DOI: 10.6086/d11d5d
title: Wetting‐induced soil CO2 emission pulses are driven by interactions among soil temperature, carbon, and nitrogen limitation in the Colorado Desert

description: Warming-induced changes in precipitation regimes, coupled with
 anthropogenically-enhanced nitrogen (N) deposition, are likely to increase
 the prevalence, duration, and magnitude of soil respiration pulses
 following soil wetting via interactions among temperature and C and N
 availability. Quantifying the importance of these interactive controls on
 soil respiration is a key challenge as pulses can be large terrestrial
 sources of atmospheric CO2 over comparatively short timescales. Using an
 automated sensor system, we measured soil CO2 flux dynamics in the
 Colorado Desert—a system characterized by pronounced transitions from
 dry-to-wet soil conditions—through a multi-year series of experimental
 wetting campaigns. Experimental manipulations included combinations of C
 and N additions across a range of ambient temperatures and across five
 sites varying in atmospheric N deposition. We found soil CO2 pulses
 following wetting were highly predictable from peak instantaneous CO2 flux
 measurements. CO2 pulses consistently increased with temperature, and
 temperature at time of wetting positively correlated to CO2 pulse
 magnitude. Experimentally adding N along the N deposition gradient
 generated contrasting pulse responses: adding N increased CO2 pulses in
 low N deposition sites, whereas adding N decreased CO2 pulses in high N
 deposition sites. At the lowest N deposition site, simultaneous additions
 of C and N during wetting led to the highest observed soil CO2 fluxes
 reported globally at 299.5 µmol CO2 m-2 s-1. Our results suggest that
 soils have the capacity to emit high amounts of CO2 within small
 timeframes following infrequent wetting, and pulse sizes reflect a
 non-linear combination of soil resource and temperature interactions.
 Importantly, the largest soil CO2 emissions occurred when multiple
 resources were flushed simultaneously in historically resource-limited
 desert soils, pointing to regions experiencing simultaneous effects of
 desertification and urbanization as key players in future global C
 balance.
description: Our eight field experiments, which varied in experimental manipu-
 lations of temperature and C and N substrates, all used the same
  measurement system to capture soil temperature, moisture, and
 CO<sub>2</sub> fluxes before and after soil wetting. Prior to
 each field campaign,  pairs of polyvinyl chloride (PVC) soil collars
 measuring 20-cm in diam- eter were installed to 5-cm depth and adjacent to
 each other under  L. tridentata canopies or in interspaces between shrubs.
 One collar  per pair was used for trace gas measurements and the other was
 used  for soil temperature, moisture, and ancillary soil measurements.
 This  secondary collar was necessary because temperature and moisture
 probes contained wiring attachments that would interfere with the
 chamber's ability to seal around the soil collar; however, we assumed
 that collar pairs would experience similar climate and edaphic conditions
 given their proximity to each other. Both collars in each pair received
 identical wetting treatments and collar pairs were situated at least 2 m
 apart from other pairs. Beginning at 15 min following wetting, soil
 temperature, moisture, and fluxes of CO<sub>2</sub>  were
  measured  at  30-min  intervals  over  24-45 h. Soils were also measured
 for up to 24 h prior to wetting as an assessment of dry conditions. For
 these instantaneous measurements, we used a robotic chamber array and
 sampling procedure previously described in Andrews et al. (2022 AGEE). In
 this system, eight automated long-term chambers (LI- 8100-104; LI-COR
 Biosciences) with  soil  temperature  (LI-8150-203 thermistor  probe;
 LI-COR Biosciences) and moisture (LI-GS1 probe; LI-COR Biosciences) probe
 attachments were installed on soil PVC collars; probes were inserted to 5
 cm below the soil surface and provided integrated measurements of 3–5  cm
 soil depth. Each chamber collected measurements on a 30-min interval. Air
 collected from an actively measuring chamber was passed through a
 multiplexer (LI-8150; LI-COR Biosciences) and delivered to a gas analyzer
 suite, including a CO<sub>2</sub> infrared gas analyzer system
 (LI-8100A; LI-COR Biosciences), in a closed loop. Each chamber measurement
 sequence included a 30-s pre-measurement purge, a 2.5-min active
 measurement period of trace gas concentrations and soil climate status,
 and a 30-s post-measurement purge. All eight chambers were sampled at
 30-min intervals over a 24- to 45-h measurement period.
 Raw CO<sub>2</sub> concentration and soil probe
 measurements were batch processed into fluxes and associated soil
 temperature and moisture data using algorithms adapted from previous work
 using this chamber array (Andrews et al., 2022 AGEE; Krichels et al.,
 2022). Instantaneous fluxes of CO<sub>2</sub> were calculated
 as the regression coefficient of linear increase in gas concentration data
 during the 2.5-min active chamber measurement period, accounting for soil
 collar dimensions and atmospheric parameters following the Ideal Gas Law
 (Davidson et al., 2000). Instantaneous fluxes of
 CO<sub>2</sub> were compiled and integrated with instantaneous
 soil temperature and moisture measurements using a publicly-available R
 script (Andrews & Krichels,  2021). Additional post-processing
 filtering steps were conducted when data failed to cross a threshold of
 data quality and control due to chamber or analyzer malfunctions. Our
 final eight-campaign dataset consisted of 22,553
 CO<sub>2</sub> fluxes and corresponding soil temperature
 and/or moisture measurements. From continuous measurements of each
 chamber, we constructed 24-h time series following wetting and extracted
 the magnitude and timing of maximum instantaneous flux. We also calculated
 24-h cumulative CO<sub>2</sub> fluxes using linear trapezoidal
 integration of chamber observations which occurred at 30-min
 intervals.
description: Statistical analyses were conducted  in JMP 14 (2021) and data
 management and visualization were performed in R 4.1.3 (2021) and RStudio
 (RStudio Team, 2020).