Soil nitrate production in lawns and adjacent forests in the White Mountains, NH

Background/Question/Methods

Increases in combustion of fossil fuels and crop fertilization, as well as land use change, have more than doubled available nitrogen in the biosphere.  The increasing suburbanization of the United States is making turfgrass lawns one of the most rapidly expanding ecosystems. Previous studies suggest turfgrass systems produce high rates of nitrate, which is easily leached into waterways and can have detrimental effects to ecosystems downstream. There have been few attempts to quantify the differences between microbial production in lawns and the ecosystems they are replacing. In our experiment, 10 volumetric soil cores were collected in from 58 randomly selected suburban lawns and adjacent forested lands in the White Mountain region of New Hampshire. These samples were assayed to determine potential nitrification and mineralization rates using laboratory incubations.  We conducted vegetation surveys to accompany the soil sampling at all sites.  All property owners/tenants were interviewed about their lawn care practices.

Results/Conclusions

Mean nitrification and mineralization rates for lawns were significantly higher than for adjacent forests (p < 0.01). The mean nitrification rate from lawns was more than double the mean of forests, however, the variation among lawn sites was considerably higher than the variation found in adjacent forest patches.  The rates seen in the forested areas were not a significant predictor of what was seen in lawns adjacent to them, supporting the idea that human decision making is controlling these processes. This was due to vegetation structure within the lawns, and differences in lawn care practices.  Average C:N ratios in soil were significantly higher in forests (p > 0.0001). Further research will focus on how the land use history of these sites combined with human decision making (fertilization rates, watering, pet ownership) affect the soil carbon and nitrogen storage, and potential nitrate losses of these systems.