Is the missing nitrogen source (or sink) at Hubbard Brook statistically significant? Quantifying Uncertainty in Ecosystem Studies (QUEST)
Poster Number:
306
Presenter/Primary Author:
Ruth Yanai Nutrient budgets for forested ecosystems have rarely included error analysis, making it difficult to establish the significance of the results. At the Hubbard Brook Experimental Forest, the N budget for the period 1965 – 1977 showed more N accumulating in living biomass than was deposited from the atmosphere, and the "missing source” of 14.2 kg N ha-1 y-1 was attributed to N fixation. More recently (1992–2007), biomass accumulation has been negligible, and streamwater export of N has fallen to ~1 kg-N ha-1 y-1 despite chronically elevated atmospheric N deposition, resulting in a “missing sink” of ~6 kg N ha-1 y-1. When estimating forest biomass, researchers commonly report sampling uncertainty but rarely propagate the uncertainty in the allometric equations used to estimate tree biomass. The uncertainty in change over time in biomass N at Hubbard Brook is ± 0.6 – 4.7 kg N ha-1 yr-1, depending on the time interval. The uncertainty in change in the forest floor is -21 to +24 kg N ha-1 y-1 and in the mineral soil ± 53 kg N ha-1 y-1. Clearly, to attribute missing sources or sinks to N fixation or denitrification would require more precise estimates of change in soil stores. Changes in the net hydrologic flux of N (streamflow – precip) have much less uncertainty.
