The effect of sulfur and nitrogen additions on nutrient cycling and vegetative cover composition in sandplain grassland restoration plots in Edgartown, MA
By Fiona Jevon
Harvard College ‘13
Ecosystems Center, Marine Biological Laboratories
Sandplain grasslands of the northeastern United States are early successional, coastal ecosystems that support many rare and uncommon species. The conversion of these ecosystems for agriculture in the 19th and 20th centuries resulted in the loss of many of these important species. Because of the enduring effects of cultivation on soil characteristics, abandoned farmland tends to be dominated by non-native vegetation. One hypothesized method for restoring these systems to native sandplain grassland is using elemental sulfur to lower the pH of the soil. This study is based on an experiment in Edgartown, MA, in which many different treatments were applied to an abandoned agricultural field with the aim of finding the most effective method for restoring the area to a sandplain grassland ecosystem dominated by historically native plant species. Soil was collected from these experimental plots and tested for pH and inorganic nitrogen. Non-native fescue grass was also grown in pH-manipulated soil in growth chambers to determine a pH response curve. Sulfur additions in the experimental plots were found to lower the pH of the soil and increase the total cover of native species. Nitrogen treatments increased the cover of non-native species but had no significant effects on the native species. Total biodiversity was unchanged by either type of treatment. Non-native fescue grass grown in growth chambers were inherently inhibited by low pH soils. Based on these results, sulfur addition is an effective method for restoring agricultural land to native-dominated sandplain grassland systems.
Sandplain grasslands are early successional ecosystems found primarily in the coastal northeastern United States. These systems support a diverse group of uncommon plant and animal species (Eberhardt et al., 2003). In Massachusetts, for example, sandplain grasslands are home to 24 species listed as endangered, threatened, or of special concern (Lezberg et al., 2006, MNHESP, 2004). Conservation and restoration of these systems has become a priority, not only because of their unique species composition but also because they maintain the heterogeneity of the land. Historically, they have maintained earlier succession stages by disturbances such as fires, salt water spray, and forest clearing, while other systems follow natural, linear succession and become forests (Neill, draft paper). Unfortunately, many of these ecosystems have been lost over the past 200 years as they were converted to agricultural lands (Motzkin and Foster, 2003). Today, they are still in rapid decline; sandplain grasslands are naturally early succesional systems, but due to human intervention such as fire suppression and reforestation, the early successional stage cannot be maintained (Neill, draft paper). Currently, the only remaining systems exist on small fragmented areas in coastal New England and New York (Neill, draft paper).
On Martha’s Vineyard, much of the sandplain grassland was converted to farmland, but even though agriculture has been abandoned in the area, the system has not returned to its historical state. This result is observed because formerly cultivated lands are easily invaded by non-native plants due to the long lasting effect that cultivation has on soil chemistry (Von Halle and Motzkin, 2007). For example, many farms use nitrogen and lime fertilizers, which increase the pH and available nitrates in the soils. The effects of these fertilizers are still present even long after cultivation of the soil has ended (Von Halle and Motzkin, 2007). The active restoration of the native sandplain grassland species will allow for this unique ecosystem to expand and provide critical habitats for rare species.
Restoring sandplain grasslands from agricultural fields requires a method of returning a non-native dominated system to a native dominated one. One of the theories behind restoring agricultural lands to native sandplain grasslands is based on the concept that non-native species are better adapted for nutrient rich soils, as many of them are fast growing and have high nitrogen requirements (Wilson and Tilman, 2002). They therefore tend to dominate abandoned farmland, which has residual high nutrient levels due to historical fertilizer use. Native species in the northeast tend to be the opposite; they are adapted for the naturally more nutrient poor soil (Davis et al., 2000). In consequence, in order to restore native species, it is necessary to lower the nutrient levels in the soil. One method to achieve this is to lower the pH of the soil; this method has been found to decrease nitrification rates, therefore decreasing the available nitrogen in the form of nitrate (Ste Marie and Pare, 1999). A technique used for restoration by lowering the pH of the soil is the addition of elemental sulfur fertilizer (Weiler, 2011). Theoretically, the sulfur will increase the acidity of the soil, decreasing nutrient availability and thus favoring the establishment of native species over non-natives.
With this study, I tried to determine how both sulfur and nitrogen additions to a previously cultivated field affect the available nutrient levels, particularly nitrate. In theory, nitrogen additions should increase nitrate concentrations, and sulfur additions should decrease nitrate concentrations due to lowering the pH. I also wanted to determine if the sulfur and nitrogen additions affect the number and cover of native species. If the above hypotheses were true, I would expect he nitrogen additions to have lower native cover and higher non-native cover, while the opposite should occur in the sulfur addition plots. Finally, I wanted to find the growth response curve of a non-native grass species. If non-natives are less dominant in the lower pH plots, this could be due either to the fact that native plants outcompete them for the limited nutrient resources, or due to an inherent inhibition of the non-natives by low pH soils. A growth response curve showing if and at what pH a non-native grass responds negatively to low pH in the absence of competition will show which of these hypotheses is true.
The experimental restoration plots at Herring Creek in Edgartown, MA began in 2007, when Neill et al. set up 180 plots on a previously agricultural field adjacent to native sandplain grassland (Katama airfield). The plots were set up in 5 different blocks, which were each broken into a 6×6 grid of 36 square plots (Figure 1). These small plots were randomly assigned to treatments and treated in the summer of 2007. Thus each treatment has 5 repetitions. The treatments consist of various methods for decreasing the prevalence of non-native species and increasing native species. For the purpose of this project I considered only the following treatments: control, control tilled, low sulfur addition, medium sulfur addition, high sulfur addition, low nitrogen addition, medium nitrogen addition, and high nitrogen addition (Figure 1). The control plots were plots left entirely untouched. The tilled control plots were tilled and re-seeded with native seed from the Katama Airfield in 2008 but then left untreated. The treated plots were also tilled and re-seeded and then treated with various levels of either nitrogen or elemental sulfur fertilizer, also in 2008. Data on the soil characteristics (such as pH, bulk density, nutrient stocks and mineralization rates) and vegetative cover (measured by determining the percent cover of each species found within a plot) of these plots has been collected since 2007. Therefore, data from 2007 is pre-treatment data and data from 2008 is the first year of data post-treatment. The purpose of these experimental plots is to determine the most effective method for restoring previously agricultural land to native sandplain grassland.
Field and lab methods
For the purposes of this project I sampled only from the sulfur addition, nitrogen addition, control and tilled control plots (other treatments also exist). In the field, I used a 10 cm deep corer to collect 2 soil samples. In the lab, I homogenized each soil sample, weighed out 10 grams of wet soil, dried these samples in an oven at 60oC for 2 days, and weighed them again to determine the wet:dry ratio of soil from each plot. I then measured the initial pH and initial nitrate and ammonium stocks. This was done by extracting a 10 g subsample with 100 mL of 1 M KCl in a sealed cup. These 64 cups were then placed on a shaker table for 1.5 hours then allowed to settle for 24 hours. Then they were filtered using 25 mm GF/F swinnex filters into scintillation vials and frozen until analysis. A 0-100µM standard curve was then run on both the Lachat and Shimadzu 1601 analyzers, and the samples were analyzed for nitrate on the Lachat and ammonium on the Shimadzu 1601. The remaining samples were incubated in closed plastic bags at 25oC. I extracted another 10 g of each sample and analyzed for nitrate and ammonium concentrations after 8 days and again after 16 days. Net mineralization and nitrification rates for each of the plots were then calculated based on the change in concentration over the incubation period. I also measured pH at each of these three time points.
I compared my data to data collected on the pH, inorganic nitrogen concentrations in the soil and mineralization and nitrification rates over the past 5 years. I also compared these soil characteristics to data collected on the overlying vegetation. In particular I was interested in the total number of species per plot, the number of natives and non-natives, and the total cover of natives and non-natives.
Plant growth experiment
In addition to collecting samples from the experimental plots, I grew the non-native fescue grass (Festuca arundinacea) in a growth chamber. I tilled 1 gram (about 438 seeds) of a store bought mix of fescue seed (Black Beauty) into the top inch of soil, which was a 1:1 mix by volume of sterile potting soil and sand. I used a control and 4 different treatments of dilute sulfuric acid in order to manipulate the pH of the soil to determine the response of a non-native plant to different levels of acidity. Each pot was leached through with 350 ml of the given dilution of sulfuric acid initially, and then watered with that dilution as necessary in order to maintain the acidity level. The control pots were watered with distilled water, the S1x pots (4 reps) were watered with 1:10,000 dilute sulfuric acid, the S2x pots (4 reps) were watered with 1:2000 dilute sulfuric acid, the S3x pots were watered with 1:600 dilute sulfuric acid and the S4x pots were watered with 1:200 dilute sulfuric acid. The pH of the pots was checked 5 times over the 18 day growing period and if necessary additional acid was added to pots. If the pH was stable, I watered the pots with DI water to equalize the amount of liquid I gave each pot. After 18 days, I harvested the aboveground and belowground biomass of each pot. I dried and weighed the aboveground and belowground biomass and then counted the blades that sprouted in each pot to determine a percent germination.
The experimental plot data on soil characteristics showed that nitrification rates across all plots regardless of treatments were not significantly related to pH (Figure 2). However, nitrification rates from previous years did have a weak positive correlation (Figure 3). Pools of extractable nitrate tended to be higher in less acidic soils (Figure 4). Lower soil pH also correlated weakly with higher native species across all plots (Figure 5). Looking over time at the tilled plots collectively, three dominant, non-native grass species (Anthoxanthum odorata, Bromus inermis and Dactylis glomerata) initially decrease sharply, then slowly increase again over time (Figure 6). Ragweed (Ambrosia artemisiifolia), a native ruderal species, increases initially then decreases. Little bluestem (Schizachyrium scoparium), a native species that is one of the targets for sandplain restoration, doesn’t appear until after 2009 but increases steadily. In the untilled plots, Little bluestem never appears, and ragweed only appears in small amounts. The non-native grasses are highly variable, but stay dominant over the entire time period. Looking specifically at the treatments, the average pH is lower in the sulfur plots, and about equal in the nitrogen addition and control plots (Figure 7). Nitrate concentrations are positively correlated with nitrogen addition, and negatively correlated to sulfur addition (Figure 8). Total species number is approximately the same across the six treatments and similar to the tilled control, and all of these are higher than untouched control (Figure 9). Additionally, there is a large increase in total number of species between 2008 and 2009 in all plots, and a small decrease in 2011 in the high sulfur treatment. The medium and high nitrogen addition plots have a stable number of native species from 2009 to 2011, and slightly more than the low nitrogen and the controls (Figure 10). The sulfur addition plots have more highly variable numbers of native species across the years, and native species numbers decrease slightly in 2011 in the medium and high sulfur treatments. Total native cover, on the other hand, is highest in the high sulfur addition plots, followed by the medium and low sulfur additions (Figure 11). The native cover in the sulfur addition plots also increases over time. In the nitrogen addition plots, native cover is highly variable and all three treatments have similar cover to the controls. The number of non-natives is not significantly different across the treatments or from the tilled control, but it does seem to be decreasing over time in the sulfur addition plots (Figure 12). Non-native cover in 2011 is slightly lower in the nitrogen addition plots than in the controls, but nitrogen addition and non-native cover are positively correlated. Non-native cover is correlated negatively with sulfur addition (Figure 13).
Plant growth experiment
The percent germination of the pots approximately followed a second-degree polynomial curve; at an average soil pH of around 5, the percent germination of the grass began to decline rapidly (Figure 14). Aboveground biomass also responded sharply to pH levels lower than 5, but at pH levels between 5 and 7, aboveground biomass did not seem to show much difference (Figure 15). Belowground biomass had a very similar curve to aboveground biomass (Figure 16). The root:shoot ratio seemed to have a optimal pH of around 5.5, with a higher root:shoot ratio at low pH.
The lack of correlation between nitrification rates and pH possibly shows that in this case, the acidic soils do not affect the nitrification process significantly. However, the correlation between the same variables in previous years at the same sites indicates that we cannot conclusively disprove this relationship. In fact, there is a correlation between the pools of extractable nitrate and the pH of the soil, which implies that pH is in fact related to the available nutrient levels in the soil. Additionally, the pH also correlates with total native vegetative cover. This indicates that the theory of restoration through pH manipulation is manifest in these plots.
The vegetation dynamics of the experimental plots showed some of the expected dynamics. The initial tilling of the treated plots significantly decreased the cover of non-native “old grasses”, but they began to re-emerge again after a few years. In the first year after treatment, the fast-growing, ruderal, native species Ragweed spiked but decreased in the following years. As the ragweed decreased, Little bluestem began to increase and now continues to grow. Little bluestem is a target species for sandplain grassland restoration, as it was a historically dominant species in these ecosystems, so the steady increase over the last two years is an indicator that the treatment is having the anticipated effect.
The specific treatments had the expected effect on the soil. The sulfur addition plots had significantly lower pH when compared to pretreatment levels and when compared to present control plots (even though these have become more acidic). This shows that the treatment of sulfur impacted the soil in the expected manner. Additionally, the nitrate concentration was much lower in the sulfur treatments than in the other plots, showing that the decrease in pH in these plots seemed to decrease the nitrate as well. The nitrogen treatments have also had the expected effects on the soil; the nitrogen treatment plots had higher levels of nitrate than the control.
The changes in soil chemistry also affected the vegetative cover. The nitrogen treatments seemed to slightly increase biodiversity (measured by total number of species) compared to the control plots. This is somewhat surprising, as we might expect the additional nitrogen to favor only fast-growing, non-native plants. The sulfur additions also seemed to increase biodiversity slightly compared to the controls, but the diversity seemed to be decreasing over time. This decrease is likely due to a shift in dominance, as a few species become established as the dominant players in these plots while species that are not well established are out-competed. Little bluestem, for example, became dominant in some of these plots.
The total number of native species had the same trend as the total number of all species. Again, the recent drop in number of native species in the sulfur addition plots is likely due to the increased dominance of certain native species, such as Little bluestem. The total native cover in the nitrogen addition plots did not meet the expected hypothesis; increasing nitrate levels should decrease total native cover, but the 3 nitrogen treatments all had similar native cover to each other and to the controls. This is possibly due to the fact that these plots already had high nutrient levels due to their previous cultivation, so the addition of nitrogen did not significantly affect the vegetation even though it increased the available nutrients in the soil. The sulfur plots did follow the expected hypothesis, as higher sulfur additions had higher native cover. Additionally, the native cover in the sulfur plots increased over time, which relates again to the idea that certain native species are becoming dominant and increasing their total cover over time.
The number of non-native species was approximately the same across the treatments, which is not what was expected. I predicted that the treatments should have caused higher numbers in the nitrogen addition plots and lower numbers in the sulfur addition plots. However, there was a downward trend of non-natives in the sulfur addition plots over time, so it seems that over time the treatments are working as anticipated. More notably, the total non-native cover in 2011 was significantly lower in the sulfur addition plots than in the control or the nitrogen addition plots. This suggests that the pH manipulation favors the establishment of native species over non-natives. The opposite was true in the nitrogen and control plots. The positive correlation between nitrogen addition and non-native cover shows that the nitrogen additions were having some of the expected effects, promoting an increase in non-native cover.
The apparent negative correlation between sulfur addition and non-native cover could be explained either by the natives being able to outcompete the non-natives for the already low levels of nutrients or by an inherent pH inhibition of the non-natives. Based on data from the pot experiment, non-native fescue grass is inherently inhibited by low pH soils. The percent germination in the pots shows that below a threshold pH of about 5, the non-native grass is strongly inhibited, as less seeds are able to germinate. This result is supported by a general literature value of about 5.3 for a crucial soil pH turning point, below which nitrification rates are strongly inhibited (Ste-Marie and Pare, 1999). The aboveground and belowground biomass had a similar threshold level of about 5, below which the total biomass decreased significantly. This also compares to the average pH of the sulfur plots, which in 2011 were 4.46, 4.18, and 3.86. The control plots had a pH of approximately 5.4. This implies that it is not only competition that is limiting the establishment of non-natives in the sulfur addition plots, but also an inherent inhibition of non-natives to grow in low pH soils. The root: shoot ratio relationship seems to show that plants growing in a pH between about 5.2 and 6.2 expend the least energy on building roots relative to shoots, implying that this is the pH range at which the plant feels least stressed. Particularly at lower pH soil, the grass tends to expend relatively more energy building roots, implying that it is more nutrient-stressed. However, it is not clear whether this is a significant trend. Overall, the pot experiment shows a strong inhibition of non-native growth below a pH of 5. While this is possibly due to the changes in nitrification, it could also be a simple pH inhibition. Further research on the mechanism of pH inhibition could show this conclusively.
Regardless of the mechanism, elemental sulfur fertilizer seems to be a very effective method of achieving sandplain restoration goals. Without significantly effecting overall biodiversity, the lowering of the soil pH favors the establishment of target native vegetative species, such as Little bluestem. It also seems to create an environment that is less favorable for the non-native species, so that over time the treatment becomes more effective in recreating historical vegetative dynamics.
I would like to thank Chris Neill for all of his help and support during the process of completing this project. I also want to thank Rich McHorney, Stef Strebel and Carrie Harris for their endless patience with me in helping me with lab work. Finally I want to thank Emily Rogers for her assistance with my fieldwork.
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