Land Use Change from Natural Tropical Forests to Managed Ecosystems Reduces Gross Nitrogen Production Rates and Increases the Soil Microbial Nitrogen Limitation.

Understanding the underlying mechanisms of soil microbial nitrogen (N) utilization under land use change is critical to evaluating soil N availability or limitation and its environmental consequences. A combination of soil gross N production and ecoenzymatic stoichiometry provides a promising avenue for nutrient limitation assessment in soil microbial metabolism. Gross N production via 15N tracing and ecoenzymatic stoichiometry through the vector and threshold element ratio (Vector-TER) model were quantified to evaluate the soil microbial N limitation in response to land use changes. We used tropical soil samples from a natural forest ecosystem and three managed ecosystems (paddy, rubber, and eucalyptus sites). Soil extracellular enzyme activities were significantly lower in managed ecosystems than in a natural forest. The Vector-TER model results indicated microbial carbon (C) and N limitations in the natural forest soil, and land use change from the natural forest to managed ecosystems increased the soil microbial N limitation. The soil microbial N limitation was positively related to gross N mineralization (GNM) and nitrification (GN) rates. The decrease in microbial biomass C and N as well as hydrolyzable ammonium N in managed ecosystems led to the decrease in N-acquiring enzymes, inhibiting GNM and GN rates and ultimately increasing the microbial N limitation. Soil GNM was also positively correlated with leucine aminopeptidase and ß-N-acetylglucosaminidase. The results highlight that converting tropical natural forests to managed ecosystems can increase the soil microbial N limitation through reducing the soil microbial biomass and gross N production.

Twelve-year conversion of rice paddy to wetland does not alter SOC content but decreases C decomposition and N mineralization in Japan.

Land-use change worldwide has been driven by anthropogenic activities, which profoundly regulates terrestrial C and N cycles. However, it remains unclear how the dynamics and decomposition of soil organic C (SOC) and N respond to long-term conversion of rice paddy to wetland. Here, soil samples from five soil depths (0-25 cm, 5 cm/depth) were collected from a continuous rice paddy and an adjacent wetland (a rice paddy abandoned for 12 years) on Shonai Plain in northeastern Japan. A four-week anaerobic incubation experiment was conducted to investigate soil C decomposition and N mineralization. Our results showed that SOC in the wetland and rice paddy decreased with soil depth, from 31.02 to 19.66 g kg-1 and from 30.26 to 18.86 g kg-1, respectively. There was no significant difference in SOC content between wetland and rice paddy at any depth. Soil total nitrogen (TN) content in the wetland (2.61-1.49 g kg-1) and rice paddy (2.91-1.78 g kg-1) showed decreasing trend with depth; TN was significantly greater in the rice paddy than in the wetland at all depths except 20-25 cm. Paddy soil had significantly lower C/N ratios but significantly larger decomposed C (Dec-C, CO2 and CH4 production) and mineralized N (Min-N, net NH4+-N production) than wetland soil across all depths. Moreover, the Dec-C/Min-N ratio was significantly larger in wetland than in rice paddy across all depths. Rice paddy had higher exponential correlation between Dec-C and SOC, Min-N and TN than wetland. Although SOC did not change, TN decreased by 14.1% after the land-use conversion. The Dec-C and Min-N were decreased by 32.7% and 42.2%, respectively, after the12-year abandonment of rice paddy. Conclusively, long-term conversion of rice paddy to wetland did not distinctly alter SOC content but increased C/N ratio, and decreased C decomposition and N mineralization in 0-25 cm soil depth.

Global soil nitrogen cycle pattern and nitrogen enrichment effects: Tropical versus subtropical forests.

Tropical and subtropical forest biomes are a main hotspot for the global nitrogen (N) cycle. Yet, our understanding of global soil N cycle patterns and drivers and their response to N deposition in these biomes remains elusive. By a meta-analysis of 2426-single and 161-paired observations from 89 published 15 N pool dilution and tracing studies, we found that gross N mineralization (GNM), immobilization of ammonium ( I NH 4 ) and nitrate ( I NO 3 ), and dissimilatory nitrate reduction to ammonium (DNRA) were significantly higher in tropical forests than in subtropical forests. Soil N cycle was conservative in tropical forests with ratios of gross nitrification (GN) to I NH 4 (GN/ I NH 4 ) and of soil nitrate to ammonium (NO3 - /NH4 + ) less than one, but was leaky in subtropical forests with GN/ I NH 4 and NO3 - /NH4 + higher than one. Soil NH4 + dynamics were mainly controlled by soil substrate (e.g., total N), but climatic factors (e.g., precipitation and/or temperature) were more important in controlling soil NO3 - dynamics. Soil texture played a role, as GNM and I NH 4 were positively correlated with silt and clay contents, while I NO 3 and DNRA were positively correlated with sand and clay contents, respectively. The soil N cycle was more sensitive to N deposition in tropical forests than in subtropical forests. Nitrogen deposition leads to a leaky N cycle in tropical forests, as evidenced by the increase in GN/ I NH 4 , NO3 - /NH4 + , and nitrous oxide emissions and the decrease in I NO 3 and DNRA, mainly due to the decrease in soil microbial biomass and pH. Dominant tree species can also influence soil N cycle pattern, which has changed from conservative in deciduous forests to leaky in coniferous forests. We provide global evidence that tropical, but not subtropical, forests are characterized by soil N dynamics sustaining N availability and that N deposition inhibits soil N retention and stimulates N losses in these biomes.

[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.

[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.

Initial evidence on the effect of copper on global cropland nitrogen cycling: A meta-analysis.

Copper (Cu) is a key cofactor in ammonia monooxygenase functioning responsible for the first step of nitrification, but its excess availability impairs soil microbial functions and plant growth. Yet, the impact of Cu on nitrogen (N) cycling and process-related variables in cropland soils remains unexplored globally. Through a meta-analysis of 1209-paired and 319-single observations from 94 publications, we found that Cu (Cu addition or Cu-polluted soil) reduced soil potential nitrification by 33.8% and nitrite content by 73.5% due to reduced soil enzyme activities of nitrification and urease, microbial biomass content, and ammonia oxidizing archaea abundance. The response ratio of potential nitrification decreased with increasing Cu concentration, soil total N, and clay content. We further noted that soil potential nitrification inhibited by 46.5% only when Cu concentration was higher than 150 mg kg-1, while low Cu concentration (less than 150 mg kg-1) stimulated soil nitrate by 99.0%. Increasing initial soil Cu content stimulated gross N mineralization rate due to increased soil organic carbon and total N, but inhibited gross nitrification rate, which ultimately stimulated gross N immobilization rate as a result of increased the residence time of ammonium. This resulted in a lower ratio of gross nitrification rate to gross N immobilization rate, implying a lower potential risk of N loss as evidenced by decreased nitrous oxide emissions with increasing initial soil Cu content. Our analysis offers initial global evidence that Cu has an important role in controlling soil N availability and loss through its effect on N production and consumption.

Carbon and nitrogen fractions control soil N2O emissions and related functional genes under land-use change in the tropics.

Converting natural forests to managed ecosystems generally increases soil nitrous oxide (N2O) emission. However, the pattern and underlying mechanisms of N2O emissions after converting tropical forests to managed plantations remain elusive. Hence, a laboratory incubation study was investigated to determine soil N2O emissions of four land uses including forest, eucalyptus, rubber, and paddy field plantations in a tropical region of China. The effect of soil carbon (C) and nitrogen (N) fractions on soil N2O emissions and related functional genes was also estimated. We found that the conversion of natural forests to managed forests significantly decreased soil N2O emissions, but the conversion to paddy field had no effect. Soil N2O emissions were controlled by both nitrifying and denitrifying genes in tropical natural forest, but only by nitrifying genes in managed forests and by denitrifying genes in paddy field. Soil total N, extractable nitrate, particulate organic C (POC), and hydrolyzable ammonium N showed positive relationship with soil N2O emission. The easily oxidizable organic C (EOC), POC, and light fraction organic C (LFOC) had positive linear correlation with the abundance of AOA-amoA, AOB-amoA, nirK, and nirS genes. The ratios of dissolved organic C, EOC, POC, and LFOC to total N rather than soil C/N ratio control soil N2O emissions with a quadratic function relationship, and the local maximum values were 0.16, 0.22, 1.5, and 0.55, respectively. Our results provided a new evidence of the role of soil C and N fractions and their ratios in controlling soil N2O emissions and nitrifying and denitrifying genes in tropical soils.

[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 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 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 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 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 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.

[N2O Emissions from a Tea Field with Deep Application of Nitrogen Fertilizer and Intercropping with White Clover].

In recent years, the area of tea fields in China has expanded. The application rate of nitrogen fertilizer is usually high in tea fields, which causes high N2O emissions. Tea fields are important sources of N2O emissions; thus, it is necessary to research N2O emission reduction in tea fields. A three-year field study was conducted to investigate soil N2O emissions and influencing factors under different fertilization measurements in a typical tea field in a subtropical hilly region of China. Three treatments-conventional fertilization, deep application of nitrogen fertilizer, and intercropping with clover-were studied to measure the soil N2O fluxes and the related soil and environmental properties. The results showed that the subtropical hilly tea field had high N2O emissions, and the cumulative annual emissions of N2O-N were as high as 5.1-10.1 kg·hm-2. The N2O emissions occurred mainly in spring and summer. When the soil temperature was lower than 15℃, the N2O flux shown mainly a positive correlation with the soil temperature. However, when the soil temperature was higher than 15℃, the positive correlation was mainly with the soil water, soil NH4+-N, and NO3--N contents. Compared with conventional fertilization, the intercropping of white clover did not significantly reduce N2O emissions, although deep application of fertilizer increased annual N2O emissions when the rainfall was high. Neither intercropping of white clover or deep application of fertilizer affected the tea yield or the yield-scaled N2O emissions compared with conventional fertilization. Our study indicates that both intercropping of white clover and deep application of fertilization without reducing the nitrogen application rate did not reduce the soil N2O emissions in subtropical tea fields. Further studies are needed to determine the effects of deep fertilization application combined with a reduction in the nitrogen application rate on N2O emissions from tea fields.

Effect of controlled-release fertilizer on N2O emissions and tea yield from a tea field in subtropical central China.

Tea (Camellia sinensis L.), a perennial leaf-harvested crop, favors warm/humid climate and acidic/well-drained soils, and demands high nitrogen (N) fertilizer inputs which lead to significant emissions of N2O. Potential mitigation options should be adopted to improve N use efficiency (NUE) and reduce environmental pollution in tea field system. A 3-year field experiment was carried out in a tea field in southern China from January 2014 to December 2016 to investigate the effect of controlled-release fertilizer (CRF) application on N2O emissions in tea field system. Three practices, namely conventional treatment (CON, 105 kg N-oilcake ha-1 year-1 + 345 kg N-urea ha-1 year-1), treatment with a half amount of the N fertilizer (CRF50%, 105 kg N-oilcake ha-1 year-1 + 120 kg N CRF ha-1 year-1) and full amount of N fertilizer (CRF100%, 105 kg N-oilcake ha-1 year-1 + 345 kg N CRF ha-1 year-1) were used. Compared with the CON, our results showed that CRF50% reduced the N2O emissions by 26.2% (p > 0.05) and increased the tea yield by 31.3% (p > 0.05), while CRF100% significantly increased the N2O emissions by 96.7% (p < 0.05) and decreased the tea yield by 6.77% (p > 0.05). Overall, yield-scaled N2O emissions of tea were reduced by 44.5% (p > 0.05) under CRF50% and significantly increased by 100% (p < 0.05) under CRF100%, compared with CON. Based on the gross margin analysis, CRF50% obtained the highest net economic profit. Our findings suggest that reducing N input of CRF (CRF50%) is necessary and feasible for adoption in the current tea plantation system.

[Effect of Dicyandiamide on N2O Emission in Fallow Paddy Field and Rape Cropping].

The emissions of greenhouse gas in winter are often neglected, and the latest research results showed that N2O emissions in fallow paddy field and winter oilseed rape are still large, research on mitigating the N2O flux and the mechanism behind them is of significance for mitigating N2O emissions from agricultural soil. By using static chamber techniques and molecular biology techniques, the N2O emission as well as the community composition and abundance of ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) from fallow paddy field, rape cropping with and without DCD treatment in Taoyuan agricultural ecological experiment station of the Chinese Academy of sciences were measured. The results showed that the addition of DCD significantly inhibited N2O emissions in fallow paddy field and rape cropping by 36.7% and 23.6%, respectively. The application of DCD in fallow paddy field inhibited the abundance of AOA and AOB by 59.3% and 73.7%, respectively, but only changed the community structure of AOA. The addition of DCD in rape cropping only changed the community structure and inhibited the abundance of AOB. This research showed that DCD application could effectively mitigate the N2O emissions in fallow paddy field and winter rape cropping under different mitigation mechanisms.

[Effects of rice straw returning on the community structure and diversity of nitrogen-fixing gene (nifH) in paddy soil].

Taking a long-term fertilization experiment in Taoyuan Agro-ecosystem Research Station under Chinese Academy of Sciences as the platform, and selecting four treatments (no fertilization, CK; rice straw returning, C; nitrogen, phosphorus and potassium fertilization, NPK; and NPK+C) as the objects, soil samples were collected at the tillering, booting and maturing stages of rice, and the abundance, composition and diversity of nifH-containing bacterial community were measured by real-time quantitative PCR and terminal restriction fragment length polymorphism (T-RFLP), aimed to understand the effects of rice straw returning on the nifH-containing bacterial community in paddy soil. Compared with CK, treatments NPK+C and NPK increased the abundance of nifH-containing microorganisms significantly (except at tillering stage), and NPK+C had the highest abundance of nifH-containing microorganisms. Under the effects of long-term fertilization, the composition of nifH gene community in CK differed obviously from that in the other three treatments. The nifH composition had definite difference between C and NPK, but less difference between NPK and NPK+C. Long-term fertilization did not induce significant changes in nifH diversity. Therefore, long-term rice straw returning not only induced the changes of nifH gene community composition, but also resulted in a significant increase in the abundance of nifH-containing community, and hence, the increase of soil nitrogen fixing capacity.