[Effects of Straw Returning and Biochar Addition on Greenhouse Gas Emissions from High Nitrate Nitrogen Soil After Flooding in Rice-vegetable Rotation System in Tropical China].

In rice-vegetable rotation systems in tropical areas, a large amount of nitrate nitrogen accumulates after fertilization in the melon and vegetable season, which leads to the leaching of nitrate nitrogen and a large amount of N2O emission after the seasonal flooding of rice, which leads to nitrogen loss and intensification of the greenhouse effect. How to improve the utilization rate of nitrate nitrogen and reduce N2O emissions has become an urgent problem to be solved. Six treatments were set up [200 mg·kg-1 KNO3 (CK); 200 mg·kg-1 KNO3 + 2% biochar addition (B); 200 mg·kg-1 KNO3+1% peanut straw addition (P); 200 mg·kg-1 KNO3 + 2% biochar + 1% peanut straw addition (P+B); 200 mg·kg-1 KNO3 + 1% rice straw addition (R); 200 mg·kg-1 KNO3 + 2% biochar+1% rice straw addition (R+B)] and cultured at 25℃ for 114 d to explore the effects of organic material addition on greenhouse gas emissions and nitrogen use after flooding in high nitrate nitrogen soil. The results showed that compared with that in CK, adding straw or combining straw with biochar significantly increased soil pH (P<0.05). The B and P treatments significantly increased the cumulative N2O emissions by 41.6% and 28.5% (P<0.05), and the P+B, R, and R+B treatments significantly decreased the cumulative N2O emissions by 14.1%, 24.7%, and 36.7% (P<0.05), respectively. The addition of straw increased the net warming potential of greenhouse gases (NGWP). The addition of coir biochar significantly reduced the effect of straw on NGWP (P<0.05). The combined application of straw and biochar decreased NGWP, and P+B significantly decreased NGWP, but that with R+B was not significant (P>0.05). Adding straw or biochar significantly increased soil microbial biomass carbon (MBC) (P<0.05), and that of P+B was the highest (502.26 mg·kg-1). The combined application of straw and biochar increased soil microbial biomass nitrogen (MBN), and that of P+B was the highest. The N2O emission flux was negatively correlated with pH (P<0.01) and positively correlated with NH4+-N and NO3--N (P<0.01). The cumulative emission of N2O was negatively correlated with MBN (P<0.05). There was a significant negative correlation between NO3--N and MBN (P<0.01), indicating that the reduction in NO3--N was likely to be held by microorganisms, and the increase in the microbial hold of NO3--N also reduced N2O emission. In conclusion, the combined application of peanut straw and coconut shell biochar could significantly inhibit N2O emission and increase soil MBC and MBN, which is a reasonable measure to make full use of nitrogen fertilizer, reduce nitrogen loss, and slow down N2O emission after the season of Hainan vegetables.

[Effects of Biochar Application Two Years Later on N2O and CH4 Emissions from RiceVegetable Rotation in a Tropical Region of China].

The effects of biochar application on soil nitrous oxide （N2O） and methane （CH4） emissions in a typical rice-vegetable rotation system in Hainan after two years were investigated. The aim was to clarify the long-term effects of biochar on greenhouse gas emissions under this model， and it provided a theoretical basis for N2O and CH4 emission reduction in rice-vegetable rotation systems in tropical regions of China. Four treatments were set up in the field experiment， including no nitrogen fertilizer control （CK）； nitrogen， phosphorus， and potassium fertilizer （CON）； nitrogen， phosphorus， and potassium fertilizer combined with 20 t·hm-2 biochar （B1）； and nitrogen， phosphorus， and potassium fertilizer combined with 40 t·hm-2 biochar （B2）. The results showed that： ① compared with that in the CON treatment， the B1 and B2 treatments significantly reduced N2O emissions by 32% and 54% in the early rice season （P < 0.05， the same below）， but the B1 and B2 treatments significantly increased N2O emissions by 31% and 81% in the late rice season. The cumulative emissions of N2O in the pepper season were significantly higher than those in the early and late rice seasons， and the B1 treatment significantly reduced N2O emissions by 35%. There was no significant difference between the B2 and CON treatments. ② Compared with that in the CON treatment， B1 and B2 significantly reduced CH4 emissions by 63% and 65% in the early rice season， and the B2 treatment significantly increased CH4 emissions by 41% in the late rice season. There was no significant difference between the B1 and CON treatments. There was no significant difference in cumulative CH4 emissions between treatments in the pepper season. ③ The late rice season contributed to the main global warming potential （GWP） of the rice-vegetable rotation system， and CH4 emissions determined the magnitude of GWP and greenhouse gas emission intensity （GHGI）. After two years of biochar application， B1 reduced the GHGI of the whole rice-vegetable rotation system， and B2 increased the GHGI and reached a significant level. However， the B1 and B2 treatments significantly reduced GHGI in the early rice season and pepper season， and only the B2 treatment increased GHGI in the late rice season. ④ Compared with that in the CON treatment， the B1 and B2 treatments significantly increased the yield of early rice by 33% and 51%， and the B1 and B2 treatments significantly increased the yield of pepper season by 53% and 81%. In the late rice season， there was no significant difference in yield except for in the CK treatment without nitrogen fertilizer. The results showed that the magnitude of greenhouse gas emissions in the tropical rice-vegetable rotation system was mainly determined by CH4 emissions in the late rice season. After two years of biochar application， only low biochar combined with nitrogen fertilizer had a significant emission reduction effect， but high and low biochar combined with nitrogen fertilizer increased the yield of early rice and pepper crops continuously.

[Runoff Simulation and Its Response to Extreme Precipitation in the Yangtze River Basin].

Studies on runoff are crucial for the scientific allocation, utilization, and development of water resources and can provide an important basis for the prevention and control of flood and drought disaster, as well as water environmental pollution management. Affected by global warming, the frequency and intensity of extreme climate events, particularly extreme precipitation, have significantly changed in recent years, which can directly or indirectly impact runoff changes. In this study, we used the SWAT model to simulate the spatiotemporal variations in runoff in the Yangtze River Basin from 1965 to 2019 and analyzed the response of runoff to precipitation under extreme conditions. The results showed that the changes in total runoff in the Yangtze River Basin were not significantly different from 1965 to 2019. The total runoff and the mid-lower runoff in the basin experienced four stages of "dry-wet-dry-wet." Simulations revealed that under the 50-year extreme precipitation event, the increase in daily average runoff was 6200%, 21%, and 15% for the typical sub-basins of the upper, middle, and lower reaches of the Yangtze River, respectively. Additionally, the increase in monthly and annual average runoff was 355%, 5%, and 1.3% and 78%, 1%, and 0.24%, for upper, middle, and lower reaches of the Yangtze River, respectively. Moreover, under the 100-year extreme precipitation, the average daily runoff increasing rates were 8000%, 25%, and 17% for upper, middle, and lower reaches of the Yangtze River, respectively, compared to the monthly increase of 437%, 7%, and 1.5% and annual increase of 96%, 1.2%, and 0.28%, respectively. Our findings may improve the understanding of hydrological responses to climate change and provide valuable inferences to decision-makers and water managers for better allocation and management of water resources.

[Effects of Biochar Amendment on N2O Emission and Its Functional Genes in Pepper Growing Soil in Tropical Areas].

Biochar application may mitigate N2O emissions and increase crop yield, yet little is known about microbial dynamics variation. To investigate the potential of increasing yield and reducing emissions of biochar in tropical areas and the dynamic mechanism of related microorganisms, a pot experiment was conducted to investigate the biochar application on pepper yield, N2O emissions, and dynamic variation of related microorganisms. Three treatments were applied:2% biochar amendment (B), conventional fertilization (CON), and no nitrogen (CK). The results showed that the yield of the CON treatment was higher than that of the CK treatment. Compared with that of the CON treatment, biochar amendment significantly increased the yield of pepper by 18.0% (P<0.05), and biochar amendment could increase the content of NH+4-N and NO-3-N in soil in most periods of pepper growth. Compared with that in the CON treatment, the B treatment significantly reduced cumulative N2O emissions by 18.3% (P<0.05). Ammonia oxidizing archaea (AOA)-amoA and ammonia oxidizing bacteria (AOB)-amoA were very significantly negatively correlated with N2O flux (P<0.01). N2O flux was significantly negatively correlated with nosZ gene abundance (P<0.05). This indicated that N2O emission may have mainly resulted from the denitrification process. In the early stage of pepper growth, biochar significantly reduced N2O emissions by reducing the value of (nirK+nirS)/nosZ, whereas in the late stage of pepper growth, the value of (nirK+nirS)/nosZ of the B treatment was higher than that of the CON treatment, resulting in higher N2O flux in the B treatment. Therefore, biochar amendment could not only increase vegetable production in tropical areas but also reduce N2O emissions, which can be used as a new strategy to improve soil fertility in Hainan Province and other tropical areas.

[Effects of Land-use Conversion on Soil Nitrification and NO & N2O Emissions in Tropical China Under Different Moisture Conditions].

Rain and heat conditions are abundant in tropical areas, and rubber and tea are widely planted in this region; the nitrification process produces nitrate content, which is not conducive to the maintenance of nitrogen nutrients, and has negative environmental effects (nitrogen oxide emissions). The characteristics of soil nitrification rate and nitrogen oxide emission under different land use patterns remain unclear. An incubation experiment was conducted under the 5 a (T5) and 15 a (T15) tea plantation soils and the nearby typical rubber plantation (XJ) soils in Baisha county of Hainan province under two moisture contents (50% WFPS-L and 80% WFPS-H) for 71 d at 25℃. The results showed that:① after the rubber plantation was converted to a tea plantation, the net nitrification and soil NO and N2O emissions were significantly reduced under high moisture content. The overall trend was in the order of XJH>T15H>T5H, and the values of soil net nitrification and NO and N2O emissions were as high as 4.2 mg·(kg·d)-1, 1.4 mg·kg-1, and 14.3 mg·kg-1 in the XJH treatment, respectively. Under the low moisture content, soil NO emissions in tea field soil were significantly reduced relative to those in rubber plantation soil, N2O emissions had no significant difference among different treatments, and net nitrification had no significant difference between the XJ and T15 treatments. There was a significant positive correlation between NO emissions and net nitrification rate (P<0.01). ② The net nitrification of XJH was higher than that of XJL, but the net nitrification values under different moisture contents in tea field soil was in contrast to that in rubber plantation soil. The NO emissions of XJ and T15 under different moisture contents were consistent with the trend of net nitrification, and the high nitrification promoted NO emissions, whereas NO emissions of T5 were not significantly affected by moisture content. The high moisture content treatment significantly promoted N2O emissions relative to those under the low moisture content treatment. The results showed that SOM, TN, pH, and moisture content were the key factors affecting soil net nitrification rate, NO, and N2O emissions. The conversion of the rubber plantation to a tea plantation significantly reduced the net nitrification rate and negative impact on the environment under high moisture content.

[Effect of Different Fertilization Treatments on Methane and Nitrous Oxide Emissions from Rice-Vegetable Rotation in a Tropical Region, China].

The study of the effects of different fertilization treatments on soil methane (CH4) and nitrous oxide (N2O) emissions in rice-vegetable rotation systems is of great significance to supplement the research gap on greenhouse gas emissions in tropical regions of China. In this study, four fertilization treatments were set up during the pepper season:phosphorus and potassium fertilizer application (PK); nitrogen, phosphorus, and potassium (NPK) application; half application of nitrogen, phosphorus, and potassium plus half application of organic fertilizer (NPK+M); and application of organic fertilizer (M). There was no fertilizer application during the following early rice season. The objective of our study was to investigate the rules of CH4 and N2O emissions under different fertilization treatments in the pepper growth season, and the effects of different fertilization treatments in the pepper growth season on rice yield, and CH4 and N2O emissions in the following early rice growth season. The close static chamber-gas chromatography method was applied to determine soil CH4 and N2O emissions. We measured crop yield, estimated global warming potential (GWP), and calculated greenhouse gas emission intensity (GHGI). Our results showed that:① the cumulative CH4 emission under the four fertilization treatments ranged between 0.9 kg·hm-2 to 2.7 kg·hm-2 during the pepper growth season and between 5.5 kg·hm-2 to 8.4 kg·hm-2 during the early rice growth season. Compared with NPK, NPK+M and M reduced the cumulative CH4 emission in the pepper growth season by 35.3% and 7.6%, respectively; however, NPK+M and M increased the cumulative CH4 emission in the early rice season by 37.5% and 55.1%, respectively. There was a significant difference in cumulative CH4 emission between M and NPK in the early rice growth season. ② The cumulative N2O emission under the four fertilization treatments varied from 0.5 kg·hm-2 to 3.0 kg·hm-2 in the pepper growth season and from 0.3 kg·hm-2 to 0.5 kg·hm-2 in the early rice growth season. The cumulative N2O emission was significantly decreased by 33.7% in NPK+M and by 16.0% in M, compared with that in NPK. In the early rice growth season, the cumulative N2O emission was decreased by 23.5% by NPK+M but was increased by 9.1% by M. There was no significant difference in the cumulative N2O emission among the four fertilization treatments. ③ The yields of pepper and early rice under the four fertilization treatments were 3055.6-37722.5 kg·hm-2 and 5850.9-6994.4 kg·hm-2, respectively. Compared with that in NPK, NPK+M and M significantly increased pepper yield. The GWP under the four fertilization treatments in the pepper-early rice rotation system varied from 508.0 kg·hm-2 to 1864.4 kg·hm-2. Compared with NPK, NPK+M significantly decreased GWP by 25.7% and M insignificantly decreased GWP by 5.7%. The pepper growth season with the four fertilization treatments contributed to 69.2%-78.1% of the total GWP, and N2O contributed to 77.3%-85.3% of the total GWP. The GHGI ranged between 0.03 kg·kg-1 and 0.09 kg·kg-1 in the pepper growth season and between 0.04 kg·kg-1 and 0.24 kg·kg-1 in the early rice growth season. Compared with that in NPK, both M and NPK+M significantly reduced the GHGI by 71.5% and 54.7%, respectively, in the pepper growth season. In the early rice season, NPK+M significantly decreased the GHGI by 44.0%, but M non-significantly decreased the GHGI by 20.8%. The peak in N2O emission in the tropical pepper-early rice rotation system appeared after fertilization, and N2O emissions primarily occurred in the pepper growth season. However, CH4 emission was mainly concentrated in the early rice season. Considering the overall enhancing effects on crop yield and mitigation of greenhouse gas emissions, the co-application of chemical and organic fertilizers (NPK+M) can be recommended as an optimal fertilization practice to mitigate greenhouse gas emissions and maintain crop yield in pepper-rice rotation systems of Hainan, China.

[Effects of manure substituting chemical nitrogen fertilizer on rubber seedling growth and soil environment.] / 有机肥替代化学氮肥对橡胶幼苗生长和土壤环境的影响.

The substitution of manure for chemical nitrogen fertilizers has great impacts on the growth of rubber seedlings and soil environment, with implications for rubber cultivation and transplantation and soil environment improvement. In this study, rubber seedlings of thermal research '7-33-97' strain were cultivated under four treatments: No fertilizer application (CK), only application of chemical fertilizer (N), manure replacing 50% chemical fertilizer (M+N), and manure replacing 100% chemical fertilizer (M). Plants parameters (including plant height, basal diameter, biomass, and chlorophyll), soil physicochemical properties (including pH, soil organic carbon and nitrogen, soil enzyme activities), and their relationships were investigated. The results showed that plant height, basal diameter, biomass, and chlorophyll content in the M+N and M treatments were significantly higher, while underground biomass and root-shoot ratio were significantly lower than those of in N treatment. Compared with CK, soil pH was significantly increased in the M treatment, decreased in the N treatment, and was not changed in the M+N treatment. Soil ammonium and nitrate content in the M+N and M treatments were significantly lower, while soil organic carbon content, the activity of ß-1,4-glucosidase (BG), ß-1,4-N-acetylglucosaminidase (NAG) and leucine aminopeptidase (LAP) were significantly higher than those of in N treatment. Results of correlation analysis showed that soil pH was negatively correlated with soil ammonium and nitrate content, but positively correlated with BG and NAG activities. The structural equation model analysis showed that soil pH had significant positive effects on seedling quality index, while nitrate content had significant negative effects, and soil enzyme activities had no significant effect. Those results indicated that soil pH and nitrate content were the important driving factors on the growth of rubber seedlings. The manure replacing of 50% and 100% chemical nitrogen fertilizer could promote rubber seedlings growth, improve soil environment, and promote sustainable development of rubber production in Danzhou City, Hainan Province.

[Effects of Nitrogen Fertilizer Application Times and Nitrification Inhibitor on N2O Emission from Potted Maize].

Rational application of nitrogen is an important strategy for increasing yield while reducing environmental pollution due to nitrogen. Pot experiments were conducted to study the effects of different application times on maize yield and soil N2O emission under conditions of equal nitrogen content, and to explore the relationship between the abundance of nitrogen conversion functional genes and N2O emission. Four treatments were used, namely a control (CK, no urea), one-time application (S1, one application of 0.5 g·kg-1 urea+nitrification inhibitor), two separate applications ï¼»S2, two applications of 0.5 g·kg-1 urea (40% and 60% respectively)ï¼½ and three separate applications (S3, 0.5 g·kg-1 urea was divided into three different applications: 20%, 40% and 40% respectively). The results showed that: ① nitrogen application promoted soil acidification, and the degree of soil acidification varied significantly with different application times. More applications of nitrogen led to stronger soil acidification. Nitrogen application significantly increased the ear yield and stem biomass of fresh table maize, but different nitrogen application times may alter soil pH, leading to differences in the degree of nitrogen uptake and utilization in plants. While the S3 treatment significantly reduced soil pH, it also reduced the cumulative nitrogen uptake and utilization in the plants, resulting in a high cumulative N2O emission. Compared with the S3 treatment, the yield was 40.21% and 42.55% higher in the S1 and S2 treatments, and the cumulative N2O emission decreased by 79.4% and 20.9%, respectively. ② N2O emission was positively correlated with the abundance of AOB and nirK genes, which were the main contributors to N2O emission. S1 significantly decreased the abundance of AOB and nirK genes and N2O emissions, while S2 and S3 significantly increased the abundance of nirK and nirS genes and decreased the abundance of nosZ genes after fertilization, promoting N2O emissions. Nitrogen application times affect the functional genes of the nitrogen transformation process, and thus affect N2O emissions. In conclusion, a one-time application of urea combined with DCD only guarantees high maize yield and improves the efficient use of nitrogen, but also reduces greenhouse gas emissions. Thus, it is the recommended nitrogen fertilization mode for the cultivation of fresh corn in Hainan.

[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.

[Effects of Biochar Addition Under Different Water Management Conditions on N2O Emission From Paddy Soils in Northern Hainan].

Alternating dry and wet conditions affect the main processes of N2O production, such as nitrification and denitrification. Such conditions are very common in tropical rice-growing areas, such as Hainan. As a type of soil amendment, biochar is widely used to improve physical and chemical properties of soil and to reduce soil greenhouse gas emissions. However, there is a lack of existing in-depth research on the emission reductions of biochar when used in tropical soils that undergo frequently alternating dry and wet conditions. In this experiment, typical paddy soil from northern Hainan was used as the test soil, and corn stalk biochar, carbonized under anaerobic conditions at 400℃, was used as the test biochar. This experiment explored the effects of adding biochar on soil greenhouse gas emissions and microbial-related functional genes under different water management conditions. The experiment comprised a 30 d culture, kept in the dark at 25℃, and a total of six treatments:alternating dry-wet conditions without adding biochar (AWD1), alternating dry-wet conditions with 2% biochar (AWD2), alternating dry-wet conditions with 4% biochar (AWD3), continuous flooding without adding biochar (CF1), continuous flooding with 2% biochar (CF2), and continuous flooding with 4% biochar (CF3). The results showed that:① the addition of biochar under different moisture conditions can reduce N2O emissions in acidic paddy soil (P<0.05, the same below), as the total N2O emissions with the AWD3 treatment were 0.43 mg ·kg-1, which indicates an approximate reduction of 68%, relative to the AWD1 treatment; ② Corn stalk biochar can significantly increase the soil pH under different water management conditions. Compared to the no-biochar treatment, the soil pH increased by 0.5 units on average after cultivation with the addition of biochar, and as the soil NH4+-N content increased, it led to a decrease in Eh. ③ Corn stalk biochar significantly reduces the abundance of ammonia oxidizing bacteria and significantly increases the nosZ gene abundance. However, it decreases the ratio of (nirK+nirS)/nosZ, inhibits the nitrification process, and promotes the reduction of N2O in the denitrification process. Thereby, the addition of corn stalk biochar can reduce N2O emissions. These results show that alternating dry-wet conditions, combined with the addition of corn stalk biochar, are beneficial for reducing N2O emissions in paddy soil, which may have further application in the reduction of agricultural greenhouse gas emissions in northern Hainan.

[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.

[Effects of Optimizing Fertilization on N2O and CH4 Emissions in a Paddy-Cowpea Rotation System in the Tropical Region of China].

In-situ measurement of nitrous oxide (N2O) and methane (CH4) emissions in a typical paddy-cowpea rotation system in Southern Hainan was conducted to determine the characteristics of greenhouse gas emissions under different optimum fertilization treatments. The experiment consisted of 5 treatments:conventional farming fertilization (CON), optimized fertilization (OPT), organic-inorganic fertilization (ORG), slow-controlled optimization fertilization (SCOPT), and no nitrogen as the control (CK). The N2O and CH4 emissions were measured using static chamber-gas chromatography during the all the paddy-cowpea rotation seasons. Global warming potential (GWP) and greenhouse gas intensity (GHGI) were also estimated in this study. The cumulative N2O emission during the rice growth season was 0.19-1.37 kg·hm-2. Compared with the CON treatment, other treatments reduced N2O emission by 50% to 86%. The cumulative N2O emission during the cowpea growth season was 1.29-3.55 kg·hm-2. In addition, N2O emission increased by 14% as a result of the ORG treatment, whereas that of the remaining treatments decreased by 16% to 59%. The cumulative CH4 emissions during the paddy growth season were 4.67-14.23 kg·hm-2. The CH4 emissions following the CK, OPT, and ORG treatments were higher by 116%, 22%, and 102%, respectively, whereas that of SCOPT was lower by 29%, than that following the CON treatment. Moreover, the cumulative CH4 emission during the cowpea growth season was 0.03-0.26 kg·hm-2, and CH4 absorption occurred during the same period. With regard to the contribution rate of different periods to GWP, the cowpea growth season still had a proportion of 44.7%-54.5%, despite extremely low CH4 emission. Regarding the two greenhouse gases, N2O contributed 66.7%-77.2%. During the entire rotation system, both GWP and GHGI processed by SCOPT were significantly lower than those of the CON treatments. To sum up, the SCOPT treatment was determined to be the optimal fertilization scheme in this study and had the most significant effects on increasing production and reducing greenhouse gas emissions.

[Effects of Different Fertilization Modes on Greenhouse Gas Emission Characteristics of Paddy Fields in Hot Areas].

Greenhouse gas emissions studies commonly focus on temperate and subtropical regions. As a result, greenhouse gas emissions from agricultural soils in tropical areas are often neglected. Therefore, greenhouse gas fluxes in a Hainan paddy field under different fertilization regimes were studied. This research provides an accurate assessment of CH4 and N2O emissions from paddy fields in China and sound mitigation measures. Through static chamber/gas chromatography techniques, CH4 and N2O emissions, global warming potential (GWP), and greenhouse gas emissions intensity (GHGI) in late rice season under five fertilizer treatments were measured. The treatments included:control (CK), conventional treatment (CON), optimized fertilization treatment (YH), optimized fertilization combined with controlled slow-release fertilizer treatment (ZHY1), optimized fertilization combined with controlled slow-release fertilizer and organic fertilizer treatment (ZHY2). The results showed that the cumulative CH4 emissions in the CK, CON, YH1, ZYH1, and ZYH2 treatments were 175.70, 60.30, 63.00, 62.80, and 56.60kg·hm-2, and the cumulative N2O emissions were 0.78, 3.40, 1.03, 1.44, and 0.44kg·hm-2, respectively. Correlation analysis showed that soil temperature and Eh were the main factors driving CH4 emission. Compared with CK, CON, YH, and ZYH1, the yield of rice in ZYH2 treatment increased by 29.69%, 11.81%, 6.74%, and 10.36%, respectively. While GWP of ZYH2 decreased by 64.80%, 43.23%, 12.93%, and 15.15%, and GHGI decreased by 76.49%, 52.52%, 20.54%, and 23.87%, respectively. Therefore, in terms of yield and greenhouse gas emissions, optimal fertilization combined with sheep manure and slow release fertilizer treatment (ZYH2) is feasible in this region.

[Effects of long-term fertilization on paddy soils organic nitrogen, microbial biomass, and microbial functional diversity].

Soil samples were collected from the plow layers at two long-term experiment sites in Xinhua and Ningxiang counties of Hunan Province, China to study the effects of long-term fertilization on organic nitrogen, microbial biomass, and microbial functional diversity of paddy soils. Long-term fertilization showed great effects on the soil N content. Compared with CK, treatments NPK plus manure or straw increased the contents of soil total acid-hydrolysable N and its fractions amino sugar N, amino acid N, and ammonium N. Treatment NPK had no significant effects on soil microbial biomass C and N, but treatments NPK plus manure increased the contents of soil microbial biomass C and N significantly. BIOLOG test showed that treatments NPK plus manure enhanced the carbon utilization efficiency of soil microbes, and improved the functional diversity of soil microbial communities, compared with CK. Long-term different fertilizer treatments resulted in the differences of carbon substrate utilization patterns of soil microbial communities.