Relationship between soil solution electrochemical changes and methane and nitrous oxide emissions in different rice irrigation management systems

Reduced nitrogen fertilization under flooded conditions cut down soil N2O and CO2 efflux: An incubation experiment

Unreasonable water (W) and inorganic nitrogen (N) fertilization cause an intensification of soil greenhouse gas (GHGs) emissions. W-N interactions (W × N) patterns can maximise the regulation of soil GHGs efflux through the rational matching of W and N fertilization factors. However, the effects of W × N patterns on soil GHGs efflux and the underlying mechanism remain unclear. In this study, urea fertilizers were applied to paddy soils in a gradient of 100 (N100), 80 (N80), and 60 mg kg^-1 (N60) concentrations. Flooding (W1) and 60% field holding capacity (W2) was set for each N fertilizer application to observe the effects of W × N patterns on soil properties and GHGs efflux through incubation experiments. The results showed that W significantly affected soil electrical conductivity and different N forms (i.e., alkali hydrolyzed N, ammonium N, nitrate N and microbial biomass N) contents. Soil organic carbon (C) content was reduced by 14.40% in W1N60 relative to W1N100, whereas microbial biomass C content was increased by 26.87%. Moreover, soil methane (CH₄) fluxes were low in all treatments, with a range of 1.60-1.65 μg CH₄ kg^-1. Soil nitrous oxide (N₂O) and carbon dioxide (CO₂) fluxes were significantly influenced by W, N and W × N. Global warming potential was maintained at the lowest level in W1N60 treatment at 0.67 g CO₂-eq kg^-1, suggesting W1N60 as the preferred W × N pattern with high environmental impact. Our findings demonstrate that reduced N fertilization contributes to the effective mitigation of soil N₂O and CO₂ efflux by lowering the soil total N and organic C contents and regulating soil microbial biomass C and N.

Plants are a natural source of nitrous oxide even in field conditions as explained by 15N site preference

Plants are either recognized to produce nitrous oxide (N₂O) or considered as a medium to transport soil-produced N₂O. To date, it is not clear whether in their habitat plants conduit N₂O produced in soil or are a natural source. We aimed to understand role of plants in N₂O emissions in field conditions. Therefore, rubber plants (Ficus elastica) were planted in the field; then plant and soil chambers were deployed simultaneously to collect gas samples, and ¹⁵N site preference (SP) of N₂O was evaluated. The mean SP values of plant and soil emitted N₂O were -20.85 ± 2.8‰ and -8.85 ± 1.08‰, respectively, and were significantly different (p < 0.0001); while bulk ¹⁵N of plant and soil emitted N₂O were -10.83 ± 3.33‰ and -22.56 ± 3.37‰, respectively and were similar (p = 0.06). In the current study, soil always acted as a source of N₂O, while plants were both source and sink. Plant and soil N₂O fluxes had significant positive exponential relationship with both soil and air temperature. Soil water-filled pore space (WFPS) had significant negative linear relationship with only soil N₂O fluxes. Plant N₂O fluxes had significant positive linear relationship with plant respiration rates and negative linear relationship with plant surface areas. Based on the relationship between plant respiration rates and N₂O fluxes, we suggest that mitochondria are the possible sites of N₂O formation in plant cells while the relationship between plant surface areas and N₂O fluxes suggests that roots are the parts of its formation in natural and field conditions. Our results suggest that plants are a natural source of N₂O even at field conditions and challenge a view that plants are a medium to transport soil-produced N₂O into the atmosphere.