||Terrestrial ecosystems are an important and dynamic component of the Earth’s climate system, and models are the primary basis for understanding the future interactions between terrestrial ecosystems and the climate. Although temperate forests in the northern mid-latitude of the United States are important net sinks for anthropogenic CO2 , the balance between sinks (GPP) and sources (respiration) may change under future climate change scenarios. Two key facts about temperate forests, that they sequester substantial carbon, but that their long-term trends in sequestration are poorly predicted by models, alludes to the importance of understanding these ecosystems. Model simulations of belowground processes are difficult to test, largely because of the intractable difficulties associated with making the necessary observations. Integrating traditional observations (plot trenching and root rhizotron observations), new experimental methods (artificial roots that can trickle isotopically labeled model “exudate” into soil), and recent technology (highly sensitive laser absorption spectrometers) that brings new capability for real time observations of the isotopologues of CO2, will bring new and more reliable insights into belowground processes. These will in turn enable new and more rigorous tests of model-simulated processes belowground, thereby providing new data and tools to the broader modeling community. We will integrate a newly developed instrumentation (Aerodyne Research Quantum Cascade Laser, QCL) for continuous insitu observations of the isotopologues of CO2, together with the trenching technique to partition components of belowground carbon flux (autotrophic and heterotrophic). The calculated 13CO2 in the trenched plot will provide an estimate of the 13C signature of the heterotrophic soil CO2 source endmember in undisturbed soil. The difference in 13CO2 between the trenched and control plots will provide an indication of the root/rhizosphere/ mycorrhizal endmember of respiration of fresh photosynthate, which will be compared with more direct measurements obtained from the root respiration chambers. The relative contributions of heterotrophic and root/rhizosphere/mycorrhizal to total soil respiration can be solved from estimating the 13C endmember signature of each and knowing the total flux and its 13C signature. These measurements, in conjunction with measurements made by our colleagues, of the isotopic composition of NEE using the eddy covariance technique will provide a new method for partitioning NEE between GPP and ecosystem respiration, as well as partitioning of respiration between aboveground and belowground, and partitioning of belowground respiratory fluxes between components influenced by autotrophic and heterotrophic processes. This data will provide valuable insight into belowground carbon cycling processes.