Effect of Soil Transplantation to Abandoned Paddy Field on the Conservation of Threatened Hydrophyte Species

Threatened or near threatened hydrophytes, Ottelia alismoides, Monochoria korsakowii, Najas graminea, Najas minor and Chara braunii, appeared in an inundated paddy field after the 2011 Tohoku-oki Tsunami in Japan. Due to the reconstruction of roads and agricultural restoration efforts implemented following the disaster, the top soil of the paddy field was transplanted to another abandoned paddy field in 2014 to avoid extirpation of the aforemen-tioned species. We then conducted vegetation surveys in July and September from 2014 to 2016. Monochoria korsakowii appeared at the transplantation site from 2014 to 2016, forming a large community in 2016. The volume of this species was significantly higher than that in July 2014 and 2015. Although Ottelia alismoides and Chara braunii appeared in 2014, they were not observed in 2015. Najas graminea and Najas minor were not observed during the vegetation survey, and Salvinia natans and Alisma plantago-aquatica newly appeared at the transplantation site. Our findings suggest that transplantation of surface soil and the seed bank therein to an abandoned paddy field is well suited for the conservation of hygrophytes such as Monochoria korsakowii, Ottelia alismoides and Chara braunii. Preventing disturbances that suppress the growth of herbaceous perennial plants is considered necessary for maintaining the habitats of threatened plant species.


Introduction
March 2011, a devastating earthquake and tsunami struck the northern Pacific coast of Japan, devastating the coastal areas of Iwate, Miyagi and Fukushima prefectures. Although the tsunamis have long-lasting impacts on vegetation [1], several threatened plant species appeared in many of the areas that were inundated by the 2011 Tohoku-oki Tsunami in Miyagi Prefecture, Japan [2] [3] [4]. For example, threatened or near threatened hygrophyte species, such as Ottelia alismoides, Monochoria korsakowii, Najas graminea, Najas minor and Chara braunii appeared along coastal areas of Miyagi Prefecture in August 2013 [5]. Of these species, Monochoria korsakowii was widely distributed in swampy paddy fields from Iwate to Miyagi prefectures after the tsunami [6] [7] [8]. This increase in the distribution of these species was promoted by ground subsidence, which inundated paddy fields, and subsequent disturbance of surface soils containing buried seeds, which enhanced the germination of hygrophytes and promoted the establishment of hygrophyte communities after the tsunami. Many of these hygrophyte species are classified as near threatened or vulnerable in the Red Data Book of Plants published by the Ministry of Environment, Japan [9]. However, the areas in which these hygrophytes became newly established were redeveloped when the roads and paddy fields in the affected areas were repaired and reclaimed, respectively. Soil including buried seeds contribute greatly to vegetation restoration [10] [11]. Transplantation of surface soil containing seeds to wetlands is considered to be effective for the conservation and reestablishment of hygrophyte communities [12] [13]. We therefore transplanted surface soil from these hygrophyte communities to an abandoned paddy field to conserve these threatened and near threatened plant species. Although previous studies have investigated plant succession in original habitat after tsunami [1] [14] [15], plant succession in abandoned paddy fields areas using soil transplanted from original habitat has not yet been clarified. This study therefore provides information on how threatened and near threatened species conservation can be effectively combined with using abandoned paddy fields following a natural disaster, like a tsunami.
We hypothesized that the transplantation of donor soil from original habitat to an abandoned paddy field can be effective for the conservation of threatened or near threatened hygrophyte species. The present study aims to investigate whether threatened or near threatened species emerge and grow in abandoned paddy fields, and whether methods could be developed for managing abandoned paddy fields to conserve hygrophyte species.

Vegetation Surveys
The surface soil at the soil transplant site was tilled vigorously using an agricultural tractor on April 2016. The tilling was conducted on the entire aban-

Data Analysis
To compare differences between years, a one-way ANOVA was performed to compare the volume of threatened or near threatened species, medium and tall herbaceous plants such as Phragmites australis, Typha spp., Bidens frondosa and Echinochloa spp., followed by multiple comparison tests (Tukey HSD post hoc test). We verified statistically whether period and tillage affect the succession and plant volume of threatened or near-threatened plant species and medium and tall herbaceous plants. All statistical analyses were performed using the R software package [20].

The Emergence of Threatened and Near Threatened Plant Species
Monochoria korsakowii, Ottelia alismoides and Chara braunii all emerged at the soil transplant site. However, Najas graminea and Najas minor were not observed at the soil transplant site or in the control plots during 2014-2016 (Table   1). Although Monochoria korsakowii and Chara braunii were observed in one of the control plots in 2014, we considered that these plants were derived from the donor soil. Although Salvinia natans and Alisma plantago-aquatica were not observed at the donor soil site, these species were observed at the soil transplant site.

Plant Volumes of Threatened, Near-Threatened and Tall Herbaceous Perennial Herbs
The

Water Depth and Water Quality at the STUDY Site
The water depth in July 2016 (3.08 ± 0.32) was significantly higher than that in

Discussion
Our results showed that the recovery of species such Ottelia alismoides, Monochoria korsakowii and Chara braunii can be achieved by transplanting donor soil to an abandoned paddy field. In addition, other threatened herb species, such as Salvinia natans and Alisma plantago-aquatica also appeared in the abandoned paddy field. Intense physical disturbance of sediments, such as by tilling with an agricultural tractor, appears to play a role in determining the emergence and growth of Ottelia alismoides, Monochoria korsakowii and Chara braunii because initial germination and subsequent growth of annual herbs are the primary driving forces underlying vegetation recovery. To promote the growth of annual herb species, site maintenance is required in order to suppress the germination and growth of herbaceous perennial herbs, such as Phragmites australis and Typha spp. Monochoria korsakowii is an emergent, summer annual, aquatic plant that occurs in pools, ditches, canals and rice fields in East Asia [21] [22]. Continuous growth in this species is promoted by tilling in original habitat [23] [24] [25] [26] [27]. For example, disturbance with bulldozers promotes germination and growth of Monochoria korsakowii and other threatened species in wetlands [28]. Our results suggest that intense disturbance caused by an agricultural tractor (depth: 20 cm) promoted the growth of Monochoria korsakowi in the abandoned paddy field. Consequently, the volume of Monochoria korsakowii was five-fold that observed in 2014 and 2015 ( Figure 2). Conversely, perennial herb species, such as Phragmites australis and Typha spp. decreased significantly in 2016 compared to 2014 and 2015. These results are consistent with a previous study by [17], who reported that tilling in an abandoned paddy field promoted the growth of Monochoria korsakowii but suppressed the growth of Phragmites australis and Typha spp.
The water temperature at the soil transplant site was suitable for Monochoria korsakowii germination and growth. The germination rate of this species has been reported to increase at temperatures of 15˚C to 29˚C (Wan et al. 2004). In the present study, the water temperature at the soil transplant site from June to September was 20.4˚C to 25.1˚C (Figure 4). The reason for the water temperature being maintained at 20.4˚C -25.1˚C might be due to the springs from the secondary forest surrounding the site and groundwater, as the site is in a valley bottom. The germination rate of Monochoria korsakowii has been shown to be 100% at water depths from 3 to 5 cm [24]. The water depth at the soil transplant site was approximately 1 cm in July 2015. Although the plant volume of Monochoria korsakowii was small at lower water depths, the population can expand when water depth exceeds 5 cm. Ottelia alismoides and Chara braunii were ob- alismoides needs sufficient light in order to germinate (May to July), which is why the growth of tall herbaceous plants needs to be managed [31].
Oospores of Charales are abundant in the soil of submerged plants area [32] [33] [34]. Transplantation of soil oospore banks into other areas has been shown to be effective for regenerating stands of the endangered Chara braunii [35]. In the present study, transplantation of donor soil was shown to be effective for the conservation of Chara braunii in abandoned paddy fields. Charales plantas colonizes shallow to deep water and needs abundant sunlight and clear water in order to thrive [36]. The reasons why Chara braunii was not widespread in 2015 may have been because the tall herbaceous plants obstructed the sunlight and covered the available soil surface, preventing the germination of Chara braunii oospores. Therefore, continuous soil disturbance by agricultural tractors is considered essential for the maintenance of Ottelia alismoides and Chara braunii populations. [37] reported that the optimal water temperature for anthesis and growth in Ottelia alismoides was 20˚C -27˚C in a lotus paddy field. The water temperature at our study site from August to September was 20.4˚C to 25.3˚C, which is suitable for anthesis and growth in Ottelia alismoides. Seeds of Alisma plantago-aquatica germinate and grow at water depths below 3 cm [38]. At our study site, Alisma plantago-aquatica was observed to have emerged in July at a water depth of 1.08 cm in 2015, 1.33 cm in 2014 and 3.80 cm in 2016 ( Figure 3). Thus, the water depth at the soil transplant site was considered to be suitable for the germination and growth of Alisma plantago-aquatica.
Najas graminea and Najas minor were not found at the soil transplant site during 2014-2016. [39] reported that Najas graminea is sensitive to some herbicides and is therefore found frequently in paddy fields free of such herbicides.
The field used for transplantation had been abandoned for 3 years (i.e. since 2011), and the EC was low. Therefore, Najas graminea growth is not directly affected by water quality. Water depth at sites colonized by Najas graminea and Najas minor at the donor soil site were approximately 25 cm. It is thus possible that Najas graminea and Najas minor may prefer deeper water than the 5 cm at the study site.

Conclusions
Our results describe how transplanting a donor soil can be applied to the recovery of Ottelia alismoides, Monochoria korsakowii and Chara braunii communities in an abandoned paddy field. Maintenance to regulate tall herbaceous perennial plants, such as Phragmites australis and Typha spp. is necessary to promote the germination and growth of threatened annual plant species. In the present study, the maintenance work was performed by hand in 2014 and 2015, and a tractor in 2016. The intensity of the disturbance achieved by a machine is sufficient to suppress the growth of Phragmites australis and Typha spp. Sustained and intense disturbance by tilling has been shown to be effective for sup- The water temperature at the site remained at 20.4˚C to 25.1˚C due to the study site being located in a swampy paddy field. It is considered desirable to use swampy or continuously inundated abandoned paddy fields for the transplantation of donor soil. Furthermore, the depth of the water in field should be increased to 25 cm to encourage colonization (and transplantation) by Najas graminea and Najas minor. Long-term field monitoring is needed to elucidate the frequency, intensity and period of suitable disturbance and water depth that is required in order to maintain threatened or near threatened hygrophytes.