Assessing the role of winter flooding on baseline greenhouse gas fluxes from corn fields in the Sacramento – San Joaquin Bay Delta

Science Center Objects

Understanding the magnitude and variability of baseline greenhouse gas (GHG) emissions from the Sacramento – San Joaquin Bay Delta is critical for current and future land management. For example, strategies that maximize carbon sequestration in soils and plants while minimizing unintended consequences such as GHG emissions are likely to produce both economic and environmental benefits for the people of California. 

Diagram showing hypothesized changes in GHG emission pathways due to winter flooding of corn fields.

Conceptual diagram showing hypothesized changes in GHG emission pathways due to winter flooding of corn fields.

(Public domain.)

Establishing baseline emissions requires a clearer understanding of the driving factors – natural and anthropogenic – that affect both the timing and magnitude of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) emissions to the atmosphere over seasonal to annual time scales.

Previous studies in agricultural fields and wetlands have shown that soil moisture is a key determinant of GHG emission rates and fluxes from soils. For example, rice field and wetland studies have reported high fluxes of CH4 due to changes in soil oxygen concentrations and redox conditions during long-term inundation (Li et al., 2005). Similarly, anaerobic conditions brought about by soil saturation resulted in a significant flux of N2O, particularly during transition periods such as flood up and drawdown (Li et al., 2005). Therefore, a critical first step in quantifying the baseline GHG emissions from Delta soils is to develop a clearer understanding of the timing, duration and seasonality of soil saturation and flooding in the Bay – Delta system. Future steps to define how hydrology interacts with soil types, crop management, and temperature gradients will allow for a broader understanding of GHG variability and drivers in the Bay – Delta.

Seasonal flooding of agricultural lands has occurred for more than 60 years in the Sacramento – San Joaquin Bay Delta to provide wintering areas for migratory birds on the Pacific Flyway (Delta Protection Commission, 1994). In addition, winter flooding also provides benefits to farmers such as winter weed control, prevention of soil oxidation and subsidence, improved crop yields through changes in soil texture and faster germination of spring crops following winter drainage (Ivey et al., 2003). In the Bay – Delta, harvested corn fields make up nearly 80% of the total winter flooded area (37,236 acres), an area equivalent to approximately 25 % of the total area planted in corn in the basin (Central Valley Joint Venture, 2006). This is in contrast to the broader Central Valley where rice fields are the primary location for winter flooding, making up greater than 90 % of flooded habitat (CJVJ, 2006). Given the importance of corn fields for seasonal flooding and crop production in the Bay - Delta, additional information is clearly needed to assess the importance of changes in soil moisture on GHG emissions from Delta soils.

Here we propose to assess the impact of winter flooding on baseline GHG emissions from harvested corn fields in the Sacramento – San Joaquin Bay Delta. Corn is a significant crop in the Bay Delta (aerially and economically) and provides a unique opportunity to study the impact of seasonal water management on active agricultural areas. While studies are currently underway to assess the role of soil moisture on GHG emissions from rice fields and wetlands on Twitchell Island, little data is currently available to assess timing and magnitude GHG from corn fields influenced by flooding. This study represents the first step in a broader goal to quantify the magnitude and variability of baseline GHG fluxes in the Delta by providing model input and validation data for the DNDC model (Li et al., 2005), as well as providing field data that are directly comparable to carbon sequestration and GHG studies on Twitchell Island and Sherman Island.

In addition to soil moisture, the magnitude and timing of GHG fluxes from agricultural landscapes is influenced by soil tillage practices, cover cropping, crop residue management and fertilizer applications (Li et al., 2005). For example, studies have reported that changes in soil organic carbon content and fertilizer nitrogen additions strongly influence CO2, CH4 and N2O fluxes from agricultural soils (Lee et al., 2006; Bouwman et al., 2002). Several biogeochemical parameters related to land use – along with the timing, frequency and duration of soil saturation – likely influence the GHG fluxes from Delta corn fields. Additional data are needed for field and modeling studies to assess if management approaches meet multiple objectives including bird habitat, carbon storage, and agricultural productivity. 

The objectives of this study are: (1) evaluate changes in the timing, magnitude and form of GHG fluxes (CO2, CH4 and N2O) from Delta corn fields due to winter flooding, (2) determine the changes in biogeochemical properties (soil organic carbon content and lability, nitrogen species and availability, and redox conditions) associated with winter flooding, and (3) provide a dataset of input and calibration / validation parameters for modeling the role of winter flooding and other hydrologic alterations on GHG fluxes from Delta agricultural fields. This will allow us to better assess the role of winter flooding – an important water management practice in the Delta – on the variability and release of GHG’s from Delta agricultural landscapes. This represents a step towards developing a broader understanding of the timing, magnitude and variability of GHG release from Delta ecosystems, and fills a critical gap in our baseline understanding of the Delta ecosystem. 

We propose a 2 year study at Staten Island to quantify GHG fluxes and biogeochemical parameters from Delta corn fields on peat soils with different winter water management strategies (e.g. flooded vs. non-flooded). Staten Island is located within the Delta near Walnut Grove in San Joaquin County, and includes 9,200 acres owned by The Nature Conservancy. The island lies below sea level due to subsidence of organic peat soils behind levees, and supports a high number of wintering sandhill cranes and waterfowl. In 2002-2003, approximately 2,271 acres of cropland were flooded around September (wheat) or October (corn, tomato) and drained several weeks prior to planting in the spring (Ivey et al., 2003).

Study sites will be selected in adjacent fields to minimize differences due to original soil type, climate, and crop management. GHG fluxes are known to be highly spatially and temporally variable in most systems, while soil characteristics such as carbon content and carbon-to-nitrogen (C:N) ratios tend to be more homogenous across individual agricultural fields and change more slowly over time. Therefore, GHG flux measurements will require higher frequency sampling and greater replication than some of the soil characteristics measurements. The goals of our field sampling plan are to 1) provide a reasonable comparison of GHG fluxes from flooded and non-flooded Delta corn fields over the course of the flooding season and compare this to fluxes during the summer, 2) characterize and compare the biogeochemical conditions in the soils and flooding waters associated with GHG flux, and 3) compare shallow soil characteristics during flooding and non-flooding seasons and between flooded and non-flooded corn fields in order to determine how flooding management may impact long term soil characteristics.

GHG measurements: We will measure GHGs (CO2, CH4 and N2O) in static chambers at time intervals corresponding to water management. While we will collect samples throughout the year, a comprehensive characterization of total annual GHG fluxes from these agricultural sites is outside the scope of this project. Therefore, intensive spatial and temporal sampling will take place during flood-up, near the middle of the flooding season, during draw-down, and during two comparison time periods in non-flooding seasons. This approach will allow us to determine if flooding is associated with significant increases or reductions in GHG fluxes in comparison to non-flooded fields, and will be used for guiding sampling efforts in future Delta GHG studies.

GHG fluxes will be measured using small static flux chambers. This technique is cost effective and allows for a large number of measurements to be made across different spatial and temporal scales. Three measurement locations will be chosen within each field, and triplicate chambers will be deployed at each of the measurement locations in order to address both small and large scale spatial variability. Locations will be chosen near the side and center of each field, with a third location in between, in order to estimate spatial variation throughout the field. It is possible that slight differences in topography within the winter-flooded field may result in uneven flooding of the field. If this occurs, a fourth flux chamber location will be added in order to provide duplicate flooded sites, a transitional low-water site, and a non-flooded site within the flooded field. We hypothesize that a non-flooded location within a mostly flooded field will present different GHG behavior in comparison to an entirely non-flooded site due to greater soil water saturation in the mostly flooded site. We will use chambers that have a soil sample area of at least 175 cm2 and are up to 15 cm high based on protocols used by USDA and other studies. However, we are planning to test a few different types of static chambers and dimensions in the early part of the project.

Measurements will be daily or every other day during a two-week period spanning flood up, daily for one week during the middle of flooding, and again daily or every other day during a two week period around draw-down. Frequency and total sampling period may be adjusted depending on the speed of draw-down. One week of daily sampling will be conducted in both the early summer and the later summer to provide comparisons between GHG fluxes, biogeochemistry and soil characteristics in the summer and winter seasons. Additionally, diurnal fluctuations in GHG fluxes and biogeochemical parameters will be measured early in the study, and the results will be used to determine whether diurnal fluctuations are significant enough to require further characterization. In order to assess diurnal variability, GHG flux chamber measurements and biogeochemical parameters will be measured every two hours over two full diurnal cycles (48 hours total) at one site in the flooded field and one site in the non-flooded field. The diurnal studies will take place once in the middle of the flooding season and once during one of the summer sampling events. 

For all soil flux measurements, two piece chambers consisting of a base and a chamber top which can be temporarily sealed onto the base will be used in order to minimize the effects of soil disturbance on GHG flux measurements (Matthias et. al., 1980; Rochette and Eriksen-Hamel, 2005). Once the water levels become too deep for these chambers (during or after flood-up) we will switch to floating chambers of similar size. The chambers will be relatively small in order to maximize the minimum detectable flux for each of the gases measured. Gas aliquots for measurement of all three gases (CO2, CH4 and N2O) will be collected simultaneously from the chambers directly into Exetainers (Rochette and Eriksen-Hamel, 2005). Chambers will be deployed in the field at least 24 hours prior to sampling and measurements will be made over short time scales (15 to 30 minute total deployments, with samples collected every 5 to 10 minutes) in order to minimize changes in air temperature, pressure, and light conditions over the soil. Chambers will be equipped with air temperature probes, small circulation fans, and vent tubes in order to monitor and control for environmental effects caused by the chambers. 

Biogeochemical measurements: Biogeochemical parameters for soil and water samples will be collected in conjunction with the chamber measurements in order to identify critical drivers of GHG flux in the flooded and non-flooded fields, and to serve as key input or calibration/ validation parameters for new and ongoing modeling efforts in the Delta and Central Valley (DNDC; Li et al., 2005). This data will also support future efforts for broad scale understanding of GHG fluxes in the Delta and will be used to design future GHG flux studies. 

Winter flooding is expected to have a significant impact on GHG fluxes due to changes in water saturation of the soil column and changes in both N and C (Figure 1). Physical and chemical characteristics of the overlying water will be measured routinely during flooding for several parameters (Table 1). In addition, we plan to install soil moisture and redox probes at several depths and in several locations in each field coincident with chamber locations for the measurement of field conditions throughout the flooding period. We also plan to install shallow groundwater wells to identify the water table elevation throughout the year, a key parameter for DNDC modeling of GHG emissions from fields and wetland (to be completed as part of a separately funded study).

Soil characteristics: Soil cores will be collected near each of the chamber sites within the flooded and non-flooded fields at the beginning and end of the flooding season, and once during the summer chamber measurements. Bulk soil characteristics change slowly over time, and therefore do not require high-frequency sampling. Approximately 5-10 soil cores per site will be divided into several depth increments (0-5 cm, 5-15 cm, and 15-30 cm), homogenized, and measured for the characteristics listed in Table 1. These measurements will be used to identify soil characteristics associated with different levels of GHG fluxes, determine if winter flooding results in measureable seasonal changes to shallow bulk soil composition, compare differences in field soil composition associated with flooded and non-flooded management practices, and provide the necessary data for inclusion of these sites in the DNDC model.