Dispersion has a large effect (Cohen's d) on crop yield in crop residue application [version 1; peer review: awaiting peer review]

Background: Crop residue application can maintain soil fertility and sustain agriculture. However, the effects of residue application are unstable because of variable weather conditions and the residual effects of crop residue application. Residue application often reduces crop yields. I tried to clarify effective residue application factors in an environment which was has stable weather conditions and low residual effects. Methods: Majuro atoll, a coral sand atoll near the equator, was selected for the experiment site because of its stable weather and low residual effect of coral sand. A factorial design experiment using sweet corn was conducted based on the following four factors: fungi propagation before application, cutting residue into pieces, dispersion (or accumulation) of applied residue, and placement (on the surface or incorporation) with an equal amount of crop residue. The effects of each factors on the corn yields were evaluated using Cohen’s power analysis. Results: The dispersion showed the largest effect (p = 0.045, Cohen’s d = 1.2), which exceeded the effect of incorporation (p = 0.223, Cohen’s d = 0.7). The interaction of dispersion and incorporation showed a huge effect on corn yield (p = 0.005, Cohen’s d = 4.9). Discussion: The effect of dispersion was not positive but it avoided the negative effects of residue clustering. The toxicity of the plant residue and generation of toxic substances by anaerobic decomposition are widely known. Anaerobic decomposition occurs inside the residue clusters. However, dispersion reduced the toxicity by adsorption in soil and avoiding anaerobic decomposition. Furthermore, incorporation showed an interaction effect, but surface placement did not. Conclusion: The dispersion of crop residue enhanced the positive effect of crop residue incorporation by avoiding the toxicity from crop residue. This finding adds a new viewpoint for the controversy between conventional and conservation agriculture. Open Peer Review Reviewer Status AWAITING PEER REVIEW Any reports and responses or comments on the article can be found at the end of the article. Page 1 of 8 F1000Research 2018, 7:1831 Last updated: 04 OCT 2021


Introduction
Soil degradation is a major constraint on food security (Gomiero, 2016;Lal, 2015), and the intervention points for reversing soil degradation are soil erosion and depletion of soil organic matter (Karlen & Rice, 2015). Conservation agriculture or green manure approaches are developing against this backdrop and emphasize the importance of retaining crop residue (Cherr et al., 2006;Pittelkow et al., 2015a); however, they have not yielded satisfactory results. For example, long-term trials of conservation agriculture show no particular interactions among factors such as tillage, mulch, rotation, soil texture and rainfall (Pittelkow et al., 2015b).
The main problem with crop residue application is that the decomposition of the residue can have both positive and negative effects on crop production (Kumar & Goh, 1999). Nitrogen benefits and nitrogen recovery from residues show that they have considerable potential (Kumar & Goh, 1999). However, microorganisms produce toxic substances during the decomposition of plant residue. Microorganisms metabolize these substances in aerobic soils. Nonetheless, the phytotoxic leachates of some decomposing cover crop residues have adverse effects on crops even under aerobic conditions (Bhowmik & Doll, 1982;Jessop & Stewart, 1983;Kimber, 1973a;Kimber, 1973b;Lynch, 1977;Lynch, 1978;Stirzaker & Bunn, 1996).
The application method of crop residue is a major factor in the success of the method, although the effects of organic matter application also differ by the quantity, conditions and timing (Chen et al., 2014;Tian et al., 2007). Both particle size and placement of the applied material affect the residue breakdown rate and the mineralization/immobilization process (Kumar & Goh, 1999). However, environmental factors have strong interactions with residue decomposition (Kumar & Goh, 1999). Finding effective crop residue application methods is difficult without stable weather and with the inability to control for no residual effects of organic matter, even with a focus on the application method.
The aim of this study was to clarify the most effective crop residue application method. Although, there are many factors concerning residue management (Kumar & Goh, 1999), I examined four factors (fungi propagation, cutting, dispersion and incorporation) from the viewpoint of farmers applicability.

Site description and conditions
Majuro Atoll, the capital of the Republic of the Marshall Islands, is located in the Pacific Ocean near the equator. The maximum and minimum monthly average temperatures are 29.4 to 30.2°C and 24.7 to 25.0°C, respectively. The average monthly precipitation is 169 to 356 mm, and the average annual precipitation is 3, 365 mm (CLIMATE-DATA.ORG). Coral sand, the soil of Majuro atoll, has relatively high organic matter levels in top soil due to refractory organic matter (organic carbon of 46.9 g kg −1 at 0-15 cm and 10.8 g kg −1 at 15-45 cm) (Deenik & Yost, 2006), and high percolation rates (1.4-3.5 × 10 −3 m s −1 ) (Hunt & Peterson, 1980). Thus, the climate is stable, the soil has high permeability with no waterlogging, and frequent rainfall keeps the soil moisture close to field capacity, so applied organic matter is almost decomposed in the crop period. It is known that the residual effect of organic resources is small for sandy soils in a warm and humid climate (Chivenge et al., 2011).
The experimental field was located at Laura Farm (7°8"34"N, 171°2"9"E), which belongs to the Ministry of Resources and Development. The field size was 12.5 × 7.2 m. The plot size was 1.2 × 6.0 m, and the experimental field consisted of two sets of six plots. The government forbids the use of synthetic fertilizer and chemicals on this atoll in order to protect the underground aquifer; therefore, farmers use copra cake as fertilizer. Soil water-soluble NO 3 -N (0-5 cm layer; 1:2.5 soil:water extraction) was 4 µg g soil −1 , as assessed by a nitrate ion meter (LAQUAtwin B-742, Horiba, Tokyo). No significant residual effect has been found in this field, although corn had been cropped three crops without fertilizer nor any chemicals from November 26, 2013 to July 28, 2014 (see raw data (Oda, 2018)).

Materials and crop management
Corn residue to be used as the material for the experiment was harvested on July 28, 2014, just after the harvest of the previous crop of corn in the experimental field. The residue was divided into 12 bundles of the same fresh weights of 4.7 Mg ha −1 (2.1 Mg ha −1 in dry weight) and applied to the plots by different methods (described later). Corn (Zea mays L.) was planted in two rows at 0.5 m row spacing at a population of 3.3 plants m −2 on August 4, 2014, and harvested on October 21, 2014 (78 days after seeding). Irrigation, fertilizer and chemical applications were not performed. Hand weeding was performed 2 and 5 weeks after seeding and the weed residue was left on the soil surface of the plot.

Treatments
An L 12 orthogonal array design was used in the experiment (Taguchi, 1986). This design was used to ensure robust experimental results under a wide range of conditions. Only the main effects were seen in the experiments, and all the factor levels had six replications. The 12 experiments consisted of four factors (fungi propagation, cutting, dispersion and incorporation) that were randomly assigned to the experimental plots (Table 1). The four different treatments were established as follows. In the fungi propagation factor, the residue was stored in a compost house for 7 days until propagating fungi, and the remainder of the residue stored beside the field. For the cutting factor, the residue was cut into 10 cm pieces. For the dispersion factor, the residue was scattered evenly on the plots, and for the non-dispersion, the residue was placed on the center line of the plot in a swath approximately 30 cm wide. For the incorporation factor, I removed approximately 5 cm thickness of soil of the plots, then placed the residue, and covered the residue with the soil (Figure 1).

Validation of the linearity of input and output
Strong linearity must be found between returned residue quantity and the yield if the adopted factors have a robust performance (Taguchi, 1986). For validation purposes, I returned the crop residues into the same plot using the combination of the two most effective factors (dispersion and incorporation). The plot effect can confound the yields in this plot design since the quantity (0.4-2.0 Mg ha −1 in dry weight) of applied residues was Table 1. Experimental matrix for L 12 orthogonal array. The 12 experiments consisted of four factors (fungi propagation, cutting, dispersion and incorporation), which were randomly assigned to the experimental plots.

No. Fungi propagation
Cutting Dispersion Incorporation proportional to the preceding yield in each plot. However, this conflict was considered to be small because the plot effect was very small in this field (see raw data (Oda, 2018)) and the residual effect of corn was small. I seeded corn on October 28, 2014 and harvested it on January 9, 2015 (73 days after seeding).
Determination of yields and statistical analyses I got precipitation data from the automatic weather station at the Laura Farm. I weighed the fresh weight of the whole crop and the kernels in each plot and checked the kernel/whole ratio was stable (R 2 = 0.9983). The aboveground dry matter (DM) was calculated by multiplying the fresh weight to an aboveground DM/fresh-kernel-weight ratio (2.79) of an existing study (Miura & Watanabe, 2002 The p values were calculated with an unpaired, one-sided, unequal variances t-test. I evaluated the linearity of the input and the output using the coefficient of determination (R 2 ) in a simple linear regression.

Climate conditions
There were no irregular climate conditions during the experimental periods. The number of precipitation days for each 78-day and 73-day crop period were 58 and 54 days, respectively, and total precipitation was 1111 and 542 mm, respectively. were medium and small in effect size, respectively (Table 3).

Linearity of input and output
Under the combination of dispersion and incorporation, input (the quantity of applied residue) was proportional to the output (harvested aboveground biomass) (Figure 2; R 2 = 0.8963).

Discussion
Dispersion had a very large effect on corn yield ( Table 2). The yield was influenced by the huge effect of the interaction of dispersion and incorporation (Table 3). The dispersion of crop residue application having a large effect on yield is probably a new finding; however, the main effects of dispersion or incorporation alone were not significant. This indicates that the effect of dispersion appears when it is combined with incorporation.
A positive effect for dispersion is surprising, given that dispersion typically increases nutrient loss. Intuitively, application of crop residues away from plants should be less effective. For example, farmers often incorporate compost as a cluster of organic material (Stone et al., 2000). The results of this study, however, show that clustering is not more effective than dispersion for crop residue. It will be reasonable if dispersion avoids the negative effect of residue clustering. This is because crop residue retained at the surface or incorporated into the soil may produce phytotoxic allelochemicals (Chung & Miller, 1995;Weston, 1996).
There are two aspects of avoiding phytotoxicity in dispersion. The first is aeration and the second is adsorption. Martin et al. (1990) reported that corn stover is itself phytotoxic, and the   decomposition process creates phytotoxicity due to substances produced by microorganisms. The combination of the toxicity produced by the decomposition of residue by bacteria, fungi and extract of residue results in different levels of toxicity. Anaerobic fermentation yields products that inhibit plant growth; however, increasing aeration reduces the phytotoxic effect of crop residues (Lynch et al., 1981). Aulakh et al. (1991) reported that there are no significant differences in corn decomposition speeds between incorporation and soil surface placement under conditions of adequate soil moisture. My experiment replicated the conditions and showed no significant difference in incorporation. This means that there was no difference in the aeration between the residue put on the ground and that in the coral sand. However, the inside of residue clusters must be in an anaerobic condition both on the surface and under the coral sand. For the aspect of adsorption, dispersion increases the adsorption of phytotoxic substances in soil. Adsorption is the most important soil factor controlling the fate of chemicals in the environment because it controls the chemical concentrations present in the soil solution, which is a physiological phenomenon that follows the Freundlich equation (Vidal & Bauman, 1997). This means that dispersion has a stable effect on avoiding phytotoxicity. We should pay closer attention to the dispersion of residue incorporation because tillage merely buries the residue (Staricka et al., 1991).
The positive effect of incorporating crop residue on yield is because of the nutrient addition (Kumar & Goh, 1999), especially given that this experiment was conducted without fertilizer addition. Matching the amount and timing of nitrogen release with crop nitrogen demand is a key component of crop production (Robertson, 1997). Many studies have indicated that burying residues in soils increases the decomposition rate compared with placing residues on the soil surface (Kumar & Goh, 1999). The combination of dispersion and incorporation brings a proportional yield increase to applied residue under non-fertilized conditions. In this case, the yield will reflect the nutrient applied by the residue. Furthermore, incorporation had an interactive effect, but surface placement did not. Residue incorporation, rather than surface placement, may enhance biological nitrogen fixation by reducing the inorganic N in soil (Kumar & Goh, 1999); however, further study is needed in this area.

Conclusion
A stable climate with a low residual effect of soil can reveal a stable result for crop residue management. Cohen's power analysis showed the dispersion of crop residue had a very large effect on corn yield, and the effect arose from the interaction with incorporation. Dispersion will avoid phytotoxicity from anaerobic conditions inside residue clusters and clarify the positive effect of crop residue incorporation. This finding adding a new viewpoint for the controversy between conventional and conservation agriculture.

Ethics statement
This study was conducted with the permissions from the local government.
Data are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).

Grant information
The author declared that no grants were involved in supporting this work.