Early Stage Of Rice Growth Research Articles

Correction to: Three-dimensional image analysis specifies the root distribution for drought avoidance in the early growth stage of rice

Correction to: Three-dimensional image analysis specifies the root distribution for drought avoidance in the early growth stage of rice

Three-dimensional image analysis specifies the root distribution for drought avoidance in the early growth stage of rice.

Root system architecture (RSA) plays a key role in plant adaptation to drought, because deep rooting enables better water uptake than shallow rooting under terminal drought. Understanding RSA during early plant development is essential for improving crop yields, because early drought can affect subsequent shoot growth. Herein, we demonstrate that root distribution in the topsoil significantly impacts shoot growth during the early stages of rice (Oryza sativa) development under drought, as assessed through three-dimensional image analysis. We used 109 F12 recombinant inbred lines obtained from a cross between shallow-rooting lowland rice and deep-rooting upland rice, representing a population with diverse RSA. We applied a moderate drought during the early development of rice grown in a plant pot (25 cm in height) by stopping irrigation 14 days after sowing. Time-series RSA at 14, 21 and 28 days after sowing was visualized by X-ray computed tomography and, subsequently, compared between drought and well-watered conditions. After this analysis, we investigated drought-avoidant RSA further by testing 20 randomly selected recombinant inbred lines in drought conditions. We inferred the root location that most influences shoot growth using a hierarchical Bayes approach: the root segment depth that impacted shoot growth positively ranged between 1.7 and 3.4 cm in drought conditions and between 0.0 and 1.7 cm in well-watered conditions. Drought-avoidant recombinant inbred lines had a higher root density in the lower layers of the topsoil compared with the others. Fine classification of soil layers using three-dimensional image analysis revealed that increasing root density in the lower layers of the topsoil, rather than in the subsoil, is advantageous for drought avoidance during the early growth stage of rice.

Assemblages of rhizospheric and root endospheric mycobiota and their ecological associations with functional traits of rice.

The assembly processes and functions of root-associated mycobiota are among the most fascinating yet elusive topics in microbial ecology. Our results revealed that stochastic forces (dispersal limitation or ecological drift) act on fungal community assembly in both the rice rhizosphere and root endosphere at the early stage of plant growth. In addition, high covariations between the rhizosphere fungal community compositions and plant functional trait profiles were clearly demonstrated in the present study. This work provides empirical evidence of the root-associated fungal assembly principles and ecological relationships of plant functional traits with rhizospheric and root endospheric mycobiota, thereby potentially providing novel perspectives for enhancing plant performance.

Interaction between water, crop residue and fertilization management on the source-differentiated nitrogen uptake by rice

Alternate wetting and drying (AWD) is an effective water-saving practice for rice cultivation that may however promote nitrogen (N) losses compared to continuous flooding (CF). The interaction between water, crop residue and N fertilization management can influence the contribution of different N sources to plant uptake. We hypothesized that microbial processes driving the source-differentiated N supply for rice uptake during the early growth stages will depend on the interaction between water management, the timing of straw incorporation with respect to flooding and the temporal distribution of mineral N application. Rice was grown for 60 days in mesocosm experiment involving a factorial design with (i) two water regimes (CF vs. AWD) and (ii) three straw and fertilizer managements, during which soil N, porewater chemistry, plant growth and N uptake were evaluated. Source partitioning of plant N between fertilizer-, straw- and soil-derived N was achieved by means of a dual-stable isotope 15N tracing approach. Although AWD reduced total N uptake by about 4–25% with respect to CF, this could only be partly attributed to a lower uptake of fertilizer-N (and lower fertilizer-N use efficiency), suggesting that other N sources were affected by water management. Our findings evidence how the interaction between soil redox conditions and the availability of labile C and inorganic N strongly determined the supply of soil-derived N through microbial feedback and priming responses. Although incorporated straw contributed only minimally to rice N, it represented the primary driver controlling plant N nutrition through these microbial responses. These insights may contribute to identify suitable fertilization practices that favour plant N uptake during the early stages of rice growth under AWD.

Applying Nitrogen Fertilizer at the Full Heading Stage Has the Potential to Decrease Brown Rice Cd Accumulation

Soil contamination by cadmium (Cd) has presented a major challenge in China. The objective of the field experiments in this study was to examine the influence of nitrogen fertilizer application at the full heading and milky stages on minimizing the absorption of Cd in rice. This was achieved by affecting the distribution of Cd in root plaques and subcellular compartments of the root and flag leaf. The hydroponic culture experiments aimed to examine the effect of nitrogen and Cd interaction or deficiency on Cd accumulation in rice during the late growth stage. The findings revealed that adequate nitrogen supply during the early growth stage, coupled with nitrogen application during the full heading and milky stages, led to a notable increase in Fe concentration in the root plaques during the milk and mature stages. Furthermore, it elevated the Cd proportion in the soluble fraction of the flag leaves at the milky stage. Conversely, nitrogen deficiency during the early growth stage resulted in a significant increase in Fe concentration in the root plaques, along with a decrease in Cd concentration. Additionally, the proportion of Cd in the flag leaf cell walls increased significantly, while the proportion in the soluble fraction decreased notably. Irrespective of nitrogen supply during the early growth stage, applying nitrogen at the full heading stage significantly reduced Cd transport from shoots to brown rice, leading to a considerable reduction in the Cd concentration in brown rice. Under hydroponic culture conditions, combined Cd exposure with nitrogen supply significantly increased the Cd concentration in brown rice. Nitrogen supply had no impact on the Cd concentration in brown rice in the absence of Cd. The study showed that applying nitrogen fertilizer at the full heading stage effectively decreased the brown rice Cd concentration. This was achieved by elevating the concentration of Fe in the root plaque, thereby influencing the adsorption of Cd by the roots. Additionally, nitrogen application at the full heading stage can influence the distribution of Cd in flag leaf cells during the filling stage. Ensuring ample nitrogen supply in the early stage of rice growth is crucial, and nitrogen application at the full heading stage can effectively reduce the Cd concentration in brown rice.

Weed target detection at seedling stage in paddy fields based on YOLOX

Weeds are one of the greatest threats to the growth of rice, and the loss of crops is greater in the early stage of rice growth. Traditional large-area spraying cannot selectively spray weeds and can easily cause herbicide waste and environmental pollution. To realize the transformation from large-area spraying to precision spraying in rice fields, it is necessary to quickly and efficiently detect the distribution of weeds. Benefiting from the rapid development of vision technology and deep learning, this study applies a computer vision method based on deep-learning-driven rice field weed target detection. To address the need to identify small dense targets at the rice seedling stage in paddy fields, this study propose a method for weed target detection based on YOLOX, which is composed of a CSPDarknet backbone network, a feature pyramid network (FPN) enhanced feature extraction network and a YOLO Head detector. The CSPDarknet backbone network extracts feature layers with dimensions of 80 pixels ⊆ 80 pixels, 40 pixels ⊆ 40 pixels and 20 pixels ⊆ 20 pixels. The FPN fuses the features from these three scales, and YOLO Head realizes the regression of the object classification and prediction boxes. In performance comparisons of different models, including YOLOv3, YOLOv4-tiny, YOLOv5-s, SSD and several models of the YOLOX series, namely, YOLOX-s, YOLOX-m, YOLOX-nano, and YOLOX-tiny, the results show that the YOLOX-tiny model performs best. The mAP, F1, and recall values from the YOLOX-tiny model are 0.980, 0.95, and 0.983, respectively. Meanwhile, the intermediate variable memory generated during the model calculation of YOLOX-tiny is only 259.62 MB, making it suitable for deployment in intelligent agricultural devices. However, although the YOLOX-tiny model is the best on the dataset in this paper, this is not true in general. The experimental results suggest that the method proposed in this paper can improve the model performance for the small target detection of sheltered weeds and dense weeds at the rice seedling stage in paddy fields. A weed target detection model suitable for embedded computing platforms is obtained by comparing different single-stage target detection models, thereby laying a foundation for the realization of unmanned targeted herbicide spraying performed by agricultural robots.

Effect of mulch weight on temperature, soil moisture and vegetative growth of Sigambiri Merah in lowlands

Rice is staple food source for most Indonesians. However, highland rice development on dry land is regularly isolated by action makers who find it simple to development paddy construction. One of the cultivation techniques to achieve maximum production of upland rice in dry land is by modifying the microclimate in the roots zone using organic mulch. The study was carried out at the Experimental Farm of Meteorology, Climatology and Geophysics Agency, Sempali, Medan. The research method is a non-factorial randomized block design with three replications. Mulch thickness (M) used as treatment, consisted of without mulch (M0), 20 t/Ha (M1), 25 t/Ha (M2), and 30 t/Ha (M3). Observations were invented insanely by expansive two fragment for each analysis 28 days after planting (DAP). As a result, we found that multi-weight is efficient in reducing soil temperature and increasing soil moisture compared to without mulch, but there is no improvement in the vegetative growth of Sigambiri Merah because it was still in the early stages of rice growth.

The effects of organic fertilizer on loss risk of nitrogen and phosphorus in paddy ponded water.

There is a great challenge globally for both achieving high crop yields via fertilization and minimizing environmental pollution from nutrient losses. Organic fertilizer (OF) application has been extensively reported to effectively improve arable soil fertility and mitigate nutrient losses. However, few studies are available that accurately quantified the substitution rates of OF for chemical fertilizers (CF) that affect rice yield, the nitrogen/phosphorus in ponded water, and its loss potential in paddy field. Here, an experiment with 5 levels of CF nitrogen substituted by OF nitrogen was performed during the early stage of rice growth in paddy field of Southern China. The results showed that the first 6days and 3days following fertilization generally were risky period for N losses and for P losses, respectively, due to corresponding high concentrations in ponded water. Compared to CF treatment, over 30% of OF substitution rates significantly decreased the daily mean TN concentrations by 24.5-32.4%, while TP concentrations unaffected and rice yield maintained comparative levels. OF substitution also improved acidic paddy soils, with the increment of 0.33-0.90 unit for ponded water pH compared to CF treatment. Conclusively, the 30-40% of CF substituted by OF based on N amounts can be considered an ecological fertilization practice for rice production to mitigate environmental pollution due to lower N losses and without significant effect of grain yield. However, the attention also must be paid concerning the rise of environmental pollution risk from NH3 volatilization and P runoff after long-term OF application.

Application of Bacterial Endophytes to Control Bacterial Leaf Blight Disease and Promote Rice Growth.

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial leaf blight (BLB) disease in rice (Oryza sativa L.) and it is among the most destructive pathogen responsible for severe yield losses. Potential bacterial biocontrol agents (BCAs) with plant growth promotion (PGP) abilities can be applied to better manage the BLB disease and increase crop yield, compared to current conventional practices. Thus, this study aimed to isolate, screen, and identify potential BCAs with PGP abilities. Isolation of the BCAs was performed from internal plant tissues and rhizosphere soil of healthy and Xoo-infected rice. A total of 18 bacterial strains were successfully screened for in vitro antagonistic ability against Xoo, siderophore production and PGP potentials. Among the bacterial strains, 3 endophytes, Bacillus sp. strain USML8, Bacillus sp. strain USML9, and Bacillus sp. strain USMR1 which were isolated from diseased plants harbored the BCA traits and significantly reduced leaf blight severity of rice. Simultaneously, the endophytic BCAs also possessed plant growth promoting traits and were able to enhance rice growth. Application of the selected endophytes (BCAs-PGP) at the early growth stage of rice exhibited potential in suppressing BLB disease and promoting rice growth.

Microbial iron reduction compensates for phosphorus limitation in paddy soils.

Limitation of rice growth by low phosphorus (P) availability is a widespread problem in tropical and subtropical soils because of the high content of iron (Fe) (oxyhydr)oxides. Ferric iron-bound P (Fe(III)-P) can serve as a P source in paddies after Fe(III) reduction to Fe(II) and corresponding H2PO4- release. However, the relevance of reductive dissolution of Fe(III)-P for plant and microbial P uptake is still an open question. To quantify this, 32P-labeled ferrihydrite (30.8 mg P kg-1) was added to paddy soil mesocosms with rice to trace the P uptake by microorganisms and plants after Fe(III) reduction. Nearly 2% of 32P was recovered in rice plants, contributing 12% of the total P content in rice shoots and roots after 33 days. In contrast, 32P recovery in microbial biomass decreased from 0.5% to 0.08% of 32P between 10 and 33 days after rice transplantation. Microbial biomass carbon (MBC) and dissolved organic C content decreased from day 10 to 33 by 8-54% and 68-77%, respectively, suggesting that the microbial-mediated Fe(III) reduction was C-limited. The much faster decrease of MBC in rooted (by 54%) vs. bulk soil (8-36%) reflects very fast microbial turnover in the rice rhizosphere (high C and oxygen inputs) resulting in the mineralization of the microbial necromass. In conclusion, Fe(III)-P can serve as small but a relevant P source for rice production and could partly compensate plant P demand. Therefore, the P fertilization strategies should consider the P mobilization from Fe (oxyhydr)oxides in flooded paddy soils during rice growth. An increase in C availability for microorganisms in the rhizosphere intensifies P mobilization, which is especially critical at early stages of rice growth.

Comprehensive effects of integrated management on reducing nitrogen and phosphorus loss under legume-rice rotations

Paddy fields-based cropping systems are usually located in areas with dense water networks, so nitrogen (N) and phosphorus (P) losses from rice paddy fields caused by runoff might pose a serious pollution risk to their surrounding water bodies. However, current research over establishing a low fertilizer input paddy-based cropping system with comprehensive management for reducing runoff losses of N and P is insufficient. A five-year field trial was carried out with a rice-fava planting system in the Erhai Lake watershed from 2015 to 2019. Three treatments were designed namely chemical fertilizer plus manure (CMF), reduced chemical fertilizer combined with manure at the same application rate under CMF treatment (RF), and returning straw plus chemical and organic fertilizers at the same rate with RF treatment (RFS). The mean annual runoff losses of total N (TN) and total P (TP) range from 0.99 to 1.39 kg ha −1 yr −1 and 0.09–0.12 kg ha −1 yr −1 . Compared with CMF treatment, RF had lower runoff losses of TN and TP by 20.86% and 23.05%, respectively, but the yields and soil fertility were also reduced. After returning straw, soil organic carbon (SOC), TN, and crop yields were maintained or even increased, and runoff losses of TN and TP were further reduced by 9.48% and 7.41%, respectively, compared with RF. Therefore, the runoff losses of N and P decreased by selecting the rice-fava rotation system and reducing chemical fertilizer input through manure application and straw returning, which greatly reduced the surplus N and P in this system. Moreover, the temporal variation in the concentration of TN and TP, along with the N:P ratio approved the existence of potential risk caused by TN and TP losses to the watershed in the early stage of rice growth. Overall, in addition to taking field measures to reduce the N and P runoff losses from paddy fields, the runoff water with high N and P concentrations in early rice growth stage cannot be ignored. • Rice-fava system with low fertilizer maintained yield and had low environment risk. • Straw returning ensured crop yields under substituting chemical fertilizer by manure. • Combined of straw returning and manure substitution reduced nutrient runoff loss. • Runoff N/P concentration and N:P ratio in early stage of rice may need interception.

Effect of reed-biochar application on ammonia volatilization from different types of soils

The application of straw biochar in farmland in Dongting Lake area can realize the resource utilization of straw and reduce environmental risk. In 2020, a rice pot experiment was conducted to investigate the effects of different biochar application levels on ammonia volatilization rate, cumulative ammonia volatilization, pH value, and NH4+-N concentration in surface water. Six levels of reed (Miscanthus lutarioriparius) biochar amount, i.e., 0%, 1%, 2%, 4%, 6% and 8% of soil weight of the 0-20 cm column, were applied in two typical paddy soils in sou-thern China, i.e., reddish clayey soil derived from quaternary red soil and granitic sandy soil derived from granite. Compound fertilizer was applied at a rate of 200 kg N·hm-2. The results showed that biochar application resulted in significant differences in the rate and cumulative amount of ammonia volatilization between the two soils and among different biochar treatments. For the granitic sandy soil, peak ammonia volatilization under different biochar treatments appeared at the second day after fertilization, which was decreased by 23.6%-53.4%. For the reddish clayey soil, peak ammonia volatilization appeared between the 7th to 13th day after fertilization, which increased with biochar addition level. The rate of ammonia volatilization from the granitic sandy soil was generally higher than that from the reddish clayey soil. For the granitic sandy soil, addition of <4% biochar could inhibit the ammonia volatilization and cumulative volatilization amount, with the greatest reduction (46.9%) at the treatment with 2% biochar addition. The addition of biochar did not affect the pH value of surface water at the early stage of rice growth. For the reddish clayey soil, the pH value and NH4+-N concentration in the surface water increased with biochar addition level, resulting in the increases of ammonia volatilization rate and cumulative volatilization amount by 1.3-10.5 times. Biochar addition level was the key factor affecting ammonia volatilization from the two soils. Elovich equation could well fit the variation trend of cumulative ammonia volatilization with time for the two soils, with the correlation reaching extremely significant level for each treatment. Overall, the application of reed biochar could suppress ammonia volatilization from the granitic sandy soil which was nearly neutral in acidity, while would promote ammonia volatilization via increasing pH value and NH4+-N concentration of surface water for the acidic reddish clayey soil. Therefore, appropriate dosages of reed biochar should be selected for different types of soil in order to reduce nitrogen loss.

Effect of Time and Method of Sowing in Wet Direct Seeded Rice

Background: Direct wet seeding offers the advantage of faster and easier sowing, reduced labour and less drudgery, 7-10 days earlier crop maturity, more efficient water use and higher tolerance to water deficit, less methane emission, and often higher profit in areas with assured water supply. There is a possibility of intercropping green manures during early growth stage of rice with less interference on the crop growth, which also smothering the weed growth to enhance the productivity of wet direct seeded rice. Methods: A field experiment was conducted during Kharif, 2019 and 2020 at the Department of Rice, Tamil Nadu Agricultural University, Coimbatore, to study the time of sowing and method of sowing under wet direct seeded rice. The experimental trial was laid out in split plot design with three replications. Treatments comprising of time of sowing in main plot viz., normal sowing time in first fortnight of July and delayed sowing time in second fortnight of July. In sub-plot, method of sowing viz., broadcasting method, line sowing method, paddy drum seeder method, paddy + dhaincha drum seeder method and direct planting method. Result: The results revealed that, sowing of wet direct seeded rice during first fortnight of July month by using Paddy + Dhaincha drum seeder method recorded higher grain yield of 5707 kg/ha which resulted 33 per cent higher yield than that of broadcasting method.

Development of a mechanical weeder and experiment on the growth, yield and quality of rice

Weed control in paddy fields is a worldwide problem and mechanical weeding will become a major weeding method in non-chemical weeding. A mechanical weeder was designed and tested to improve the efficiency of mechanical weeding and reduce the application of herbicides in this study. The weeding equipment is equipped with three sets of inter-row weeding parts which include ground-contoured-following pressing-grass float (GPF), weeding roller, and so on. The weeding principle of weeding part is that the weeding equipment enters the inter-row area with weeder moving forward, then the GPF overwhelms the weeds in the inter-row area to improve the probability that the weeding roller behind the GPF pressed the overwhelmed weeds into the soil. The effects of weeding methods on the plant height, grain yield and quality attributes were investigated in a two-seasonal field experiment. Three weeding methods were applied in the present study: no-weeding, chemical weeding and mechanical weeding. The results of the two-seasonal field experiment showed that the average weeding rates of mechanical weeders were 87.10% and 87.61% respectively. In both seasons, there was no significant difference among weeding methods on plant height at the early growth stage of rice after weeding, but weeding methods had significant effect on plant height at the late growth stage. The plant height of mechanical weeding was higher at the late growth stage. Weeding methods had significant effects on grain yield, grain number per panicle, seed-setting rate, 1000-grain weight and shoot dry matter accumulation (p 0.05). Grain yield of mechanical weeding was significantly higher than that of no-weeding. There was no significant difference between chemical weeding and mechanical weeding. It can be concluded from the two-seasonal experiment that mechanical weeding had the same effect as chemical weeding on grain yield and rice quality. Keywords: weeder, paddy field, mechanical weeding, grain yield, rice quality DOI: 10.25165/j.ijabe.20221503.6978 Citation: Jiao J K, Wang Z M, Luo H W, Chen G L, Liu H L, Guan J J, et al. Development of a mechanical weeder and experiment on the growth, yield and quality of rice. Int J Agric & Biol Eng, 2022; 15(3): 92–99.

Effect of microcystins at different rice growth stages on its yield, quality, and safety.

Microcystins (MCs) in water for irrigation may damage crop growth and enter food chains to threaten human health. To evaluate the potential risk of irrigation water contaminated with MCs, we exposed rice at each of the seedling, booting, and filling stages to irrigation water spiked with MCs at 1, 10, 100, and 1000μg/L for 7days. Afterwards, all rice underwent a recovery (without MCs) till the harvest. Low MCs (1 or10μg/L) during different rice growth stages did not affect its yield and nutritional quality and had no risk to human health. High-concentration MCs (100 or 1000μg/L) during the seedling or booting stage caused a larger decrease in the nutritional quality and yield of rice grains than that during the filling stage. In addition, MCs at 100μg/L during the booting stage or at 1000μg/L during the filling stage potentially threatened human health. The effect of MCs on rice yield, quality, and health risk was associated with the MC concentration and rice growth stage. Irrigation water contaminated with moderate-concentration MCs should be of concern at the early growth stage of rice.

Co-application of poultry-litter biochar with Azolla has synergistic effects on CH4 and N2O emissions from rice paddy soils

Poultry-litter biochar and Azolla as green manure amendments are reported to enhance paddy soil fertility and rice yields. However, whether their co-application in lowland rice paddies has synergistic effects and whether those benefits are accompanied by greenhouse gas (GHG) emissions remains unknown. The objective of this study was to determine the effects of poultry-litter biochar (hereafter: biochar) and its co-application with Azolla as green manure (hereafter: Azolla), on the simultaneous methane (CH4) and nitrous oxide (N2O) emissions from a lowland paddy soil planted with rice during a single rice growing season in Tsuruoka, Yamagata, Japan. Biochar and Azolla amendments were applied once before rice was transplanted at a density of 20 t ha−1 and 133.9 kg N ha−1, respectively. Compared with NPK, NPK + biochar, and Azolla only treatments, Azolla and biochar co-application (i.e., Azolla + biochar) significantly increased CH4 emissions by 33%–197.6% in the early stages of rice growth (before 63 days after transplanting, DAT), but did not significantly influence CH4 emissions at both late rice growth stages (after 63 DAT,) and whole rice growth period (112 DAT). Conversely, Azolla + biochar significantly reduced N2O emissions by 83.0%–97.1% before 63 DAT, and by 76.4%–95.9% during the whole rice growth period at 112 DAT, with a significantly high interaction between biochar and fertilizer amendments. There were no significant N2O emission differences among all treatments after 63 DAT. Additionally, Azolla + biochar significantly increased rice grain yield by 27.3%–75.0%, and consequently, decreased both yield-equivalent CH4 emissions by 24.7%–25.0% and N2O emissions by 81.8%–97.7%. Our findings suggest that the co-application of poultry-litter biochar and Azolla as green manure offers a novel approach to increase rice yield while reducing the emissions of non-carbon dioxide greenhouse gases.

Long-term impacts of nitrogen fertilization and straw incorporation on rice production and nitrogen recovery efficiency under plastic film mulching cultivation

The application of nitrogen (N) fertilizer as base fertilizer in plastic film mulching cultivation (PFMC) system easily leads to soil N deficiency in the late growth period of rice. The combination of N fertilization and straw incorporation was put forward and their impacts on rice yield and grain N recovery efficiency (NRE) based on a long-term experiment in a PFMC system were evaluated. Five N application levels included 0, 45, 90, 135, and 180 kg ha−1 and two subplots were rice straw removal (RSR) and rice straw incorporation (RSI). Moreover, RSR and RSI in a conventional flooding cultivation (CFC) system with 135 kg ha−1 N application rate were also included. The results showed that the optimum N application rate was 135 kg ha−1. At this N application level, the promotion effect on yield and grain N uptake over years in PFMC system was more obvious than that in CFC system. Continuous PFMC might accelerate the mineralization of soil organic matter and the increase of soil N availability. However, RSI had no obvious promotion effect on the increment of yield, grain NRE, and soil available N with years compared with RSR in PFMC system. In conclusion, film mulching with the N fertilizer application rate of 135 kg ha−1 improved soil thermal condition in early growth stage of rice, and thus enhanced the availability of soil N, grain N uptake, and yield. This provides a reference for PFMC application in water-saving rice production system.

Periphyton has the potential to increase phosphorus use efficiency in paddy fields

The phosphorus (P) supply is mismatched with rice demand in the early and late stages of rice growth, which primarily results in low P use efficiency and high environmental risk. In recent years, the use of the natural periphyton in nutrient regulation in paddy fields has attracted much research interest. However, a mechanistic understanding of the action of periphyton on P biogeochemical cycling during the pivotal stages of rice growth has received little attention. In this study, the influence of periphyton proliferation on the soil surface and its consequential decomposition on P migration and bioavailability were investigated in two paddy soils using two microcosm experiments. The results showed that periphyton rapidly accumulated fertilizer P when it proliferated on the soil surface under favorable light condition, which led to more fertilizer P being stored on the soil surface and less P being fixed by soil particles or transported via runoff into the water bodies. The decomposition of periphyton under unfavorable light condition not only increased soil soluble reactive P, but also increased the amount of easily available P species, such as labile P, AlP, FeP, and mobilized OP. Thus, periphyton colonizing the soil surface in the early stage of rice growth could act as a P sink and decrease the P environmental risk, and its decomposition in the late stage of rice growth could act as a P source and activator. Phosphorus bioavailability regulated by periphyton could be synchronous with rice needs. Thus, periphyton has the potential to increase P use efficiency in paddy fields.

Negative and Positive Impacts of Rape Straw Returning on the Roots Growth of Hybrid Rice in the Sichuan Basin Area

Incorporating oilseed-rape straw in soil is one of the effective methods for enhancing the use efficiency of agricultural resources in the rape-rice rotation system. However, the impacts of oilseed-rape straw incorporation on root growth and dynamic changes in soil are still unclear. In order to provide a deeper understanding of the oilseed rape straw return on rice growth and productivity, the experiment was conducted in the field and in a specially-designed pots system from 2016 to 2017 by means of two straw returning methods and four straw returning amounts. In the early stage of rice growth (0–36 days after rice transplanting) the straw returning treatments decreased 1.0–8.6 mg/plant in bleeding density and 0.10–6.11, 0.06–0.31, and 0.52–0.84 μmol/(g h) in the activity of glutamine synthetase (GS), glutamic-oxalacetic transaminase (GOT), and glutamic-pyruvic transaminase (GPT), respectively. Oilseed rape straw returned by mulching induced negative impacts on new germinating roots in 0–10 cm of soil, while the negative impacts were observed on roots in 10–30 cm of soil for straw returned by plowing. In the later stage of rice growth (56–75 days after rice transplanting), oilseed rape straw returning produced some positive impacts on rice roots, which enhanced the yield of rice. Conclusively, our findings suggested that dynamic root growth and the activity of root enzymes are two major factors behind the slow reviving of rice after transplanting in the straw returning field. Plowing is a more appropriate method of straw returning than mulching in the rape-rice rotation system in the Sichuan basin area, with a straw incorporation rate of 3.0 t/hm2.

Optimising rice mapping in cloud-prone environments by combining quad-source optical with Sentinel-1A microwave satellite imagery

Optical satellite images are prone to cloud contamination in the tropics and subtropics especially during the monsoon season which is contemporaneous with rice cultivation. Producing rice field maps in such areas using a single optical satellite source is therefore very challenging. To obtain an adequate number of usable optical images that captures the seasonal profile of rice fields for their discrimination, this study explored an operational approach to combining quad-source optical satellite data that include Landsat-8, Sentinel-2A, China’s environment and disaster monitoring and forecasting satellite constellation (HJ-1 A and B), and Gaofen-1 over a test site located in southeast China at two rice growing seasons (2016 and 2017). To minimise inter-sensor differences, a spectral index (SI) dataset containing cross-calibrated Enhanced Vegetation Index (EVI) images from all optical sensors and Modified Normalised Difference Water Index (MNDWI) images from Landsat-8 and Sentinel-2, was derived. As more accurate rice maps have been obtained with the synergistic use of optical and microwave data, the aforementioned datasets were combined with the vertical transmitted and horizontal received (VH) and vertical transmitted and vertical received (VV) polarisation images of Sentinel-1A. The resultant optical, microwave, and combinations of optical and microwave data were used as inputs to the Support Vector Machine (SVM) and Random Forest (RF) algorithms available in ENVI 5.3 and ENMAP Box 2.0, respectively to discriminate rice fields from other land-cover types. Results showed that Sentinel-1A data produced higher mapping accuracies than the quad-source optical datasets. This is attributed to the higher number of Sentinel-1A images at the early stages of rice growth (vegetative phase), a period where changes in rice satellite signals are most abrupt and therefore most diagnostic for discriminating the rice crop from other land-cover types. Additionally, combinations of optical and microwave data produced higher overall accuracies than when used separately, and the highest overall accuracies for both years were achieved with the application of RF on a combination of VH, VV and SI (VHVVSI), yielding 98.43% in 2016 and 96.73% in 2017. Moreover, combining VH with SI (VHSI) produced overall mapping accuracies that are more equivalent to the above (98.40% in 2016 and 96.53% in 2017) than with the combination of VV and SI (VVSI) which yielded 97.22% in 2016 and 93.92% in 2017. This indicates that VH is not only more complementary to optical satellite imagery in rice discrimination, but its combination with SI alone can produce rice distribution maps of the highest possible accuracies in cloud-prone environments.