Late-season Rice Research Articles

Impact of historical climate variability on rice production in Mainland Southeast Asia across multiple scales

Climate change poses significant pressure on global food production. Although previous work has explored impacts of climate, management, and genetics on food production, additional research is needed to examine the effects of large-scale climate modes at local and regional scales. This study explores the impact of climate variability on rice yield in Mainland Southeast Asia from 1961-2017 at three different spatial scales: the whole Mainland Southeast Asia region, country-level (Cambodia, Laos, Myanmar, Thailand, and Vietnam), and province-level for Vietnam. Annual rice yields over this period have nearly tripled with Vietnam experiencing the largest increases. Correlations between annual rice yield anomalies at the regional and country levels and climate data reveal clear influences of tropical climate variability associated with the El Niño-Southern Oscillation and the Pacific Meridional Mode. At the provincial level in Vietnam, many provinces show similar correlation patterns for the spring-summer season of rice (e.g., a co-occurring La Niña and positive phase of the Pacific Meridional Mode in the preceding boreal winter and spring are associated with increased yields in spring-summer rice). However, the late summer-fall season rice yield anomalies show much weaker correlations with tropical climate patterns. Variations across provinces were also noted, particularly between the Red River and Mekong River Deltas. The history of this 56-year period, which included the Vietnam-American War and changes in land management policies, makes it challenging to disentangle the effects of climate variability and social factors on rice yields in these areas. However, these results highlight the importance of using a multidisciplinary and multiscale approach to help inform local and regional decision-making. • We examined climate and rice yields in Mainland Southeast Asia at 3 scales. • Correlations revealed clear influences of tropical climate variability. • Provinces in Vietnam showed varying results by rice season and location. • Future climate change may shift planting timing and threaten national exports. • A multidisciplinary and multiscale approach can help disentangle effects on rice.

Precise delivery of nitrogen at tillering stage enhances grain yield and nitrogen use efficiency in double rice cropping systems of South China

Nitrogen fertilizer management substantially affects the growth, productivity, and nitrogen use efficiency (NUE) of rice production systems. To assess the effects of precise application of nitrogen fertilizer at tillering stage as ‘tillering-fertilizer’ on grain yield and nitrogen use efficiency, field trials were conducted in the late season of 2019 and 2020, in addition to a radioisotope 15 N tracer experiment. The experiment was comprised of four N fertilizer treatments i.e., no fertilizer application as a control (T1), traditional surface broadcast fertilization (90 kg N/ha as basal fertilizer and 60 kg N/ha as tillering fertilizer, T2), deep placement of tillering fertilizer (surface broadcast at 90 kg N/ha as basal fertilizer and 45 kg N/ha as tillering fertilizer at 10 cm depth, T3), deep placement of tillering fertilizer (surface broadcast at 90 kg N/ha as basal fertilizer and 30 kg N/ha as tillering fertilizer at 10 cm depth, T4) and two rice cultivars i.e., Wufengyou 615 and Yuxiangyouzhan . Results showed that grain yield remained 18.8% and 12.4% higher owing to increase in productive panicles and grain filling, 42.2% and 38.9% higher nitrogen agronomic efficiency, 4.6% and 10.4% higher nitrogen recovery efficiency, and 10.2% and 7.8% higher nitrogen physiological efficiency in T3 than T2 for Wufengyou 615 and Yuxiangyouzhan , respectively. Moreover, nitrogen derived from fertilizer in T3 was increased by 8.0% and 4.4% compared to T2 for Wufengyou 615 and Yuxiangyouzhan , respectively whereas the residual rate of T3 was increased by 31.5%. In addition, the T3 treatment substantially enhanced the leaf area index and leaf chlorophyll content that led to improved photosynthesis and the activities of enzymes related to nitrogen metabolism. Overall, 45 kg N/ha as tillering fertilizer at 10 cm depth and surface broadcast at 90 kg N/ha as basal fertilizer can improve grain yield as well as nitrogen use efficiency in late season rice production system of South China. • Precise delivery of nitrogen (PDN) at tillering stage (TS) can enhance grain yield in rice. • PDN at TS can improve nitrogen use efficiency in rice. • PDN at TS can increase nitrogen residual rate in the soil and decrease nitrogen unaccounted rate.

Effects of Applying Different Organic Materials on Grain Yield and Soil Fertility in a Double-Season Rice Cropping System

Double-cropping rice cultivation reduces soil fertility, and the extensive use of chemical fertilizers has harmful effects on both the environment and grain yield. The application of organic materials could be used as a practical strategy to maintain soil fertility and improve grain yield in a double-season rice cropping system. For this purpose, field experiments with six growing seasons over three years, from 2016 to 2018, were conducted to assess the effects of five organic materials (biochar, Chinese milk vetch, rice straw, rapeseed cake fertilizer, and manure) on the grain yield and soil fertility, aiming to save about 25% of the chemical nitrogen (N) fertilizer required for all rice growing stages. The result showed that, compared with CK (the most common dose of fertilizer in this study region; 100% chemical fertilizer without organic fertilizer), the grain yield and soil fertility of double-cropped rice were increased after applying organic fertilizers for three consecutive years. Specifically, the CRC treatment (Chinese milk vetch (10.77 t ha−1 in fresh)/rice straw (26.51 t ha−1 in fresh) + 75% chemical fertilizer) showed significantly higher rates of effective panicles (4.65–10.92%) and annual grain yield (8.00–8.82%). The total N, total phosphorus (P), total potassium (K), alkaline N, and available P content in the CRC soil were significantly increased by 11.85%, 12.22%, 15.08%, 23.32%, and 41.04%, respectively, relative to CK. The decomposition of the applied Chinese milk vetch and rice straw combined with 75% chemical fertilizer resulted in more soil humus (9.50 g kg−1), humic acid (3.19 g kg−1), fulvic acid (3.26 g kg−1), and active organic carbon (5.78 g kg−1) and a significantly higher carbon pool management index (13.5%), as well as significantly higher soil urease activity (18.10%) and acid phosphatase activity (17.64%). Therefore, in this study, Chinese milk vetch (10.77 t ha−1 in fresh) in the early rice season/rice straw (26.51 t ha−1 fresh) in the late rice season + 75% chemical fertilizer treatment was the optimal dose for the double-season rice cropping system. It resulted in higher rice yields and has the potential to be used for more sustainable soil fertility.

Organic cultivation induced regulation in yield formation, grain quality attributes, and volatile organic compounds of fragrant rice

Organic cultivation is an eco-friendly planting strategy. To explore the effects of organic cultivation on yield, grain quality, and volatile organic compounds of fragrant rice, the present study conducted a field experiment with organic cultivation and inorganic cultivation treatments over two subsequent growing seasons. The late season rice was sent for the determination of untargeted flavoromics using GC × GC-ToF-MS. Meanwhile, the GC × MS were used for the quantification of 2-acetyl-1-pyrroline. Our results showed that compared with inorganic cultivation, organic cultivation increased grain 2-AP and amylose contents by 21.19–26.10 % and 4.51–7.69 %, respectively. 5.89 %-15.55 % lower crude protein contents were recorded in organic cultivation. The results of untargeted flavoromics showed that organic cultivation caused the absence of 56 volatiles and the exclusive presence of 10 new volatiles compared with inorganic cultivation. In conclusion, organic cultivation produced fragrant rice with lower yield and higher 2-AP content as well as different flavor profiles.

Quantifying the effects of advection on single crop coefficients over a humid paddy field for sustainable irrigation

Understanding the variation and magnitude of crop coefficient (Kc) is important to accurate determine crop evapotranspiration (ETc) and optimal irrigation scheduling. Sensible heat advection is expected to increase Kc by providing additional energy, while its effect on Kc over paddy field is rarely featured. A three-year experiment was conducted over a paddy field to develop local Kc using the eddy covariance technique (EC) in the Poyang River basin, southern China. The local Kc curve, the characteristics of advection and its contribution to Kc were investigated. The three-year average local Kc values during the initial, mid-season and late-season stages were 1.12, 1.29 and 1.13 for the early rice and 1.11, 1.39 and 1.02 for the late rice, respectively. The advection was more likely to occur during the late rice season than the early rice season. The three-year average advective days were 8.0 d for the early rice season and 29.7 d for the late rice season, respectively. The contribution of advection to daily Kc ranged from 0.6% to 13.2% for the early rice season and 1.1% to 37.7% for the late rice season, respectively. The upwind city was found to be a possible energy source for advection. The occurrence of advection was closely related to air temperature gradients (ΔT) between the city and paddy field and wind speed (u2). Our study indicated advection also existed in the humid paddy field and had considerable effects on Kc. These results are helpful for decision-makers to quantify crop water consumption and improve irrigation water efficiency.

Effects of mixed fertilizers formed by the compounding of two targeted controlled-release nitrogen fertilizers on yield, nitrogen use efficiency, and ammonia volatilization in double-cropping rice

One-time application of mixed fertilizer formed by the compounding of two controlled-release nitrogen fertilizers (CRUs) with targeted N supply during the periods from transplantation (TS) to panicle initiation (PI) and from PI to heading (HS) is expected to synchronize the double-peak N demand of rice. However, its effects on the yield and N use efficiency (NUE) of labor-intensive double-cropping rice were unknown. Two targeted CRU (CRUA and CRUB) were compounded in five ratios (CRUA: CRUB = 10:0, 7:3, 5:5, 3:7, and 0:10) to form five mixed fertilizers (BBFs): BBF1–5. A field experiment was performed to investigate the characteristics of N supply in early and late seasons under different BBFs and their effects on N uptake, yield, and ammonia volatilization (AV) loss from paddy fields of double-cropping rice. Conventional high-yield fertilization (CK, three split applications of urea) and zero-N treatments were established as controls. The N supply dropped significantly with the increased compound ratio of CRUB during the period from TS to PI, but increased during the period from PI to HS. With the exception of the period from TS to PI in the late rice season, the N uptake of early and late rice maintained close synchronicity with the N supply of BBFs during the double-peak periods. Excessive N supply (BBF1 and BBF2) in the late rice season during the period from TS to PI increased N loss by AV. The effect of BBF on grain yield increase varied widely between seasons, irrespective of year. Among the BBFs, the BBF2 treatment of early rice not only stabilized the spikelets per panicle but also ensured a high number of effective panicles by promoting N uptake during the period from TS to PI and a high grain-filling percentage by appropriately reducing the N supply at the later PI stage, resulting in the highest rice yield. While stabilizing the effective panicle number, the BBF4 treatment of late rice increased the number of spikelets per panicle by promoting N uptake during the period from PI to HS, resulting in the highest rice yield. The two-year average yield and apparent N recovery efficiency of the BBF2 treatment during the early rice season were 9.6 t ha−1 and 45.3%, while those of late rice in BBF4 were 9.6 t ha−1 and 43.0%, respectively. The yield and NUE indexes of BBF2 in early rice and BBF4 in late rice showed no significant difference from those of CK. The AVs of BBF2 during the early rice season and of BBF4 during the late rice season were 50.0% and 76.8% lower, respectively, than those of CK. BBF2 and BBF4 could effectively replace conventional urea split fertilization in early and late rice seasons, ensuring rice yield and NUE and reducing AV loss in paddy fields.

Long-term green manuring to substitute partial chemical fertilizer simultaneously improving crop productivity and soil quality in a double-rice cropping system

Rotation with Chinese milk vetch (MV) as winter green manure in double-rice cropping system is an efficient approach to substitute chemical fertilizer (CF). However, the optimal substitution rate of MV for CF and its long-term effects on crop productivity and soil quality remain unknown. We conducted a 13-year field experiment with different substitution rates of MV for CF (22.5 t ha−1 fresh MV incorporation with 100%, 80%, 60%, and 40% of the recommended nitrogen (N) and potassium (K) supply, represented by MV+F100, MV+F80, MV+F60 and MV+F40, respectively) to investigate its long-term effects on yield sustainability and soil quality in double rice cropping system. Compared with the sole application of CF, the MV+F100 and MV+F80 treatments significantly increased the early rice yields by 15.6% and 11.3% and the late rice yields by 9.3% and 6.3%, respectively. Utilization of MV to substitute 0–40% of the N and K in early rice season and 0–20% of the N and K in late rice season significantly increased the soil quality index (SQI). However, further increasing the substitution rate decreased the sustainability yield index (SYI) and SQI. The soil available phosphorus had a stronger effect on the early and late rice grain yield than the other soil physicochemical properties. Meanwhile, rice grain yields significantly increased with increasing SQI, and the apparent N, P and K balances positively affects SQI. The SYIs of early and late rice crops linearly increased with increasing SQIs, while the coefficient of variance in grain yield (CVy) decreased with increasing SQI. Consequently, long-term utilization of MV to substitute 0–40% of the N and K in early rice season and 0–20% of the N and K in late rice season can simultaneously improves yield sustainability and soil quality.

Product Type, Rice Variety, and Agronomic Measures Determined the Efficacy of Enhanced-Efficiency Nitrogen Fertilizer on the CH4 Emission and Rice Yields in Paddy Fields: A Meta-Analysis

Enhanced-efficiency nitrogen fertilizer (EENF) is a recommend nitrogen fertilizer for rice production because of its advantage on improving nitrogen use efficiency. However, its efficacy on CH4, the dominant greenhouse gas, emission from rice fields showed great variation under field conditions. And the factors influencing its efficacy are still unclear. We synthesized the results of 46 field studies and analyzed the impact of product type, rice variety, and primary agronomic measures (rice cropping system, nitrogen (N) application rate, and water management options) on the effectiveness of EENF on the CH4 emission and rice yield. Overall, EENF, including inhibitors (IS) and slow/control-released fertilizer (S/CRF), significantly reduced CH4 emission by 16.2% and increased rice yield by 7.3%, resulting in a significant reduction in yield-scaled CH4 by 21.7%, compared with conventional N fertilizer. Nitrapyrin, DMPP (3,4-dimethylpyrazole phosphate), and HQ (Hydroquinone) + Nitrapyrin showed relative higher efficacy on the mitigation of CH4 emission than other EENF products; and HQ showed relative lower efficacy on rice yield than other EENF products. The reduction in CH4 emission response of hybrid rice varieties to IS and S/CRF was greater than that of inbred rice varieties. IS significantly reduced the CH4 emission and increased the rice yield under all three rice cropping systems, and showed the highest efficacy in the late rice season of double rice cropping system. Whereas, S/CRF did not significantly reduce the CH4 emission from rice seasons of single rice cropping system and rice-upland crops rotation system. IS did not reduce the CH4 emission when N application rate less than 100 kg ha−1, and S/CRF did not affect the CH4 emission when N application rate less than 100 kg ha−1 or above 200 kg ha−1. Continuous flooding was unfavorable for IS and S/CRF to mitigate CH4 emission and enhance rice yield. These results emphasized the necessary to link EENF products with rice varieties and agronomic practices to assess their efficacy on CH4 emissions and rice yield.

Assessing the Genetic Improvement in Inbred Late Rice against Chilling Stress: Consequences for Spikelet Fertility, Pollen Viability and Anther Characteristics

The development of varieties with strong tolerance is one of the important strategies to diminish the negative impact of chilling stress during heading on the spikelet fertility and yield formation of late-season rice. However, whether such genetic improvement has been made in inbred late rice lines in China is not clear. In the present study, three late-season inbred rice varieties, Xiangwanxian2 (XWX2, released in 1988), Xiangwanxian8 (XWX8, released in 1998) and Xiangwanxian17 (XWX17, released in 2008) were subjected to moderate (20 °C) and extreme (17 °C) chilling stress during heading, and the grain yield components and flowering-related traits of the three varieties in response to different temperature were investigated. The results showed that the newly released inbred late rice variety XWX17, demonstrated better chilling tolerance during heading than the early released varieties with respect to higher grain filling percentage. The improved grain filling percentage in XWX17 might be the results of increased spikelet fertility, which was attributed to the increase in pollen viability, anther dehiscence length and anther volume. In addition, the SPAD value and the chlorophyll a content of the flag leaf can be used as indicators to predict the rice spikelet fertility when suffering from chilling stress during heading. The present study provides evidence that the genetic approach has been made to improve the chilling tolerance of inbred late rice lines during heading; however, further research is needed to explore the physiological and molecular mechanism underlying the relationship between leaf characteristics and function with rice spikelet fertility.

Impacts of Uptake and Accumulation of Cd on Double Rice-Paddy Soil by Silicon Fertilizer Continuous Application

The impacts of silicon (Si) fertilizers on cadmium (Cd) bioavailability in soil and Cd accumulation in paddy-rice plants were investigated in a field positioning test. The results indicated that the continuous application of 180 kg·hm-2 Si fertilizers improved soil bioavailable Si in paddy-rice fields by 108.1%-275.0% and improved pH values by 1.15-1.33 in soil. The difference in the contents of DTPA-Cd in soil by 12.3% and 15.9% was significant in early and late rice, respectively. The continuous application of silicon promoted the transformation of soil cadmium to a stable form and reduced the contents of exchangeable Cd and carbonate binding state Cd in the soil by 2.6%-5.1% and 8.6%-24.9%. The contents of ferric manganese oxide binding state Cd were increased significantly; meanwhile, the contents of organic binding state Cd and residue state Cd in soil were improved by 2.3%-12.8% and 2.3%-6.0%, respectively. The contents of Cd in the root, shoot, and rice were reduced by 38.4%, 49.7%, and 50.9% in early season rice and by 30.6%, 34.4%, and 39.2% in late season rice, respectively. The absorption factors and translocation factors were reduced significantly by 25.5%-49.6% and 13.5%-52.6%, and the average bioaccumulation factors were decreased by 6.0% and 8.0% in early and late season rice. The powder Si fertilizer had the best results overall. In conclusion, Si fertilizer could reduce the absorption and accumulation in rice; however, the continuous Cd reduction effect of continuous application Si fertilizer was not obvious.

Grain yield and nitrogen use efficiency of an ultrashort-duration variety grown under different nitrogen and seeding rates in direct-seeded and double-season rice in Central China

Nitrogen (N) and seeding rates are important factors affecting grain yield and N use efficiency (NUE) in direct-seeded rice. However, these factors have not been adequately investigated on direct-seeded and double-season rice (DDR) in Central China. The objective of this study was to evaluate the effects of various N and seeding rates on the grain yield and NUE of an ultrashort-duration variety grown under DDR. Field experiments were conducted in 2018 in Wuxue County and 2019 in Qichun County, Hubei Province, China with four N rates and three seeding rates. The results showed that the grain yield of the ultrashort-duration variety ranged from 6.32 to 8.23 t ha–1 with a total growth duration of 85 to 97 days across all treatments with N application. Grain yield was increased significantly by N application in most cases, but seeding rate had an inconsistent effect on grain yield. Furthermore, the response of grain yield to the N rates was much higher than the response to seeding rates. The moderate N rates of 100–150 and 70–120 kg N ha–1 in the early and late seasons, respectively, could fully express the yield potential of the ultrashort-duration variety grown under DDR. Remarkably higher N responses and agronomic NUE levels were achieved in the early-season rice compared with the late-season rice due to the difference in indigenous soil N supply capacity (INS) between the two seasons. Seasonal differences in INS and N response should be considered when crop management practices are optimized for achieving high grain yield and NUE in ultrashort-duration variety grown under DDR.

Sympatric genetic divergence between early- and late-season weedy rice populations.

Summary Sympatric genetic divergence is the most appealing and controversial pattern in the theory of ecological speciation. Examples that support sympatric genetic divergence in plant species are extremely rare. Solid evidence of sympatric genetic divergence will provide deep insights for revealing the underlying mechanisms of ecological speciation.We analysed the total genomic DNA sequences of 120 weedy rice (WR; Oryza sativa f. spontanea) plants, representing three WR population pairs separately from three early‐ and late‐season rice fields, in comparison with those of the co‐occurring rice cultivars and other rice materials.We detected substantial genetic divergence within the pairs of the sympatric early‐ and late‐season WR populations, although genetic divergence was unevenly distributed across the genomes. Restricted gene flow was determined between the sympatric WR populations, resulting in their distinct genetic structures. We also detected relatively low genetic diversity that was likely to be associated with stronger selection in early‐season WR populations.Our findings provide strong evidence for sympatric genetic divergence between the WR populations in the same fields but in different seasons. We conclude that temporal isolation plays an important role in creating genetic divergence between sympatric populations/species in plants.

Effects of long-term organic matter application on soil carbon accumulation and nitrogen use efficiency in a double-cropping rice field

Increasing soil carbon (C) sequestration in paddy field and improving rice nitrogen use efficiency (NUE) are vital for sustainable agriculture and environmental protection. It was a benefit practice for achieving these goals by taken rice straw and organic manure managements. However, there is still need to further investigate the effects of different long-term fertilizer managements on soil C sequestration and NUE under the double-cropping rice system in southern of China. Therefore, the effects of different long-term (36-years) fertilizer practices on soil C sequestration and NUE under the double-cropping rice system in southern of China were investigated in the present paper. The field experiment was included four different fertilizer treatments: chemical fertilizer alone (MF), rice straw residue and chemical fertilizer (RF), 30% organic manure and 70% chemical fertilizer (OM), and without fertilizer input as a control (CK). This result indicated that soil C content at plough layer in paddy field with RF and OM treatments were increased, compared with MF and CK treatments. Besides input C directly into paddy field, soil original organic C accumulation with RF and OM treatments were increased by 1.54% and 3.01%, compared with MF treatment. This result indicated that soil TOC content increase rate and annual topsoil organic C sequestration rate in paddy field with RF and OM treatments increased by 55.56%, 88.89% and 48.05%, 76.62%, compared with MF treatment, respectively. Compared with MF treatment, NUE with RF and OM treatments increased by 10.43% and 22.61%, respectively, mainly due to increasing soil organic C. Grain yield of double-cropping rice with RF and OM treatments increased by 1009.5 and 1166.5 kg ha−1, compared with MF treatment, respectively. This result indicated that there was significantly correlation between NUE/NUENPK and TOC content with RF and OM treatments, at early rice and late rice growth seasons. Therefore, it was benefit practice for increasing soil carbon sequestration and improving rice NUE in the double-cropping rice system with long-term application of rice straw and organic manure managements.

The Accumulation of Biomass Pre- and Post-Silking Associated with Gains in Yield for Both Seasons under Maize–Rice Double Cropping System

Due to relatively low yield as well as low resources use efficiency with double rice (Oryza sativa L.) cropping systems (RR), exploring new cropping systems to increase yield and resources use efficiency simultaneously has become a large challenge of the middle reaches of the Yangtze River (MRYR). Our previous study demonstrated that the maize (Zea mays L.)–rice cropping system (MR) exhibited higher superiority of yield and resource use efficiency compared with the conventional double-rice cropping system. However, the reason for the yield increases in both maize and rice and the physiological processes involved in those two crops under MR are poorly understood. A 3-year field experiment was conducted at two sites (Wuxue and Jingmen) from 2016 to 2018 to examine the differences in dry matter (DM) accumulation, soil properties, and resources use efficiency between the MR and RR cropping systems. Compared with RR, the annual yield of MR was 18.2–26.3% and 15.4–31.5% higher across three years at Wuxue and Jingmen, respectively. The average yield of maize in MR was 36.5% and 21.9% higher than that of early rice in RR at Wuxue and Jingmen, respectively. The yield increase for maize was mainly attributed to the 29.7% (Wuxue) and 28.5% (Jingmen) increases in post-silking DM accumulation due to the higher plant growth rate promoted by the higher net assimilation rate and radiation use efficiency. For the late rice in MR, the average yield was 10.9% and 14.5% higher than that of late rice in RR at Wuxue and Jingmen, respectively, which was promoted by the 7.8–23.3% increase in pre-anthesis DM accumulation due to improved soil properties. Compared with RR, the MR cropping system exhibited increased soil pH, total organic carbon, and mineral nitrogen, and decreased the bulk density in the late rice season. As a result of greater yield in both seasons under MR, the annual accumulated temperature and radiation use efficiency, partial factor productivity from applied nitrogen, and water use efficiency of MR were 17.7–26.4%, 22.2–25.5%, 5.5–7.8%, and 33.6–48.7% higher than those of RR, respectively. We conclude that the higher yield in the MR than in the RR cropping system was mainly attributed to the accumulation of post-silking biomass due to maximized use of radiation in the maize season, and the accumulation of pre-anthesis biomass due to improved soil nutrients in the late rice season.

The impacts of post-anthesis warming on grain yield and quality of double-cropping high-quality indica rice in Jiangxi Province, China

In southern China, climate warming threatens rice production in double rice cropping systems. However, the actual responses of grain yield and quality of high-quality indica rice to climate warming in double rice cropping systems are still unclear. Therefore, a 2-year field post-anthesis warming experiment was conducted by using two high-quality indica rice cultivars (e.g., Qiliangyou2012, early rice; Taiyou398, late rice). The results showed that post-anthesis warming (PAW) increased the rice canopy daily mean temperatures from heading to maturity by an average of 1.9 °C and 2.0 °C over the two years in the early and late rice seasons, respectively. The grain yield, filled grain percentage, and grain weight were not affected by PAW for either early or late rice in the two years. For early rice, PAW decreased the head rice rate and setback, whereas it increased the chalky grain rate, chalkiness, peak viscosity, breakdown, pasting temperature, protein and most amino acid contents. Similarly, the head rice rate, amylose content, and setback of late rice were decreased, whereas the chalky grain rate, chalkiness, peak viscosity, breakdown, protein and most amino acid contents were increased under PAW conditions. These results suggested that PAW worsened the grain milling and appearance qualities and improved the rice eating and nutritional qualities for both early and late rice, while it reduced the cooking quality of early rice. This study provides new insight into improving the grain yield and quality of high-quality indica rice in a double rice cropping system under future climate warming conditions.

Optimizing Irrigation and Nitrogen Management to Increase Yield and Nitrogen Recovery Efficiency in Double-Cropping Rice

Water and nitrogen are the key factors affecting the yield and nitrogen recovery efficiency of double-cropping rice, but information about optimizing nitrogen fertilizer and irrigation management to achieve high yield is still limited. The purpose was to study the effects of different nitrogen application rates (D1, D2, D3, D4 (0, 112.5, 150.0, and 187.5 kg ha−1 for early-season rice and 0, 135, 180, and 225 kg ha−1 for late-season rice)) and irrigation conditions (G1, Alternate wetting and drying irrigation, G2, flooding irrigation) on rice yield and nitrogen recovery efficiency. Field experiments were carried out in the early and late seasons of the subtropical environment in Heshan County, Hunan Province, China in 2018 and 2019. The results showed that the yield was increased by the comprehensive action of reasonable irrigation mode and nitrogen management. Under G1D4 and G1D3 treatments, the maximum grain yield of early-season rice was 7.42 t ha−1, which was 0.8~35.9% higher than other treatments, and that of late-season rice was 8.20 t ha−1, which was 13.3~67.0% higher than other treatments. The increase of yield in G1D4 and G1D3 treatments was due to the increase in dry matter accumulation, effective Panicles number, and Spikelets per panicle, whose increase was due to an increase in photosynthesis and nitrate reductase activity. Compared with other treatments, late-season rice G1D3 treatment achieved a higher yield with less nitrogen fertilizer, while early-season rice G1D4 treatment achieved the highest yield, but the nitrogen recovery efficiency decreased. We should focus on more effective nitrogen fertilizer and irrigation management to further improve the yield potential of double-cropping rice.

Effects of free-air temperature increase on grain yield and greenhouse gas emissions in a double rice cropping system

The responses of grain yield and greenhouse gas (GHG) emissions to climate warming in rice paddies are serious concerns for both global food security and climate change mitigation. However, the impact of free-air temperature increase (FATI) on grain yield and methane (CH 4 ) and nitrous oxide (N 2 O) emissions remains unclear for one of the most globally significant rice production practices: the double rice cropping system. Here, we conducted a two-year field experiment to examine the effect of FATI by infrared heaters on grain yield and CH 4 and N 2 O emissions in a double rice field in subtropical China. FATI increased rice canopy temperature by 2.0 °C and soil temperature by 1.2 °C on average but had no effect on either early or late rice yield production. FATI did not affect CH 4 emissions, but significantly increased N 2 O emissions in both the early and late rice seasons. Averaged across two years, N 2 O emissions increased by 0.3 kg ha −1 and 0.7 kg ha −1 in the warmed plots for the early and late rice seasons, respectively. Consequently, FATI enhanced the global warming potential (GWP) and GHG intensity (i.e., yield scaled GWP) in the double rice cropping system. In addition, warming increased the abundance of ammonia-oxidising bacteria, ammonia-oxidising archaea, and a nitrite reductase gene ( nirS ), but decreased that of a nitrous oxide reductase gene ( nosZ ). Our results indicate that warming by FATI does not affect grain yield and CH 4 emissions but stimulates N 2 O emissions in the double rice cropping system and therefore promotes a potential positive feedback with future climate warming. • Free-air temperature increase (FATI) with infrared heater was performed in a double rice field. • Moderate warming by FATI had no significant effect on rice yield. • Moderate warming by FATI did not significantly affect CH 4 emissions. • Moderate warming by FATI stimulated N 2 O emissions.

Nitrogen-Reduction in Intensive Cultivation Improved Nitrogen Fertilizer Utilization Efficiency and Soil Nitrogen Mineralization of Double-Cropped Rice

Under the current rice cropping system, excessive nitrogen application has become a major issue that needs to be changed, and nitrogen reduction has become a hot research topic in recent years. The use of optimum planting density is becoming a common agronomic management system in addition to nitrogen reduction, especially under double cropping rice systems. In this paper, changes in rice yield, nitrogen-use efficiency (NUE) and net N mineralization under dense planting with a reduced nitrogen rate (DPRN) were studied. By comparing DPRN with high-nitrogen sparse planting (SPHN), we found that the population tiller number (tiller number per unit area) increased by 9–27% under DPRN cultivation. Nitrogen accumulation under DPRN treatment of double-cropped rice was basically stable. NUE under DPRN was significantly higher by 1.3–22.7% compared to SPHN. The partial factor productivity of applied N (PFPN) was significantly higher than that of SPHN, with an increase of 4.3–22.8%. The net N mineralized of double-cropped rice under DPRN increased at different stages, and the increase in late-season rice (LSR) was greater than that of early-season rice (ESR). The highest net N mineralized in double cropping rice at different stages was found in the dense planting treatment (DP) and N2 (120 kg N h−1). In conclusion, DPRN cultivation of double-cropped rice could be accepted as a proper management strategy for reducing nitrogen input, improving NUE and promoting soil nitrogen mineralization under given conditions.

Dense Planting with Reducing Nitrogen Rate Increased Nitrogen Use Efficiency and Translocated Nitrogen in Grains in Double-Cropped Rice

Nitrogen fertilization and planting density are two key factors that influence the yield of rice. Reducing nitrogen fertilizer input and increasing planting density will help to improve nitrogen use efficiency and stabilize yield. Field and 15N tracer method in plot experiments were conducted to study the trends of yield, nitrogen use efficiency (NUE) and nitrogen transfer of hybrid rice and conventional rice under dense planting with a reduced nitrogen rate (DPRN) and sparse planting with a high nitrogen rate (SPHN). Among the nitrogen in rice plants, the proportion of nitrogen from fertilizer under the DPRN was reduced by 1.8–13%. The late-season rice (LSR) had a higher rate of decrease compared with the early-season rice (ESR). The uptake efficiency of nitrogen fertilizer was significantly higher under the DPRN than that under the SPHN, with an increase of 7.7–21.9%. The accumulated nitrogen and translocated ratio under the DPRN before the heading stage were 6.1–10.8% and 2.0–9.6% higher than those under the SPHN, respectively. The yield did not change under different treatments. Those findings suggest that the DPRN could guarantee a stabilized yield while increasing the NUE and the amount of translocated nitrogen in the double-cropped rice system.

Suitable varieties and high-yielding population characteristics of late season rice in the northern margin area of double-cropping rice along the Yangtze River

<p id="C2">To clarify the suitable types of rice varieties and the mechanism of high yield formation, we mainly analyzed the yield difference between<italic> indica</italic> and <italic>japonica</italic> rice in late season in the northern margin of the double-cropping rice along the Yangtze River. The experiment was conducted from 2018 to 2019 in Lujiang, Anhui province. The differences of yield and yield components, dry matter accumulation, and growth period between <italic>indica</italic> and <italic>japonica</italic> rice were compared and analyzed through 17 <italic>indica</italic> rice and 17 <italic>japonica</italic> rice varieties (strains) which were cultivated in this local area. The results showed that the overall yield level of <italic>japonical</italic> rice was similar to that of <italic>indica</italic> rice. However, compared with <italic>indica</italic> rice, the<italic> japonica</italic> rice had the characteristics of fewer yield-limiting factors, superior safety of panicle fullness, higher yield stability, and ecological safety. High-yield <italic>japonica</italic> rice mainly increased the total dry matter accumulation from transplanting to heading stage so as to improve total spikelets and yield. The effects of dry matter accumulation at different stage with different panicle types on total spikelets were quite different. Among them, the dominance period of dry matter accumulation of medium-large panicle (102.3-112.0 grains per panicle) and small panicle <italic>japonica</italic> rice (52.0-99.7 grains per panicle) are transplanting to panicle differentiation and panicle differentiation to heading stage respectively and the correlation coefficients with total spikelets were 0.709 and 0.829, respectively. Further analysis revealed that dry matter accumulation during transplanting to panicle differentiation stage of large panicle <italic>japonica</italic> rice was significantly affected by the number of tillers at tillering stage, while the dry matter accumulation during panicle differentiation to heading stage of small panicle <italic>japonica</italic> rice was affected by the number of tillers and dry weight of single stem at heading stage. In summary, this study suggested that <italic>japonica</italic> rice was a suitable late season rice variety in the northern margin area of double-cropping rice along the Yangtze River. The number of spikelets in the high-yielding <italic>japonica</italic> rice was 33.8×10³-41.0×10³ m^-2, the dry matter accumulation at maturity was 15.2-16.6 t hm^-2, and the daily yield was 58.9-64.3 kg hm^-2 d^-1. Regarding the selection of varieties, high-yield large-panicle <italic>japonica</italic> rice, such as Jia 58, Wuyujing 6571, and Changyou 5, should have strong tillering ability; while small-panicle rice varieties, such as Ningjing 7, Wuyujing 5745 and Huxiangjing 151, should have a suitable number of tillers at heading stage and dry weight of single stem. In addition, it was necessary to study the optimized cultivation techniques for high-yielding <italic>japonica</italic> rice varieties with different panicle types in order to further develop the yield potential.