Early-season Rice Research Articles

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.

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.

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.

Combining Rice Straw Biochar With Leguminous Cover Crop as Green Manure and Mineral Fertilizer Enhances Soil Microbial Biomass and Rice Yield in South China.

Whether combining rice-straw biochar (RSB) with leguminous cover crop (LCC) has synergistic effects in the rice production system or not, is still unknown. Two pot experiments were conducted to systematically explore the impacts of RSB on mass decomposition and nitrogen (N) release from LCC residues after incorporation into acidic paddy soil. Similarly, the effect of combining these two factors on soil nutrient status and microbial biomasses in the rice production system was also examined. Five treatments, namely, no N fertilizer (CK), 100% N fertilizer (150 kg N ha–1 as N100), 80% N fertilizer plus RSB (N80B), LCC (N80M), and a combination of RSB with LCC (N80BM), were included. The results indicated that biomass decomposition and N release pattern followed a double exponential decay model such that the addition of RSB slightly stimulated the rates of both mass decomposition and N release during the initial rapid phase of decomposition. Thereafter, it notably slowed down the rates of both these parameters during the relatively slower stage of incorporating LCC residues to paddy soil during early rice season. Compared to 100% N, applying 80% N in conjunction with RSB and/or LCC residue increased grain yield and its components (i.e., effective panicles, 1,000-grain weight, and fully filled grains) that subsequently increased N accumulation and its physiological use efficiency (PUEN) of rice shoot. Moreover, under 20% N, applying RSB and/or LCC residue remarkably increased the soil organic matter and total N, and soil microbial populations and biomasses, while the contents of NH4+ and NO3– were decreased in RSB-amended paddy soil (N80B and N80BM), in comparison with N100. Thus, combining RSB with LCC residue is a novel and promising management intervention for reducing mineral fertilizer use, improving soil fertility and rice production, and consequently minimizing the overall production cost in south China.

Low fertilizer inputs do not adversely affect yield or performance of Indica hybrid rice

The enhancement of rice production numbers can be achieved by using quality rice cultivars and fertilizers. The double rice cropping model has given rise to an important rice production system in southern China. Exploring the possibility of whether hybrid vigor could make a substantial contribution to early and late season rice production, and how the heterosis expression of hybrid rice functions under different levels of fertilizer application is of great significance. The objective of this study was to evaluate the grain yield and associated plant traits of popular hybrid and inbred rice varieties with large–scale promotion under conditions of customary (high) and combined (low) fertilization in the early and late seasons of 2017–18 in Changsha County, Hunan Province, China. We found that hybrid rice varieties displayed their respective advantages in the early and late rice seasons, but the advantages in their relative yield traits varied. The leading advantages of early season rice were effective panicle number per hill (EPN), 1000–grain weight (KGW), harvest index (HI), yield, and nitrogen use efficiency (NUE), whereas in late season rice, the foremost advantages were grain number per panicle (GNP), HI, yield, and NUE. The EPN was the prime advantage of early season hybrid rice with a short growth period, and the GNP was the main advantage of late season hybrid rice with a long growth period. Notably, the main yield advantage of hybrid rice was stronger under combined (low) fertilization than under customary (high) fertilization. Hence, high yield can be achieved by selecting the best hybrid rice varieties supported by combined fertilization (lower fertilizer use with higher efficiency).

Conversion from double-season rice to ratoon rice paddy fields reduces carbon footprint and enhances net ecosystem economic benefit

Ratoon rice (RR) system is an alternative to the double-season rice (DR) system in central China due to its high annual yield and relatively lower cost and labor requirement. However, the effect of conversion from DR to RR on the carbon footprint (CF) and net ecosystem economic benefit (NEEB) remains largely unknown. Here, we elucidated the effect by using two early-season rice varieties (ZJZ17, LY287) and two late-season rice varieties (WY103, TY390) for the DR system, and two RR varieties (YLY911, LY6326) for the RR system. The six varieties constituted four cropping systems, including DR1 (ZJZ17 + WY103), DR2 (LY287 + TY390), RR1 (YLY911) and RR2 (LY6326). The two-year experiment demonstrated that RR had 27.37% lower annual CF than DR, which could be attributed to the significantly lower annual CF (by 87.27%) of ratoon crop in RR relative to that of the late-season rice in DR. Direct greenhouse gas (GHG) emissions contributed the most to annual CF in both systems, accounting for 43.28% and 35.39% in DR and RR, respectively. Furthermore, conversion from DR to RR system significantly increased annual NEEB by 30.95%. This increase could be attributed to the 20.25% higher annual grain yield of main crop in RR relative to early-season rice in DR, and 75.32% and 87.27% lower annual costs for agricultural inputs and CF of ratoon crop than late-season rice in DR, respectively. Rice variety also showed certain effects on the yields and GHG emissions in different RR systems. Compared with RR1, RR2 significantly increased annual yield and annual NEEB, while decreased annual CF and annual yield-scaled CF (CFy). These findings suggest that the conversion of the DR system to LY6326 RR system may be a highly promising strategy to simultaneously reduce CF, promote NEEB and maintain high grain yield in central China.

Urea Application Rate for Crop Straw Decomposition in Temperate China

Returning straw to the field has become the most important straw utilization method in China. The aim of this research was to study the appropriate amount of nitrogen fertilizer applied when returning the straw of three major crops (wheat, rice, and corn) to the field in areas under low to high yield levels based on the demand of nitrogen for microbial decomposition of straw. Under the condition of returning 100% straw to the field, we developed the formula for calculating the nitrogen application rate and estimated the urea application rate for the three major grain crops. The results showed that returning straws of wheat, early-season rice, middle-season rice and late-season rice, and corn to the field with urea application at the rate of about 150 kg/ha, 120～135 kg/ha, 75 kg/ha, and 75～90 kg/ha, respectively, can provide sufficient nitrogen for microbial decomposition. The urea application rate for returning 100% wheat straw to the field in Huang-Huai-Hai region, Middle-Lower Yangtze region, Loess plateau region, and Northwest arid region was 135–230 kg/ha, 110–190 kg/ha, 85–145 kg/ha, and 95–165 kg/ha, respectively. 52.5–98.5 kg/ha of urea was used for 100% early rice straw returning to the field in Middle-Lower Yangtze region and South China. In addition, the urea application rate for 100% middle-late rice straw returning to the field was 95–180 kg/ha, 100–185 kg/ha, 95–175 kg/ha, and 75–140 kg/ha, respectively. The rate of urea application for 100% corn straw returning to the field in Northeast China, Huang-Huai-Hai, Northwest arid region, and Southwest China was 60–135 kg/ha, 50–115 kg/ha, 60–135 kg/ha, and 45–105 kg/ha, respectively. The amount of nitrogen fertilizer required for the total return of crop straw is not only affected by crop straw C : N, yield per unit area, and ratio of grass to grain but also affected by straw returning mode, regional nitrogen application level, and other factors. Therefore, the amount of nitrogen fertilizer should be adjusted according to the type of cropping system, soil, and climatic conditions of the specific location. This substantial N input for stimulating straw decomposition may favor N losses with nitrate leaching and nitrous oxide emissions and hold a potential for soil N eutrophication in the long term if the level is not carefully adjusted to the N requirement of the subsequent crops and changes in soil organic matter levels.

CO2 fluxes over two paddy fields with different management practices in Southern China

Management practices in paddy fields are changing in the Poyang Lake basin, southern China. Investigating CO2 fluxes in paddy fields under different management practices is essential for understanding regional carbon cycles affected by human activities. Two-year CO2 fluxes were collected from paired eddy covariance (EC) towers at two paddy fields in southern China. One site is under the traditional farmers' management (direct-seeded early rice and transplanted late rice with continuous flooding (CF) irrigation, FM site), while the other site is under the promotion of milk vetch rotation and alternate wetting and drying (AWD) irrigation (PM site). The results showed both sites were CO2 sink with annual cumulative net ecosystem exchange of CO2 (NEE) of -744±21 and -645±22 g C m−2 in 2017, and -794±22 and -766±20 g C m−2 in 2018 at the PM and FM site, respectively. After considering the grain carbon, the CO2 budgets were -142±21 and -59±22 g C m−2 in 2017, and -170±22 and -173±20 g C m−2 in 2018 at the PM and FM site, respectively. The partitioning of NEE into gross primary productivity (GPP) and ecosystem respiration (Reco) showed that the PM site had higher GPP and Reco, which was mainly caused by the milk vetch growth. The AWD irrigation did not affect the field water conditions of the early rice season, but resulted in 16 more drying days and 5.6% more Reco for the late rice season compared to the FM site. Overall, our results imply that the field with milk vetch rotation and AWD irrigation was able to fix more CO2 in 2017, while it had similar CO2 budgets to the traditional farmers’ field when the light and heat conditions during the direct seeded early rice were affluent in 2018.

Yield and quality of brown rice noodles processed from early-season rice grains

Producing rice noodles using early-season rice grains is a way to bypass difficulties in marketing early-season rice that does not meet consumer preference for soft-textured rice. In recent years, brown rice foods including noodles have attracted great attention due to their health and nutritional benefits. This study was conducted to evaluate the yield and quality of brown rice noodles processed from two early-season rice cultivars. Results showed that the yield of brown rice noodles was 12–19% higher than that of white rice noodles. Although the cooked break rate and cooking loss rate were 5–10% higher in brown rice noodles compared to white rice noodles, both were within an acceptable range for brown rice noodles. Cooked brown rice noodles had 21–27% lower hardness and chewiness than cooked white rice noodles, though differences in the elasticity parameters springiness, cohesiveness, and resilience were not significant or were inconsistent between cooked brown and white rice noodles. These results suggest that it is feasible to process early-season rice to produce brown rice noodles of desirable yield and quality.

Effects of Coconut Chaff Biochar Amendment on Methane and Nitrous Oxide Emissions from Paddy Fields in Hot Areas

Based on the rice-vegetable crop rotation model, in-situ measurements of nitrous oxide (N2O) and methane (CH4) emissions were conducted in double-cropping rice fields in Hainan to determine the impact of coconut chaff biochar on greenhouse gas emissions. The experiment involved four treatments:conventional farming fertilization (CON), nitrogen fertilizer combined with 20 t ·hm-2 biochar (B1), nitrogen fertilizer combined with 40 t ·hm-2 biochar (B2), and no nitrogen fertilizer, as the control (CK). The N2O and CH4 emissions were measured using static chamber-gas chromatography during the two paddy seasons, and the global warming potential (GWP) and greenhouse gas intensity (GHGI) were also estimated. The results show that N2O emission dynamics during the early rice season are closely related to the mineral nitrogen content of the soil. The N2O is emitted at the rice seedling and tillering stages after fertilization. The cumulative N2O emission during the early rice season was 0.18-0.76 kg ·hm-2. Compared with the CON treatment, the biochar treatments reduced N2O by 18%-43%, and the B2 treatment resulted in a significant reduction. The addition of biochar may promote the reduction of N2O at the early rice seedling stage and increase N2O emissions by improving the soil NO3--N content at the early rice tillering stage. During the late rice season, N2O is emitted during the heading and maturity stages, and the cumulative N2O emission was 0.17-0.34 kg ·hm-2. The B1 treatment reduced emissions by 37%, and B2 increased emission by only 3%, which is not a significant difference. The peak of CH4 emissions from rice fields appeared in the late phase of the early rice season and prophase of the late rice season. The cumulative emission of CH4 in the early rice season was 3.11-14.87 kg ·hm-2. Compared with CON, the CK treatment increased emission by 39%. The biochar treatment may increase soil aeration and limit the ability of CH4 production in the early rice season, as B1 and B2 treatments reduced CH4 emissions by 28% and 71%. The cumulative CH4 emission in late rice season was 53.1-146.3 kg ·hm-2, and the emission dynamics were significantly positively correlated with NH4+-N content. CK and B1 treatments increased CH4 emissions by 52% and 99%, respectively compared with CON, and the B2 treatment significantly increased CH4 emissions by 176%. Compared with CON, the B1 and B2 treatments increased the yield by 12.0% and 14.3% when applied in the early rice season and by 7.6% and 0.4% when applied in the late rice season, respectively. Due to the increased methane emissions in the late rice season, biochar amendment increased the GWP of the double-cropping rice field, in which the high amount of biochar reached a significant level; different amounts of biochar had no significant effect on the GHGI of the double-cropping rice field. Thus, the application of coconut chaff biochar for the reduction of greenhouse gas emission, from rice fields in hot areas, requires further research.

Optimizing agronomic traits and increasing economic returns of machine-transplanted rice with side-deep fertilization of double-cropping rice system in southern China

• Machine-transplanted rice with side deep fertilization (MRSF) could optimize agronomic traits and yield components. • MRSF increased grain yield by 0.5 %–20.2 % compared with conventional fertilization (CF). • MRSF increased economic return by 9.9 %–31.8 % compared with CF. • MRSF enhanced NPK fertilizer uptake but reduced their application rates. • MRSF combined with decreased nitrogen input by 20–30 % is the appropriate system. Machine-transplanted rice with side-deep fertilization (MRSF) is a recommended strategy to reduce labor cost and nitrogen (N) fertilizer loss in sustainable double-cropping rice production in China. However, the effects of MRSF on double-cropping rice profitability and agronomic traits are unclear. Here, experiments were conducted in two sites at Dongting Lake to determine the yield and economic benefits of MRSF and the optimal N application rate. Six fertilization treatments for early- and late-season rice at various N rates (0/0, 90/99, 105/116, 120/132, 135/149, and 150/165 kg N ha −1 ) were tested. The grain yield of early-season rice (ESR) was the highest (8.7 t ha −1 ) under 70 % deep fertilization (DF), whereas that of late-season rice (LSR) was the highest (6.1 t ha −1 ) under 80 % DF treatment in Miluo. The grain yield of ESR and LSR (7.5 and 7.9 t ha −1 , respectively) were the highest under 80 % DF treatment in Yiyang. Compared with conventional fertilization (CF), MRSF significantly enhanced grain yield by 0.5 %–20.2 % and 5.3 %–15.0 % in Miluo and Yiyang, respectively. The high grain yield under MRSF can be attributed to higher leaf area index, chlorophyll content, nutrient uptake, biomass accumulation, and spikelet number per m 2 . Relative to CF, MRSF increased the panicle bearing tiller rate by 8.4 %–24.6 % (Miluo) and 3.3 %–12.2 % (Yiyang), thereby improving dry matter accumulation and nutrient uptake. Furthermore, the economic benefit under MRSF increased by 9.9 %–31.8 % (Miluo) and 15.0 %–25.7 % (Yiyang) compared with that under CF. N reduction in the MRSF treatments resulted in fewer effective panicles than the 100 % DF treatment. Improved yield and nutrient uptake in double-cropping rice were associated with deep fertilizer placement rather than N application rate. The number of filled grains per panicle had the strongest influence on ESR and LSR grain yield. Overall, MRSF is recommended as an alternative approach to increase grain yield, profitability, and agronomic traits while lowering the N application rate in the double-cropped rice systems near Dongting Lake, southern China.

Water‐use efficiency and greenhouse gas emissions of rice affected by water saving and nitrogen reduction

AbstractWe investigated the effects of reduced irrigation and N application, using green manure and rice straw, on the water‐use efficiency (WUE) and greenhouse gas (GHG) emissions of machine‐transplanted rice (Oryza sativa L.) to provide theoretical and technical guidance for water and N management in double‐cropping rice. Field trials were conducted from 2014 to 2016 using two irrigation methods (flood irrigation, W0; intermittent irrigation, W1) and three N application methods (no nitrogen application, N0; chemical nitrogen constant application, N1; chemical nitrogen reduction combined with green fertilizer and straw, N2). Methane (CH4) and nitrous oxide (N2O) emissions, water consumption, and grain yield were compared for the different treatment measures. Under the same N fertilizer management, intermittent irrigation reduced cumulative emissions of CH4 of early and late season rice by 14.54–37.37% and 16.70–52.25%, respectively, whereas N2O emissions increased by 12.50–35.29% and 23.08–34.09%, compared with flood irrigation. Under identical irrigation conditions, the cumulative CH4 and N2O emissions of early and late rice were N2 > N1 > N0. Compared to W0N1 treatment, W1N2 decreased the annual average water consumption by 4.40 and 5.53% for early and late rice, whereas yield and WUE increased by 4.27 and 3.98% and 9.54 and 9.71%, respectively. In addition, the annual global warming potential and greenhouse gas intensity of double‐cropped rice increased by 47.62 and 41.77%, respectively. The incorporation of green manure and rice straw with reduced N fertilizer application using intermittent irrigation in double rice cropping systems reduced water consumption and increased grain yield but also increased GHG emissions.

Effects of Water and Fertilization Management on CH4 and N2O Emissions in Double-rice Paddy Fields in Tropical Regions

Paddy soils are widely considered a main source of methane (CH4) and nitrous oxide (N2O). Comprehensively evaluating CH4 and N2O emissions from double-rice systems in tropical regions with different water irrigation and fertilizer applications is of great significance for addressing greenhouse gas emissions from such systems in China. In this study, eight treatments were evaluated:conventional irrigation-PK fertilizer (D-PK), conventional irrigation-NPK fertilizer (D-NPK), conventional irrigation-NPK+organic fertilizer (D-NPK+M), conventional irrigation-organic fertilizer (D-M), continuous flooding-PK fertilizer (F-PK), continuous flooding-NPK fertilizer (F-NPK), continuous flooding-NPK+organic fertilizer (F-NPK+M), and continuous flooding-organic fertilizer (F-M). CH4 and N2O emissions in double-rice fields in tropical region of china were monitored in situ by closed static chamber-chromatography method and crop yields as well as global warming potential (GWP) and greenhouse gas intensity (GHGI) were determined. The results show that:① The cumulative CH4 emissions from early rice and late rice are 10.3-78.9 kg·hm-2and 84.6-185.5 kg·hm-2, respectively. Compared with F-PK and F-NPK treatments, F-NPK+M and F-M treatments significantly increased the cumulative emissions of CH4 from early rice season. Under the same fertilizer conditions, the cumulative CH4 emissions under continuous flooding condition were significantly higher than that under conventional irrigation condition. Irrigation and fertilization had extremely significant effects on CH4 emission in the early rice season. ② The cumulative N2O emissions across all treatments were 0.18-0.76 kg·hm-2 in early rice season and 0.15-0.58 kg·hm-2in late rice season, respectively. During early rice season, compared with F-PK, F-NPK significantly increased the cumulative N2O emission; however, compared with D-PK, D-NPK, D-NPK+M, and D-M treatments significantly increased the cumulative N2O emissions. Compared with F-PK, other three treatments under continuous flooding condition significantly increased N2O cumulative emission in late rice season; compared with D-PK, D-NPK, and D-M treatment significantly increased the cumulative N2O emission. Irrigation and fertilization had significant impacts on N2O emissions in late rice season, and fertilization had significant impacts on N2O emission in early rice season. ③ Early and late rice yields were 7310.7-9402.4 kg·hm-2 and 3902.8-7354.6 kg·hm-2, respectively. Early rice yields in both F-NPK and F-M treatments were significantly higher than those in F-PK, D-PK, and D-NPK treatments. Compared with PK, the other three fertilization treatments under the same irrigation condition significantly increased late rice yield. The GWP and GHGI in early rice season were 580.8-2818.5 kg·hm-2and 0.08-0.30 kg·kg-1, respectively. There was no significant difference in GWP among four fertilizer treatments under conventional irrigation condition in the early rice season. However, F-NPK+M and F-M treatments had a significant increase in GWP compared with F-PK. The GHGI in F-NPK+M and F-M treatments were significantly higher than that in other treatments. The GWP and GHGI in late rice season were 3091.6-6334.2 kg·hm-2 and 0.50-1.23 kg·kg-1, respectively. Irrigation significantly affected GWP and GHGI in both early and late rice seasons but fertilization had no significant impact on GWP and GHGI in late rice season. ④ Correlation analysis results showed that soil NH4+-N content and soil temperature below 5 cm soil layer had an extremely significant negative correlation with CH4 emissions. Soil pH was extremely significant positive correlated with CH4 emissions but significantly negatively correlated with N2O emission. Soil NH4+-N and NO3--N concentrations were extremely significantly negatively correlated with N2O emission. Given crop yield, GWP, GHGI, and D-NPK+M can be recommended for local water and fertilizer management to reduce greenhouse gas emissions while maintaining rice yields.

Stronger impact of urea application than incorporation of Chinese milk vetch (Astragalus sinicus L.) on nirK-denitrifying bacterial communities in a Chinese double-rice paddy

ABSTRACT The impact of Chinese milk vetch (Astragalus sinicus L., CMV) incorporation on the nirK-denitrifying bacteria remains unknown. Here, the diversity and the composition of nirK-denitrifying bacterial communities in a double-rice paddy soil were estimated by using Illumina MiSeq sequencing under four fertilisation treatments: no fertilisation (CK), vetch incorporation (CMV), urea (N200) and combined application of vetch and urea (CMV+N100). Compared with CK, N200 increased Shannon index by 10.5% in early rice season, whereas CMV did not change the Shannon index and Chao1 index in both seasons. Proteobacteria and Bradyrhizobium were the most abundant phylum and genus for nirK-denitrifying bacterial communities, respectively. Most variations in abundance of genus components of nirK-denitrifying bacterial communities as affected by the application of urea were detected in late rice season. Principal coordinates analysis showed that effect of CMV on the nirK-denitrifying bacterial communities was weaker than application of inorganic fertiliser (N200 and CMV+N100). Furthermore, redundancy analysis showed that soil was the key factor driving nirK-denitrifying bacterial communities. Our results indicated that the response of the nirK-denitrifying bacterial communities is more sensitive to urea than vetch, suggesting the combination of vetch and urea can be considered as an alternative N resource to inorganic fertiliser.

Soil enzymatic activities response to long-term fertilization during key growth stages of early rice

ABSTRACT This long-term fertilization experiment was conducted since 1981, included non-fertilization (NF), chemical fertilizer (CF), double chemical fertilizer (DCF) and chemicals with organic fertilizer (COF). In 2017, soil samples were collected at the tillering, heading and maturity stages of early-season rice (Oryza sativa L.). The activities of urease (UR), α-glucosidase (AG), glucosaminidase (NAG), β-1,4 glucosidase (BG), β-cellobiosidase (CBH), β-xylosidase (BXYL), acid phosphatase (ACP), phenoloxidase (POX) and peroxidase (PER) were measured. The activities of AG, NAG, BG, CBH, BXYL, ACP and POX in the maturity stage were significantly higher than in the tillering stage by 343.0%, 153.4%, 55.0%, 8.5%, 1791.5%, 1237.7% and 1179.4%, respectively. Compared with the CF treatment, the activities of UR, AG, BG, and POX in the COF treatment increased by 33.5%, 23.3%, 19.5% and 40.1%, respectively. Meanwhile, both CBH and ACP in the COF treatment were higher than in the DCF treatment by 33.4% and 66.7%, respectively. A significant relationship was identified between UR/PER activities and soil available nitrogen, phosphorus, potassium/rice biomass using the redundancy analysis method. Therefore, the application of organic and chemical fertilizers could increase most enzymatic activities and soil productivity. UR and PER were key factors indicating soil nutrient turnover and rice growth.

The effect of early season rice varieties and establishment methods on greenhouse gas emissions in southeast China

Methane (CH4) and nitrous oxide (N2O) emissions from rice fields are considered to be important sources of greenhouse gases. In order to clarify the emission characteristics of CH4 and N2O at different growth stages and to find optimal early season rice varieties and cultivation methods to control greenhouse gas emissions in southeast China, we conducted research in Jinhua City, Zhejiang Province, China in 2017 and 2018. Four different early season rice varieties were planted using three different rice establishment methods (machine transplanting, artificial transplanting, and direct seeding). The grain yield, CH4, and N2O emission rates and flux and economic profit were compared between the early varieties and establishment methods. Our results show that during the rice growth period, the CH4 and N2O emission fluxes and emission rates of the early season rice varieties increased first and then decreased, reaching the maximum at booting and the minimum at the grain filling stage. Direct seeding had the lowest cost and emission fluxes and emission rates compared to machine transplanting and artificial transplanting. The emission rates and flux of ‘Zhuliangyou609’ and ‘Zhongjiazao17’ were lower than in the other varieties. However, the yield of ‘Zhuliangyou609’ was higher than that of ‘Zhongjiazao17’ under identical conditions. Hence, direct-seeded rice shows good potential for cultivation in Zhejiang Province, and ‘Zhuliangyou609’ is the optimal early variety. This study provides theoretical support for the development of environmentally friendly early season rice establishment methods in southeast China.

Effects of green manure planted in winter and straw returning on soil aggregates and organic matter functional groups in double cropping rice area.

To explore the mechanism of exogenous organic materials enhancing soil organic carbon and soil fertility, based on a long-term experiment located in Hengyang Red Soil Experimental Station, we examined the effects of winter green manure and straw returning patterns (CK, winter fallow; MV, winter Chinese milk vetch; S, early-season rice straw total returning; DS, early-season and late-season rice straw total returning; SMV, winter Chinese milk vetch + early-season rice straw total returning; DSMV, winter Chinese milk vetch + early-season and late-season rice straw total returning) on soil aggregates and organic functional groups. The results showed that the proportion of super aggregates (>2 mm) and macroaggregates (0.25-2 mm) in double cropping rice soil was the highest with a ratio of about 72.1%-81.8%, and the organic carbon content in the two kinds of aggregates was as high as 12.1-20.7 g·kg-1, accounting for 22.7%-59.0% of the total organic carbon. The main organic functional group in paddy soil was polysaccharides, followed by aliphatic carbon and aromatic carbon. The abundance of all those groups was affected by winter Chinese milk vetch growing and straw returning. Compared with other treatments, DSMV significantly increased the proportion of super aggregates (>2 mm) and macroaggregates (0.25-2 mm) and favored the accumulation of inert carbon such as aromatic carbon in the two kinds of aggregates. DSMV could enhance the stability of soil aggregates and organic matter, which had high potential in the real agricultural production.

Contrasting effects of straw and straw-derived biochar applications on soil carbon accumulation and nitrogen use efficiency in double-rice cropping systems

Increasing cropland soil carbon (C) sequestration and improving crop nitrogen use efficiency (NUE) are vital for sustainable agriculture and environmental protection. The incorporation of straw and application of straw-derived biochar to croplands may help to achieve these goals; however, the long-term effects on soil C sequestration and NUE in double-rice cropping systems are poorly understood. Here, a 6-y field experiment was conducted in a double-rice cropping system to investigate the variations in soil C sequestration and NUE with straw and straw-derived biochar amendments. The treatments were as follows: control with zero nitrogen (N0); conventional chemical fertilizer application (NPK); NPK plus 3 t rice straw dry matter (DM) incorporated per ha−1 season−1 (NPK + LS); NPK plus 6 t rice straw DM incorporated per ha−1 season−1 (NPK + HS); NPK plus 24 t straw-derived biochar DM incorporated per ha−1 (NPK + LC) applied once; and NPK plus 48 t straw-derived biochar DM incorporated per ha−1 (NPK + HC) applied once. Both the straw and biochar additions increased the soil C content in the topsoil, and the NPK + LC treatment sequestered 2.6-fold more soil C than the NPK + HS treatment over the 6-y period, when the input amounts of straw as well as the straw derived C were similar for both treatments. Besides input C directly into the fields, biochar application also increased soil original organic C accumulation in the NPK + LC treatment. Compared with NPK, 50 % and 100 % straw incorporation to the field increased the mean NUE by 22.3 % and 39.8 %, respectively, in the late rice season of 4–6 y, mainly due to increasing soil organic C. Rice grain yields decreased in the early rice season for the first 3 y of the straw treatments but increased in the first season when the biochar was applied, when compared with NPK. However, in the second 3 y after straw application, the grain yield of NPK + HS was significantly higher than that of NPK in late rice season. This investigation has indicated that biochar application had a higher potential in soil carbon sequestration than straw incorporation, and long-term (> 3 y) straw incorporation favored for the enhancement of NUE in paddy fields.