We are interested in the nitrogen cycle because the productivity of forests is positively correlated with rates of nitrogen input and recycling in soil. Also, nitrogen outputs from forest ecosystems, including nitrate in groundwater and surface water and nitrous oxide in the air, can have important consequences for water quality and climate. Buckthorn leaves have unusually high concentrations of nitrogen so we expect that forest sites with abundant buckthorn will have greater nitrogen inputs through falling leaves, greater rates of nitrogen recycling in soil, and, at least seasonally, greater nitrogen exports into water and air. We tested these hypotheses by measuring inputs and recycling of nitrogen at the Lee and Rose Warner Nature Center near Marine on St. Croix. Since we were able to find plot locations on two different soil types (sandy and silty) within one large study area it was not necessary to set up the experiment at two site as original proposed. This made the experimental setup more efficient and logistically easier.
We identified 28 study plots at our study site. On each soil, the plots were located across a gradient of buckthorn invasion from low to high abundance. For each level of buckthorn invasion on each soil type, there were two replicate plots for each of the six different levels of buckthorn abundance (12 plots along the buckthorn gradient on each soil type). Additionally, on the silty soil type, we located four plots where buckthorn has been removed: two of these where buckthorn was removed twice in the last 6 years and two where buckthorn was removed once three years ago.
At the start of the project we surveyed each plot for vegetation characteristics including the number and stem size of each buckthorn individual, canopy species identity and basal area. We also measured slope, aspect, and elevation in each plot. In early July (2012) we initiated soil sampling from each plot to measure concentrations of soil nutrients, including ammonium, nitrate, and total carbon and nitrogen. Nitrates and ammonia represent mineralized nitrogen that is usable by plants (compared to non-mineralized organic nitrogen that is not available to plants). We used two methods to measure in situ levels of nitrogen across the gradient of buckthorn abundance. First, we installed soil incubation tubes consisting of two inch diameter PVC tubes, each 20 cm long. These were hammered into the soil, capped, and left for around 28 days. On the same day that the PVC tubes were installed, we also collected soil close to the incubation tubes and analyzed this soil for concentrations of nitrates and ammonia to serve as an initial value for mineralized nitrogen available to plants. After 28 days we removed the PVC tubes and analyzed the soil inside each one for nitrate and ammonia in the same way as for the initial soil samples. Since the soil in the incubation tubes had been isolated from plant roots and further nitrogen input we were able to subtract the amount of nitrates and ammonia in the initial soil sample from the amount in the PVC tubes and use this value as an index for the recycling of N in organic matter by soil microbes (i.e. net nitrogen mineralization). In plots with more abundant buckthorn, we would expect nitrogen mineralization to be higher since buckthorn has nitrogen-rich leaves.
Installation of incubation tubes
The second method of estimating nitrogen mineralization was by using buried bags of ion-exchange resin, a granular material onto which nitrate is adsorbed. These bags were buried 10 cm under an undisturbed soil profile so nitrate that was leached out above the bags is caught in the bag. We removed the resin bags after about 28 days and then analyzed them for nitrate and ammonia in the same manner as for the soil extracts. We performed this procedure (incubation tube and resin bag deployment) during the summer and fall of 2012, and also over the winter of 2012/2013 and spring of 2013. In this way, we were able to estimate the amount of soil mineralization during each season.
Resin bags installation
Overall, the total amount of nitrogen in the soil increased as the abundance of buckthorn increased. Whether or not this translates into higher amounts of mineralized nitrogen (ammonia and nitrate) available for plant growth will be established when we have analyzed all the data from the incubation tubes and resin bags. This analysis is ongoing and will allow us to assess whether buckthorn alters soil nutrients after invasion. This has implications for productivity and growth rates of native species and also might influence invasibility by other introduced species.
On September 7, 2012 we installed two 10 gallon pots in each survey plot for collection of leaf litterfall. This allowed us to estimate the proportion each of the major tree and shrub species (including buckthorn) contribute to total leaf litter. Every two weeks, until December 3, 2012, we collected all the leaves that had fallen into each buckets. We sorted the litter to species in order to estimate the percentage of litter each species contributed. After sorting, we dried, weighed, and ground a sample of leaves for analysis of nitrogen concentrations so we could estimate nitrogen inputs into the soil.
As expected, plots with a higher abundance of buckthorn stems, also had more buckthorn leaf litter.
The total nitrogen in all leaf litter combined was also higher in plots where buckthorn was more abundant indicating that heavy buckthorn cover does influence nitrogen inputs.