Advancing chemical lability assessments of organic matter using a synthesis of FT-ICR MS data across diverse environments and experiments

Abstract

Patterns in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) data provide perspective on how organic matter (OM) character is shaped by natural processes. Here, we reevaluate and update the molecular lability boundary (MLB) approach to assess OM lability using a synthesis of FT-ICR MS data. Now included are MLB_L (biolability) indices (i.e., the OM proportion of hydrogen-to-carbon ratios ≥ 1.5) of freshwaters, plant litter and biochar leachates, soils, and oils across the US, trends in OM and dissolved OM (DOM) lability from laboratory experiments examining isolated microbial, photochemical, and thermal degradation, and biogeochemical interpretations of biolability patterns across ecosystems influenced by human activity (e.g., fertilizer addition and agricultural management). Photo-processing increased the hydrogen saturation of OM, describing products of higher biolability (>MLB_L values). DOM MLB_L values decreased during microbial and thermal degradation processes. Laboratory experiments with isolated treatments showed larger changes in MLB_L than exhibited by field samples from systems managed by humans. We interpret changes in N- and/or S-containing DOM above the MLB as the production or use of heterogeneously biolabile material. We interpret heterogeneously labile DOM composition to indicate autochthonous production, biomass growth, nutrient accumulation, or biological degradation in stream and soil samples. The most recent data in our synthesis suggest that the MLB approach should be reframed to provide lability indices for biotic, thermal, and photochemical processes. A broader perspective on lability provides a useful tool to decompose large, complex, and process-driven DOM data into more simple and informative indicators of trends in ecosystem form and function.