Global patterns of organic matter chemistry and biogeochemical cycling in wetland soils

Soil organic matter (SOM) in wetland soils, including peatlands, is crucial for maintaining ecosystem functions such as water quality, biogeochemical cycles, and regulating greenhouse gas emissions. Water-extractable organic matter (WEOM) comprises molecular compounds that dissolve in water under natural conditions. However, molecular-level studies of WEOM across wetlands in different climates and under various agricultural use intensities remain limited. We employed ultrahigh-resolution Orbitrap mass spectrometry to analyse WEOM and integrated it with data on climate types, agricultural intensities, environmental characteristics, molecular groups, microbial functional genes, and field-measured ecosystem respiration, methane and nitrous oxide fluxes. Wetland soil samples were collected from 25 regions representing four agricultural intensities: (1) no agriculture, (2) non-intensive grassland, (3) intensive grassland, and (4) arable land. Orbitrap identified 14,890 molecular formulas with masses ranging from 100 to 950 Daltons. Correlations between agricultural intensities and formula classes containing N, S, or P was visualised using Van Krevelen diagrams. We further examined the influence of climate types (tropical, temperate, continental) and agricultural intensity on WEOM molecular composition by Principal Coordinates Analysis, and linked WEOM quality changes with gas fluxes and other available environmental and microbiome characteristics. Ecosystem respiration, nitrous oxide emission, and agricultural intensity were positively correlated with the persistence of WEOM (i.e., aromaticity vs. aliphaticity) and negatively correlated with soil water content. Diversity of bacteria and archaea, as well as methane emission, were positively correlated with soil pH, but unrelated to WEOM quality. Our findings provide new insights into how WEOM chemistry changes under varying environmental and management conditions and advance our understanding of its role in global carbon and nutrient cycling.

Keywords: Wetland, WEOM, GHG emissions, Orbitrap, climate, agricultural intensity