Soil Biogeochemical Response to Drought Conditions in the Biosphere 2 Rainforest

The direct measurement of soil gases provides insight into the biogeochemical processes responsible for micro- and macro-nutrient cycling, respiration, signaling, and environmental responses.  The concentrations and isotopic signatures of soil gases are effective messengers of the microbial pathways active in the soil.  We have developed and deployed a high frequency sensor consisting of new diffusive soil probes coupled with a Tunable Infrared Laser Direct Absorption Spectrometer (TILDAS) to monitor a range of soil gas species to investigate biogeochemical soil processes.

An array of soil probes was deployed at the Tropical Rainforest at Biosphere 2 in Arizona as part of the Water Atmosphere and Life Dynamics (WALD) experiment in 2019-2020.  Probes were located in a root zone and nearby control area, and at several depths via a soil pit.  These probes were coupled with a TILDAS to monitor isotopologues of nitrous oxide (N₂O) including ¹⁴N¹⁵NO, ¹⁵N¹⁴NO, N₂¹⁸O, and methane (¹³CH₄ and ¹²CH₄), as well as CO₂. During the WALD experiment, the probe-TILDAS system followed the impact on the soil biome of a 2 month induced drought in the rainforest and the subsequent return of rain.  The high temporal resolution of the system allowed us to monitor each probe every 2 hours and thus observe changes in the composition of soil gases that reflect biogeochemical processes and pathways.  CO₂ and thus respiration decreased significantly during the drought and was slow to recover.  Differences in N₂O mixing ratios and isotopic signatures (both site preference and bulk 15N) in the root zone versus a controlled soil region were observed during both drought and rewetting periods.  Changes in nitrogen and carbon cycles and the microbial pathways during the induced drought and rewetting as reflected in these observations will be discussed.