Environmental Science and Engineering Seminar

Scaling biological processes from the organ/organism scale at which process information is often acquired, to ecosystem or global scales relevant to quantifying feedbacks to global climate, is an outstanding challenge of earth system science.  This challenge may be addressed with isotopic tracers of organismal metabolism that integrate to larger scales, and constrain the relation between emergent ecosystem fluxes and the underlying processes governing organismal response.  This method has had limited application in studies of Net Ecosystem Exchange (NEE) of CO₂ and CH₄ due to technical limitations on ability to measure fast response (~5 Hz) isotopic composition of CO₂ on eddy covariance towers, a widely used flux acquisition method. 

Here, we used newly developed high-precision laser spectrometers to acquire measurements of the isotopic composition of atmospheric CO₂ (above a temperate forest) and CH₄ (in a thawing permafrost wetland).  Isotopic partitioning of CO₂ NEE into component GPP and Respiration in the forest revealed significant differences in both the diel and the seasonally averaged behavior of GPP as compared to conventional (non-isotopic) partitioning methods.  Isotopic partitioning is thus changing our interpretation of ecosystem CO₂ exchange at this site (Harvard Forest).

Isotopic composition of CH₄ emissions were combined with microbial community profiling via 16S rRNA amplicon sequencing along a permafrost thaw gradient in the wetland.  We found that different stages of thaw were associated with different microbial communities and isotopic compositions, and that ecosystem-scale isotopic patterns could only be statistically well-predicted when data on the changing microbial community was included.  We conclude that microbial community ecology may be a significant but neglected mechanism in earth system models of the effect of permafrost thaw on atmospheric composition and climate. 

More generally, new isotope measurement technologies may have wide applicability in advancing studies of ecosystem dynamics under climate change.