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Paddy fields play very important environmental roles in food security, water resource management, biodiversity conservation, and climate change. Therefore, reliable broad-scale paddy field maps are essential for understanding these issues related to rice and paddy fields. Here, we propose a novel paddy field mapping method that uses Sentinel-1 synthetic aperture radar (SAR) time series that are robust for cloud cover, supplemented by Sentinel-2 optical images that are more reliable than SAR data for extracting irrigated paddy fields. Paddy fields were provisionally specified by using the Sentinel-1 SAR data and a conventional decision tree method. Then, an additional mask using water and vegetation indexes based on Sentinel-2 optical images was overlaid to remove non-paddy field areas. We used the proposed method to develop a paddy field map for Japan in 2018 with a 30 m spatial resolution. The producer’s accuracy of this map (92.4%) for non-paddy reference agricultural fields was much higher than that of a map developed by the conventional method (57.0%) using only Sentinel-1 data. Our proposed method also reproduced paddy field areas at the prefecture scale better than existing paddy field maps developed by a remote sensing approach.

Rice straw and stubble burning is widely practiced to clear fields for new crops. However, questions remain about the effects of fire on soil bacterial communities and soil properties in paddy fields. Here, five adjacent farmed fields were investigated in central Thailand to assess changes in soil bacterial communities and soil properties after burning. Samples of soil prior to burning, immediately after burning, and 1 year after burning were obtained from depths of 0 to 5 cm. The results showed that the pH, electrical conductivity, NH4-N, total nitrogen, and soil nutrients (available P, K, Ca, and Mg) significantly increased immediately after burning due to an increased ash content in the soil, whereas NO3-N decreased significantly. However, these values returned to the initial values. Chloroflexi were the dominant bacteria, followed by Actinobacteria and Proteobacteria. At 1 year after burning, Chloroflexi abundance decreased remarkably, whereas Actinobacteria, Proteobacteria, Verrucomicrobia, and Gemmatimonadetes abundances significantly increased. Bacillus, HSB OF53-F07, Conexibacter, and Acidothermus abundances increased immediately after burning, but were lower 1 year after burning. These bacteria may be highly resistant to heat, but grow slowly. Anaeromyxobacter and Candidatus Udaeobacter dominated 1 year after burning, most likely because of their rapid growth and the fact that they occupy areas with increased soil nutrient levels after fires. Amidase, cellulase, and chitinase levels increased with increased organic matter levels, whereas β-glucosidase, chitinase, and urease levels positively correlated with the soil total nitrogen level. Although clay and soil moisture strongly correlated with the soil bacterial community’s composition, negative correlations were found for β-glucosidase, chitinase, and urease. In this study, rice straw and standing stubble were burnt under high soil moisture and within a very short time, suggesting that the fire was not severe enough to raise the soil temperature and change the soil microbial community immediately after burning. However, changes in soil properties due to ash significantly increased the diversity indices, which was noticeable 1 year after burning.

Aim There is an ongoing discussion of potential effect of controlled-release fertilizer (CRF) on nitrous oxide (N₂O) emission from paddy fields. Method A four-year field experiment was launched in a major rice cultivation region of China in 2008 and completed in 2011 to explore effects of CRF on N₂O emissions from paddy fields. The experimental field followed a water regime pattern typical of China, that is, flooding, mid-season aeration (MSA), reflooding and drying-wetting alternation in sequence. Three treatments, i.e. Treatment CK (no N-fertilizer applied), Treatment U (urea applied), and Treatment C (CRF applied), were laid out in a randomized block design. Results Over the 4 years, the estimated average of N₂O emissions during the rice seasons were 19.5±5.5, 81.4±17.8 and 69.6±18.7 mg N m⁻² in Treatments CK, U and C, respectively. On average, CRF reduced N₂O emissions by 13 % and grain yields by 5 %, too, when compared to urea. With normal MSA, Treatment C was 50 %, 11 %, and 25 % lower in N₂O emission than Treatment U in 2009, 2010 and 2011 N ( N normal aeration) (p<0.05), respectively, while the former was 31 % higher (p<0.05) in 2008 and 7 % lower (p>0.05) in 2011 D ( D delayed aeration) than the latter with delayed MSA. Conclusions CRF inhibited not only the N₂O concentration trapped in the soil pore-water but also the production of N₂O during the MSA period. Timing of MSA affected the efficiency of CRF mitigating N₂O from the paddy field. The findings suggest that MSA starting around D30 (30 days after rice transplanting), the way the local farmers do, would optimize the effect of CRF mitigating N₂O from rice fields in China. However, optimization of rice yield using CRF requires further research.

Unmanned aerial vehicle (UAV) photogrammetry was used to monitor crop height in a flooded paddy field. Three multi-rotor UAVs were utilized to conduct flight missions in order to capture RGB (RedGreenBlue) and multispectral images, and these images were analyzed using several different models to provide the best results. Two image sets taken by two UAVs, mounted with RGB cameras of the same resolution and Global Navigation Satellite System (GNSS) receivers of different accuracies, were applied to perform photogrammetry. Two methods were then proposed for creating crop height models (CHMs), one of which was denoted as the M1 method and was based on the Digital Surface Point Cloud (DSPC) and the Digital Terrain Point Cloud (DSPT). The other was denoted as the M2 method and was based on the DSPC and a bathymetric sensor. An image set taken by another UAV mounted with a multispectral camera was used for multispectral-based photogrammetry. A Normal Differential Vegetation Index (NDVI) and a Vegetation Fraction (VF) were then extracted. A new method based on multiple linear regression (MLR) combining the NDVI, the VF, and a Soil Plant Analysis Development (SPAD) value for estimating the measured height (MH) of rice was then proposed and denoted as the M3 method. The results show that the M1 method, the UAV with a GNSS receiver with a higher accuracy, obtained more reliable estimations, while the M2 method, the UAV with a GNSS receiver of moderate accuracy, was actually slightly better. The effect on the performance of CHMs created by the M1 and M2 methods is more negligible in different plots with different treatments; however, remarkably, the more uniform the distribution of vegetation over the water surface, the better the performance. The M3 method, which was created using only a SPAD value and a canopy NDVI value, showed the highest coefficient of determination (R2) for overall MH estimation, 0.838, compared with other combinations.

As a step toward conserving rural ecosystems, evaluation of the distribution of frogs is essential. Frogs live in paddy fields and that is why its management aspects affect their distribution. Often while determining the distribution of frogs on a large scale, a method which distinguishes their calls is used. Their distribution can also be determined based on geographical aspects such as surrounding land use and distance from rivers, as well as paddy field management aspects such as the timing of rice planting. Since paddy field management methods vary from farmer to farmer, the distribution of frogs should be surveyed for each paddy field. However, the frog calls used in large-scale surveys generally confuse the frogs in multiple rice paddies. Thus, we examined the necessity of surveying individual paddy fields for a large-scale frog distribution research. Paddy fields in the three districts of Kameoka City, Kyoto Prefecture, were used as survey sites and the presence or absence of frogs was recorded. Findings show that although the three districts are geographically close in proximity, significant differences are observed in the occurrence rates of Pelophylax porosus brevipodus and Glandirana rugosa. Spatial autocorrelation of distribution varies for different species, with some being spatially clustered, such as Pelophylax nigromaculatus, Pelophylax porosus brevipodus, and Hyla (Dryophytes) japonica, and others being spatially random, such as the Glandirana rugosa and Fejervarya kawamurai. The differences in spatial autocorrelation among species and districts suggest that surveying each paddy field on a large scale is important when considering the conservation of frogs.

Terracing is the oldest technique for water and soil conservation on natural hilly slopes. In Northern Thailand, terraced paddy fields were constructed long ago, but scientific questions remain on how terraced paddy fields and upland rice (non-terraced) differ for soil organic carbon (SOC) stocks, soil nutrients and soil erodibility. Therefore, this study aims to evaluate and compare SOC stocks, soil nutrients and soil erodibility between terraced paddy fields and upland rice at Ban Pa Bong Piang, Chiang Mai Province, Thailand. Topsoil (0–10 cm) was collected from terraced paddies and upland rice fields after harvest. Results showed that SOC stocks were 21.84 and 21.61 Mg·C·ha−1 in terraced paddy and upland rice fields, respectively. There was no significant difference in soil erodibility between terraced paddies (range 0.2261–0.2893 t·h·MJ−1·mm−1) and upland rice (range 0.2238–0.2681 t·h·MJ−1·mm−1). Most soil nutrients (NH4-N, NO3-N, available K, available Ca and available Mg) in the terraced paddy field were lower than those in the upland rice field. It was hypothesized that the continuous water flows from plot-to-plot until lowermost plot caused dissolved nutrients to be washed and removed from the flat surface, leading to a short period for accumulating nutrients into the soil. An increase in soil erodibility was associated with decreasing SOC stock at lower toposequence points. This study suggested that increasing SOC stock is the best strategy to minimize soil erodibility of both cropping systems, while proper water management is crucial for maintaining soil nutrients in the terraced paddy field.

In the past decades, the application of organ fertilizer in agricultural soils has attracted wide attention. However, few studies have carefully explored the effects of humic acid organic fertilizer on soil microbial colonies, soil enzyme activities, and soil fertility. To provide a better growing environment for crops, we explore the best regulation mode of humic acid organic fertilizer in the farmland in the Songnen Plain Heilongjiang province. Through field experiment, we selected paddy as the test objective and applied humic acid organic fertilizer. Under the condition of water-saving irrigation, five fertilization levels were set up, which were NPK (local nitrogen level, 110 kg.hm-2 pure nitrogen), NPKH1 (450 kg.hm-2 humic acid organic fertilizer + 77 kg.hm-2 nitrogen), NPKH2 (750 kg.hm-2 humic acid organic fertilizer + 55 kg.hm-2 nitrogen), NPKH3 (1050 kg.hm-2 humic acid organic fertilizer + 33 kg.hm-2 nitrogen) and PKH (1500 kg.hm-2 humic acid organic fertilizer). The effects of different humic acid organic fertilizers on soil microbial colonies, soil enzyme activities, and soil fertility were discussed. The results showed that humic acid organic fertilizer could effectively change the structure of soil microbial colonies, soil enzyme activities, and soil fertility. Compared with NPK treatment, the bacteria, fungi, and actinomycete, urease, and catalase in PKH, NPKH3, NPKH2, and NPKH1 treatments increased, and significantly different under 0-10 cm layer conditions (P<0.05). With the increase of humic acid organic fertilizer application, soil organic matter and soil fertility from superior to inferior was PKH>NPKH3>NPKH2>NPKH1>NPK. Therefore, the application of humic acid organic fertilizer was an effective measure to improve soil fertility and increase the amount of soil colony structure and enzyme activities.

Japanese modern irrigation management is considered a successful model of water governance worldwide. However, debates continue over whether this success is due to natural water abundance or to water management practices. This study evaluates pre-modern water scarcity risk for six irrigation schemes, developed during that period in the Kinu River Basin (1603–1868); a period without large reservoirs, canal systems, or modern regulatory technologies. As the methodology, pre-modern river flows were reconstructed by removing the effects of four modern dams from the present-day river discharge, adjusting the conveyance efficiency, changes in paddy field area, rainfall input, and return flows. Water demand was assessed using Japanese irrigation standards of 5 mm/d (minimum water demand corresponding to evapotranspiration) and 20 mm/d (easy management), and risk was evaluated under both the prior appropriation and Equal Water Distribution rules. Results show that modern flow in the dry season is approximately 25 m3/s, whereas reconstructed natural flow during drought years declines to 10–18 m3/s, and about 15 m3/s after rainfall adjustment. Under the 20 mm/d demand scenario, scarcity occurred in four schemes (2 of 17 years in the third scheme and 7 of 17 years for the sixth scheme), while no scarcity occurred under the minimum-demand scenario (5 mm/d), even during low-flow conditions. This indicates that the available water in these schemes was at a level where drought damage could occur under extensive irrigation management, but could be avoided by intensive irrigation management to supply the minimum necessary water to all paddy fields.

There has been increasing attention toward the influence of biofertilizers on the composition of microbial communities associated with crop plants. We investigated the impact of Azospirillum sp. B510, a bacterial strain with nitrogen-fixing ability, on the structure of bacterial and fungal communities within rice plant rhizospheres by amplicon sequencing at two sampling stages (the vegetative and harvest stages of rice). Principal coordinate analysis (PCoA) demonstrated a significant community shift in the bacterial microbiome when the plants were inoculated with B510 at the vegetative stage, which was very similar to the effect of chemical N-fertilizer application. This result suggested that the inoculation with B510 strongly influenced nitrogen uptake by the host plants under low nitrogen conditions. Least discriminant analysis (LDA) showed that the B510 inoculation significantly increased the N2-fixing Clostridium, Aeromonas and Bacillus populations. In contrast, there was no apparent influence of B510 on the fungal community structure. The putative functional properties of bacteria were identified through PICRUSt2, and this hinted that amino acid, sugar and vitamin production might be related to B510 inoculation. Our results indicate that B510 inoculation influenced the bacterial community structure by recruiting other N2-fixing bacteria in the absence of nitrogen fertilizer.

The flood retention capacity of paddy fields is well-recognized in Japan, and all the existing flood control practices via paddy field management achieve reductions in peak flood discharge. However, the previous studies have not assessed the flood management potential of paddy fields in a large river basin with average paddy coverage, and the existing hydrological models are not quite suitable for simulating river discharge from closed-drainage paddy reservoir storage. We herein attempt to improve the watershed-scale version of global hydrological model H08 to simulate a reduction in the peak discharge from paddy reservoirs in the Abukuma River basin of Japan. The NSE and R2 index showed fair reliability of the H08 model during the calibration and validation stages. The simulated results from the improved model show 11% and 6% peak reductions in high paddy coverage areas for a normal year (2018) and a major typhoon year (2019), respectively. The peak-reduction percentage increased with decreasing rainfall, depending on the overflow from the paddy reservoirs. The results indicate that the paddy reservoir is not highly effective in a large river with less than 20% paddy coverage, but the peak discharge reduction capacity shows that paddy reservoirs can make some contribution when used in combination with dam operation.