The specific objectives of this research are to:
(1) Determine the maximum salinity level (osmotic and ionic) for 50% germination of different crop, agricultural weed, and restoration species in a controlled greenhouse experiment.
Hypothesis 1: Restoration species will have the highest ionic tolerance, followed by agricultural weeds.
Hypothesis 2: Agricultural weeds will have a higher osmotic tolerance than either crops or restoration species.
Hypothesis 3: Quinoa and barley will have the greatest threshold for salinity of the focal cash crops.
(2) Evaluate the productivity of crops, agricultural weeds, and restoration species in agricultural fields experiencing saltwater intrusion.
Hypothesis 1: Agricultural weeds will produce the greatest biomass in the field experiment.
Hypothesis 2: Porewater concentrations of nitrogen and phosphorus will be lowest under saltmeadow cordgrass and switchgrass.
Unlike other parts of the world that experience soil salinization due to irrigation water naturally high in salts, the Eastern Shore of Maryland soils are salinizing via saltwater intrusion (SWI). The low-lying topography of the Eastern Shore of Maryland (2.7 m above sea level) allows saline water to move into ground- and surface water. The rates of sea level rise in the region are three times the global average (Sallenger et al. 2012), pushing saltwater further inland each year. Although a great deal of research has focused on the effects of salinization on crop germination, yield, and plant performance (Zörb et al. 2019), SWI-induced salinization is a unique phenomenon comprised of high salinity levels and periods of saturation.
Three management strategies available to farmers battling SWI are (1) to resist change, and continue growing the standard crop rotation (corn, soy, and wheat) as long as they can; (2) to adapt to change, and grow salt-tolerant crops economically-appropriate for the region (sorghum, salt-tolerant soybean, and barley); or (3) restore the land by planting restoration species such as switchgrass or salt marsh hay. This research provides the groundwork for assessing the success of an alternative salt-tolerant cropping rotation and restoration planting on farms experiencing SWI.
To determine the maximum salinity level (osmotic and ionic) for 50% germination of different crop, agricultural weed, and restoration species in a controlled greenhouse experiment (Objective 1), laboratory experiments were established at George Washington University (Washington, D.C.). We designed a 9 x 7 factorial experiment to test the effect of osmotic stress on seed germination. We created ten osmotic potentials using mixtures of Polyethylene Glycol 8000 (PEG) and distilled water and ten ionic potentials using mixtures of sodium chloride (NaCl) and distilled water. The ten potentials were the same for osmotic and ionic stress: 0, -0.2, -0.5, -0.8, -1.1, -1.4, -1.8, -2, -3, -4 MPa. Germination of corn (Zea mays), wheat (Triticum aestivum), soybean (Glycine max), salt-tolerant soybean, barley (Hordeum vulgare), rapeseed (Brassica napus), sorghum (Sorghum bicolor), switchgrass (Panicum virgatum), and quinoa (Chnopodium quinoa) in PEG was conducted. Salt marsh hay (Spartina patens) and common ragweed (Ambrosia artemisiifolia) were not included in the germination trial because we were unable to collect viable seeds.
When PEG has a molecular weight greater than or equal to 6000, it cannot penetrate the cell wall of seeds (Carpita et al. 1979, Verslues et al. 1998), thus mimicking osmotic stress. The concentration of PEG used to create each solution was calculated using Eq. 1 (Hardegree and Emmerich 1990).
ψ = 0.130[PEG]2T – 13.7[PEG]2
Where, ψ is the water potential in MPa, PEG is grams of PEG per gram of water, and T is temperature in degrees Celsius. In this study, T=25°C because seeds in solution were kept in an incubator set to 30°C and 20°C for equal amounts of time. The concentration of NaCl was calculated using Eq. 2 (Lang 1967).
Where, ψ is the water potential in atm, T is temperature in Kelvin, and R is the ideal gas constant . In our case, T=25°C as the solutions were in an incubator for equal amounts of time at both 30°C and 20°C. To mimic field conditions where seeds are regularly saturated with saltwater, we placed 25 seeds per species on one 85 mm Grade 1 Whatman filter paper, 11 μm pore size, in 100 x 20 mm glass petri dishes and moistened with 3.2 mL of deionized water or PEG solution. Each species x solution combination was replicated four times (total of 100 seeds per species x solution combination). The petri dishes were tightly covered and wrapped with parafilm to prevent evaporation and were incubated in a Percival Incubator on a diurnal cycle of 30°C for 12 hours light and 20°C for 12 hours dark. If the filter paper appeared to be drying or there visibly was no solution in the petri dish, we replaced the filter paper and added 3.2 mL of solution to the petri dish. Due to the large seed size of corn, larger petri dishes and filter paper were used (150 and 125 mm diameter, respectively), and 7.8 mL of solution was added to fill the larger volume. Germination counts were made in ~6 hour intervals in the first 48 hours of experiment initiation and then in ~2-4 hour intervals for the following two days. After four days, a daily single count was made until there was no new germination observed for 14 consecutive days. A seed was considered germinated, if a radical was present.
We used germination data to calculate the time to 50% germination (Eq. 3).
Where, t50 is the median germination time, N is the final number of germinated seeds, Ni and Nj are the total number of seeds germinated in adjacent counts at time Ti and Tj, respectively (Farooq et al. 2005). The time to 50% germination standardizes germination rates so that they are comparable among studies (Scott et al. 1984, Ranal and Santana 2006). Within a species seed lot or across different species, seeds can germinate at varying rates, and data is not normally distributed, making it challenging to draw comparisons. Time to 50% germination is a measurement that provides a central tendency to the data, which is comparable to the mean of normal distribution (Scott et al. 1984, Ranal and Santana 2006).
To evaluate the productivity of crops, agricultural weeds, and restoration species on agricultural fields experiencing saltwater intrusion (Objective 2), we used recently established experimental trials on salt-intruded fields in Somerset and Dorchester Counties, MD. Our study sites are near Crisfield, Maryland in Somerset Co. (37.983436° N, -75.854527° W), Princess Anne, Maryland in Somerset Co. (38,202655° N, -75.692893° W), Quantico, Maryland in Wicomico Co. (38.374376° N, -75.742253° W) and Cambridge, Maryland in Dorchester Co. (38.5632° N, -76.0785° W); farm fields in the region have been no-till for at least 40 years (Huggins and Reganold 2008). Saltwater moves onto agricultural fields via hydrologically connected ditch networks and the groundwater table (Tully et al. 2019). The extensive agricultural ditch network designed to drain excess water from farms often serves the reverse purpose by acting as a conduit for saltwater to reach the fields during high tides and storms (Bhattachan et al. 2018, Tully et al. 2019).
Experimental plots were established in a randomized complete block design with four replicates per treatment at farms in Somerset Co., Dorchester Co., and Wicomico Co. Treatments consist of: (1) a natural recruitment control (colonized by species in the seed bank, predominantly agricultural weeds and locally sourced species); and (2-4) a sorghum-salt tolerant soybean-barley rotation with each entry point present each year (total of three plots per block); (5) switchgrass; and (6) salt marsh hay. Each plot was 3 m wide by 20 m long and established within 2 m of the field edge with evidence of SWI. Plots were made intentionally long so as to span a natural salinity gradient, from high salinity near the field edge (0-5 m) to low salinity towards the center of the field (15-20 m). There were 0.5-1 m buffers between each plot and four replicates of each treatment for a total of 24 plots per farm.
Porous cup lysimeters (22 mm diameter; Soil Solution Access Tubes, Irrometer Riverside, California, USA) were installed to 60 cm depth at 5 m (near salt source) and 15 m from the edge of the plot (far from salt source). Lysimeters were installed using a soil probe and a slurry made with the deepest soil was poured into the hole before inserting the lysimeter to ensure good soil contact. Finally, lysimeters were sealed at the soil surface with a bentonite/clay mixture to avoid preferential flow of water down the side of the tube. Pilot studies confirmed that soil solution collection was only possible following rain events that were greater than or equal to 6 mm, thus soil solution was only collected following rain events of this level.
Switchgrass and salt marsh hay were hand planted as plugs at a density of 4 plants m-2 June 2018 for a total of 240 plants per plot. Switchgrass plugs were grown in the University of Maryland Research Greenhouse complex (var. Kanlow). Salt marsh hay plugs were purchased from Environmental Concern (Cambridge, MD).
Prior to planting, plots were sprayed with glyphosate [N-(phosphonomethyl) glycine] (~0.91 kg active ingredient per acre) in early-May 2019. Sorghum (var. Dekalb DKS 2805) and salt-tolerant soybean seeds (Pioneer P42a52x; Cl–-excluder) were sown in early-June 2019 using a 1.52-m Tye drill. Sorghum was planted in 38.1 cm rows at a rate of 197,600 seeds ha-1 and salt-tolerant soybean was planted in 19 cm rows at a rate of 481,650 seeds ha-1. Sorghum received 84 kg ha-1 as urea and salt-tolerant soybean received 67 kg ha-1of K in the form of potash (K2SO4) in early-June 2019. Due to poor germination as a result of heavy rainfall, sorghum and salt-tolerant soybean were re-sown, with the same seeding practice, in late-June 2019. Barley (var. Throughbred) was planted in 19 cm rows at 1,077 kg ha-1 in mid-October.
Switchgrass, salt marsh hay, and control plots (agricultural weeds) were harvested early-October 2019. Aboveground biomass was collected along the midline of the plots from three 0.25 m2 quadrats within each 5 m segment of the plot (i.e. 0-5, 5-10, 10-15, 15-20 m) for a total 12 quadrats per plot (0-5 m is closest to the saltwater source and 15-20 m is furthest from the saltwater source). Sorghum (seed head only) was harvested in early-October 2019 and salt-tolerant soybean was harvested in late-October 2019, by running the biomass through a thrasher. Switchgrass, salt marsh hay, and agricultural weed biomass were sorted by species, dried, and weighed.
Objective 1. Across all 9 species, we observed variable tolerance to osmotic stress. Barley and wheat were the only species to germinate at osmotic stress levels below -1.1 MPa. However, there was no significant difference between wheat and barley percent germination at every osmotic stress level. Surprisingly, standard soybean and salt-tolerant soybean had very similar germination responses to osmotic stress. Quinoa was able to germinate to -0.8 MPa (osmotic stress) but did not have a higher percent germination at -0.8 MPa than corn, sorghum, and barley. When grouped, we found no difference in time to 50% germination between standard crops and alternative crops at any osmotic stress level.
Salt-tolerant soybean, soybean, and quinoa were the only species able to germinate under ionic stress levels. Salt-tolerant soybean was able to germinate at high ionic stress levels (-2 MPa; 36.9 ppt), which is equivalent to levels found in seawater. Standard soybean was also surprisingly tolerant of ionic stress; it was able to germinate at -1.8 MPa (32.9 ppt). Standard soy and salt-tolerant soybean required significantly more hours to reach 50% germination at -0.2 MPa and -0.5 MPa compared to quinoa.
Objective 2. Harvestable biomass (kg ha-1) of switchgrass, salt marsh hay, sorghum, and salt-tolerant soybean has been dried and weighed for two of the four farms so far. Interestingly, at the farm in Crisfield, Maryland, switchgrass biomass increased as we moved further from the saltwater source. Sorghum at the farm in Crisfield consistently had greater biomass compared to the farm in Cambridge, Maryland.
We were curious to document how quinoa would germinate under the same salinity conditions imposed on the standard and alternative crop species because quinoa is a halophyte and tolerant of salinity levels approaching those of seawater (Adolf et al. 2013, Adolf et al. 2012, Shabala et al. 2012). Furthermore, quinoa can grow in drought-prone and marginal soils (Jacobsen et al. 2003, 2005, 2007, Sun et al. 2014) such as those found on the Eastern Shore, MD. However, we did not include quinoa in the field experiment because our farmer partners stated they would not plant quinoa due to the cost of new equipment (e.g. new seed plates) and the non-existent market. Further research is needed to assess if there could be a market for quinoa in the region and if it would be advantageous for farmers to plant quinoa on their salt-intruded farm fields.
Markets on the Eastern Shore, MD support the cultivation of sorghum, salt-tolerant soybean, and barley, all of which past research indicated should have a higher salt tolerance than corn, soybean, and wheat (Rani et al. 2012, Munns et al. 2006, Munns and Tester 2008). For example, barley seeds are capable of absorbing Na+, which facilitates imbibition and germination under salt stress as water is able to pass through the cell wall (Zhang et al. 2010). Based on the controlled environment data alone, one might assume there would be no reason for a farmer to switch from the standard crops to alternative crops. However, farmers on the Eastern Shore, MD are already switching to sorghum on salt-intruded farm fields instead of corn because corn cannot produce profitable yields (Jarrod Miller, personal communication), further highlighting the importance of pairing controlled environment experiments with field trials.
Education & Outreach Activities and Participation Summary
On 11 Nov 2019, I presented preliminary data at the SSSA-CSSA-ASA meeting in San Antonio, Texas.
I wrote a blog post for the Sustainable, Secure Food blog, sponsored by ASA and CSSA, for my presentation at the SSSA-CSSA-ASA meeting.
Saltwater intrusion stakeholder meeting was held on 14 Feb 2020 (22 attendees). This audience consists of farmers, landowners, natural resource managers (Maryland Department of Natural Resources, Natural Resources Conservation Service, Soil Conservation District), and Extension agents. This meeting included a Public Q&A for press, local residents, etc and was well-attended (24 additional attendees).
My data was presented at the Delmarva Soil Summit in Georgetown, Delaware, which allowed farmers, scientists, and other relevant stakeholders to learn about the work I’m conducting.
My research was presented at a stakeholder meeting in 2020 where our farmer participants were able to hear how my work has been progressing on what my findings are.
The purpose of this project is to examine the effect of saltwater intrusion (SWI) on plant germination and productivity. Saltwater intrusion, the landward movement of sea salts, is driven by sea level rise, frequency and intensity of storms and tides, drought, groundwater withdrawals, and connectivity to saline water bodies. We have observed extensive SWI on coastal farms in the Lower Eastern Shore of Maryland. As the standard corn-full season soybean-wheat rotation holds little promise as SWI moves inland due to their low salt tolerance, farmers across the Atlantic Coastal Plain face a tough decision: continue farming with the standard three-year crop rotation, switch to salt-tolerant crops, or abandon their fields and way of life. My research is testing the salinity thresholds of standard and salt-tolerant crops, as well as restoration species. This data will help inform farmer management decisions given economic and environmental objectives and tradeoffs.
This project has highlighted the complexity and challenges of identifying crops capable of growing on salt-intruded farm fields as every crop responds differently at different life stages. Additionally, understanding the germination success is critical in developing farm management strategies in coastal agricultural regions.