Double Rice Cropping System Research Articles

Substitution of Inorganic Nitrogen Fertilizer with Green Manure (GM) Increased Yield Stability by Improving C Input and Nitrogen Recovery Efficiency in Rice Based Cropping System

A long-term field experiment was carried out (since 2008) for evaluating the effects of different substitution rates of inorganic nitrogen (N) fertilizer by green manure (GM) on yield stability and N balance under double rice cropping system. Treatments included, (1) N0 (no N fertilizer and no green manure); (2) N100 (recommended rate of N fertilizer and no green manure); (3) N100-M (recommended rate of N fertilizer and green manure); (4) N80-M (80% of recommended N fertilizer and green manure); (5) N60-M (60% of recommended N fertilizer and green manure); and (6) M (green manure without N fertilization). Results showed that, among all treatments, annual crop yield under N80-M treatment was highest. Crop yield did not show significant differences between N100-M and N80-M treatments. Substitution of different N fertilizer rates by GM reduced the yield variability index. Compared to the N0 treatment, yield variability index of early rice under N100-M, N80-M, and N60-M treatments was decreased by 11%, 26%, and 36%, respectively. Compared to the N0 treatment, yield variability index of late rice was decreased by 12%, 38%, 49%, 47%, and 24% under the N100, N100-M, N80-M, N60-M, and M treatments, respectively. During period of 2009–2013 and 2014–2018, nitrogen recovery efficiency (NRE) was highest under N80-M treatment and N balance was highest under N100 treatment. NRE of all treatments with GM was increased over the time from 2009–2013 to 2014–2018. All treatments with GM showed increasing trend of SOC over the years. Substitution of N fertilizer by GM also increased C inputs and soil C:N ratio compared to the N100 and N0 treatments. Boosted regression model indicated that C input, N uptake and AN were most influencing factors of crop yield. Thus, we concluded that N fertilization rates should be reduced by 20% under GM rotation to attain high yield stability of double rice cropping system through increasing NRE and C inputs.

Substitution of chemical fertilizer by Chinese milk vetch improves the sustainability of yield and accumulation of soil organic carbon in a double-rice cropping system

The double-rice cropping system is a very important intensive cropping system for food security in China. There have been few studies of the sustainability of yield and accumulation of soil organic carbon (SOC) in the double-rice cropping system following a partial substitution of chemical fertilizer by Chinese milk vetch (Mv). We conducted a 10-year (2008–2017) field experiment in Nan County, South-Central China, to examine the double-rice productivity and SOC accumulation in a paddy soil in response to different fertilization levels and Mv application (22.5 Mg ha−1). Fertilizer and Mv were applied both individually and in combination (sole chemical fertilizers, Mv plus 100, 80, 60, 40, and 0% of the recommended dose of chemical fertilizers, labeled as F100, MF100, MF80, MF60, MF40, and MF0, respectively). It was found that the grain yields of double-rice crop in treatments receiving Mv were reduced when the dose of chemical fertilizer was reduced, while the change in SOC stock displayed a double peak curve. The MF100 produced the highest double-rice yield and SOC stock, with the value higher by 13.5 and 26.8% than that in the F100. However, the grain yields increased in the MF80 (by 8.4% compared to the F100), while the SOC stock only increased by 8.4%. Analogous to the change of grain yield, the sustainable yield index (SYI) of double rice were improved significantly in the MF100 and MF80 compared to the F100, while there was a slight increase in the MF60 and MF40. After a certain amount of Mv input (22.5 Mg ha−1), the carbon sequestration rate was affected by the nutrient input due to the stimulation of microbial biomass. Compared with the MF0, the MF100 and MF40 resulted in a dramatically higher carbon sequestration rate (with the value higher by 71.6 and 70.1%), whereas the MF80 induced a lower carbon sequestration rate with the value lower by 70.1% compared to the MF0. Based on the above results we suggested that Mv could partially replace chemical fertilizers (e.g., 40–60%) to improve or maintain the productivity and sustainability of the double-rice cropping system in South-Central China.

Reducing nitrogen surplus and environmental losses by optimized nitrogen and water management in double rice cropping system of South China

Optimized agronomic management improves nitrogen (N) use efficiency in crop production. However, limited information exists about the effect of improved agronomic practices on the N surplus in double rice cropping system. In this study, we conducted field experiments to evaluate the N surplus for the prevailing farmers’ practices (FP), optimized N management (OPTN) and optimized N and water management (OPTNW) during 2016–2017 in Guangdong province, South China. Grain yield, recovery efficiency (REN), partial factor productivity (PFPN) and agronomic efficiency (AE) of applied N in OPTN and OPTNW were substantially higher than FP. The yearly N surplus and environmental N loss in OPTN were 29.4% and 26.2% lower than FP, respectively. The N surplus in OPTNW was 32.1% lower than FP. Annual N losses resulting from runoff and leaching in OPTNW were reduced by 45.0% and 17.4%, respectively, compared with OPTN. Pooled data of 22 on-farm field trials from six sites in 2014–2017 showed that N input in OPTN and OPTNW was 16.2%–33.8% lower than FP. The tradable N output in OPTN and OPTNW was 9.9% and 9.0% greater than FP, respectively. The N efficiency of cropping systems (NUEc) in OPTN and OPTNW was increased by 39.8% and 42.0%, respectively, compared with FP. N surplus notably increased with the increasing fertilizer N input, and decreased with the increasing tradable N output and NUEc. These results suggest that through optimized N and irrigation management, N surplus and environmental risk can be practically reduced in a double rice cropping system without yield penalty.

Seasonal differences in N release dynamic of controlled-released urea in paddy field and its impact on the growth of rice under double rice cropping system

Single basal application of bulk blending urea(BBU) which consists of both controlled-released urea(CRU) and conventional urea(U) has been proposed to facilitate fertilization and lower cost of CRU. Nevertheless, its effect on the growth, yield and N use efficiency(NUE) of rice in double rice cropping system under subtropical environment are still unknown. Field experiments were conducted at two different sites(Taishan county and Wengyuan county) simultaneously in Guangdong province to investigate the effects of single basal application of two types of BBU(P-BBU which consists of parabolic pattern CRU, and S-BBU which consists of sigmodial pattern CRU) on soil available N, grain yield and NUE. Conventional high-yield fertilization treatment with four split applications of U(FP) and no-N treatment(CK) were established as controls. The results demonstrated that BBU could provide similar or more adequate N nutrient for rice with FP during late rice season. BBU performed comparably to or better than FP on soil NH4+-N, dry matter, grain yield and NUE during late rice season. During early rice season, BBU performed comparably with FP on soil NH4+-N, N content of rice, dry matter, grain yield and NUE at Taishan site, while at Wengyuan site, it significantly reduced soil NH4+-N, N content of rice, dry matter and NUE compared with FP, and reduced grain yield slightly. Release tests in water and paddy field were conducted in order to evaluate the releasing dynamics of parabolic and sigmodial pattern CRU. Results showed that the N release rate of CRU in paddy field during early rice season was significantly slower than that during late rice season. This was closely correlated with the changing pattern of accumulated temperature. The N release rate of parabolic CRU in paddy field was faster and more sensitive to changing temperature than sigmodial CRU. Non-linear regression analysis shows that the N release patterns of sigmodial CRU in paddy fields were more close to N uptake of rice during both rice seasons. The grain yield of rice and NUE in S-BBU was higher slightly than P-BBU during both rice seasons. These results demonstrated that BBU obtained similar grain yield in double rice cropping system in South China, and S-BBU treatment is better for N requirement of rice during both seasons. Future studies should focus on optimizing and customizing the blending ratio of BBU for each rice season in double rice cropping system to increase crop yield and the robustness of BBU to changing weather conditions.

Leguminous Cover Crop Astragalus sinicus Enhances Grain Yields and Nitrogen Use Efficiency through Increased Tillering in an Intensive Double-Cropping Rice System in Southern China

Chinese milk vetch (Astragalus sinicus L., vetch), a leguminous winter cover crop, has been widely adopted by farmers in southern China to boost yield of the succeeding rice crop. However, the effects of vetch on rice grain yield and nitrogen (N) use efficiency have not yet been well studied in the intensive double-cropped rice cropping systems. To fill this gap, we conducted a three-year field experiment to evaluate the impacts of the vetch crop on yields and N use efficiency in the subsequent early and late rice seasons. With moderate N input (100 kg N ha−1 for each rice crop), vetch cover significantly increased grain yields by 7.3–13.4% for early rice, by 8.2–10.4% for late rice, and by 8.6–11.5% for total annual rice production when compared with winter fallow. When rice crops received an N input of 200 kg N ha−1, vetch cover increased grain yields by 5.9–18.4% for early rice, by 3.8–10.1% for late rice, and by 6.2–11.3% for annual rice production. Moreover, comparable grain yields (11.9 vs. 12.0 Mg ha−1 for annual rice production) were observed between vetch cover with moderate N and fallow with added N fertilizer. Yield components analysis indicated that the increased tillering number was the main factor for the enhanced grain yields by vetch cover. Vetch cover with moderate and higher N input resulted in higher agronomic N use efficiency and applied N recovery efficiency compared with the fallow treatments. Here, our results showed that vetch as a winter cover crop can be combined with reduced N fertilizer input while maintaining high grain yields, thus gaining a more sustainable rice production system.

Soil physical and chemical quality as influenced by soil tillage managements under double cropping rice system of southern China

ABSTRACTThis study aimed to determine short-term effects of soil tillage and crop residues on soil quality and grain yield of rice in a double-cropping paddy field of southern China. The experiment included four soil tillage treatments: conventional tillage with crop residues incorporation (CT), rotary tillage with crop residues incorporation (RT), no-tillage with crop residues retention (NT), and rotary tillage with crop residues removed as control (RTO). The results showed that the soil bulk density with RTO treatment were higher than with CT and RT treatments. And the soil EC, aggregate mean weight diameter in both wet and dry sieving with NT and RT treatments were higher than that of RTO treatment. CT and RT treatments had the best soil quality and produced the highest rice yield of average 2016–2018. Removing the crop residues gave the lowest yield and less favourable soil physical and chemical characteristics compared to the application of crop residues treatments. It was found that conventional tillage and rotary tillage with crop residues retention is a feasible management technology for farmers producing rice in the double-cropping paddy field of southern China, resulting in a better soil quality and higher yield than with the conventional farmer practice.

Interactive effects of liming and straw incorporation on yield and nitrogen uptake in a double rice cropping system

Soil acidification and low fertility limit crop productivity in red paddy soils. Liming and straw incorporation are effective practices to alleviate soil acidification and improve soil fertility, respectively, while their interaction is still unclear. A four-year field experiment was conducted in a double rice cropping system with red paddy soil in Jiangxi province to examine the interactive effect of liming and straw incorporation on rice yield and nitrogen (N) uptake from 2015 to 2018. Either liming or straw incorporation increased grain yield and N uptake in both early and late rice seasons. Lime application increased yield and N uptake by 10.7% and 15.5% under straw incorporation, while by 4.4% and 9.7% with straw removal in the early rice season, respectively. In contrast, liming enhanced yield and N uptake by 18.7% and 24.6% in the straw-incorporated treatments in the late rice season, respectively, whereas only by 10.5% and 5.7% in the straw-removed treatments. The effect of liming on enhancing grain yield, N uptake and soil pH for both early and late rice diminished in the present of time. Neither soil organic matter nor total N content was significantly affected by liming after the four-year experiment. Straw incorporation significantly increased soil organic matter, but had no effect on total N. Therefore, limes should be applied along with straw incorporation to simultaneously increase double rice yield, alleviate soil acidification, and improve soil fertility on acidic paddies. Besides, our results suggest that limes should be applied approximately every four years in the double rice cropping system with acidic red soil.

Effects of reduced inorganic fertilization and rice straw recovery on soil enzyme activities and bacterial community in double-rice paddy soils

To decrease nutrient losses in paddy soils, use of reduced inorganic fertilization and crop straw return has gained interest in recent years. However, there is limited understanding about the complex responses of soil microbial communities to chemical fertilizers and straw return, especially in the double-rice cropping system. In this study, we report the responses of soil enzyme activities and the bacterial community structure in a double-rice cropping system in southern China after nine years of fertilizer application. Treatments included conventional high inputs of inorganic fertilization, optimum fertilization with reduced inorganic fertilizers (OPT), the combination of inorganic and organic fertilization with rice straw to partially substitute the inorganic fertilizers and keep the inputs of pure N, P, and K the same as OPT (OPT + S), and a no-fertilization control. The soil bacterial communities were examined using Miseq sequencing. The fertilization after nine years significantly increased the soil N, P, K and organic matter contents, but showed little effect on soil pH. The soil enzyme activities were also largely enhanced by fertilization, which were mostly the highest in the OPT + S treatment. The results of Miseq sequencing indicated that the bacterial diversity and community composition were not significantly changed among different fertilization treatments. Nevertheless, they selectively enriched and inhibited the growth of certain bacterial taxa. The OPT + S treatment enriched the highest number of operational taxonomic units (OTUs), most of which were from the phyla Proteobacteria, Acidobacteria, Chlorobi and Bacteroidetes. Correlation analyses suggested that the available potassium, available phosphorus and soil organic matter emerged as the major determinants of the bacterial community composition. Overall, the OPT + S treatment can be more efficient in improving soil nutrient availability without excessive chemical fertilization.

Variable effects of 2°C air warming on yield formation under elevated [CO2] in a Chinese double rice cropping system

Elevated atmospheric CO2 concentration ([CO2]) and temperature can strongly affect rice (Oryza sativa L.) production. However, the impact of warming is variable depending on ambient temperature and cropping system. Little is known about the effect of elevated temperature and its interaction with elevated [CO2] in the Chinese double rice cropping system. To study this issue, field experiments were conducted using open-top chambers (OTC) with varying [CO2] (ambient, ambient+60 μmol mol−1) and varying temperature (ambient, ambient+2 °C) for 4 rotations of double rice from 2013 to 2016. Elevated [CO2] significantly increased grain yield in both early rice and late rice. This could be attributed to greater aboveground biomass, higher panicle number (m-2), spikelet number (per panicle) and 1000-grain weight. Elevated temperature decreased yield in early rice (except for 2016) due to decreases in % filled grains and 1000-grain weight. Elevated temperature increased yield in late rice due to increases in aboveground biomass, number of panicles and spikelets, although % filled grains decreased. We found that warming had positive effects on tiller development and dry matter accumulation during the pre-heading period both in early and late rice, which contributed to yield formation. When elevated temperature and [CO2] were combined, yield changed by −1.9–23.7% in early rice compared with ambient condition as warming partly offset the benefits of CO2 enrichment, while yield increased by 14.3–18.8% in late rice due to a synergy between warming and CO2 enrichment. The benefits of CO2 enrichment for fertile spikelets (m-2) and aboveground biomass could reduce or eliminate the negative impacts of warming. However, there were no significant interactions between elevated temperature and [CO2] in most rice seasons. Our findings indicate that warming by 2 °C is not detrimental to yield formation under CO2 enrichment, and suggest that grain yield may not decrease under future climate change in the Chinese double rice cropping system.

Limited yield advantage of early-season rice hybrids over inbreds in middle-lower reaches of the Yangtze River

Early-season rice plays a significant role in double rice-cropping system in China to improve the grain yield per unit of area. But recently, the stagnation of grain yield increase has been witnessed for early-season rice varieties in middle-lower reaches of the Yangtze River, particular in the national rice regional adaption trials of southern China. Further, in the early cropping season, hybrid rice did not always perform better than inbred rice in grain production. It is relevant to question whether yield ceilings have been reached in early-season indica rice before a new breakthrough on the germplasm exploitation. Field experiments were conducted to evaluate the yield performance of five inbreds and their 12 derived hybrid rice varieties in 2017 and 2018 in Hangzhou, China. Limited yield advantage of early-season hybrids over inbreds was observed, commonly less than 5.00% and averaged as 0.62% in 2017 and 3.20% in 2018. The hybrids always produced more panicles per plant but less grains per panicle with similar growing periods compared with the inbreds. Then, the genetic distance of the hybrid parents was surveyed using 48 selected molecular markers, showing low polymorphisms. Further, they were genotyped for four critical quantitative trait loci for both yield traits and heading date and three ones only for yield traits. So did 60 major early-season indica rice varieties at the four pleiotropic loci. Evidence was shown for the high uniform of their haplotype combinations across the tested materials, which could lead to early heading and low yield potential. Our work revealed the low genetic diversity, or called genetic bottleneck, for early-season hybrids to further improve grain yield potential. Moreover, along with the popularization of low-input requirements, the inbred rice varieties of cheap seed price might be a more suitable alternative for this cropping season in middle-lower reaches of the Yangtze River.

Mitigating net global warming potential and greenhouse gas intensity by intermittent irrigation under straw incorporation in Chinese double-rice cropping systems

To clarify the effect of water management on global warming potential and crop yield with rice straw returning in typical double-rice cropping systems, a field experiment was conducted in 2013–2016. The three strategies of water management included continuous flooding (F), flooding during the rice season but with drainage during the midseason and harvest time (F–D–F), and flooding during transplanting and tillering stages, drainage during the midseason and intermittent irrigation after midseason drainage (F–D–F–M). Compared with F plots, annual topsoil organic carbon sequestration rates were reduced by 10–45% and 49–66% for F–D–F and F–D–F–M plots, respectively. However, the annual CH4 emissions were significantly decreased by 33–52% and 46–69% for the F–D–F and F–D–F–M plots. As a result, the net annual global warming potential was reduced by 34–57% for F–D–F plots and 38–63% for F–D–F–M plots, respectively. Similarly, the greenhouse gas intensity was reduced 38–60% and 39–64% for F–D–F and F–D–F–M plots, respectively. These results suggested that intermittent irrigation has the potential to mitigate GHG emissions without sacrificing grain yield under rice straw returning in Chinese rice cropping systems.

Methane emissions responding to Azolla inoculation combined with midseason aeration and N fertilization in a double-rice cropping system.

Methane (CH4) is an important greenhouse gas (GHG), and paddy fields are major sources of CH4 emissions. This pot experiment was conducted to investigate the integrated effects of Azolla inoculation combined with water management and N fertilization on CH4 emissions in a double-rice cropping system of Southern China. Results indicated that midseason aeration reduced total CH4 emissions by 46.9%, 38.6%, and 42.4%, followed by N fertilization with 32.5%, 17.0%, and 29.5% and Azolla inoculation with 32.5%, 17.0%, and 29.5%, on average, during the early, late, and annual rice growing seasons, respectively. The CH4 flux peaks and total CH4 emissions observed in the late rice growing season were significantly higher than those in the early rice growing season. Additionally, CH4 fluxes correlated negatively to soil redox potential (Eh) and dissolved oxygen (DO) concentration. Azolla inoculation and N fertilization greatly increased the rice grain yields, whereas midseason aeration had distinct effects on grain yields in both rice seasons. The highest annual rice grain yields of approximately 110gpot-1 were obtained in the Azolla inoculation and N fertilization treatments. In terms of yield-scaled CH4 emission, Azolla inoculation combined with midseason aeration and N fertilization generated the lowest yield-scaled CH4 emissions both in the early and in the late rice growing seasons, as well as during the annual rice cycle. In contrast, the highest yield-scaled CH4 emission was obtained in the treatment employed continuous flooding, without Azolla and no N application. Our results demonstrated that Azolla inoculation, midseason aeration, and N fertilization practices mitigated total CH4 emissions by 18.5-42.4% during the annual rice cycle. We recommend that the combination of Azolla inoculation, midseason aeration, and appropriate N fertilization can achieve lower CH4 emissions and yield-scaled CH4 emissions in the double-rice growing system.

Microbial mechanisms in the reduction of CH4 emission from double rice cropping system amended by biochar: A four-year study

Biochar amendment can reduce CH4 emissions from paddy soils. However, little is known about how the soil microbial communities associated with paddy soil CH4 emissions respond to biochar aging after biochar amendment. In this study, we examined the effects of biochar on CH4 emissions, soil properties, and abundance/community composition of methanogens and methanotrophs in a double rice cropping system from 2012 to 2016. Straw-derived biochar was applied once in 2012 at 24 and 48 t ha−1. Biochar application decreased the annual CH4 emissions by 20–51%. Biochar increased the abundances of both methanogens and methanotrophs, with a larger increase of methanotrophs than methanogens in the first year, mainly caused by the increases in soil dissolved organic carbon, NH4+-N, and porosity. Biochar suppressed the abundance of methanogens and had little effect on the methanotrophs in the following three years, probably due to the increased soil porosity. Eventually, the ratios of abundance of methanogens to methanotrophs decreased by 11–31% in each of the four years and were positively correlated to CH4 emissions. Biochar addition increased the relative abundances of Methanocella and Methanospirillum and reduced those of Methanoregula and Methanosaeta for methanogens, while it increased the proportion of the basophilic methanotrophs Methylomicrobium and decreased that of Methylocaldum in the growing season 2014. Our results demonstrate that biochar aging greatly alters the responses of abundances and community compositions of soil methanogens and methanotrophs to biochar addition. The reduction of CH4 emissions owing to biochar in the long run was probably mainly due to the lesser suppression of abundance and activity of methanotrophs compared with methanogens.

Influence of Biochar Amendment on Soil Denitrifying Microorganisms in Double Rice Cropping System

At present, it is not explicit how biochar regulates the microbial process of denitrification in paddy fields. Therefore, a field experiment was carried out in a double rice cropping system with three wheat straw biochar treatments:no biochar treatment (CK), added 24 t·hm-2 biochar (LC), and added 48 t·hm-2 biochar (HC). Real time PCR (qPCR) and terminal-restricted fragment length polymorphism (T-RFLP) technology were adopted to analyze the abundances and microbial community structures of denitrification functional genes (narG, nirK and nosZ) in the fallow season and rice season. Due to its alkalinity, biochar amendment increased soil pH by 0.2-0.8. Biochar amendment also increased soil NH4+-N and NO3--N contents by 21.1%-32.5% and 63.0%-176.0% in the fallow season due to the presence of soluble N. Nevertheless, it reduced NH4+-N content by 48.8%-60.1% in the rice season due to the adsorption of biochar. The amendment increased soil MBN content in the fallow season, which may be a result of the large surface of biochar supplying nutrients and a suitable survival environment for the microorganisms. In the fallow season, compared to CK treatment, the increased soil NH4+-N and NO3--N with biochar amendment promoted the conversion of NH4+-N to NO3--N, and thus increased the abundances of narG and nosZ (P<0.05). The higher soil pH with biochar addition increased the abundances of nosZ and altered the community structures of narG and nosZ in the fallow season. Biochar amendment altered the denitrification process, but it did not change N2O emissions in the fallow season, which might reduce NO3--N leaching losses. In the rice season, biochar amendment increased nosZ abundance (P<0.05). HC increased the nirK gene abundance, which contributed to increased N2O emission in the rice season (P<0.05). Biochar converted the community structures of nirK and nosZ by decreasing the NH4+-N content in the rice season. The changes of the narG community structure with HC treatment contributed to the increased N2O emission. In conclusion, biochar amendment can influence the microbes involved in soil denitrification by changing the soil properties, and thus impact the N2O emissions and NO3--N leaching.

Utilization of carbon sources in the rice rhizosphere and nonrhizosphere soils with different long-term fertilization management.

Carbon (C) plays an important role in the interaction between plant and rhizosphere microbial communities, but there is still limited information about how C source utilization soil microbial structure responds to soil fertility changes under the double-cropping rice (Oryza sativa L.) system in Southern China paddy fields. Therefore, the effects of long-term (33 years) fertilizer regimes on the characteristics of C utilization in both rhizosphere and nonrhizosphere soils under double-cropping rice fields in Southern China were investigated by using the metagenome sequencing technology. The experiment began in 1986, and included five fertilizer treatments: without fertilizer input (CK), chemical fertilizer alone (MF), rice straw residue and chemical fertilizer (RF), 30% organic matter, and 70% chemical fertilizer (LOM), and 60% organic matter and 40% chemical fertilizer (HOM). The results showed that the relative abundance of Gemmatimonadetes and Planctomycetia in both the rhizosphere and nonrhizosphere soils was increased by application of rice straw residue and organic manure, whereas the relative abundance of Gammaproteobacteria and Nitrospira was promoted by application of inorganic fertilizers. The largest group of clusters of orthologous groups of proteins categories was "amino acid transport and metabolism" with 16.46% unigenes, followed by "general function prediction only" (12.23%). Regarding the gene ontology categories, biological process were the largest category (174 949, 46.40%), followed by cellular component (126 766, 33.62%), and molecular function (110 353, 29.26%). The principal coordinate analysis indicated that different parts of the root zone were the most important factors affecting the variation of C source utilization bacteria community, and the different fertilizer treatments were the second important factor affecting the variation of C source utilization bacteria community. As a result, the application of fertilization practices had significant effects on the abundance and community composition of C source utilization microbes in paddy soils. The results showed that the combined application of rice straw residue or organic manure with chemical fertilizer practices significantly increases the C source utilization of soil microorganisms in double-cropping rice fields.

Fertilization effects on CH&lt;sub&gt;4&lt;/sub&gt;, N&lt;sub&gt;2&lt;/sub&gt;O and CO&lt;sub&gt;2&lt;/sub&gt; fluxes from a subtropical double rice cropping system

A 2-year field study was conducted in a double rice cropping system in southern China to examine the effect of fertilization on CH&lt;sub&gt;4&lt;/sub&gt;, N&lt;sub&gt;2&lt;/sub&gt;O and CO&lt;sub&gt;2&lt;/sub&gt; fluxes using static opaque chambers and gas chromatographs. Two treatments were set up including conventional fertilization with a rate of 358 kg N/ha per year in forms of compound fertilizer and urea, and a control with no fertilizer application. The results indicated that fertilization did not have a significant effect on CH&lt;sub&gt;4&lt;/sub&gt; fluxes and led to a significantly higher cumulative N&lt;sub&gt;2&lt;/sub&gt;O emission in the two years of observation period. Fertilization promoted CO&lt;sub&gt;2&lt;/sub&gt; fluxes by increasing the autotrophic respiration instead of heterotrophic respiration. By combining the global warming impact of CH&lt;sub&gt;4&lt;/sub&gt;, N&lt;sub&gt;2&lt;/sub&gt;O and CO&lt;sub&gt;2&lt;/sub&gt;, it was found that fertilization increased the area-scaled but not the yield-scaled global warming impact. These results indicated that, according to the current amount of nitrogen applied, fertilization may increase the global warming effect of paddy fields in this region. However, the appropriate dose of nitrogen fertilizer application is still a reasonable agricultural management due to the comprehensive consideration of production and environmental impacts.

Effects of different manure nitrogen input ratio on rhizosphere soil microbial biomass carbon, nitrogen and microbial quotient in double-cropping rice field

To explore the characteristics of rhizosphere soil microorganisms in paddy fields with different manure nitrogen (N) input ratios at different growth stages of early and late rice in double-cropping rice system, a field experiment was conducted with five different treatments: 1) 100% N of chemical fertilizer (M1), 2) 30% N of organic matter and 70% N of chemical fertilizer (M2), 3) 50% N of organic matter and 50% N of chemical fertilizer (M3), 4) 100% N of organic matter (M4), and 5) no N fertilizer input as a control (M0). The rhizosphere soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial quotient (SQ) of the paddy fields were measured using the fumigation-extraction and chemical analysis methods. The results showed that the rhizosphere MBC, MBN, and SQ of the paddy fields at main different growth stages of early and late rice were increased by fertilization, which increased first and then decreased with the development of rice growth period, peaked at the heading stage, and reached the minimum value at the maturity stage. The effects of different fertilization treatments were in order of M4＞M3＞M2＞M1＞M0, with no significant difference among M2, M3 and M4, but being significantly higher than M0. Therefore, the application of organic matter, and combined application of chemical fertilizer with organic matter could significantly increase the rhizosphere MBC, MBN, and SQ of the paddy fields at early and late rice growth period, while chemical fertilizer alone had little effect.

Response of chalkiness in high-quality rice (Oryza sativa L.) to temperature across different ecological regions

Field experiments were conducted to determine the relationship between chalkiness of high-quality rice and temperature during grain filling period in 2016–2017. Significant differences in chalkiness and response to temperature during grain filling period were observed among the cultivars and years of production. A positive correlation between chalkiness and temperature that includes daily minimum temperature (T-min), daily mean temperature (T-mean) and daily maximum temperature (T-max) during grain filling period was observed. The correlation between chalkiness and T-min during 7 days after heading was lower than that of other stages, while the correlation between chalkiness and T-mean during the ripening stage was the highest than other stages. The chalkiness values of some varieties such as Jiuxiangzhan, Zhendao18 and Yongyou1538 were sensitive to temperature, however, it might be reached optimum level through suitable ecological regions. Other varieties such as Huanghuazhan, Wangxiangyouhuazhan and Ganchangjing1 could maintain lower level of chalkiness regardless of different ecological regions in southern China. These high-quality rice varieties with low and relatively stable chalkiness could be moderately planted in Chinese double-rice cropping systems, and could be used as breeding materials to cope with the adverse effects of future global temperature increase (mainly night temperature) on the appearance quality of rice.

Impacts of green manure amendment on detritus micro-food web in a double-rice cropping system

Chinese milk vetch (CMV) is widely used as a green manure due to its efficiency for nutrient preservation and soil fertility improvement, as well for providing more organic substances for soil organisms. Detritus micro-food web drives carbon and nutrient cycling and maintains ecosystem productivity. The effects of CMV amendment on detritus micro-food web were investigated in a six-year field experiment. Compared to the mineral fertilizers (NPK) alone, the fertilizers combined with CMV (NPK + CMV) increased soil organic carbon content, and thus enhanced the biomass of microorganisms, as well as protozoa abundance, while did not change micro-feeding nematode abundance (MN). This indicates a substrate-driven bottom-up effect. However, the green manure application increased omnivores-predators (OP) abundance and the OP/MN ratio, suggesting a top-down effect. Higher F/B ratio indicates a shift of the food web energy structure towards to fungi-dominated channel in the manured soil. Here, the green manure amendment should be encouraged to improve soil detritus micro-food web.

Medium-term effects of different types of N fertilizer on yield, apparent N recovery, and soil chemical properties of a double rice cropping system

Controlled-release, slow-release and stabilized nitrogen (N) fertilizers can potentially increase crop production and reduce N losses. However, information is limited regarding their medium- and long-term effects on rice productivity and soil chemical properties. Therefore, we investigated the effects of these three types of N fertilizer on rice grain yield, apparent N recovery efficiency (ANR), and soil chemical properties in a double cropped rice system over a 7-year period in Zhejiang Province, China. This study examined six N treatments: no N fertilizer (CK), split application of prilled urea (Urea), a single basal application of polymer-coated sulfur-coated urea (PSCU, formed by coating SCU with a polymer coating), urea with a nitrification inhibitor (UNI), urea formaldehyde (UF), and a mixture of PSCU and prilled urea (Mix). The effect of N treatment varied over the 7 years, but annual grain yields of double cropped rice with UNI, UF, and Mix were significantly higher than with Urea in 6/7, 4/7 and 3/7 years, respectively, and similar to yield with Urea in the other years. Performance of PSCU was variable, with significantly higher yields than Urea in 3/7 years, but lower yields in 2/7 years. Compared to Urea, the UNI treatment increased the grain yield and ANR of early rice by averages of 9.2 and 24%, respectively, and increased the grain yield and ANR of late rice by averages of 8.6 and 22%, respectively.Total N in the topsoil (0–20 cm) of the CK declined gradually during years 2-4. At the end of each of years 4–7, total soil N in all N fertilized treatments exceeded that in the CK. After 7 years, the five N fertilized treatments had 8–17% higher topsoil total N than the initial value, with no significant difference between these treatments. Available P increased over time in all treatments, with higher values in the CK than the five N fertilized treatments after the first year. After 7 years, available P was 387–529% higher in the N fertilized treatments than the initial value, and with no significant differences between these treatments. However, available K declined considerably in the five N fertilizer treatments during the first four years, but then steadily increased during the next three years when the application rate was increased from 120 to 200 kg K ha−1. Soil pH declined over the seven years, more so in PSCU and Mix when pH declined from 5.2 to 4.5, compared with declines to 4.7–4.9 in the other treatments. Although fertilizer for the UNI treatment cost $ 29 ha−1 more than for the Urea treatment, the higher yield/revenue and lower fertilization labor cost of UNI led to increases in net profit of $ 326 and 329 ha−1 year−1 in early and late rice, respectively. Overall, application of UNI was the superior alternative to prilled urea in the double rice cropping system because of consistently higher yield and profitability, while maintaining soil fertility at similar levels to the other N fertilizer options.