Management options for farmers facing saltwater intrusion along the Chesapeake Bay’s Eastern Shore

Progress report for GNE21-268

Project Type: Graduate Student
Funds awarded in 2021: $14,999.00
Projected End Date: 08/01/2023
Grant Recipient: University of Maryland - College Park
Region: Northeast
State: Maryland
Graduate Student:
Faculty Advisor:
Dr. Katherine Tully
University of Maryland
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Project Information

Project Objectives:

The specific objectives of this research are to:

(1) Quantify how soil total P and available Na and P concentrations and pools vary among adaptation, remediation, restoration, and abandonment species along a salinity gradient.

Q1.A Does field management (e.g. adapt, remediate, restore, abandon) affect soil total P and available Na and P concentrations and pools?

Q1.B How do field management practices change soil total P and available Na and P concentrations and pools along a salinity gradient?

 

(2) Quantify how leaf tissue total Na and P concentrations and pools vary among adaptation, remediation, restoration, and abandonment species along a salinity gradient. 

Q2.A Does field management (e.g. adapt, remediate, restore, abandon) affect plant biomass pools of Na and P? 

Q2.B How do field management practices change plant biomass Na and P pools along a salinity gradient?

Introduction:

The purpose of this project is to identify best management practices for farmers facing saltwater intrusion (SWI). I will do this by determining the effects of adaptation, remediation, restoration, and abandonment treatments on soil and plant Na and P levels. These results will be used to determine the potential of each management practice to remove Na and P from soils to increase land productivity and reduce nutrient pollution of the Chesapeake Bay, contributing to the economic and environmental sustainability of systems in the region.

 

Climate change is often viewed as an impending threat. However, on the Eastern Seaboard of the U.S., SWI is already damaging agricultural systems, negatively impacting plant productivity and soil health [1]. Unfortunately, SWI will only worsen as sea levels continue to rise [2, 3]. Saltwater intrusion is the movement of sea salts into freshwater aquifers and shallow groundwater tables. Sea level is projected to rise by 0.2-0.5 meters by 2050 [4], concurrently with increased frequency of storm events and drought. Saltwater intrusion encroaches on coastal agricultural land connected to saline water bodies (e.g. tidal channels) through ditch networks [5]. This phenomenon causes an alteration in biogeochemical cycling, leading to eutrophication through the loss of nutrients (e.g. P), decreased land productivity through soil aggregate dispersion (e.g. due to Na), and overall loss of farmland through the flooding of fields and conversion of uplands into tidal marshes [3]. 

 

Agriculture is a way of life on the Eastern Shore of Maryland. Centuries of cultivation have drastically altered soil physical and chemical properties in the region. This history of land management is now clashing with the new threat of SWI. Soil P levels are high in the region due to decades of fertilizer applications in the form of poultry manure (known as “legacy P”) [6, 7]. At the same time, SWI is changing soil chemistry through the introduction of elements like Na. Sodium can lead to swelling and soil aggregate dispersion, reducing drainage and infiltration [8]. Saltwater intrusion may also release legacy P due to competition between phosphate (PO4) and sulfate for iron (Fe) on soil mineral surfaces [9]. As soils become anoxic, Fe is reduced and releases PO4 into solution. With SWI, the sulfate in saltwater competes with PO4 for Fe binding sites as the soils dry, leaving P in solution and prone to loss [10, 11].

 

My team has observed soil salinity levels ranging from 5 to 20 ppt in farm soils across the Lower Eastern Shore of Maryland [12]. However, most agronomic crops in the region do not tolerate salinity levels above 1-3 ppt [13, 14]. Elevated salinity levels have forced farmers to reconsider management practices. Landowners in coastal regions facing SWI have several choices: continue farming with salt-tolerant crops, remediate fields for a few years before returning to agriculture, restore the land to native marshes (which creates wildlife habitat that supports recreational activities like duck hunting), or abandon salt-damaged fields entirely [3].

Research

Materials and methods:

The following objectives will be accomplished via field experiments established in Somerset and Dorchester Counties, Maryland. Soil and plant samples will be processed and analyzed at the University of Maryland, College Park (UMCP).

 

Field research

I will leverage three experimental sites and one no-salt control site established by Drs. Tully, Gedan, and Miller (see Experience and Roles) on the Lower Eastern Shore of Maryland in 2018. The experiment was designed to examine the survival of different treatments under SWI. My research will add a new element - the potential for these treatments to remove Na and P. Thus, my work will add a novel element, which is highly relevant to the NE SARE objective of good stewardship, profitability, and quality of life for farmers and the agricultural community by guiding best management practices and reducing nutrient losses to the Bay.

 

Field sites are located on three saltwater-intruded agricultural fields planted with four replicate blocks of six treatments: (1-3) salt-tolerant crop rotation (salt-tolerant soybean, Glycine max; sorghum, Sorghum bicolor; and barley, Hordeum vulgare), (4) perennial remediation species (switchgrass, Panicum virgatum), (5) perennial restoration species (saltmarsh hay, Spartina patens), and (6) abandoned (natural recruitment control; native switchgrass, Panicum dichotomiflorum). Each plot is 3 m wide and 20 m long to capture a natural salinity gradient from the edge of the field (visible intrusion) to the center of the field (no visible intrusion). There is a 0.5 m buffer between treatments and a 1 m buffer between each block (Figure 1). The no-salt control field site has a layout of 16 plots with the same treatments (1-3) and a salt-tolerant crop rotation (salt-tolerant soybean, sorghum, and barley) with each entry point present every year, a remediation species (switchgrass) (treatment 4), and an abandonment species (native grasses) (treatment 6). As the field is not affected by saltwater, we did not install the restoration species (treatment 5; saltmarsh hay). Plots in the no-salt control are also 3 m x 20 m with a 0.5 m buffer between treatments and 1 m buffer between blocks. Soils at all sites are predominantly silt loams (Table 1) with little to no slope. The climate in the region is humid subtropical with a mean annual minimum temperature of 9.8 ºC, a mean annual maximum temperature of 20.6 ºC, and annual precipitation of 1270 mm [24, 25]. 

Proximity of ag field to ditch; zoomed in 2021 plot layout
Figure 1. Greenhead Farm. Proximity of agricultural field to ditch. Zoomed in 2021 Plot Layout. Fields are planted with four replicate blocks of six treatments: (1-3) salt-tolerant crop rotation (salt-tolerant soybean; sorghum; and barley), (4) switchgrass, (5) saltmarsh hay, and (6) weeds. Each treatment is represented in a plot that is 3 m wide and 20 m long. Each plot is separated into three sections to capture a natural salinity gradient from the edge of the field (visible intrusion) to the center of the field (no visible intrusion). The bare, saline edge in red (Section 1; S1), mid-field in orange (Section 2; S2), and the fresh, center of the field in yellow (Section 3; S3). S1 includes the first 5 m of the plot closest to the ditch, S2 covers 5 m to 15 m away from the ditch, and S3 includes the last 5 m of the plot, 15 m away from the ditch. There is a 0.5 m buffer between plots (treatments) and a 1 m buffer between each block (replicate).

 

Table 1. Soil characteristics of field sites

Site

Location

Texture

Clay (%)

Silt (%)

Sand (%)

Taxonomic Classification

1

Dorchester Co., MD

Nearest town: 

Cambridge

Elkton silt loam 

23.57 

71.59

4.84

mesic Typic Endoaquults

2

Somerset Co., MD

Nearest town:

Princess Anne

Queponco silt loam

16.74

38.31

44.95

mesic Typic Hapludults

3

Somerset Co., MD

Nearest town:

Crisfield

Fallsington sandy loam 

Othello silt loam 

8.50

21.83

69.83

mesic Typic Endoaquults

mesic Typic Endoaquults

Control

Wicomico Co., MD

Nearest town:

Quantico

Mattapex silt loams

22.93

64.75

12.32

mesic Aquic Hapludults

 

Soil collection: Soil cores were collected using a 22-mm diameter push probe (AMS, Idaho Falls, ID, USA) from three SWI fields and one no-salt control field in November and March of 2018, November of 2019, and November of 2020 from each plot to 60 cm (0-10, 10-20, 20-30, 30-60 cm; three replicates for each depth) at 5, 10, and 15 m from the edge of the field. I will also collect soils in November of 2021 using the same method. 

 

Plant collection: Biomass was collected from the saltmarsh hay and switchgrass plots in August 2018, 2019, and 2020. Plants will also be collected in 2021. Saltmarsh hay and switchgrass are collected from three replicate 0.25 m^2 quadrats in four subplots: 0-5, 5-10, 10-15, and 15-20 m from the brackish water ditch that borders the study sites to capture the effects of salinity. Total biomass samples are clipped, weighed wet, allowed to air-dry, and then clipped again into sub-samples for processing. Sub-samples were oven-dried for at least 48 h at 60 ºC and weighed for conversion of total plant biomass to dry-weight-equivalent. Mature sorghum seed heads were collected from the sorghum plots in late September to early October and soybean plots in late October 2018, 2019, and 2020 along the SWI gradient as described above (0-5, 5-10, 10-15, 15-20 m). Soybean yields were collected within a single 0.25 m^2 quadrat in each subplot. Both sorghum and soybean seeds were passed through a plot combine (Massey-Ferguson 8XP), bagged, and weighed on a scale. As there was no salt gradient in the control farm, we collected biomass and yields at the plot-level. In the abandonment treatments, we will collect dominant weed species for additional analysis. I will determine harvestable yields (sorghum, soybean) and non-crop productivity and species composition measurements (switchgrass, saltmarsh hay; abandonment species) in mid to late September of this year (2021) using the same methods described above.

 

Objective 1: Soil analysis

All soil samples will be air-dried for at least three days before homogenization by grinding. Ground soils are oven-dried for two days prior to analysis. Each depth will be analyzed for texture (hydrometer method), pH, total C and N (elemental analysis), inorganic N (potassium chloride extraction), total P (modified Kjeldahl digestion) [26], and bioavailable Na and P (Mehlich-3 extraction) [27]. We will determine soil electrical conductivity (EC; proxy for salinity) on a 2:1 soil slurry in topsoils (0-10 cm) from all sites and treatments between 2018 to 2021. I will then perform a modified Kjeldahl digestion and a Mehlich-3 extraction on the 2018 and 2021 soil samples. Soils will be digested (modified Kjeldahl) [26] at 160 ºC with sulfuric/salicylic acid solution to break the carbon bonds between organic P compounds, which converts organic elements to a measurable inorganic form (e.g. total P). I will analyze the digestate solution for total P using colorimetry. Bioavailable Na and P will be extracted from soils using a Mehlich-3 extraction [27]. Bioavailable Na will be analyzed using atomic absorption spectroscopy (AAS) and bioavailable P will be analyzed using colorimetry. The data will be used to compare Na and P concentrations in soils among the four treatments at each agricultural field site along a salinity gradient.

 

In order to calculate soil nutrient pools, I will use the nutrient concentration from each depth increment (0-10, 10-20, 20-30, and 30-60 cm) and multiply by the depth of the segment and the bulk density. Bulk density will be collected by inserting cylinders of a known volume (5 cm diameter aluminum core) into the side of a 55 cm deep pit at 0-10 cm, 10-20 cm, 20-30 cm, and 30-50 cm). Soils will be returned to the UMCP and dried at 105 ºC for seven days. Core volume will be used to calculate bulk density (g cm-3).

 

Statistical analysis: I will use a linear mixed-effects (LME) model (lme4 package for R) [28] to examine the effect of management strategy (e.g. adapt, remediate, restore, abandon) on soil Na and P pools averaged at the plot-level (Q1.A). I will examine each year and site individually with management strategy and depth as the main effects and block as the random effect. I will use Tukey post-hoc comparisons to determine significant differences among management strategies. I will use a split-plot design (lme4 package for R) [28] to examine the effect of saltwater intrusion on soil Na and P concentrations (in the topsoil; 0-10 cm) (Q1.B). I will examine each site and year separately with management strategy as the main plot effect, section (saltwater intrusion) as the split-plot effect, and block as the random effect. I will use Tukey post-hoc comparisons to determine significant differences among management strategies and salinity levels. I will also use univariate regression to determine if there is a relationship between topsoil EC (0-10 cm) and topsoil Na and P concentrations. All statistics will be run in the R environment for Mac [29].

 

Objective 2: Plant tissue analysis

Oven-dry plant sub-samples from each treatment and year will be homogenized by grinding through a 2-mm mesh screen (Wiley Mill, Swedesboro, New Jersey)and analyzed for total C, N, Na, and P. The Gedan Lab at George Washington University will analyze leaf tissues for total Cand N using dry combustion (LECO TruMac N, St. Joseph, MI). Plant tissue samples will be digested as above (modified Kjeldahl) [26]. I will analyze the digestate solution for total P using colorimetry and total Na using AAS.

 

Aboveground biomass will be harvested close to the soil surface, oven-dried for at least 48 h at 60 ºC, and weighed. Then, I will calculate biomass Na and P pools by multiplying nutrient concentrations by aboveground biomass and yields (kg/ha) collected from the plots. 

 

Statistical analysis: I will use a linear mixed-effects (LME) model (lme4 package for R) [28] to examine the effect of management strategy (e.g. adapt, remediate, restore, abandon) on leaf tissue Na and P concentrations and pools averaged at the plot-level (Q2.A). I will examine each year and site individually with management strategy as the main effect and block as the random effect. I will use Tukey post-hoc comparisons to determine significant differences among management strategies. I will use a split-plot design (lme4 package for R) [28] to examine the effect of SWI on leaf tissue Na and P concentrations and pools (Q2.B). I will examine each site and year separately with management strategy as the main plot effect, section (SWI) as the split-plot effect, and block as the random effect. I will use Tukey post-hoc comparisons to determine significant differences among management strategies. All statistics will be run in the R environment for Mac [29].

Research results and discussion:

Figure 1 A-D. Baseline (2018) Soil Total Phosphorus. Baseline soil total P data at the three SWI field sites (A-C) and one no-salt control (D). This figure is a depth profile of soil P. The axis is flipped, so the top of the Y-axis is the soil surface and the X-axis is the concentration of soil P as we move down the profile. Baseline soil total P concentrations decrease with depth. Both A and C have higher P concentrations than B and the control, D, especially in shallow depths 0-20. Error bars represent standard error of the mean. Differences in depth were tested from each site by ANOVA. Values were significantly different at all sites (p < 0.05).
Figure 2 A-D. Baseline (2018) Soil Cations: Calcium (Brookside). Baseline soil available salt concentrations, specifically Ca, from three SWI field sites (A-C) and one no-salt control (D). The x-axis is Mehlich-3 available Ca concentration in mg per kg. The concentrations range from 400-900 mg Ca/kg. The y-axis is decreasing depth in cm. Calcium was separated from the other cations because of the high concentrations, especially seen in the shallower depths in C. Error bars represent standard error of the mean.
Figure 3 A-D. Baseline (2018) Soil Cations: Magnesium, Potassium, and Sodium (Brookside). Baseline soil available salt concentrations (Mg, K, and Na) from three SWI field sites (A-C) and one no-salt control (D). The x-axis is Mehlich-3 available cation (Mg, K, and Na) concentration in mg per kg. The concentrations range from 0-300 mg/kg. The y-axis is decreasing depth in cm. Mg and K concentrations were similar across sites (including D) despite there being a lack of Na, but the K concentrations were highest in A. Error bars represent standard error of the mean.
Figure 4 A-B. First-year Plant Leaf Tissue Phosphorus. “Baseline” plant leaf tissue P data from two SWI field sites (A and B) established and planted in 2018. The x-axis is section in meters. The y-axis is aboveground biomass P concentration. Surprisingly, the weeds had the highest concentrations of P, followed by switchgrass, then saltmarsh hay. Error bars represent standard error of the mean. Differences in treatment (managment practice) were tested from each site by ANOVA. Values were significantly different at both sites (p < 0.05).
Figure 5 A-C. Pilot Study (2019) Soil Mehlich-3 Available Phosphorus (Saltmarsh Hay). Pilot study conducted on 2019 soils from three SWI field sites (A-C) to determine the significance of Mehlich-3 available P in soil depths under the specific restoration treatment (saltmarsh hay). The x-axis is Mehlich-3 available P concentration in mg per kg. The y-axis is decreasing depth in cm. A and C have the highest concentrations of available soil P under saltmarsh hay treatment compared to B. There appears to be no strong effect of section on available P concentration in the topsoils.
Figure 6 A-C. Pilot Study (2019) Soil Mehlich-3 Available Phosphorus (Switchgrass). Pilot study conducted on 2019 soils from three SWI field sites (A-C) to determine the significance of Mehlich-3 available P in soil depths under the specific restoration treatment (switchgrass). The x-axis is Mehlich-3 available P concentration in mg per kg. The y-axis is decreasing depth in cm. A and C have the highest concentrations of available soil P under switchgrass treatment compared to B, but overall concentrations are less than under saltmarsh hay treatment. There appears to be no strong effect of section on available P concentration in the topsoils.

Participation Summary
3 Farmers participating in research

Education & Outreach Activities and Participation Summary

1 Webinars / talks / presentations
1 Workshop field days
2 Other educational activities: Supervised four undergraduate students who leveraged my data to create mini-projects to express the knowledge they learned; TA for Independent Research in Agroecology class: leverage my data, do all grading, present topics in biogeochemistry and relate topics to my research

Participation Summary:

70 Farmers
48 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

I will target farmers who are experiencing saltwater intrusion at local farmer field days and workshops (e.g. The Maryland Sea Grant Coastal Farming Challenges Workshop), so they are able to make informed decisions on how to manage their land. I will reach landowners through Soil Conservation District events in Somerset, Dorchester, and Wicomico counties and work with farmers to generate applied research questions and effective land management practices. The annual research team Stakeholder Meetings are another outreach venue; the next one is planned for February 2022 in Princess Anne, MD. The annual stakeholder meetings help inform farmers, extension agents, and other agricultural professionals of our findings and seek input on new research directions. We also discuss the advantages and disadvantages of enrolling in easement programs or using alternative best management practices. I will target extension personnel and other farm advisors through the previously mentioned methods as well as by sharing findings at the Mid-Atlantic Crop Management School. I will also participate in a Mid-Atlantic SWI Conference (2023) that will include other regional (VA, NC, GA) research results from stakeholders. I will produce at least one Fact Sheet and at least two Extension short communications providing updates on the project. These will be disseminated to the farming community through the UMD extension network.

 

I will reach the scientific community through presentations at The American Society of Agronomy, the Crop Science Society of America, and the Soil Science Society of America  (ASA-CSSA-SSSA) annual meeting in Baltimore, MD in 2022. I will share the results of my research with a large scientific audience through presentations at conferences and research symposiums and by publishing at least two articles in peer-reviewed journals such as Agriculture, Ecosystems, & Environment. This work builds on a larger project [funded by the National Institute of Food and Agriculture (NIFA)] that has an extensive outreach component. I will also leverage a new National Science Foundation Coastal Critical Zone Network (CZN) plan to disseminate my findings.

 

Finally, I will target policy-makers by sharing my findings with the Maryland Department of Agriculture, Maryland Department of Planning, and Maryland Department of Natural Resources, which are collaborating with Drs. Tully and Gedan to develop best management practices and potential easement strategies for salt-intruded lands. This proposal addresses pressing needs to better understand how agricultural landscapes will change in the coastal zone and how this research can inform current and future decision-makers and researchers. The knowledge we gain on Na and P uptake will inform best practices for adaptive farming, remediation, restoration, or abandonment of agricultural land impacted by SWI. For example, the Maryland Department of Natural Resources (DNR) is working to develop Coastal Resiliency pilot projects on salt-intruded farm fields to improve coastal communities’ resilience to the effects of climate change [30]. Our team has active collaborations with Maryland DNR, and I will present my results to the Director of the Office of Science and Stewardship and her team in an effort to inform policies that can support both farmers and the Chesapeake Bay.

 

Updates:

  1. University of Delaware Crop Management School Presentation - Recorded a 45-minute presentation on saltwater intrusion, how the soil and plants respond to saltwater intrusion (soil dispersion, sodium toxicity in plants), and possible new strategies to adapt to or mitigate the effects of saltwater intrusion (through the planting of salt-tolerant crops or transitioning fields to marshes) to farmers and extension agents.
  2. Research Recap Nutrient Management Sessions - Presenting for 45 minutes on saltwater intrusion, how the soil and plants respond to saltwater intrusion, possible new strategies to adapt to or mitigate the effects of saltwater intrusion, and some preliminary data. Talking with/answering questions for farmers about my research.
  3. Delmarva Soil Summit Presentation - Presenting for 15 minutes on the poster of my project with preliminary data for farmers, extension agents, and other researchers.

Project Outcomes

57 Farmers reporting change in knowledge, attitudes, skills and/or awareness
1 Grant applied for that built upon this project
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.