Molecular Chemodiversity of Dissolved Organic Matter in Paddy Soils

Organic
matter (OM), and dissolved organic matter (DOM), have a
major influence upon biogeochemical processes; most significantly,
the carbon cycle. To date, very few studies have examined the spatial
heterogeneity of DOM in paddy soils. Thus, very little is known about
the DOM molecular profiles and the key environmental factors that
underpin DOM molecular chemodiversity in paddy soils. Here, Fourier-Transform
Ion Cyclotron Resonance Mass Spectrometry was applied to unambiguously
resolve 11 361 molecular formulas in 16 paddy soils; thereby
elucidating the molecular characteristics of paddy soil DOM. Soil
pH, iron complexing index (Fep/FeR) and C/N
ratio were established to be key factors controlling DOM profiles.
Polycyclic aromatics (derived from combustion) and polyphenols (derived
from plants) increased with increasing pH, while polyphenols molecules,
pyrogenic aromatics, and carboxylic compounds decreased with increasing
iron complexing index. Patterns in molecular profiles indicated DOM
in paddy soils to become more recalcitrant at higher soil C/N ratio
and higher pH. Furthermore, plant-derived polyphenols and pyrogenic
DOM were retained favorably by iron and the chemodiversity of DOM
in paddy soil increased with increasing soil C/N ratios. This study
provides critical information about DOM characteristics at a molecular
level and will inform better global management of soil carbon in paddy
soil ecosystems.