Global nitrogen input on wetland ecosystem: The driving mechanism of soil labile carbon and nitrogen on greenhouse gas emissions

Abstract

Greenhouse gas emissions from wetlands are significantly promoted by global nitrogen input for changing the rate of soil carbon and nitrogen cycling, and are substantially affected by soil labile carbon and nitrogen conversely. However, the driving mechanism by which soil labile carbon and nitrogen affect greenhouse gas emissions from wetland ecosystems under global nitrogen input is not well understood. Working out the driving factor of nitrogen input on greenhouse gas emissions from wetlands is critical to reducing global warming from nitrogen input. Thus, we synthesized 72 published studies (2144 paired observations) of greenhouse gas fluxes and soil labile compounds of carbon and nitrogen (ammonium, nitrate, dissolved organic carbon, soil microbial biomass nitrogen and carbon), to understand the effects of labile carbon and nitrogen on greenhouse gas emissions under global nitrogen input. Across the data set, nitrogen input significantly promoted carbon dioxide, methane and nitrous oxide emissions from wetlands. In particular, at lower nitrogen rates (<100 kg ha^-1·yr^-1) and with added ammonium compounds, freshwater wetland significantly promoted carbon dioxide and methane emissions. Peatland was the largest nitrous oxide source under these conditions. This meta-analysis also revealed that nitrogen input stimulated dissolved organic carbon, ammonium, nitrate, microbial biomass carbon and microbial biomass nitrogen accumulation in the wetland ecosystem. The variation-partitioning analysis and structural equation model were used to analyze the relationship between the greenhouse gas and labile carbon and nitrogen further. These results revealed that dissolved organic carbon (DOC) is the primary factor driving greenhouse gas emission from wetlands under global nitrogen input, whereas microbial biomass carbon (MBC) more directly affects greenhouse gas emission than other labile carbon and nitrogen.

Keywords: Fertilization; Greenhouse gas; Nitrogen deposition; Soil labile compounds; Wetland.

Graphical abstract

Fig. 1

Geographical distribution of the study sites.

Fig. 2

Standardized mean difference for greenhouse gases emissions from different wetland environments under nitrogen inputs. The numbers in the figure represent the number of case studies. A standardized mean difference >0 reveals a positive effect on greenhouse gas emissions, whereas values < 0 reveal negative effects. Error bars are the bootstrap confidence intervals (CIs). CIs that do not include 0 and do not overlap indicate a significant effect on greenhouse gas emissions and significant differences among groups, respectively. SMD represents the standardized mean difference, which is a type of effect size. The unit of nitrogen input rate is kg·ha⁻¹·yr⁻¹.

Fig. 3

Standardized mean difference for DOC, NH₄⁺–N, NO_3⁻–N from different wetland environments under nitrogen inputs. The numbers in the figure represent the number of case studies. For details on effect size interpretation, refer to Fig. 2.

Fig. 4

Mean effect size for MBC and MBN from different wetland environments under nitrogen inputs. The numbers in the figure represent the number of case studies. For details on effect size interpretation, refer to Fig. 2.

Fig. 5

Variation-partitioning analysis of the effects of soil labile carbon and nitrogen on greenhouse gas emissions. NH₄⁺-N, ammonium; NO_3⁻-N, nitrate; DOC, dissolved organic carbon; MBN, microbial biomass nitrogen; MBC, microbial biomass carbon.

Fig. 6

Structural equation model (SEM) evaluating the direct effects on greenhouse gases (a–c) and the standardized total effect (direct plus indirect effects) derived from the SEM (d–f) on a global scale. The number represents the direct effects on greenhouse gas emissions. The various widths of the gray lines represent p < 0.001, p < 0.005, p<0.01 and p>0.01.

Fig. 7

The driving mechanisms of labile carbon and nitrogen on greenhouse gas emissions from wetland ecosystems under nitrogen input. The black numbers represent the effect size of various parameters, the red numbers represent the total effects of ammonium on greenhouse gas emissions, the brown numbers represent the direct effects of labile carbon and nitrogen on greenhouse gas emissions and the direct effects between various labile carbon and nitrogen.

Fig. 8

Regression analysis for the effect size of greenhouse gas emission and mean annual precipitation, mean annual temperature at the global level. The red short dash represents the regression analysis of the effect size and mean annual precipitation and mean annual temperature among all studies. The black short dash represents the regression analysis of the effect size and mean annual precipitation in the range from 400 to 700 mm. d-CO₂ is the effect size of carbon dioxide, d-CH₄ is the effect size of methane, d-N₂O is the effect size of nitrous oxide.

Fig. 9

The effect size of greenhouse gas emission among different nitrogen compounds and the nitrogen input rate. d-CO₂ is the effect size of carbon dioxide, d-CH₄ is the effect size of methane, and d-N₂O is the effect size of nitrous oxide. NH₄⁺-N represents ammonium, NO_3⁻-N represents nitrate, NH₄NO₃ represents ammonium nitrate, ONF represents organic nitrogen fertilizer. The red point represents the mean effect size of greenhouse gas emission under different nitrogen inputs and nitrogen input rates.