test results.
Full size table

Soil cutting forces
The rippled disc required an average draft force of 675 N, which was numerically the highest among the discs (Fig. 1a). The notched disc had a minimal draft force demand of 579 N. Increasing the working depth from the shallow (63.5 mm) to deep (127 mm) resulted in the draft force increasing from 291 to 965 N. This more-than-tripled increase was significant and can be explained by the soil dynamics theory that draft force varies with the contact area between soil and tool22. The rippled edge slightly increased the surface area as compared to the smooth edge, while the notched edge slightly decreased the contact area due to the notches. As for the depth effect, a deeper operation significantly increased the portion of the disc in contact with soil regardless of the disc type.
Figure 1

Soil cutting forces of different discs at different working depths: (a) draft force, (b) vertical force, and (c) lateral force; means followed by different lower case letters or upper case letters are significantly different according to Tukey’s test at the significance level of 0.05; error bars are standard deviations.

Full size image

All the vertical forces measured were positive, which indicated that they were acting on the disc in the downward direction (Fig. 1b) that favored the disc penetration into the soil. The rippled disc had the maximal vertical force of 289 N, which will help to maintain its working depth as compared to the other two discs. As was expected, the notched disc experienced the minimal vertical force of 164 N, which was lower than that of the rippled disc. The plain disc had a medium vertical force, which was not different from the other two discs. The lower vertical force of the notched disc may not necessarily affect its superior ability of soil penetration. The deep working depth created a 65.9% higher vertical force as compared to the shallow depth. The vertical forces of similar magnitudes were also observed in previous studies, such as approximately 200 N in Nalavade et al.23.
There were no significant differences among the discs in terms of the lateral force (Fig. 1c). The notched disc had the minimal lateral force of 215 N. The lateral force increased roughly twofold from 171 to 347 N as the disc was operated from the shallow to deep depths, which was significant. The insignificant difference in the lateral force among the discs was partially attributed to their identical disc angles and similar concavity. Lower lateral forces are usually desired in terms of the frame stability of the implement. The increase of the lateral force as the depth increased indicated a great deal of attention must be paid on the frame strength when designing the disc for deep tillage application.
The soil cutting forces were resultant forces of passive cutting reaction on the concave face and the scrubbing reaction on the convex face for a concave disc24. Both the cutting force and scrubbing force acted at some angle between the horizontal and vertical directions. The projected soil cutting force was against the travel direction in the horizontal direction and downward in the vertical direction; on the other hand, the projected scrubbing force is along the travel direction and upward. The resultant draft force was against the travel direction and the resultant vertical force was downward, which was the same as that of the cutting force. This agreed with the literature that the scrubbing force on the trailing convex side of the disc tends to be minor compared to the cutting force on the leading concave side of the disc22. However, the soil cutting forces were smaller than those reported in Godwin et al.25. The combination of shallow concavity and small disc angle used in this study possibly helped in reducing the soil cutting forces in all three directions. The results agreed with that in Choi and Erback20, where the notched disc had the least forces and the forces were more dependent on the working depth than the disc shape.
Soil displacements
The soil forward displacement was maximized with the rippled disc and was minimized with the notched disc (Fig. 2a). The plain disc resulted in a medium soil forward displacement of 264 mm. During the operation of the notched disc, some soil particles might not be pushed forward, but being passed over by the notches. This could explain the small soil displacements observed for the notched disc. However, statistical analysis did not show any significant differences among the three discs with regard to soil forward displacement. The soil tracers were dislodged 184 mm on average when the discs were used at the shallow depth, which was increased by 73.4% at the deep depth.
Figure 2

Soil displacements of different discs at different working depths in three directions: (a) forward, (b) lateral, and (c) upward; means followed by different lower case letters or upper case letters are significantly different according to Tukey’s test at the significance level of 0.05; error bars are standard deviations.

Full size image

The rippled disc moved the soil tracers the furthest in the lateral direction at 197 mm (Fig. 2b). The notched disc created the minimal soil lateral displacement of 109 mm, which was less than that of the other two discs. The soil lateral displacement was increased by 42.0% as the working depth changed from the shallow to deep depth. The soil lateral displacement was the average displacement of all the tracers in the lateral direction and a positive value denoted the direction pointing toward the concave face of the disc. It was worth noting that soil tracers on the convex side tended to be pushed away in the opposite direction as compared to other tracers as observed in the experiment. This was related to the scrubbing action as described above.
No significant difference was found in the soil vertical displacement among the treatments (Fig. 2c). All the soil vertical displacements were less than 20 mm with an average of 10.6 mm. Similar to the lateral displacement, not all tracers were dislodged in the same direction. However, the majority of them were in an upward direction including the average value. The small upward displacements indicated moderate soil swelling and elevating movements and minimal soil overturning effect of the discs. This was supported by the soil failure pattern study in Nalavade et al.23, which observed that the dominating compressive shear failure pattern of the free-rolling disc discouraged soil inversion actions.
Residue mixing
The rippled disc had the highest residue mixing rate of 23.1%, which was higher than that of the notched disc, being the lowest at 14.7% (Fig. 3). The residue mixing of the plain disc was medium among the three discs. As for the working depth, the shallow depth created a residue mixing of 16.7%, which was lower than that of the deep depth.
Figure 3

Residue mixing of different discs at different working depths; means followed by different lower case letters or upper case letters are significantly different according to Tukey’s test at the significance level of 0.05; error bars are standard deviations.

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The residue mixing could be used to estimate the amount of residue being incorporated into the soil, given the surface residue before tillage was 7500 kg/ha. Therefore, the rippled disc was the most effective in terms of the residue incorporation at a rate of 2746 kg/ha. The residue incorporation increased by 606 kg/ha as the working depth increased from shallow to deep. Also, deducting the residue mixing from the original residue cover of 63.1% would be the residue cover remaining. None of the treatments resulted in a residue cover less than 30%, which suggested that all treatments would satisfy the requirement of conservation tillage.
Residue cutting
The residue cutting effectiveness of the discs varied from the highest to the lowest as the rippled, notched, and plain with no significant differences were found (Fig. 4). The total residue cutting of the notched disc consisted of one-third of partially cut while no partially cut was observed for the rippled disc. As for the plain disc, roughly a quarter of the total residue cutting was partially cut. The shallow working depth had a numerically higher residue cutting rate than the deep depth: 32.8% versus 22.2%. One in every four residue tracers being cut was partially cut when the discs were operated at the shallow depth. As a comparison, less than one residue was partially cut for every ten residue tracers being cut at the deep depth. The results suggested that the most effective treatment in cutting residues was the rippled disc at the shallow depth. On average, only 27.5% of the residue tracers were being cut, either partially or completely, by the discs. Partial cuts tended to be pushed into the soil and damaged by the discs. The majority of the remaining residue was pushed aside by the discs through disturbed soil.
Figure 4

Residue cutting including completely cut and partially cut of different discs at different working depths; means followed by different lower case letters or upper case letters are significantly different according to Tukey’s test at the significance level of 0.05; error bars are standard deviations.

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The effects of disc type and working depth on the residue cutting efficiency of the discs differed from the previous studies of disc openers. For example, the plain disc was found to have a much higher residue cutting efficiency than the notched and serrated discs and the efficiency increased as the working depth increased17. The primary cause of the difference was due to the difference in residue cutting mechanism between the angled tillage discs and relatively straight disc openers. The concaved discs disturbed a fair amount of soil ahead of the disc and relied on the edge to “hook” lying residues in order to cut them. Therefore, the rippled and notched discs had numerically higher residue cutting rates than the plain disc thanks to their hooking edges. The shallower the working depth, the less the soil disturbance and the higher the residue cutting efficiency is. On the other hand, a straight disc opener would ride over all possible residues on the path and penetrate the soil without causing significant disturbance to the seedbed. The difference in residue cutting effectiveness can also be accounted for in part by the difference in residue characteristics such as type, percent cover, and moisture content. For instance, wet rice residue with a moisture content of 41.4% at 2000 kg/ha15 versus dry corn stalk with a moisture content of 4.5% at 7500 kg/ha in this study. Previous studies have shown that the cutting performance of the disc openers was significantly affected by the mechanical properties of the residue26 and residue density17.
The numerically higher portion of surface residue being cut at the shallow depth was attributed to a smaller cutting angle. This cutting angle was the angle of absolute velocity vector acting on the residue with the vertical axis in Kushwaha et al.16, whose analytical model showed that the angle of absolute velocity vector for a disc is smaller at a shallower depth. The disc tended to cut or bend the residues at a smaller cutting angle, while the disc tended to push the residue ahead at a larger cutting angle. The notched disc had the numerically highest portion of partially cut among the three discs, which agreed with the results in Bianchini and Magalhaes21. Kushwaha et al.17 also observed that residue pieces were held into the notches and serrations of the discs instead of being cut, being thrown backward as the disc exited from the soil. More