Versatile soil gas concentration and isotope monitoring: optimization and integration of novel soil gas probes with online trace gas detection

[摘要] Gas concentrations and isotopic signatures can unveil microbialmetabolisms and their responses to environmental changes in soil. Currently,few methods measure in situ soil trace gases such as the products ofnitrogen and carbon cycling or volatile organic compounds (VOCs) thatconstrain microbial biochemical processes like nitrification,methanogenesis, respiration, and microbial communication. Versatile tracegas sampling systems that integrate soil probes with sensitive trace gasanalyzers could fill this gap with in situ soil gas measurements thatresolve spatial (centimeters) and temporal (minutes) patterns. We developeda system that integrates new porous and hydrophobic sintered polytetrafluoroethylene (sPTFE) diffusivesoil gas probes that non-disruptively collect soil gas samples with atransfer system to direct gas from multiple probes to one or more centralgas analyzer(s) such as laser and mass spectrometers. Here, we demonstratethe feasibility and versatility of this automated multiprobe system forsoil gas measurements of isotopic ratios of nitrous oxide ( δ 18 O, δ 15 N, and the 15 N site preference of N 2 O), methane, carbon dioxide( δ 13 C), and VOCs. First, we used an inert silica matrix to challengeprobe measurements under controlled gas conditions. By changing andcontrolling system flow parameters, including the probe flow rate, weoptimized recovery of representative soil gas samples while reducingsampling artifacts on subsurface concentrations. Second, we used this systemto provide a real-time window into the impact of environmental manipulationof irrigation and soil redox conditions on in situ N 2 O and VOCconcentrations. Moreover, to reveal the dynamics in the stable isotoperatios of N 2 O (i.e., 14 N 14 N 16 O, 14 N 15 N 16 O, 15 N 14 N 16 O, and 14 N 14 N 18 O), wedeveloped a new high-precision laser spectrometer with a reduced samplevolume demand. Our integrated system – a tunable infrared laser direct absorption spectrometry (TILDAS) in parallel with Vocus proton transfer reaction mass spectrometry (PTR-MS), in line with sPTFEsoil gas probes – successfully quantified isotopic signatures for N 2 O, CO 2 ,and VOCs in real time as responses to changes in the dry–wetting cycle and redoxconditions. Broadening the collection of trace gases that can be monitored in thesubsurface is critical for monitoring biogeochemical cycles, ecosystemhealth, and management practices at scales relevant to the soil system.

[学科分类] 大气科学