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.

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 km²) 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⁻¹ y⁻¹ (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 CO₂, CH₄, and N₂O 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 CO₂ 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 CO₂e budgets. For the alfalfa site, we utilized annual net ecosystem exchange (NEE) estimates from a nearby (< 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 CO₂, CH₄, 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 N₂O and CH₄ flux measurements to CO₂e, we used the IPCC AR5 100-year GWP values of 28 CO₂e for CH₄ and 298 CO₂e for N₂O (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 O₂ 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).