Effects of drying-rewetting cycles on the fluxes of soil greenhouse gases

Irregular precipitation caused by climate changes has resulted in frequent events of soil drying-rewetting cycles (DWC), which can strongly affect soil carbon (C) and nitrogen (N) cycling, including the fluxes of greenhouse gases (GHGs). The response of soil carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes to DWC events may differ among different ecosystem types and vary with experimental settings and soil properties, but these processes were not quantitatively assessed. Here, we evaluated the responses of soil GHG fluxes to DWC, compared with consistent moisture, as well as the associated driving factors with 424 paired observations collected from 47 publications of lab incubation experiments. Results showed that: (1) DWC significantly decreased soil CO₂ emissions by an average of 9.7%, but did not affect the emissions and uptakes of soil CH₄ and N₂O; (2) DWC effects on soil GHG emissions varied significantly among different ecosystem types, with CO₂ emissions significantly decreased by 6.8 and 16.3% in croplands and grasslands soils, respectively, and CH₄ and N₂O emissions significantly decreased and increased in wetlands and forests soils, respectively; (3) the effects of DWC on CO₂ emissions were also positively regulated by organic C and N concentrations, pH, clay concentration, and soil depth, but negatively by C:N ratio and silt concentration, while DWC effects on N₂O emissions were negatively controlled by C:N ratio, silt concentration, and soil depth. Overall, our results showed that CO₂ emissions were significantly decreased by DWC, while the fluxes of CH₄ and N₂O were not affected, indicating an overall decrease of GHGs in response to DWC. Our results will be useful for a better understanding of global GHG emissions under future climate change scenario.

Reducing nitrous oxide emissions from irrigated maize by using urea fertilizer in combination with nitrapyrin under different tillage methods

The aim of this study was to evaluate the effectiveness of nitrification inhibitor (nitrapyrin; NI) as a mitigation option for yield-scaled emissions of nitrous oxide (N₂O) under tillage management and urea fertilization in the irrigated maize fields in northern Iran. A split-plot experiment was performed based on a randomized completed blocks design with three replicates. The main plots were the levels of tillage practices (conventional tillage (CT) and minimum tillage (MT), and the subplots were the fertilizer treatments (control, urea, and urea + NI). The gas samples for measuring N₂O emissions were collected during the maize growing season from June to September, using opaque manual circular static chambers. Soil samples were taken at 0-10 cm to determine water-filled pore space, ammonium (NH₄⁺), and nitrate (NO₃^-) concentrations in the soil. When the crop reached physiological maturity, maize was harvested to measure grain yield, biomass production, N uptake of aboveground, and nitrogen use efficiency (NUE). The results showed that the applying NI in combination with urea reduced the total N₂O emissions by up to 58% and 64% in MT and CT, respectively. In the urea + NI treatment, mean soil concentrations of NH₄⁺ and NO₃^- were significantly higher (20%) and lower (23.5%), respectively, compared with other treatments. The NI reduced the yield-scaled N₂O-N emission up to 79% and 55% for CT and MT, respectively. Furthermore, compared to treatment with urea alone, the application of NI increased the NUE of the MT and CT systems by an average of 55% and 46%, respectively. This study emphasized that the application of nitrapyrin should be encouraged in irrigated maize fields, in order to minimize N₂O emissions and improve NUE and biomass production.

Effects of straw returning and feeding on greenhouse gas emissions from integrated rice-crayfish farming in Jianghan Plain, China

Great efforts have been devoted to assessing the effects of straw managements on greenhouse gas (GHG) emissions, global warming potential (GWP), and net economic budget in rice monoculture (RM). However, few studies have evaluated the effects of straw managements on GHG emissions and net ecosystem economic budget (NEEB) in integrated rice-crayfish farming (RC). Here, a randomized block field experiment was performed to comprehensively evaluate the effects of aquatic breeding practices (feeding or no feeding of forage) and straw managements (rice straw returning or removal) on soil NH₄⁺-N and NO^-₃-N contents, redox potential (Eh), CH₄ and N₂O emissions, GWP, and NEEB of fluvo-aquic paddy soil in a rice-crayfish co-culture system in Jianghan Plain of China. We also compared the differences in CH₄ and N₂O emissions, GWP, and NEEB between RM and RC. Straw returning significantly increased CH₄ and N₂O emissions by 34.9-46.1% and 6.2-23.1% respectively compared with straw removal. Feeding of forage decreased CH₄ emissions by 13.9-18.7% but enhanced N₂O emissions by 24.4-33.2% relative to no feeding. Compared with RM treatment, RC treatment decreased CH₄ emissions by 18.1-19.6% but increased N₂O emissions by 16.8-21.0%. Moreover, RC treatment decreased GWP by 16.8-22.0% while increased NEEB by 26.9-75.6% relative to RM treatment, suggesting that the RC model may be a promising option for mitigating GWP and increasing economic benefits of paddy fields. However, the RC model resulted in a lower grain yield compared with the RM model, indicating that more efforts are needed to simultaneously increase grain yield and NEEB and decrease GWP under RC model.

[Effect of Nitrification on N2O Emissions and Their Environmental Factors in Saline-alkali Wetlands]

Wetlands are important sources and sinks for N₂O. Exploring the role of N₂O emissions in saline-alkali wetlands has great significance in understanding the nitrification mechanism of N₂O production and assessing the role of saline-alkali wetlands in the greenhouse effect. The present study examined the N₂O fluxes and environmental factors of a typical Zhalong reed wetland during the growing season. The results suggested that the N₂O fluxes tended to decrease in volatility, with the highest value in mid-July. The mean flux of N₂O was (37.49±15.75) μg·(m²·h)^-1, indicating that the typical Zhalong reed wetland was a source of N₂O. The N₂O fluxes exhibited a significantly positive correlation with soil temperature at different depths (P<0.05), and the impact of the upper soil temperature on N₂O flux was higher than that of deep soil. In the flooding period, the relationship between N₂O fluxes and water table depth was negatively correlated (P<0.05). Meanwhile, the TOC and TN contents were lower, and the N₂O flux was significantly positively correlated with the NH₄⁺-N content in the 0-40 cm soil layer (P<0.05), but it was not related to NO₃^--N content. Nitrification was stronger than denitrification. There was a significant positive correlation between ammonia-oxidizing bacterial activity and soil temperature in 0-20 cm layer (P<0.01). Additionally, the activity of ammonia-oxidizing bacteria also presented significantly positive linear correlation with the N₂O fluxes (P<0.001), which indicated that the release of N₂O in saline-alkali wetlands was greatly affected by nitrification.

[Characteristics of N2O Release and Influencing Factors in Grass-type and Algae-type Zones of Taihu Lake During Summer]

Spatial heterogeneity of N₂O generation and emissions in multi-ecotype lakes limited the accurate estimation of the N₂O fluxes in lakes, but few studies on the characteristics of N₂O generation and emissions have been conducted. In this study, N₂O flux at the water-gas interface, dissolved N₂O concentration in the water column, and N₂O flux at the sediment-water interface in typical grass-type and algal-type zones of Taihu Lake were analyzed during summer, and indoor micro-environment experiments were conducted to illustrate the main factors affecting the generation and emissions of N₂O. The results showed that the N₂O fluxes at the water-gas interface, dissolved N₂O concentration, and N₂O fluxes at the sediment-water interface of the emergent macrophyte type area was higher than the algae-type area and submerged macrophyte area during the summer., with N₂O fluxes at the water-gas interface of (115.807±7.583), (79.768±1.842), and (3.685±0.295) μmol ·(m² ·h)^-1, respectively. The dissolved N₂O concentration in the water column were (0.051±0), (0.029±0.001), and (0.018±0) μmol ·L^-1, respectively; and the N₂O fluxes at the sediment-water interface were (178.275±3.666), (160.685±0.642), and (75.665±1.016) μmol ·(m² ·h)^-1, respectively. The spatial difference could be attributed to dominant plants and the concentration of inorganic nitrogen in the water column. The results of micro-environment experiments showed that nitrate and organic carbon sources could significantly increase the N₂O production potential of sediments, the high concentration of NH₄⁺-N in the water column might inhibit the N₂O production in sediments, and the production rates of N₂O in the sediment increased remarkably when the incubation temperature increased, suggesting that the generation and emissions of N₂O were mainly restricted by nitrate, organic carbon, and temperature in summer.

[Effects of warming and precipitation exclusion on soil N2O fluxes in subtropical forests.]

In order to explore how soil warming and precipitation exclusion influence soil N2O fluxes, we used related functional genes as markers, and four treatments were set up, i.e. , control (CT), soil warming (W, 5 ℃ above the ambient temperature of the control), 50% precipitation reduction (P), soil warming plus 50% precipitation reduction (WP). The results showed that precipitation exclusion reduced soil ammonium nitrogen concentration significantly. Soil warming decreased soil N2O flux and soil denitrification potential significantly. Soil microbial biomass nitrogen (MBN) in warming treatment (W) and precipitation exclusion treatment (P) was significantly lower than that in the control. The amoA gene abundance of AOA was negatively correlated with MBN and ammonium nitrogen contents, but neither soil nitrification potential nor soil N2O flux was correlated with the amoA gene abundance of AOA. Path analysis showed that the denitrification potential affected soil N2O flux directly, while microbial biomass phosphorus (MBP) and warming affected soil N2O flux indirectly through their direct effects on denitrification potential. Temperature might be the main driver of N2O flux in subtropical forest soils. Global warming would reduce N2O emissions from subtropical forest soils.