Rice fields are found commonly in monsoon Asia and are distributed widely in the world. Rice fields are generally flooded during the growing season, and this provides a unique agricultural ecosystem that consists of soil, water, plants and the atmosphere. The first paper published in Journal of Agricultural Meteorology was titled “Micrometeorological studies on the rice field” (Suzuki et al., 1943). Since then, a variety of agro-meteorological studies on rice fields have been made to understand characteristics of rice fields in comparison with other non-flooded croplands. As early as the 1950s and the 1960s, micrometeorological studies on water temperature, turbulence above and within rice canopy, heat and water balance and leaf/canopy photosynthesis were made (e.g. Uchijima, 1976), and those fundamental studies were synthesized into a systematic rice canopy model, which was later utilized in northern Japan to reduce cool summer damage of rice. In the late 1990s, new experimental platforms such as FACE (Free-Air CO2 Enrichment) and tower flux monitoring sites were made in rice fields to cope with climate change. Those platforms are utilized for various studies such as micrometeorology of rice canopy under elevated CO2 conditions, long-term changes of carbon and water exchange and methane emission. To survey recent micrometeorological and biogeochemical studies in rice fields and to discuss direction of further studies, a small meeting sponsored by JapanFlux was held in Okayama, Japan, March 2016 as an organized session of the Annual Meeting of the Society of Agricultural Meteorology of Japan. Based on the meeting, this special issue “Rice paddy environment and productivity under changing climate” was planned. Although this special issue is a small collection of agro-meteorological studies on rice fields, it covers a wide range of topics from phenological modelling to morphological sensing and a variety of approaches from laboratory experiment to satellite remote sensing. Five of the seven articles are related to Mase paddy flux site (Fig. 1). We hope this special issue will help readers to realize that various agro-meteorological studies are going on in rice fields, as in other croplands, to cope with climate change. Publication of this special issue owes much to Dr. Keisuke Ono.
Water temperature (Tw) plays a key role in growth and development of plants inhabiting flooded environment, but most phenology models use air temperature (Ta) for phenology prediction. Gaps between Tw and Ta are known to differ regionally, but regional differences in the importance of using Tw for phenology prediction are not known. This study attempts to determine whether the use of Tw improves the prediction of heading time, by using 758 field observations across Japan and estimated Tw from a nationwide weather database (MeteoCrop Database). We have confirmed that the use of Tw improved the accuracy of prediction by 1.0-2.4 days as measured by the root-mean-square error, but the degree of improvement was similar at 23-41% across different latitudes, altitudes, or planting times, likely because the Tw-Ta difference is highly variable even at similar latitudes or altitudes. The models proposed here and the nationwide database of future climate projection will help to reduce the uncertainty in predicting crop calendar for a range of climatic conditions.
To mitigate CH4 emission from paddy fields, accelerating the decomposition of the incorporated rice straw during the fallow season would be a practical strategy. Various ways to accelerate the straw decomposition rate have been proposed, but their effectiveness in cold regions has not been confirmed. In this study, the use of shallow autumn tillage to incorporate straw in the soil and its potential to mitigate CH4 emission during the following rice growing season were evaluated in an Andisol paddy field in Morioka, a cold region in Japan. A japonica rice cultivar, ‘Akitakomachi’, was planted and grown from 2012 to 2014 under consistent conditions, but with two autumn tillage and straw incorporation treatments: conventional (15 cm) and shallow (7 cm). CH4 fluxes from the plots were measured using a closed-chamber method throughout the 3 years. Overall, CH4 emission did not differ between the conventional and shallow tillage plots in this study. However, CH4 emission differed greatly among the years, especially during early rice growth stages, and the differences were related to the temperatures but not the soil moisture content during the previous fallow season. Simulation of water contents during the fallow period suggested that the percolation rate was sufficiently high to create more aerobic soil conditions during the fallow season in both the conventional and the shallow autumn tillage treatments. These results suggest that the soil water was neither so high nor so low that it retarded rice straw decomposition in the fallow season. The results suggest that shallow autumn tillage will not necessarily reduce CH4 emission during the following growing season in an Andisol rice paddy in a cold region in Japan.
Plant height is an important trait linked to yield potential and phenotyping; hence, a method for measuring plant height with sufficient temporal and spatial resolutions is required. Furthermore, a low-cost and easy-to-use method is desirable for practical application in agricultural fields. In previous studies, methods for measuring plant height have only fulfilled one or a few of these requirements, and there have been very few studies that have attempted to fulfill all the above requirements in one method. The current study proposes a low-cost plant height measurement system using a commercial time-lapse camera to capture seasonal and year-to-year variations in plant height that represent plant heights at the site scale. The system's performance was tested in a rice field in Japan for two growing seasons. Plant heights were determined to the nearest 1-cm in height from the camera images by referencing a scale bar that was erected next to the four target plants. The obtained plant heights were corrected for vertical distortion (referred to as the 'displacement effect' in the main text), and those were compared with the direct height measurements for each the target plant and site-scaled plant heights that were averaged from 10 samples. For both cases, good agreement between the captured and referenced plant heights was obtained. The system was able to reflect detailed seasonal variations in the plant heights including the maximum heights and the times at which they occurred as well as year-to-year differences. Consequently, the developed system appears to fulfill all the necessary requirements for practical measurement of plant height on-site in agricultural fields.
Evapotranspiration (ET) in rice paddy fields has been studied over the past half-century, but despite that long history of rice ET study, not much is known about their inter-annual variation. This study investigates ET in rice paddy fields during the growing season for 13 years (2002-2014) at one of the longest flux monitoring sites in Japan. The cumulative ET estimated for each growing season (120 days after transplanting) was 419±45 SD (standard deviation) mm/season (3.5 mm d−1), which increased to 534±64 mm/season (4.4 mm d−1) when energy balance was forced to close. This study determined that ET was correlated with meteorological conditions defined by potential evaporation (EP), and the inter-annual variability of seasonal ET/EP (coefficient of variation (CV) = 4.6%) was less than half the variability of ET (CV = 10.7%). For most years, the seasonal ET was reasonably estimated as the product of EP and the overall average ET/EP (=0.89), including years that experienced both extremely warm and cold summers. However, the errors in estimation were relatively large for 2008 (which experienced high precipitation) and 2012 (which was characterized by an extremely low leaf area index). Although the seasonal ET/EP was relatively invariable among years, both the seasonal trends of ET and ET/EP greatly differed between years. The inter-annual variability in ET/EP was particularly high at the ripening stage (defined as 91-120 days after transplanting). The ET/EP at this stage was well correlated to precipitation, and the variation was possibly caused by evaporation from intercepted rainfall. The variation in ET/EP associated with precipitation at the ripening stage accounted for 21% of the inter-annual variation of seasonal ET/EP.
Gross primary production (GPP) capacity is defined as GPP under low stress, and the algorithm for its estimation was developed by Thanyapraneedkul et al. (2012) using a light-response curve. The idea behind this algorithm is that the light response curve under low stress is related to chlorophyll content. The parameter is estimated from a vegetation index derived from satellite observations of the green chlorophyll index (CIgreen) for seven vegetation types, including rice paddy. These previous studies included 1 year of data for the flux site and MODIS reflectance data. Recently, long-term data have become publicly available for flux data covering a period of 6 years, and MODIS reflectance data covering a period of more than 16 years. This study determined the parameters in the GPP capacity estimation algorithm for rice paddies using 6 years of Mase paddy flux site data and clear daytime reflectance data observed using MODIS. The fitted parameter-related initial slopes of the light-photosynthesis curves for each year were identical within the fitting error. Using the averaged parameter-related initial slope over 6 years, we were able to determine a linear relationship between CIgreen and the maximum photosynthesis rate at 2000 PAR (μmol m−2 s−1), the slope of which was slightly higher than has been reported previously. Using the parameters for the period 2001-2006, we investigated how GPP capacity varied for irrigated rice paddy. The ratio of the average GPP capacity to the GPP after transplanting until harvesting was 0.91 for the period 2001 to 2006. This result shows that GPP capacity provides a useful first approximation of GPP for irrigated rice paddies as a framework of the global GPP estimation algorithm.
It is needed to accurately evaluate the methane emission from paddy fields at a national scale for both scientific and political purposes. The existing approaches have shared a common issue with obtaining the realistic information on spatiotemporal variations of crop management. Satellite sensor could possibly detect them, but the methodology to link the satellite observations to the methane emission has not been established. In this study, we enhanced the existing diagnostic satellite-driven paddy ecosystem model, the Biosphere model integrating Eco-physiological And Mechanistic approaches using satellite data for regional Cropland (BEAMS-C) (Sasai et al., 2012), by integrating the methane processes and examined the potential of this approach by comparing the estimated nation-wide methane emission with that by the existing approaches. The carbon flux estimations had good agreements with the measurements at the Mase paddy flux site. In regional-scale analyses in Japan, methane emission averaged from 2001 to 2010 was 15.0 gC m-2 year-1, which was similar to the Tire-1 and -2 results. If we have a continuing improvement of the diagnostic approach, it could be one of the most efficient tools for estimating the methane flux at national scales.
To improve our understanding of the carbon dioxide (CO2) efflux dynamics in paddy fields during the drained fall-to-spring fallow season, we conducted incubation experiments with paddy soil to determine the dependence of the soil CO2 emission rate on temperature and moisture. The 37-day integrated CO2 emission rate per unit soil weight was 1.45 μmol g-1 d-1 at a soil temperature of 15°C and a soil moisture content of 0.65 g g-1. The temperature dependence of the soil CO2 emission rate was expressed using the Arrhenius equation, but its activation energy was likely to be underestimated if the data obtained at a high temperature were included. Soil CO2 emissions were linearly dependent on the soil moisture. We occasionally observed inverse relationships between the CO2 efflux estimated using the incubation experiments and those determined using the eddy covariance method, which was probably due to the effects of CO2 storage in the soil. However, when the data were averaged over a relatively long-time scale (10-15 days), the empirical model based on the incubation experiments successfully reproduced the soil CO2 efflux determined by the eddy covariance method, probably because the long-time scale canceled the CO2 storage effect.
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