CONTINUOUS MONITORING OF SOIL AND ECOSYSTEM RESPIRATION IN A SEMI-ARID HIGH-ELEVATION CRITICAL ZONE OBSERVATORY

Atmospheric CO₂ concentrations have risen approximately 40% since the Industrial Revolution, significantly impacting both the terrestrial carbon cycle and environmental feedbacks involved in its regulation. Soil respiration represents a major component in the global carbon cycle and is considered one of the largest CO₂ fluxes from land to the atmosphere. Accurate understanding of temporal and spatial variability of soil respiration has been limited by the techniques employed to soil CO₂ efflux in the field. The application of a gradient method using automated systems allows for a continuous data record and the tracking of the up regulation and gradual change in soil respiration. Here, we obtain continuous time series of soil CO₂, O₂, moisture, temperature, matrix potential, and aboveground carbon allocation and ecosystem respiration by installing arrays of instruments in soil pits at various depths near an eddy covariance in the Jemez River Basin Critical Zone Observatory (a semi-arid high-elevation site).

Soil pO₂ anti-varied with CO₂ efflux and pCO₂, with its minimum lagging slightly behind peak CO₂ efflux. However, variations in soil pO₂ are more fluctuated than CO₂ efflux/pCO₂ during the non-monsoon season. Low soil moisture evidently limited the positive response of soil respiration to increased soil temperature. Soil respiration peaked rapidly after relatively heavy monsoonal precipitation, when the summer drought periods were interrupted. Within the periods of relatively high soil water contents, however, soil temperature played an important role in regulating CO₂ efflux. In addition, the results suggest that gross primary productivity and net ecosystem exchange are primarily dependent on soil moisture, whereas ecosystem respiration is mainly influenced by moisture and secondarily by temperature, which is quite similar in nature to soil respiration. Our results highlight the tight couplings among physical, biological, and chemical processes, displayed on event time scales during the incremental co-evolution of the CZ.