Mitigation of nitrous oxide emissions from agricultural soils by fungivorous mesofauna

概要

Chapter 1: General Introduction
Nitrous oxide (N2O) is an important long-lived greenhouse gas and ozone-depleting substance. Agricultural soils are the largest anthropogenic sources of global N2O emissions. Mitigation of N2O emissions from agricultural soils is significant for the alleviation of global warming. It is already known that N2O emissions from soils are primarily resulted from microbial processes including denitrification and nitrification of soil bacteria, archaea, and fungi. However, it is still difficult to control these processes and mitigate soil N2O emissions at a field scale. Soil mesofauna are soil invertebrates between 0.1 mm and 2 mm in size. They interact with soil microbes in various ways. A lot of mesofauna, such as mites and springtails, are microbial grazers and can directly affect the abundance of soil microbes by their feeding behaviors. In this study, I find for the first time that fungivorous mites, a major group of mesofauna in agricultural soils, can feed on a wide range of N2O-producing fungi, can suppress the growth of N2O- producing fungi, and can be used to mitigate N2O emissions from agricultural soils.

Chapter 2: A decreased soil N2O emissions in a corn-field after applying coconut husk to the soil
In a field experiment performed in the spring of 2016, N2O emissions from a sweet corn field were dramatically decreased after coconut husk (45 g/ m2, from 0 ~ 10 cm layer of the soil) application to the soil. The coconut husk used is chips with a few centimeters in size and is made from the dried husk of coconut fruit. The application of coconut husk almost halved the accumulative N2O emissions during the whole cultivation period (Fig. 1) without impacting the yield of sweet corns (Fig. 2). Under the stereomicroscope, it was observed that coconut husk recovered from soil contained a large amount of oribatid mites, in brown color and of around 0.4 mm diameter. Since soil oribatid mites were wildly known predators of soil fungi, and soil fungi were important producers of N2O, a hypothesis was proposed that the coconut husk application increased the abundance of oribatid mites, then the increased mites consumed more soil fungi including N2O-producing fungi, and the decreased abundance of N2O-producing fungi produced less N2O and led to the decrease in the N2O emission (Fig. 3).

Chapter 3: Linking the soil N2O emission with soil oribatid mite abundance
To verify the hypothesis, miticide was employed in the field experiment in the following autumn. The miticide was used for offsetting the possible enrichment effect of coconut husk on oribatid mites. As a result, coconut husk application increased the mite abundances and decreased the N2O emissions (Fig. 4, Fig. 5). In contrast, when coconut husk was applied together with the miticide, the mite abundances were not increased and the N2O emissions were not significantly decreased compared to the control (Fig. 4). These results strongly supported that, it was the increased mite abundance by the coconut husk application that resulted in the decreased N2O emissions If the increased mite abundance truly caused the mitigated N2O emissions, it would be conceivable that applying mites directly to the soil should also mitigate N2O emissions. Accordingly, a laboratory-scale soil microcosm experiment was performed to make a further confirmation. In the microcosm experiment, an application of ca. 300 mite individuals to 200 g of soil dramatically decreased the N2O emissions (Fig. 6). This result straightly confirmed the mitigation effect of mites on N2O emissions. More interestingly, the addition of coconut husk to the mite-applied soils increased the abundance of mites (Fig. 7) and further decreased the N2O emissions (Fig.6), indicating coconut husk application enhanced the mitigation effect of mites on N2O emissions by increasing their abundances. Since the soil microcosm was a relative closed system and there was only one time of mite introduction to the system, a higher mite abundance observed after incubation should indicate a higher mite reproductivities, suggesting coconut husk could not only attract mites but also enhance their reproduction.

Chapter 4: Feeding behaviors of oribatid mites against N2O-producing fungi
The next question was how mites mitigated soil N2O emissions. The hypothesis was mites mitigate soil N2O emissions by feeding on N2O-producing fungi and decreasing their abundance in soil. To verify if the mites in this study have abilities to feed on N2O-producing fungi, mites were placed on agar plates cultured with isolates of N2O-producing fungi. Under the stereomicroscopic observation, mites showed feeding behaviors against various genus of N2O- producing fungi. An example was showed in Fig. 8.

Chapter 5: Soil N2O emissions were mitigated by fungal feeding behaviors of oribatid mites
Another soil microcosm experiment was performed to verify if the abundance of N2O-producing fungi was decreased by mite application to the soil. The abundance of N2O-producing fungi was estimated by q-PCR targeting nirK genes of fungi. In the microcosm experiment, both the N2O emissions and the copy numbers of fungal nirK genes were significantly decreased (Fig. 9) by mite application to the soil. Combining the confirmation of mites’ feeding behaviors in chapter 4, these results strongly supported that oribatid mites could mitigate N2O emissions by their feeding behaviors against N2O- producing fungi. Furthermore, in situ evidences were found to support mites’ feeding behaviors against N2O- producing fungi by investigating the fungal community compositions in mite bodies (Fig. 10). Five genera of fungi including Trichocladium, Fusarium, Pseudaleuria, Actinomucor, and Mortierella were detected both from the DNA extracted from mite bodies and the soils. Four of them were reported to contain N2O- producing fungal species, indicating soil N2O-producing fungi belonging to these genera could be on the food list of mites. However, fungal feeding behaviors of the oribatid mites in this study could be unspecific but board-spectrum since the fungal community compositions in soils were not significantly affected by mite application (Fig. 10). This could be an advantage for the application of this method, since less disturbance of the microbial community compositions suggested there might be less risks to disturb the original function of the soil.

Chapter 6: Potential to mitigate soil N2O emissions from urea fertilized soils by fungivorous mesofauna
Urea is one of world’s most widely used fertilizer. It is worthwhile to test the efficacy of the N2O mitigation method developed in this study. In the field experiment, coconut husk was applied to a urea fertilized field. In this field experiment, the variation of the data was large and the difference between N2O emissions from coconut husk applied plots and control were not statistically significant. However, regardless of the abnormal value from one of the replicates, the N2O emissions from urea fertilized soils were decreased from 16% to 63% by the coconut husk application. Therefore, coconut husk might be also effective on the mitigation of N2O emissions from soils applied with urea. To confirm this, more experiments should be performed in the future.

Chapter 7: General Overview
In this study, N2O soil emission was linked with soil fungivorous mite abundance based on the predator-prey interactions between soil fungivorous mites and N2O-producing fungi. This finding revealed the potential to mitigate the N2O emission by controlling the mite abundance. Moreover, an easy method, applying coconut husk to the soil, was developed to increase the mite abundance in agricultural field. This made it practical to use mites’ feeding behaviors to mitigate soil N2O emissions at a field scale. This study suggests a possibility to use ecological methods to control microbial processes that produce N2O and provides a new option for the mitigation of N2O emissions from agricultural soils.

この論文で使われている画像