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

Chapter 1 Results

Soil extractable P concentrations

In the soil, concentrations of Mehlich-3 extractable P ([M3P]) decreased by 30-50% in the topsoil (0-10 cm) at two sites after three years of planting (33.9% at Farm 3 and 52.8% at Farm 6; Figure 1.3). At Farm 6, [M3P] decreased from 109.3 mg kg^-1 in 2018 to 51.6 mg kg^-1 in 2021 in the top 10 cm of the soil. There was a significant effect of distance from the ditch at Farm 6 (p<0.05; Figure 1.3), where the section closest to the ditch (0-5 m) had higher [M3P] than the section furthest from the ditch (15-20 m). There was a significant effect of depth at all sites ([M3P] decreases with depth; p<0.01), but there was no effect of treatment.

Soil total P concentrations

Overall, soil total [P] decreased at all SWI sites after three years of planting, and concentrations decreased by 20% in the topsoil at Farm 1 and Farm 6 (19.6% and 18.3%, respectively; Figure 1.4). The total [P] in 2018 at Farm 6 was 507.4 mg kg^-1 and decreased to 414.5 mg kg^-1. There was a significant effect of distance from the ditch at Farm 3 and Farm 6 (p<0.05; Figure 1.4), where the section closest to the ditch (0-5 m) had higher [M3P] than the section furthest from the ditch (15-20 m) at Farm 6 and the opposite was seen at Farm 3. Similar to the soil extractable [P], there was no significant effect of treatment, and there was a significant effect of depth (total [P] decreases with depth; p<0.01).

Soil available P pools

Soil Mehlich-3 extractable P (M3P) pools decreased by 30-50% in the topsoil at two sites after three years of growth (33.9% at Farm 3, 52.9% at Farm 6; Figure 1.5). There was a significant effect of distance from the ditch at Farm 3 and Farm 6, where the section closest to the ditch (0-5 m) had higher M3P levels than the section furthest from the ditch (15-20 m) at Farm 6 (p<0.05). In Farm 3, we observed the opposite, with higher M3P levels in the section closest to the ditch (0-5 m; p<0.01; Figure 1.5).

Figure 1.5. Mehlich 3-extractable soil P pools (kg ha-1) in topsoil (0-10 cm) at three saltwater-intruded field sites on the Lower Eastern Shore of Maryland in 2018 (baseline) and 2021 by treatment and section at (a) Farm 1, (b) Farm 3, and (c) Farm 6.

There was a significant effect of soil depth on soil M3P pools at all farms (p<0.01). In 2018, soil M3P pools in the 0-10 cm depth had the highest M3P pools, the next 10 cm (10-20 cm) had intermediate levels of M3P pools, and finally the deeper depth (20-30 cm) had the lowest M3P pools (p<0.01 in all cases). In 2021, soil M3P pools in the topsoil (0-10 cm) were greater than the 10-20 cm depth and the 20-30 cm depth (p<0.01 in all cases). We observed a decrease in soil M3P after three years of growth in all soil depths with a few exceptions in the 20-30 cm depth under all treatments at Farm 1 and 3 (p<0.05).

Farm 3 had the highest M3P levels across the saltwater-intruded farms (p<0.01; medium risk group; FIV = 425), and Farm 6 had the lowest M3P levels of all farms (p<0.01; low risk group; FIV = 214). Farm 1 had similar M3P levels to Farm 3 in the topsoil in year 1 (p<0.01; medium risk group; FIV = 365). The FIV decreased at Farm 1 by 33%, at Farm 3 by 37%, and at Farm 6 by 54% under all treatments (Figure 1.6). Under remediation, restoration, and abandonment treatments at Farm 6, FIV levels in the soil were reduced to reach optimum levels (< 100-150).

Figure 1.6. Conceptual diagram of saltmarsh hay (A-C), switchgrass (D-F), and weeds (G-I) aboveground biomass P and respective belowground total and available P pools from three depths (0-10, 10-20, and 20-30 cm) in year 1 (baseline) compared to year 4 at three saltwater-intruded field sites on the Lower Eastern Shore of Maryland.

Soil total P pools

Soil P pools decreased by approximately 20% in the topsoil at two sites over the three study years (Figure 1.7). There was a significant effect of distance from the ditch at all SWI sites, where the section furthest from the ditch (15-20 m) had higher P pools than the section closest to the ditch (0-5 m) at Farm 1 and Farm 3 (p<0.05; Figure 1.7); the opposite was seen at Farm 6 (p<0.05; Figure 1.7). There was a significant effect of soil depth on total P pools in baseline soils, where total soil P pools were highest in the 0-10 cm depth intermediate at 10-20 cm, and smallest at 20-30 cm depth (p<0.01 in all cases). After three years of growth, there was still a significant effect of depth on total soil P pools, although the size of the pools was smaller across the soil profile.

Figure 1.7. Soil total P pools (kg ha-1) in topsoil (0-10 cm) at three saltwater-intruded field sites on the Lower Eastern Shore of Maryland in 2018 (baseline) and 2021 by treatment and section at (a) Farm 1, (b) Farm 3, and (c) Farm 6.

We found similar patterns in soil available and total P concentrations among the three saltwater-intruded sites across all study years where the both the soil closest to the ditch and the soil furthest from the ditch were strongly positively correlated with topsoil EC (0-10 cm; r² > 0.5 in year 1 and r² > 0.2 in year 4; p<0.001 and p<0.01, respectively; Table 1.2).

Table 1.2. Topsoil EC (0-10 cm) and topsoil total and available P concentration correlations in year 1 and year 4 for the section closest to the ditch (0-5 m) and the section furthest from the ditch (15-20 m) with each saltwater-intruded site on the Lower Eastern Shore of Maryland averaged. Values are r² and symbols are p-values (^ = p<0.05, * = p<0.01, ** = p<0.005, *** = p<0.001).

Porewater P concentrations

After two years of growth, there was an overall decrease in porewater soluble reactive P (SRP) concentrations under all treatments. There was no effect distance from the agricultural ditch on SRP concentrations at any farm in any year of the study; there was no significant difference in SRP concentrations among the saltwater-intruded sites. Overall, there was not a strong treatment effect on porewater SRP concentrations. However, in 2019, we observed higher concentrations of SRP in the saltmarsh hay treatment compared to the weeds and switchgrass treatments (p<0.05; Figure 1.8). Porewater SRP concentrations differed significantly through time (p<0.01). Overall, porewater SRP concentrations ranged from 0.01-0.10 mg L^-1 and decreased significantly from 2018 to 2020 (< 0.01 mg L^-1; p<0.01; Figure 1.8). There was a slight increase in SRP concentrations under all treatments from the end of 2018 (November) to the start of 2019 (June, July, August), which decreased in October/November of 2019 and increased again in December of 2019. In May 2020, SRP decreased to below 0.01 mg L^-1.

Figure 1.8. Soluble reactive phosphorus concentrations (mg L-1) in soil porewater in (a) 2018, (b) 2019 and (c) 2020 by treatment.

Trends in plant P concentrations and pools

We found similar patterns in plant leaf tissue P concentrations among the three saltwater-intruded sites across all study years where the highest P concentrations were always found in weed tissue and the lowest P concentrations were found in saltmarsh hay tissue with intermediate concentrations in switchgrass tissues (p<0.03 in all cases; Figure 1.9). There was no clear relationship between plant leaf tissue P concentrations and distance from the salinity source although in some cases, we observed a significant effect of distance from the ditch (p<0.01; Figure 1.9).

 Plant biomass drove patterns in plant pools of P across sites, the SWI gradient, and study years. In the first study year, at Farm 1 and Farm 3, where weeds accumulated the largest biomass, weed P pools were the highest compared to switchgrass and saltmarsh hay (p<0.003; Figure 1.10). This trend continued into year 2 at Farm 1, however at Farm 3 and Farm 6, switchgrass biomass levels were highest, therefore switchgrass P pools were highest compared to saltmarsh hay and weeds (p<0.01; Figure 1.10; Figure 1.11). In the third and fourth years of growth, switchgrass P pools were the highest at all saltwater-intruded field sites (p<0.001; Figure 1.10). At Farm 1 and Farm 6, switchgrass biomass levels were highest compared to saltmarsh hay and weeds in year 3 and 4 (p<0.01; Figure 1.11), however at Farm 3, saltmarsh hay biomass was highest in year 3 and 4. Despite saltmarsh hay having higher biomass levels than switchgrass at our saltiest field site, switchgrass P pools still had higher P pools than saltmarsh hay (although they were both higher than weeds; p<0.01; Figure 1.10; Figure 1.11).

The SWI gradient had an effect in year 1 and 2 at Farm 6, where biomass increased with distance from the ditch (p<0.01 in both cases; Figure 1.11), however, at Farm 1 and Farm 3, there was no significant effect of distance from the ditch on aboveground plant biomass for any of the study years.

At the no-salt control site, we only planted switchgrass, which showed P concentrations and biomass levels lower than the saltwater-intruded sites (Figures 1.9, 1.10, 1.11). Aboveground biomass P pools in the no-salt control showed a similar pattern to Farm 1, where switchgrass P pools were low in years 1 and 2 but increased in year 3 (Figure 1.10).

Figure 1.10. Plant aboveground biomass P pools (kg ha-1) by distance from salty ditch at three saltwater-intruded farms on the Lower Eastern Shore of Maryland in year 1 (a-c), year 2 (d-f), year 3 (g-i), and year 4 (j-k) by treatment.

We found similar patterns in soil available and total P concentrations among the three saltwater-intruded sites across all study years where both the soil closest to the ditch and the soil furthest from the ditch were strongly positively correlated with each other (0-10 cm; r² > 0.8; p<0.001 in all cases; Table 1.3). There were no clear relationships between plant leaf tissue P and topsoil available and total P, however, there was a strong positive correlation in the saltmarsh hay treatment only in the section furthest from the ditch (r² > 0.3; p<0.005 in both cases; Table 1.3).

Table 1.3. Plant leaf tissue P and topsoil total and available P concentration correlations in year 4 for saltmarsh hay, switchgrass, and weeds in both the section closest to the ditch (0-5 m) and the section furthest from the ditch (15-20 m) with each saltwater-intruded site on the Lower Eastern Shore of Maryland averaged. Values are r2 and symbols are p-values (^ = p<0.05, * = p<0.01, ** = p<0.005, *** = p<0.001).

Grain P concentrations

We found similar patterns in grain [P] among all saltwater-intruded sites across all study years with significantly higher [P] in soybean grain (4.5-9.0 g kg^-1) than sorghum grains (5.1-7.0 g kg^-1; p=0.01 in all cases; Figure 1.12). Soybean grain P concentrations were highest at Farm 1 in 2021 (6.1-8.0 g kg^-1; p<0.001). There was no significant effect of distance from the salinity source at any of the sites each year (2019-2021). Finally, soybean P concentrations were lower (4.5-6.1 g kg^-1) at the no-salt control site (LESREC) than the SWI-impacted sites in 2021.

Grain yield

The grain yield at Farm 1 in 2019 showed sorghum having higher yield (79.5-123.9 bu acre^-1) than the county and no-salt control site averages (76 and 66 bu acre^-1, respectively; Table 1.4), but in the following two years, sorghum yield decreased to below average (0.78-23.5 bu acre^-1). Soybean yield did not meet county and LESREC averages (55 and 58 bu acre^-1, respectively) in any years at the saltwater-intruded sites (1.6-34.5 bu acre^-1). Soybean grain was not collected at Farm 1 and Farm 3 in 2020 likely because of birds or deer eating the crops, so the values were zero. In 2021, the weeds at Farm 3 took over the field, and soybeans were unable to grow, and deer likely ate the grain at Farm 6. 

Table 1.4. Sorghum and soybean grain yields in bushels per acre from each research site on the Lower Eastern Shore of Maryland
from 2019 to 2021 compared to Dorchester and Somerset County (MD) averages calculated in 2021.

Grain P pools

Sorghum and soybean grain P pools showed different patterns than grain yield and grain [P] at all SWI sites for all years. In year 2, sorghum grain P was slightly higher (2.71-9.81 kg ha^-1) than soybean grain (3.46-5.95 kg ha^-1; p=0.01; Figure 1.13), but there was no significant effect of distance from the ditch. There was no significant effect of treatment or distance from the ditch at Farm 3. Since there was only one species collected at Farm 1 and Farm 3 in year 3, there was no significant difference between species, but there was no significant effect of distance from the ditch either. Similarly, at Farm 6, there was no significance of distance from the ditch or treatment on grain P pools. The following year, there was only a significant difference between species at Farm 1 (p=0.01). There was a significant effect of distance from the ditch at Farm 3 (p=0.01) on grain P pools, but not at Farm 1 and Farm 6. Finally, the no-salt control (LESREC) soybean grain P pools were higher than pools at the SWI sites (22.5 kg grain-P ha^-1).

Chapter 1 Discussion

Our results have implications for how planting remediation and restoration species can draw down legacy soil P in coastal agricultural soils experiencing SWI. After analyzing the aboveground biomass and soil beneath five proxy species for management practices (adapt, remediate, restore, abandon) on three farms experiencing SWI, we were able to determine if soil and biomass P pools changed along a SWI gradient. We show that remediation and restoration management practices are efficient in taking up soil and porewater P that may ultimately decrease P runoff into the Chesapeake Bay. Overall, we saw significant differences in plant and soil P pools among each of the SWI sites, with relatively consistent trends across years (Figs. 1.5, 1.7, 1.8, 1.10), which indicates that even with differences among our saltwater-intruded farm fields (degree of SWI, precipitation, soil type, etc.), patterns of P pools were influenced by biomass levels, and the uptake of P in biomass was consistent with the decrease in soil P and M3P concentrations and pools after three years of planting. Legacy P in our region due to historic application of poultry litter has led to high levels of both total and available P in the soil that could be mobilized and released when fields are exposed to saltwater (Weissman et al. 2019). Previous work showed that agricultural soils have a greater release of nutrients (e.g., SRP) than wetlands when fields are allowed to flood with saltwater (Weissman et al. 2021), therefore it is important to determine best management practices that will draw down soil P. In this study, after three years of planting both remediation and restoration species (switchgrass and saltmarsh hay), there was an increase in aboveground biomass P pools as biomass accumulated over time, and this was associated with a decline in soil total and available P pools.

Switchgrass is efficient at taking up soil and porewater P

We found switchgrass biomass had higher P pools than both saltmarsh hay and weeds from saltwater-intruded fields, suggesting that switchgrass could be used as a P removal tool. Previous research has shown switchgrass can accumulate large quantities of biomass after the establishment year and can last over 5-10 years (McLaughlin & Kszos 2005; Wullschleger et al. 2010; Richner et al. 2014). We found that high levels of switchgrass biomass were associated with large aboveground P pools, and soil total and available P pools decreased after three years of planting (Figs. 1.5, 1.7, 1.8, 1.10), which indicates that species with high levels of biomass and deep root systems can uptake high levels of soil P (Schmer et al. 2011; Basyal & Emery 2020). Porewater P was also lowest under the switchgrass plots after the second year and continued to decrease in the third year, and this was likely because of the decrease in soil P pools, so there is less P to be released as SRP (Meyerson et al. 1999). The difference between treatments on soil and porewater P were likely not significant because of switchgrass roots extending into nearby plots. Therefore, planting switchgrass on coastal agricultural fields could be a mitigation strategy to remove soil P, if harvested, and may ultimately decrease the amount of P available to runoff into the Chesapeake Bay (Schmer et al. 2011; Ashworth et al. 2017; Rivera-Chacon et al. 2022).

Switchgrass is a practical option for planting in the mid-Atlantic. First, switchgrass is more tolerant of saline and dry conditions due to its deep root system and strong associations with mycorrhizal fungi, so it is able to take up more water and nutrients than most agronomic crops (Di Virgilio et al. 2007; Schmer et al. 2011; Meyer et al. 2014; USDA NRCS 2020). Additionally, the deep, extensive root system can be as deep as three meters and as wide as six meters, allowing switchgrass to act as a buffer to take up soil N and P and prevent nutrient loss to nearby waterbodies (Blanco-Canqui et al. 2004; Lemus et al. 2008; Kibet et al. 2016; Kumar et al. 2019). As a perennial that does not need to be replanted each year with relatively low seed cost and no need for fertilizer amendments (UMD Extension 2010; AGMRC 2018), switchgrass incurs lower annual input costs than other crops. Finally, switchgrass can be used as a cellulose biofuel or poultry bedding, so landowners could continue to make a profit from planting, albeit at a reduced price per acre than typical corn and soybean crops ($265 per acre compared to $749 and $402 per acre, for corn and soybean respectively; Moyle et al. 2016; Purswell et al. 2020; UMD Extension 2022). In sum, this study showed switchgrass would provide a great tool to buffer against P loss, maintain low input costs, and be used as a biofuel or poultry bedding.

Saltmarsh hay is efficient at taking up P in soil with high salinity

We found saltmarsh hay biomass had intermediate biomass P pools compared to other treatments from saltwater-intruded fields, suggesting the potential to be used as a P removal tool. Previous research indicated that fields experiencing SWI will naturally transition to host more native marsh species such as saltmarsh hay (Gedan et al. 2019). Our work shows that the increase in saltmarsh hay biomass and aboveground P pools provide an opportunity to harvest this biomass to remove both sodium and P in the soil. At Farm 3 where salinity was the highest, saltmarsh hay had the highest biomass levels because of the higher tolerance to salt concentrations and was able to uptake similar levels of P as switchgrass (Figure 1.10). This uptake was associated with decreased levels of soil P pools and porewater [SRP] (Figs. 1.5, 1.7; Figure 1.8). Similar to switchgrass, our work indicates that allowing the natural conversion of fields to saltmarsh hay, or deliberately planting this species can serve as a tool for reducing P losses to nearby waterbodies by utilizing their high root porosity to maintain nutrient uptake (Burdick & Mendelssohn 1990; DeLaune et al. 2005). 

Saltmarsh hay is a practical option for planting in high salinity soils on the Eastern Shore. First, saltmarsh hay is a halophytic C₄ grass that can create a new wildlife habitat for waterfowl hunting and aid in restoring the land to native marsh. In addition to maintaining a profit by allowing hunting activity, landowners can receive financial assistance and guidance from state- and federal-level conservation programs that can support restoration practices (e.g., Conservation Reserve Enhancement Program; CREP 2022, Environmental Quality Incentives Program; EQIP 2022, Wetland Reserve Easements; WRE 2022). Finally, planting saltmarsh hay (Spartina patens) may facilitate the transition to tidal marshes that can protect coastlines from flooding by reducing strong winds and storm surges with their strong-stems and other physical properties (Hu et al. 2015; Leonardi et al. 2018). The restoration approach has been shown to save landowners in the Delaware, Maryland, Virginia (DMV) area between $4.5-23.8 million annually. For example, in Maryland, coastal wetlands reduced property damages from $20 million to $15.5 million from severe weather and flooding (Smith & Katz 2013; Narayan et al. 2017; USGCRP 2017; Li et al. 2020). Salt marsh presence reduces average annual flood losses by 18-70% (Narayan et al. 2017). In sum, restoring farm fields has benefits for nearby waterbodies by taking up soil P, landowners by maintaining revenue, and communities by protecting coastlines.

Grain crops will not maintain high yield with increasing SWI

The alternative grain crops tested in our field trials demonstrated efficient grain yield P uptake rates when the crops were able to survive. Sorghum is a salt-tolerant crop (relative to corn and soybeans) that maintains a good yield under increasing saline conditions (Grieve et al. 2012). Our data suggest that sorghum is efficient at taking up soil P pools, and we predict that soil P levels may decrease if the total aboveground biomass of the grain crop was removed at harvest. Utilizing this mitigation strategy would allow continued farming operations on the land because there is a market for sorghum as feed on the Lower Eastern Shore of MD (J. Miller, personal communication). Unfortunately, the value for the rest of the plant beyond the grain and the cost for specialized equipment to harvest the whole plant does not outweigh the option to grow alternative species (e.g., switchgrass, saltmarsh hay). Additionally, we planted Cl-excluding soybeans, which had lower yield than expected even under increasing saline conditions. Both sorghum and soybean total aboveground biomass is usually returned to the field as chaff during the harvesting process, so significant removal of P from the system is not likely with the adaptation management strategy. Finally, our research shows that these alternative grains, although efficient in taking up P, will not maintain high yield with increasing SWI.

Chapter 2 Results

Soil available cation concentrations

In the soil, concentrations of Mehlich-3 extractable Ca ([M3Ca]) decreased by 50-60% in the 0-10 cm depth at two of the sites after three years of planting (51.6% at Farm 1 and 63.1% at Farm 3; Figure 2.1). There was no change in Mehlich-3 extractable Mg ([M3Mg]) over time at any of the saltwater-intruded sites, however, Farm 6 there was an increase in soil [M3Mg] in the 0-10 depth after three years of planting (149 to 275 mg Mg kg^-1; p<0.01; Figure 2.1). There was no clear relationship between [M3Ca] and [M3Mg] and treatment across all saltwater-intruded sites. At Farm 1, distance from the ditch affected how much [M3Ca] and [M3Mg] was in the soil, where the section closest to the ditch had higher concentrations of M3Ca and M3Mg than the section further from the ditch (p<0.005 in both cases). At Farm 3, distance from the ditch and depth were significant (p<0.05 in both cases) where the soil closer to the ditch had higher [M3Ca] than the section further from the ditch and the 0-10 cm depth had higher [M3Ca] than the deeper depths (10-30 cm). There was also a depth effect at Farm 3, where [M3Mg] decreases with depth (p=0.01). At Farm 6, there was no change in [M3Ca] over time, however, there was a significant effect of depth where [M3Ca] increased with depth (p<0.01; Figure 2.1), but there was no effect of SWI gradient. Finally, Farm 6 was the only site that had more [M3Mg] than the no-salt control.

Concentrations of Mehlich-3 extractable K ([M3K]) decreased by 25-30% at all saltwater-intruded sites after three years of planting (34.3% at Farm 1, 27.8% at Farm 3, 31.6% at Farm 6; Figure 2.3). Treatment was only significant at Farm 1 where saltmarsh hay and switchgrass had similar [M3K] in all soil depths (71.8-126.3 mg K kg^-1 and 85.5-133.8 mg K kg^-1, respectively) compared to the weeds (63.7-119.1 mg K kg^-1; p<0.05; Figure 2.1).

Across all years among the SWI sites, soil depth was significant, where the 0-10 cm had the highest [M3K] (72.4-137.9 mg K kg^-1) compared to the deeper soil depths (10-20 and 20-30 cm; 37.9-102.3 mg K kg^-1 and 35.0-88.9 mg K kg^-1; p<0.005 in all cases). In 2021, distance from the ditch was significant at all saltwater-intruded sites, where Farm 1 and Farm 6 showed high [M3K] in the section closest to the ditch, but the opposite was seen at Farm 3, where [M3K] was actually higher further from the ditch.

Contrastingly, concentrations of Mehlich-3 extractable Na ([M3Na]) increased by 35-60% in the section closest to the ditch (43.8% at Farm 1, 59.3% at Farm 3, 36.8% at Farm 6) and by 60-90% furthest away from the ditch (88.3% at Farm 1, 90.1% at Farm 3, 59.7% at Farm 6; Figure 2.1) at all saltwater-intruded sites from year 1 to year 4. Distance from the ditch was significant at all SWI sites in 2021, where the section closest to the ditch had the highest [M3Na] in the soil (589.9-936.5 mg Na kg^-1; p<0.05 in all cases). Treatment was only significant at Farm 6 where saltmarsh hay and weeds had similar [M3Na] in all soil depths (468.4-689.7 mg Na kg^-1 and 424-764.7 mg Na kg^-1, respectively) compared to switchgrass (385.3-589.9 mg Na kg^-1; p<0.05; Figure 2.1). Across all years, depth was significant at all SWI sites where the deepest depth (20-30 cm) had the highest [M3Na] in 2018, and the shallow depth (0-10 cm) had the highest [M3Na] in 2021 (p<0.05 in all cases). Finally, the saltwater-intruded sites had significantly more [Na] in the soil than the no-salt control (450 mg Na kg^-1 and 31 mg Na kg^-1, respectively; p<0.001).

Figure 2.1. Topsoil (0-10 cm) available cation concentrations at three saltwater-intruded field sites on the Lower Eastern Shore of Maryland from year 1 (baseline) to year 4. The x-axis is treatment and the y-axis is Mehlich-3-extractable Ca (A-C); K (D-F); Mg (G-I); and Na (J-L) in milligrams per kilogram. Treatments are represented by colors, where the saltmarsh hay treatment is blue, the switchgrass treatment is green, and the weeds treatment is purple and section is represented by open or closed circles, where open circles are 0-5 m away from the ditch and closed circles are 15-20 m away from the ditch.

Soil available cation pools

In the soil, Mehlich-3 extractable Ca pools decreased by 50-60% in the 0-10 cm depth at two of the sites after three years of planting (49.8% at Farm 1 and 63.6% at Farm 3; Figure 2.2). There was an increase in Mehlich-3 extractable Mg pools over time at all of the saltwater-intruded sites, especially at Farm 6 where there was an increase in soil M3Mg pools in the 0-10 depth after three years of planting (225 to 444 kg Mg ha^-1; p<0.01; Figure 2.2). There was no clear relationship between M3Ca and M3Mg pools and treatment across all saltwater-intruded sites. At Farm 1, distance from the ditch affected M3Ca and M3Mg pools, where the section closest to the ditch had higher pools of M3Ca and M3Mg than the section further from the ditch (p<0.005 in both cases). At Farm 3, distance from the ditch and depth were significant (p<0.05 in both cases) where the soil further from the ditch had larger M3Ca pools than the section closer to the ditch and the 0-10 cm depth had larger M3Ca pools than the deeper depths (10-30 cm). There was also a depth effect at Farm 3, where M3Mg pools decrease with depth (p=0.01). At Farm 6, there was no change in M3Ca pools over time, however, there was a significant effect of depth where M3Ca pools increased with depth (p<0.01; Figure 2.2), but there was no effect of distance from the ditch. Finally, Farm 6 was the only site that had greater M3Mg pools than the no-salt control.

Pools of Mehlich-3 extractable K decreased by 25-30% at all saltwater-intruded sites after three years of planting (34.3% at Farm 1, 27.8% at Farm 3, 31.6% at Farm 6; Figure 2.2). Treatment was only significant at Farm 1 where saltmarsh hay and switchgrass had similar M3K pools in the soil below (85.8-155.8 kg K ha^-1 and 110.3-161.8 kg K ha^-1, respectively) compared to the weeds (81.8-147.9 kg K ha^-1; p<0.05; Figure 2.2). Across all years among the SWI sites, depth was significant, where the 0-10 cm had the highest M3K pools (95.2-168.2 kg K ha^-1) compared to the deeper soil depths (10-20 and 20-30 cm; 54.4-129.8 kg K ha^-1 and 50.6-114.9 kg K ha^-1; p<0.005 in all cases). In 2021, distance from the ditch was significant only at Farm 3, where high M3K pools were found in the section furthest from the ditch.

Contrastingly, Mehlich-3 extractable Na pools increased by >100% in the section closest and furthest away from the ditch (Figure 2.2) at all saltwater-intruded sites from year 1 to year 4. Distance from the ditch was significant at all SWI sites in 2021, where the section closest to the ditch had the largest M3Na pools in the soil (436.0-999.7 kg Na ha^-1 at Farm 1, 401.9-1264.3 kg Na ha^-1 at Farm 3, and 701.9-1235.3 kg Na ha^-1 at Farm 6; p<0.05 in all cases). Treatment was only significant at Farm 6 where saltmarsh hay and weeds had similar M3Na pools in the soil below (714.4-1112.2 kg Na ha^-1 and 649.1-1235.3 kg Na ha^-1, respectively) compared to switchgrass (588.5-955.5 kg Na ha^-1; p<0.05; Figure 2.2). Across all years, depth was significant at all SWI sites where the deepest depth (20-30 cm) had high M3Na pools in 2018, and the shallow depth (0-10 cm) had the largest M3Na pools in 2021 (p<0.05 in all cases). Finally, the saltwater-intruded sites had significantly higher Na pools in the soil than the no-salt control (646 kg Na ha^-1 and 47.7 kg Na ha^-1, respectively; p<0.001).

Figure 2.2. Soil extractable cation pools in topsoil (0-10 cm) at three saltwater-intruded field sites on the Lower Eastern Shore of Maryland in 2018 (baseline) and 2021. The x-axis is treatment and the y-axis is Mehlich-3 extractable Ca (A-C); K (D-F); Mg (G-I); and Na (J-L) pools in kilograms per hectare. Treatments are represented by colors, where the saltmarsh hay treatment is blue, the switchgrass treatment is green, and the weeds treatment is purple and section is represented by open or closed circles, where open circles are 0-5 m away from the ditch and closed circles are 15-20 m away from the ditch.

Topsoil electrical conductivity

In 2018 (baseline), topsoil (0-10 cm) electrical conductivity (EC) was low at all saltwater-intruded sites, where distance from the ditch was significant (0-5 m from ditch > 15-20 m from ditch; p<0.01; Figure 2.3). After three years, EC increased under all plant treatments, but there was no significant difference between them. At Farm 3, baseline soil EC farthest from the ditch was closer to the 2021 soil EC under all treatments. At Farm 6, in the soil under the weeds, there was a significant difference between the distance closest to and farthest from the ditch, where section closer to the ditch had higher EC than section further from the ditch (p<0.01; Figure 2.3). Soil EC was higher at all SWI sites compared to the no-salt control where EC in 2018 was approximately 0.076 dS m^-1 and increased to 0.094 dS m^-1 in 2021.

Plant leaf tissue cation concentrations

Across all saltwater-intruded sites across all years, there was no clear relationship between plant leaf tissue cation (Ca²⁺, K⁺, Mg²⁺, and Na⁺) concentrations and distance from the salinity source. At the no-salt control site, we only planted switchgrass, which showed [Ca²⁺], [K⁺], and [Mg²⁺] that were similar to the saltwater-intruded sites (2.1-4.0 g Ca kg^-1; 2.1-2.8 g Mg kg^-1; 10.3-11.5 g K kg^-1) in both 2019 and 2021. Across all study years and all sites, plant leaf tissue Na concentrations ([Na⁺]) were highest in saltmarsh hay (2.1-3.8 g Na kg^-1) compared to weeds (0.2-2.0 g Na kg^-1) and switchgrass (0.07-0.3 g Na kg^-1; p<0.001). 

In 2019 at Farm 1, saltmarsh hay had the highest [Ca²⁺] (2.5 g Ca kg^-1; p<0.01; Figure 2.4). However, two years later, Ca concentrations in leaf tissue increased in the switchgrass and weed treatments, such that there were no longer differences among the treatments. At Farm 3 and Farm 6, switchgrass had the highest concentrations (2.3-2.6 g Ca kg^-1), saltmarsh hay had intermediate concentrations (2.15-2.18 g Ca kg^-1), and weeds had the lowest concentrations (2.0-2.1 g Ca kg^-1). In 2021, switchgrass tissues had the highest [Ca²⁺] (4.2-5.0 g Ca kg^-1; p<0.01; Figure 2.4) in Farms 3 and 6.

We found similar patterns among all saltwater-intruded sites across all study years where the highest concentrations of Mg were always found in the weed tissue (2.1-2.9 g Mg kg^-1) and the lowest [Mg²⁺] were found in saltmarsh hay tissue (1.2-1.7 g Mg kg^-1) with intermediate levels in switchgrass tissues (1.7-3.1 g Mg kg^-1; p<0.01 in all cases; Figure 2.4).

We found similar patterns among all saltwater-intruded sites across all study years where the highest concentrations of K were always found in the weed tissue (10.0-21.2 g K kg^-1) and the lowest [K⁺] were found in saltmarsh hay tissue (5.0-9.0 g K kg^-1) with intermediate levels in switchgrass tissues (8.1-12.0 g K kg^-1; p<0.01 in all cases; Figure 2.4).

Figure 2.4. Plant leaf tissue cation concentrations along the salinity gradient at three saltwater-intruded farms on the Lower Eastern Shore of Maryland in year 1 (A-C) and year 4 (D-F). The x-axis is treatment at each of the three farm sites, and the y-axis is cation concentrations in plant leaf tissue in milligrams per kilogram. Alkaline earth metals (Mg [circles] and Ca [squares]) are represented by the color green and alkali metals (Na [diamonds] and K [triangles]) are represented by the color pink.

Plant biomass cation pools

Across all saltwater-intruded sites across all years, there was no clear relationship between plant biomass cation (Ca, K, Mg, and Na) pools and distance from the salinity source. At the no-salt control site, we only planted switchgrass, which showed Ca, K, and Mg pools that were similar to the saltwater-intruded sites in both 2019 and 2021. Across all study years and all sites, plant biomass Na pools were highest in saltmarsh hay (1.3-26.8 kg Na ha^-1) compared to weeds (0.09-3.6 kg Na ha^-1) and switchgrass pools (0.009-2.5 kg Na ha^-1; p<0.001; Figure 2.5). 

In 2019 at Farm 1, weeds had the highest Ca, K, and Mg pools (1.6-2.5 kg Ca ha^-1; 10.3-19.7 kg K ha^-1; 2.1-4.7 kg Mg ha^-1; p<0.01; Figure 2.5). However, two years later, Ca, K, and Mg in plant biomass increased in the switchgrass, so the pools were highest (8.3-34.2 kg Ca ha^-1; 27.9-98.2 kg K ha^-1; 9.2-36.4 kg Mg ha^-1), weeds had the lowest pools (1.3-5.0 kg Ca ha^-1; 16.6-64.6 kg K ha^-1; 1.9-7.3 kg Mg ha^-1), and saltmarsh hay had intermediate pools (5.8-8.7 kg Ca ha^-1; 18.4-26.1 kg K ha^-1; 3.8-4.2 kg Mg ha^-1; p<0.001; Figure 2.5). 

Across all years at Farm 3 and Farm 6, switchgrass had the highest Ca, K, and Mg pools (14.2-64.5 kg Ca ha^-1; 53.9-165.9 kg K ha^-1; 13.8-49.8 kg Mg ha^-1), saltmarsh hay had intermediate pools (6.4-22.1 kg Ca ha^-1; 25.2-57.9 kg K ha^-1; 4.2-14.2 kg Mg ha^-1), and weeds had the lowest pools (0.6-7.6 kg Ca ha^-1; 14.0-48.4 kg K ha^-1; 0.4-9.9 kg Mg ha^-1; p<0.001; Figure 2.5).

Figure 2.5. Plant aboveground biomass cation pools at three saltwater-intruded field sites on the Lower Eastern Shore of Maryland in year 1 (A-C) and year 4 (D-F). The x-axis is treatment at each of the three farm sites, and the y-axis is cation pools in aboveground biomass in kilograms per hectare. Alkaline earth metals (Mg [circles] and Ca [squares]) are represented by the color green and alkali metals (Na [diamonds] and K [triangles]) are represented by the color pink.

Correlations among cations in soils

In 2018, across all sites, Ca and K were highest in the topsoil (0-20 cm; r² = -0.41 and r² = -0.56; p<0.05 and p<0.01, respectively; Table 2.1) where Mg and Na concentrations were highest in the deepest depth of the soil (20-30 cm; r² = 0.2; p=0.1 in both cases). Ca and Mg and Ca and K concentrations in the soil were positively correlated (r² = 0.4; p<0.01 in both cases). Mg and Na were also positively correlated (r² = 0.5; p<0.001). In 2021, across all sites, K, Mg, and Na were highest in the topsoil (0-20 cm; r² < -0.2; p<0.05 in all cases; Table 2.1) and Mg and Na were highest closer to the ditch (r² = -0.3; p<0.001 in both cases). Ca and Mg and Ca and Na concentrations in the soil were positively correlated (r² = 0.6 and r²  = 0.3, respectively; p<0.001 in both cases). Mg and Na (r² = 0.7; p<0.001) and Mg and K (r² = 0.5; p<0.001) were also positively correlated.

Table 2. 1. Soil cation concentration correlations in year 1 (baseline) and saltmarsh hay, switchgrass, and weeds in year 4 in both the section closest to the ditch (0-5 m) and the section furthest from the ditch (15-20 m) on three-saltwater-intruded fields on the Lower Eastern Shore of Maryland. Values are r² and symbols are p-values (^ = p<0.05, * = p<0.01, ** = p<0.005, *** = p<0.001).

Topsoil EC vs. soil cation concentrations

In 2018, EC was negatively correlated with [M3Ca] and [M3K] (r² = -0.27, p<0.05; r² = -0.32, p<0.01; Table 2.2). Contrastingly, EC was strongly positively correlated with [M3Mg] and [M3Na] (r² = 0.54, p<0.001; r² = 0.75, p<0.001; Table 2.2). In 2021, EC was positively correlated with [M3K], [M3Mg], and [M3Na] (r² = 0.30, p<0.05; r² = 0.58, p<0.001; r² = 0.94, p<0.001; Table 2.2).

Table 2. 2. Soil EC and soil cation concentration correlations in year 1 (baseline) and year 4 in both the section closest to the ditch (0-5 m) and the section furthest from the ditch (15-20 m) on three-saltwater-intruded fields on the Lower Eastern Shore of Maryland. Values are r² and symbols are p-values (^ = p<0.05, * = p<0.01, ** = p<0.005, *** = p<0.001).

Correlations among cations in plant leaf tissue

In 2019, across all saltwater-intruded sites, K and Mg concentrations in plant leaf tissue were the highest in weeds, Na was the highest in saltmarsh hay, and Ca was higher in the saltmarsh hay and switchgrass than weeds. In saltmarsh hay tissue, all cations were positively correlated, where K, Ca, Mg, and Na were strongly positively correlated with each other (r² > 0.5; p<0.05 in all cases). In switchgrass plant leaf tissue, all cations were positively correlated (r² = 0.68; p=0.001, r² = 0.65; p=0.002), except K~Ca which was weakly negatively correlated (r² = -0.1; p=n.s.). In the weed tissue, all cations were positively correlated with each other (r² > 0.4; p<0.05 in all cases; Table 2.3).

In 2021, salt concentrations in plant tissues largely followed the same patterns within species: across all saltwater-intruded sites, again K and Mg concentrations in leaf tissue were highest in the weeds, again Na was highest in the saltmarsh hay, but in 2021, Ca was highest in the switchgrass rather than switchgrass and saltmarsh hay. In saltmarsh hay tissue, all cations were positively correlated with each other (r² > 0.5; p<0.05 in all cases). In switchgrass plant leaf tissue, K was negatively correlated with Ca and Mg, and Na was negatively correlated with Ca and Mg. In the weeds, all cations were positively correlated with each other (r² = 0.75; p<0.001; Table 2.3).

Table 2. 3. Plant leaf tissue cation concentration correlations for saltmarsh hay, switchgrass, and weeds from three-saltwater-intruded fields on the Lower Eastern Shore of Maryland in year 2 and year 4. Values are r² and symbols are p-values (^ = p<0.05, * = p<0.01, ** = p<0.005, *** = p<0.001).

Topsoil EC vs. plant cation concentrations

Correlations between soil EC and plant leaf tissue cation concentrations were mostly insignificant except for in the section closest to the ditch in switchgrass and weeds where EC was positively correlated with [Na] (r² = 0.60, p<0.005; r² = 0.89, p<0.005; Table 2.4). There was also a weak positive correlation between soil EC and [Mg] (r² = 0.43, p<0.05).

Table 2. 4. Soil EC and plant leaf tissue cation concentration correlations in year 4 for saltmarsh hay, switchgrass, and weeds in both the section closest to the ditch (0-5 m) and the section furthest from the ditch (15-20 m) from three-saltwater-intruded fields on the Lower Eastern Shore of Maryland. Values are r² and symbols are p-values (^ = p<0.05, * = p<0.01, ** = p<0.005, *** = p<0.001).

Soil and plant cation concentration relationships (topsoil 0-10 in 2021)

In 2021, topsoil [M3Na] and plant leaf tissue [Na] were strongly positively correlated among switchgrass and weeds in both the area closest to the ditch and furthest from the ditch (r² > 0.5, p<0.05 in all cases; Table 2.5). There was no correlation between any of the cations in the topsoil and saltmarsh hay plant leaf tissue in either section of the field. Switchgrass [Ca] in tissue closest to the ditch was negatively correlated with [M3K] in topsoils, while [Na] in tissues were positively correlated with [M3Mg] in topsoils (r² = -0.44, p<0.01; r² = 0.37, p<0.05). Switchgrass [K] and [Mg] in tissue furthest from the ditch was positively correlated with [M3Na] in topsoils (r² =0.46, p<0.01; r² = 0.39, p<0.05). The concentrations of Na in weed tissues in the 0-5 m section was strongly positively correlated with [M3K], [M3Mg], [M3Na] in soils (r² = 0.75, p<0.005; r² = 0.77, p<0.005). Weed tissue [K], [Mg], and [Na] concentrations furthest from the ditch was positively correlated with [M3Ca] (r² = 0.84, p<0.005; r² = 0.80, p<0.01; r² = 0.64, p<0.05).

Table 2. 5. Plant leaf tissue and topsoil cation concentration correlations in year 4 for saltmarsh hay, switchgrass, and weeds in both the section closest to the ditch (0-5 m) and the section furthest from the ditch (15-20 m) from three-saltwater-intruded fields on the Lower Eastern Shore of Maryland. Values are r² and symbols are p-values (^ = p<0.05, * = p<0.01, ** = p<0.005, *** = p<0.001).

Chapter 2 Discussion

Our results from this study have implications for improved understanding of cation dynamics in plant leaf tissues and soils in saltwater intruded fields. After analyzing the aboveground biomass and soil under four management practices (adapt, remediate, restore, abandon) on three farms experiencing SWI, we determined that saltmarsh hay is efficient at taking up soil Na. We showed the relationships between plant leaf tissue and soil available cations are complex and differ by our variables (e.g., distance from the ditch, year) under saltwater-intruded conditions. Not surprisingly, concentrations of available K, Mg and Na in soils were higher close to the ditch than near the center of the agricultural field as the result of SWI and SLR (Arslan & Demir 2013; Figure 2.4). After three years of planting, available Ca and K decreased in topsoils and available Mg and Na increased in topsoils at all saltwater intruded sites (Figure 2.4). The significant decrease in soil available Ca in the topsoil (0-10 cm) over 2 years suggests that Ca was leached below 10 cm or that it was assimilated into plant tissues. While Ca is normally bound to soil colloids, with an excess of Na from saltwater, there is more opportunity for Na to exchange with Ca, causing Ca to release (Chidambaram et al. 2015). Soil available K decreased in the topsoil after 2 years likely due to plant uptake as K does not readily leach from soils (Mayland & Wilkinson 1989; Lehmann & Schroth 2002). On the other hand, soil available Mg and Na increased over 2 years which was expected as these cations are the main constituents that make up seawater and can be increased by SWI (Kester et al. 1967; Figure 2.4; Table 2.2). Overall, restoration (e.g. saltmarsh hay) management practices showed no increase in aboveground biomass Na concentrations as soil available Na accumulated over time, and this was indicative of the species’ ability to co-regulate cations in their leaf tissue.

Plant tissue cation concentrations varied among the different species

We expected that as Na concentrations in the soil increased, it would prevent the uptake of Ca into plant tissues (Lazoff & Bernstein 1999). While there were no strong correlations between soil Na and leaf tissue Ca in our data (Table 2.5), we observed an increase in Ca concentrations in switchgrass and weeds after two years (p<0.01; Figure 2.4A-C). Increased soil Na has also been known to prevent the uptake of K into plant tissues (Munns & Tester 2008), which could have caused leaching from surface flooding, but this would cause an increase or no change in soil available K concentrations, which is not what we saw (observed decrease in soil available K; Figure 2.4D-F). However, we found similar patterns among all saltwater-intruded sites across all study years where the highest concentrations of K were always found in the weed tissue and intermediate levels in switchgrass tissues, and there was an increase in K concentrations for both species in 2021 (Figure 2.4D-F). Similarly, we found the highest concentrations of Mg were in the weed tissue, the lowest [Mg²⁺] were found in saltmarsh hay tissue, and intermediate levels were found in switchgrass (Figure 2.4G-I). Previous literature has shown excess uptake of K in grasses lowers Mg content (Robinson et al. 1989). The differing results from our expectations could be due to how different species are able to regulate salt uptake and tolerate different levels of salinity (Grieve et al. 2012). Switchgrass is known to have a higher tolerance to limiting water and nutrient conditions and may be able to uptake more soil nutrients due to their deep root system and relationship with mycorrhizal fungi (Di Virgilio et al. 2007; Xu et al. 2017). The weed species in 2019 was a native switchgrass species (Panicum dichotomiflorum) with extensive roots similar to our planted switchgrass. In 2021, the dominant weed was a marsh species, crabgrass (Digitaria sanguinalis), that is similar to saltmarsh hay that is able to regulate nutrient uptake under saline conditions and maintain biomass regardless of Ca and Mg additions (Grieve et al. 2012; Pierce et al. 2017). 

Saltmarsh hay is able to regulate uptake of Na

We predicted that plant biomass would be suppressed by excess Na from SWI due to the osmotic and toxic effects on plant growth and cell function (Flowers et al. 1991). Topsoil [M3Na] and plant leaf tissue [Na] were strongly positively correlated in the switchgrass and weed treatments in both the area closest to the ditch and furthest from the ditch (Table 2.5) meaning both took up more Na when there was more Na in the soil (Kim et al. 2012; Rashidi et al. 2020). Because switchgrass and weeds were taking up more Na in their leaf tissue, this may have caused the inhibition of uptake of important nutrients that contribute to their biomass (Greenway & Munns 1980; Hasegawa et al. 2000; Netondo et al. 2004). There were no relationships between saltmarsh hay tissue and soil cations due to the salt-tolerant species’ ability to regulate salt intake (Grieve et al. 2012), specifically taking in high concentrations of Na. A change in soil cation concentrations did not affect leaf tissue cation concentrations (Table 2.5), but interestingly, saltmarsh hay plant leaf tissue cation concentrations were very strongly positively correlated with each other (Table 2.3). 

Across all study years and all sites, plant leaf tissue Na concentrations [Na⁺] and biomass Na pools were highest in saltmarsh hay (p<0.001). Saltmarsh hay is efficient at taking up soil Na and, if planted along the saltier edges of the field, could be a mitigation strategy to decrease soil Na that is toxic to most plants in the area. Planting saltmarsh hay could facilitate the transition to native marsh which might be a better option if the field experiences more flooding (Gedan et al. 2011; Hu et al. 2015; Leonardi et al. 2018). Previous research indicated that fields experiencing SWI will naturally transition to host more native marsh species such as saltmarsh hay (Gedan et al. 2019). Federal reserve programs could provide landowners with financial assistance, or there are opportunities to use the land for other activities such as waterfowl hunting that could be an alternative source of revenue (e.g., Conservation Reserve Enhancement Program; CREP 2022, Environmental Quality Incentives Program; EQIP 2022, Wetland Reserve Easements; WRE 2022). Overall, restoring farm fields has benefits for the soil by taking up Na, landowners by maintaining revenue, and communities by protecting coastlines.