DOI: 10.5061/dryad.qz612jmnx
title: Hot spots and hot moments of greenhouse gas emissions in agricultural peatlands

description: Drained agricultural peatlands occupy only 1% of agricultural land but are
 estimated to be responsible for approximately one-third of global cropland
 greenhouse gas emissions. However, recent studies show that greenhouse gas
 fluxes from agricultural peatlands can vary by orders of magnitude over
 time. The relationship between these hot moments (individual fluxes with
 disproportionate impact on annual budgets) of greenhouse gas emissions and
 individual chamber locations (i.e. hot spots with disproportionate
 observations of hot moments) is poorly understood but may help elucidate
 patterns and drivers of high greenhouse gas emissions from agricultural
 peatland soils. We used continuous chamber-based flux measurements across
 three land uses (corn, alfalfa, and pasture) to quantify the
 spatiotemporal patterns of soil greenhouse gas emissions from temperate
 agricultural peatlands in the Sacramento-San Joaquin Delta of California.
 We found that the location of hot spots of emissions varied over time and
 were not consistent across annual timescales. Hot moments of nitrous oxide
 (N2O) and carbon dioxide (CO2) fluxes were more evenly distributed across
 space than methane (CH4). In the corn system, hot moments of CH4
 flux were often isolated to a single location but locations were not
 consistent across years. Spatiotemporal variability in soil moisture, soil
 oxygen, and temperature helped explain patterns in N2O fluxes in the
 annual corn agroecosystem but was less informative for perennial alfalfa
 N2O fluxes or CH4 fluxes across ecosystems, potentially due to
 insufficient spatiotemporal resolution of the associated drivers. Overall,
 our results do not support the concept of persistent hot spots of soil
 CO2, CH4, and N2O emissions in these drained
 agricultural peatlands. Hot moments of high flux events generally varied
 in space and time and thus required high sample densities. Our results
 highlight the importance of constraining hot moments and their controls to
 better quantify ecosystem greenhouse gas budgets.
description: Site description We used data from three
 agricultural peatland sites in the Sacramento-San Joaquin Delta region of
 California. The sites experienced a similar climate over a relatively
 small regional scale (~60 km<sup>2</sup>) but had contrasting
 land uses and soil conditions (Table 1). The datasets included over 98,000
 chamber flux measurements from an organic-rich annual maize agroecosystem
 (38.1091°N, 121.5351°W; hereafter referred to as corn). Preliminary data
 on temporal flux patterns from years 1–3 were reported in Anthony and
 Silver (2021); here we added an additional year of observations and
 explored spatial dynamics not previously considered. We also included data
 from a continuous perennial alfalfa agroecosystem (chambers: 38. 1076°N,
 121.5021°W, tower: 38.0992°N, 121.4993°W; hereafter referred to as
 alfalfa) with 103,000 flux measurements; as with the corn site previous
 work explored the temporal dynamics of greenhouse gas emissions from this
 site, but did not report spatial patterning (Anthony et al. 2023).
 Finally, we included data from a continuously grazed irrigated pasture
 38.0402 °N, -121.7272 °W; hereafter referred to as pasture) with 33,000
 flux measurements. These are the three dominant land uses for drained
 agricultural peatlands in the region, accounting for 53% of crop acreage
 (The Delta Protection Commission 2020). Importantly, grazed pastures often
 occur on degraded peatland soils where other agricultural activity is less
 likely to be profitable (Hatala et al. 2012; Buschmann et al. 2020). Corn
 was the only system to receive N fertilizer inputs, which were applied at
 a rate of 118 kg N ha<sup>−1</sup> y<sup>−1</sup>
 (Anthony and Silver 2021). Here we used the three datasets, where we
 include both fluxes to the atmosphere from soil and fluxes from the
 atmosphere to the soil, to explore spatial dynamics both within and across
 agricultural peatland ecosystems and relationships to temporal patterns
 which previous studies did not address.  Greenhouse
 gas flux measurements All datasets collected soil
 fluxes of CO<sub>2</sub>, CH<sub>4</sub>, and
 N<sub>2</sub>O continuously using an automated chamber system
 for 1 to 4 years. Each system consisted of nine opaque, automated gas flux
 chambers (eosAC, Eosense, Nova Scotia, Canada) connected to a multiplexer
 (eosMX, Eosense, Nova Scotia, Canada). The multiplexer allowed for
 dynamically signaled chamber deployment and routed gases to a cavity
 ring-down spectrometer (Picarro G2508, Santa Clara, CA, USA). Chambers
 were measured sequentially over a 10-min sampling period with a 1.5-min
 flushing period before and after each measurement. Chambers were deployed
 in a 10 × 10 m grid design, with each chamber approximately 5 m from other
 chambers. Vegetation was included in all pasture chambers and randomly
 assigned to bare soil (n = 4) or plants (n = 5) in alfalfa. In corn,
 chambers were randomly assigned to beds (n = 3) or furrows (n = 6) without
 vegetation included due to the height of the stalks. Data filtering and
 detection limits are described in detail in Anthony & Silver
 (2021) and Anthony et al. (2023). Site-level net
 ecosystem CO<sub>2</sub> exchange (NEE) and calculated
 site-level global warming potential (GWP) utilizing annual flux
 measurements from paired Ameriflux towers in corn (US-Bi2) and pasture
 (US-Snf) sites (Kasak et al. 2020; Rey-Sanchez et al. 2021) was employed
 to contextualize the importance of hot moments on net ecosystem
 CO<sub>2</sub>e budgets. For the alfalfa site, we utilized
 annual net ecosystem exchange (NEE) estimates from a nearby (&lt; 1
 km) Ameriflux tower (US-Bi1) in an alfalfa agroecosystem with identical
 management practices and soil type (Anthony and Silver 2020; Rey-Sanchez
 et al. 2022b). The eddy covariance technique captures continuous,
 long-term exchange of CO<sub>2</sub>,
 CH<sub>4</sub>, water, and energy fluxes between the landscape
 and the atmosphere, along with measurements of environmental drivers
 (Eichelmann et al. 2018). Fluxes were measured at a frequency of 20 Hz
 using open-path infrared gas analyzers (LI-7500 for CO2, LI-7700 for CH4,
 LiCOR Inc., Lincoln, NE, USA). Sonic anemometers measured sonic
 temperature and three-dimensional wind speed at 20 Hz (WindMaster Pro
 1590, Gill Instruments Ltd, Lymington, Hampshire, England). To convert
 N<sub>2</sub>O and CH<sub>4</sub> flux
 measurements to CO<sub>2</sub>e, we used the IPCC AR5 100-year
 GWP values of 28 CO<sub>2</sub>e for
 CH<sub>4</sub> and 298 CO<sub>2</sub>e for
 N<sub>2</sub>O (Myhre et al. 2013).
 Environmental sensors In both the corn and
 alfalfa systems, two sets of soil sensors per site were installed at
 depths of 10 cm, 30 cm, and 50 cm and included O<sub>2</sub>
 and soil temperature sensors (SO-110, Apogee Instruments, Logan, UT) and
 moisture sensors (CS616, Campbell Scientific, Logan, UT) connected to a
 datalogger (CR1000, Campbell Scientific, Logan, UT) from September
 2018-February 2021 in alfalfa and October 2018-July 2021 in corn. Sensors
 were installed as close to corresponding chamber grid locations as
 possible while minimizing soil disturbance and limiting interference with
 agricultural activities. Except for periodic agricultural events, sensors
 remained installed throughout the year. Data filtering and data gaps are
 described in detail in Anthony & Silver (2021) and Anthony et al.
 (2023).
description: # Hot spots and hot moments of greenhouse gas emissions in agricultural
 peatlands
 [https://doi.org/10.5061/dryad.qz612jmnx](https://doi.org/10.5061/dryad.qz612jmnx) <br> 2017-2021 Automated chamber (Eosense eosAC) and Picarro G2508 GHG analyzer flux data for CO2, CH4, and N2O from corn, pasture, and alfalfa, and 2018-2021 continuous soil sensing data (oxygen, moisture, and temperature) from corn and alfalfa ## Description of the data and file structure Alfalfa\_Chamber, Corn\_chamber, and Pasture\_chamber flux data tab: Alfalfa: Continuous soil flux measurements from January 2017-February 2021 Corn: Continuous soil flux measurements from July 2017-October 2021 Pasture: Continuous soil flux measurements from April 2019-July 2022 * Chamber * ChamberPressure (kPa) * ChamberTemperature (K) * CO2 flux: CO2\_umol/m2/s * CH4 flux: CH4\_nmol/m2/s * N2O flux: N2O\_nmol/m2/s * Site Year Alfalfa, Corn, Soil Sensor Data tab: Measurements at 10, 30, and 50 cm soil depths from October 2018-February 2021 * Temp = Temperature in Celsius * VWC= volumetric water content in m3/m3 * O2 = Oxygen concentration in % * TIMESTAMP: Date and Time * Temp\_10cm (C) * Temp\_30cm (C) * Temp\_50cm (C) * VWC\_10cm (m3/m3) * VWC\_30cm (m3/m3) * VWC\_50cm (m3/m3) * O2\_10cm (%) * O2\_30cm (%) * O2\_50cm (%) NEE: Net Ecosystem Exchange (µmolCO2 m-2 s-1) data can be found in Ameriflux datasets available at URLs below ## Sharing/Access information Links to other publicly accessible locations of the data: Was data derived from another source? All Eddy covariance data (Net Ecosystem Exchange (NEE)) from Ameriflux tower sites. If yes, list source(s): https://ameriflux.lbl.gov/sites/siteinfo/US-Bi1 https://ameriflux.lbl.gov/sites/siteinfo/US-Bi1 https://ameriflux.lbl.gov/sites/siteinfo/US-Snf