.|  Baltimore Ecosystem Study
Long-term biogeochemical study plots
Long-term biogeochemical study plots: Urban forests and grasslands
One of the earliest efforts in BES was the establishment of long-term plots for comparative analysis of soil biogeochemical variables in lawns and forests. We established eight plots in urban (Leakin, Hillsdale) and rural (Oregon Ridge) forested parks and four grassland plots on two school campuses (McDonogh School, University of Maryland Baltimore County (UMBC)). The forest plots provided two contrasts; urban versus rural atmospheric conditions, and soils, as they encompassed the two most common soil types in the region (Groffman et al. 2006). The lawn plots provided a gradient of management intensity, with the McDonogh plots receiving no fertilizer or pesticides, one of the UMBC plots receiving moderate fertilizer (~100 kg N ha-1 y-1) and occasional herbicide applications and the other UMBC plot receiving more intensive management with higher fertilizer (~ 200 kg N ha-1 y-1) and more regular pesticide application. This network of plots is clearly not an ideal experimental design providing controlled contrasts of multiple factors in representative components of the urban environment. Rather, these were plots that we considered to be "representative" of the major ecosystem types in our study region where we could have access and control for long-term studies. The limitations of these plots have driven us to constantly compare them with a wider range of actual lawns in the region to ensure that the information that they produce is relevant.
The long-term study plots were instrumented with lysimeters (tension and zero tension) to sample water that "leaches" through the soil profile, with chambers that allow for quantification of soil:atmosphere fluxes of gases (carbon dioxide - CO2, nitrous oxide - N2O, methane - CH4) and probes for measurements of soil temperature and moisture (Groffman et al. 2006, Groffman and Pouyat 2009, Groffman et al. 2009, Savva et al. 2010). This instrumentation has produced some interesting and surprising results. First, leaching of nitrate (NO3-) the most mobile form of nitrogen, was not as high as expected. Lawns definitely had higher leaching than forests (Figure 1), but the differences were not as large as expected given the differences in input. Even more interestingly, there were no systematic relationships between inputs and outputs. While the McDonogh grass plots received less fertilizer than the UMBC plots, they had higher NO3- leaching. And the two UMBC plots differed markedly in fertilizer input, but had similar leaching. These data immediately signaled to us that the nitrogen cycle in these urban grasslands was more complex and retentive than we had originally thought.

Figure 1. Volume-weighted nitrate concentrations in leachate in zero tension lysimeters in four forest and four grass plots in the Baltimore metropolitan area. Values are mean (standard error) of three water years (2002 - 2004). Bars with different superscripts are significantly different at p < 0.05. From Groffman et al. (2009).

It is important to note that the lawns still had significant hydrologic losses of NO3-. If the concentration data from Figure 1 are combined with estimates of water flow, the forest plots consistently yielded less than 3 kg N ha-1 y-1 of leaching, which is considerably less than estimates of atmospheric deposition in the region (8 - 12 kg N ha-1 y-1) (Bettez and Groffman 2013). The lawns produced from 1.4 (in a very dry year) to 25 kg N ha-1 y-1. So even though the differences between lawns and forests were not as big as expected, and there is evidence for significant retention of nitrogen in the lawns, they are still important sources of reactive nitrogen to the environment.
Surprising results were also evident in the soil:atmosphere gas flux data that we collected. We had expected to find high fluxes of N2O, a potent greenhouse gas, from the grass plots. Studies in other areas, especially irrigated lawns in the arid U.S. West, have found very high N2O fluxes from lawns (Kaye et al. 2004, Bijoor et al. 2008, Hall et al. 2008, Townsend-Small et al. 2011). Instead, we found that N2O fluxes were generally lower in lawns than in forests (Figure 2a). And as with the leaching data, there was no systematic response to fertilizer input, i.e. the McDonogh plots which received less fertilizer than the UMBC plots had higher N2O fluxes and the two UMBC plots that had very different fertilizer input had very similar (and low) N2O fluxes.

Figure 2. Soil:atmosphere fluxes of N2O (A) and CO2 (B) from forest and grass plots in the Baltimore metropolitan area. Values are means of all fluxes measured in 3 chambers per plot from June 2001 through May 2004. Sites followed by different superscripts are significantly different at p < 0.05. From Groffman et al. (2009).

The long-term data series of N2O flux was also interesting and surprising. We observed an increase in flux in 2003 and 2004 (Figure 3), which were wet years with very dynamic nitrogen cycling and loss in our study watersheds (Kaushal et al. 2008). However, the increase was much more marked in the forest plots than the grass plots, suggesting the nitrogen cycle in lawns is less susceptible to hydro-climatic disruption than in forests.
A major factor underlying the complexity of nitrogen cycling and retention in the lawn plots is likely high flux of carbon from the atmosphere into plants and then into soil microbial populations. High carbon flux in lawns is evident from the long-term patterns of soil:atmosphere CO2 flux (Figure 2b), i.e. this flux was consistently higher from lawns than forests, suggesting that there is dynamic cycling of both carbon and nitrogen in lawn soils. High carbon cycling is also likely driven by the higher temperatures in lawns than in forest, i.e. mean annual average temperature at 10 cm depth ranged from 13.5 - 15.0 oC in lawns and 12.2 to 12.6 oC in forests over an 8 year period (Savva et al. 2010).
The major data stream that behaved as expected in our long-term study plots was soil:atmosphere CH4 flux. Upland soils are known to have the capacity of remove CH4, a potent greenhouse gas, from the atmosphere. However, this capacity is known to be susceptible to inhibition by soil disturbance and especially by nitrogen inputs (Mosier et al. 1991, Dutaur and Verchot 2007). We found that CH4 uptake was reduced by approximately 50% in the urban forest plots compared to the rural forest plots and was completely eliminated in the lawn plots (Groffman and Pouyat 2009) (Figure 4). Subsequent studies determined that this inhibition was linked to long-term increases in nitrogen enrichment and cycling in the urban forest and lawn soils (Costa and Groffman 2013).

Figure 3. Mean annual soil:atmosphere N2O flux from in two urban forests (1998 - 2010) and two urban lawns (2001 - 2010) in the Baltimore metropolitan area.


Figure 4. Mean annual soil:atmosphere CH4flux from in two urban and two rural forests (1998 - 2010) and two urban lawns (2001 - 2010) in the Baltimore metropolitan area.

Literature Cited:
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Costa, K. H. and P. M. Groffman. 2013. Factors regulating net methane flux by soils in urban forests and grasslands. Soil Science Society of America Journal 77:850-855.
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Mosier, A., D. Schimel, D. Valentine, K. Bronson, and W. Parton. 1991. Methane and nitrous-oxide fluxes in native, fertilized and cultivated grasslands. Nature 350:330-332.
Savva, Y., K. Szlavecz, R. V. Pouyat, P. M. Groffman, and G. Heisler. 2010. Effects of land use and vegetation cover on soil temperature in an urban ecosystem. Soil Science Society of America Journal 74:469-480.
Townsend-Small, A., D. E. Pataki, C. I. Czimczik, and S. C. Tyler. 2011. Nitrous oxide emissions and isotopic composition in urban and agricultural systems in southern California. J. Geophys. Res. 116:G01013.

This research was supported by funding from the NSF Long-term Ecological Research (LTER) Program. This material is based upon work supported by the National Science Foundation under Grant No. 1027188. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.